Reflectance measurements: forward and inverse problem
The precise measurement of optical properties in the EUVregime around 91.85 eV / 13.5 nm is still a challenge for the development of optical components such as photomasks or mirrors for EUV lithography. Due to the short wavelength in this spectral regime, contamination, oxidation, or surface roughness in the nanometer range will affect the optical response of a sample much stronger than, for instance, at optical wavelengths. The most common way to determine optical constants in the Xray range is through transmission experiments, which measure the absorption of the radiation directly. Using the KramersKronig relation, the complex refractive index can be determined from a measurement of the absorption. Since the penetration depth in the EUV regime is very low, ultrathin and freestanding films are required for this type of measurements, which has an impact on the accuracy. Another major problem for transmission mode measurements is that the optical properties extracted from freestanding thin films are not always identical to thin layers within stratified systems as they are used in optical components. Measurements on samples that represent the structure of a real optical component, like thin films on a substrate, are therefore much more promising. Such can be studied by EUV reflectometry where the reflectance is measured as a function of wavelength and angle of incidence.
A model for the reflectometry experiment, taking into account the effects of interdiffusion, surface contamination and roughness, is being used. It is based on a transfer matrix approach, similar to the M¨ullermatrix approach for data analysis in spectroscopic ellipsometry in the optical regime. Given the model of the experiment and the noise distribution, our prior statistical knowledge can be updated based on the measurement via the Bayes formula. In practice, however, only a pointwise value of the density of the Bayesian posterior, up to a multiplicative constant, is accessible. Thus, an efficient algorithm is required to sample or evaluate expectations of quantities of interest with respect to the posterior distribution. Relatively high dimension (possibly hundreds of parameters for multilayered samples, just as possible concentration, nonconvexity and multimodality of the posterior are also to be taken into account.
Bayesian inverse problems via optimal transport and Wasserstein gradient flows
Standard approaches based on representing the posterior density in some fixed functional basis (Eulerian view) quickly become practically infeasible due to the curse of dimensionality . Thus, it is common to resort to methods that generate a sequence of approximations that, in the limit, is distributed as the posterior (Lagrangian view). Notable examples of such methods include Markov chain MonteCarlo (MCMC) and Langevin dynamics. Such methods can be trivially parallelized but suffer from long mixing times. Additionally, the sampling procedure has to be redone if the problem changes.
The field of optimal transport (OT) and gradient flows has been flourishing in the last 20 years and recently has been brought into attention of the machine learning community. The application in Bayesian inverse problems is still quite new. Thus, the scope of our work lies in transfering theoretical knowledge in this field into practically applicable algorithms. The overall idea is to view the space of probability measures as a metric space with respect to an optimaltransportbased Wasserstein distance. The posterior can then be approached by succesively minimizing some dissimilarity functional, such as the KL divergence. A proximal method with respect to the Wasserstein distance, also called the JordanKinderlehrerOtto (JKO) scheme, plays the role of the (implicit) gradient descent method. The dynamic reformulation of the OT problem, due to Benamou and Brenier, additionally gives a vector field that interpolates between the two measures and can be used to propagate samples, thus bringing together the Eulerian and Lagrangian perspectives. The discretization of the scheme can be tackled with Artificial Neural Networks, sparse polynomial systems, tensor networks and possibly other modern compression techniques.
One of the particular schemes that is being developed is a regularized Wasserstein proximal method. It can be interpreted as an Fischer information regularization of the aforementioned JKO scheme and considers entropic optimal transport between the iterates. The minimization problem is reformulated as a coupled system of PDEs in a new set of variables, which resemble the exponentials of the potentials from the Schrödinger bridge problem. Each of the variables encodes information on both the subsequent measure and on the interpolating vector field. The coupled system is solved as a fixedpoint problem with Anderson Acceleration method, which drastically decreases the numerical burden. The spatial discretization is tackled with a tensor train decomposition. This lowrank method allows for compressed storage of the highdimensional tensor and efficient computations of contractions and certain linear operators. Approximate lowrank reconstruction from pointwise observation is also possible by cross approximation. Solving the resulting ODE with the interpolating vector field as a righthandside gives a deterministic methods of approximate sampling without additional posterior calls. A Tensor Train model of the target density can possibly be reused in further experiments as a new prior in case when additional measurements are made (e.g. for more reflection angles) or new parameters added (e.g. optical parameteres at a new wavelength).
Publications
Monographs

D. Hömberg, G. Hu, eds., Issue on the workshop ``Electromagnetics  Modelling, Simulation, Control and Industrial Applications'', 8, no. 3 of Discrete Contin. Dyn. Syst. Ser. S, American Institute of Mathematical Sciences, Springfield, 2015, 259 pages, (Collection Published).

L.I. Goray, G. Schmidt, Chapter 12: Boundary Integral Equation Methods for Conical Diffraction and Short Waves, in: Gratings: Theory and Numerical Applications, Second Revisited Edition, E. Popov, ed., Université d'AixMarseille, Institut Fresnel, Marseille, 2014, pp. 12.112.86, (Chapter Published).

P. Deuflhard, M. Grötschel, D. Hömberg, U. Horst, J. Kramer, V. Mehrmann, K. Polthier, F. Schmidt, Ch. Schütte, M. Skutella, J. Sprekels, eds., MATHEON  Mathematics for Key Technologies, 1 of EMS Series in Industrial and Applied Mathematics, European Mathematical Society Publishing House, Zurich, 2014, 453 pages, (Collection Published).
Articles in Refereed Journals

G. Hu, A. Rathsfeld, Radiation conditions for the Helmholtz equation in a half plane filled by inhomogeneous periodic material, Journal of Differential Equations, 388 (2024), pp. 215252, DOI 10.1016/j.jde.2024.01.008 .
Abstract
In this paper we consider timeharmonic acoustic wave propagation in a halfplane filled by inhomogeneous periodic medium. If the refractive index depends on the horizontal coordinate only, we define upward and downward radiating modes by solving a onedimensional SturmLiouville eigenvalue problem with a complexvalued periodic coefficient. The upward and downward radiation conditions are introduced based on a generalized Rayleigh series. Using the variational method, we then prove uniqueness and existence for the scattering of an incoming wave mode by a grating located between an upper and lower half plane with such inhomogeneous periodic media. Finally, we discuss the application of the new radiation conditions to the scattering matrix algorithm, i.e., to rigorous coupled wave analysis or Fourier modal method. 
X. Yu, G. Hu, W. Lu, A. Rathsfeld, PML and highaccuracy boundary integral equation solver for wave scattering by a locally defected periodic surface, SIAM Journal on Numerical Analysis, 60 (2022), pp. 25922625, DOI 10.1137/21M1439705 .
Abstract
This paper studies the perfectlymatchedlayer (PML) method for wave scattering in a half space of homogeneous medium bounded by a twodimensional, perfectly conducting, and locally defected periodic surface, and develops a highaccuracy boundaryintegralequation (BIE) solver. Along the vertical direction, we place a PML to truncate the unbounded domain onto a strip and prove that the PML solution converges to the true solution in the physical subregion of the strip with an error bounded by the reciprocal PML thickness. Laterally, we divide the unbounded strip into three regions: a region containing the defect and two semiwaveguide regions, separated by two vertical line segments. In both semiwaveguides, we prove the wellposedness of an associated scattering problem so as to well define a NeumanntoDirichlet (NtD) operator on the associated vertical segment. The two NtD operators, serving as exact lateral boundary conditions, reformulate the unbounded strip problem as a boundary value problem over the defected region. Due to the periodicity of the semiwaveguides, both NtD operators turn out to be closely related to a Neumannmarching operator, governed by a nonlinear Riccati equation. It is proved that the Neumannmarching operators are contracting, so that the PML solution decays exponentially fast along both lateral directions. The consequences culminate in two opposite aspects. Negatively, the PML solution cannot converge exponentially to the true solution in the whole physical region of the strip. Positively, from a numerical perspective, the Riccati equations can now be efficiently solved by a recursive doubling procedure and a highaccuracy PMLbased BIE method so that the boundary value problem on the defected region can be solved efficiently and accurately. Numerical experiments demonstrate that the PML solution converges exponentially fast to the true solution in any compact subdomain of the strip. 
G. Hu, A. Rathsfeld, W. Lu, Timeharmonic acoustic scattering from locallyperturbed periodic curves, SIAM Journal on Applied Mathematics, 81 (2021), pp. 25692595, DOI 10.1137/19M1301679 .
Abstract
We prove wellposedness for the timeharmonic acoustic scattering of plane waves from locally perturbed periodic surfaces in two dimensions under homogeneous Dirichlet boundary conditions. This covers soundsoft acoustic as well as perfectly conducting, TE polarized electromagnetic boundary value problems. Our arguments are based on a variational method in a truncated bounded domain coupled with a boundary integral representation. If the quasiperiodic Green's function to the unperturbed periodic scattering problem is calculated efficiently, then the variational approach can be used for a numerical scheme based on coupling finite elements with a boundary element algorithm.
Even for a general 2D roughsurface problem, it turns out that the Green's function defined with the radiation condition ASR satisfies the Sommerfeld radiation condition over the half plane. Based on this result, for a local perturbation of a periodic surface, the scattered wave of an incoming plane wave is the sum of the scattered wave for the unperturbed periodic surface plus an additional scattered wave satisfying Sommerfeld's condition on the half plane. Whereas the scattered wave for the unperturbed periodic surface has a far field consisting of a finite number of propagating plane waves, the additional field contributes to the far field by a farfield pattern defined in the halfplane directions similarly to the pattern known for bounded obstacles. 
W.M. Klesse, A. Rathsfeld, C. Gross, E. Malguth, O. Skibitzki, L. Zealouk, Fast scatterometric measurement of periodic surface structures in plasmaetching processes, Measurement, 170 (2021), pp. 108721/1108721/12, DOI 10.1016/j.measurement.2020.108721 .
Abstract
To satisfy the continuous demand of ever smaller feature sizes, plasma etching technologies in microelectronics processing enable the fabrication of device structures with dimensions in the nanometer range. In a typical plasma etching system a plasma phase of a selected etching gas is activated, thereby generating highly energetic and reactive gas species which ultimately etch the substrate surface. Such dry etching processes are highly complex and require careful adjustment of many process parameters to meet the high technology requirements on the structure geometry.
In this context, realtime access of the structure's dimensions during the actual plasma process would be of great benefit by providing full dimension control and film integrity in realtime. In this paper, we evaluate the feasibility of reconstructing the etched dimensions with nanometer precision from reflectivity spectra of the etched surface, which are measured in realtime throughout the entire etch process. We develop and test a novel and fast reconstruction algorithm, using experimental reflection spectra taken about every second during the etch process of a periodic 2D model structure etched into a silicon substrate. Unfortunately, the numerical simulation of the reflectivity by Maxwell solvers is time consuming since it requires separate timeharmonic computations for each wavelength of the spectrum. To reduce the computing time, we propose that a library of spectra should be generated before the etching process. Each spectrum should correspond to a vector of geometry parameters s.t. the vector components scan the possible range of parameter values for the geometrical dimensions. We demonstrate that by replacing the numerically simulated spectra in the reconstruction algorithm by spectra interpolated from the library, it is possible to compute the geometry parameters in times less than a second. Finally, to also reduce memory size and computing time for the library, we reduce the scanning of the parameter values to a sparse grid. 
C. Brée, D. Gailevičius, V. Purlys, G.G. Werner, K. Staliunas, A. Rathsfeld, G. Schmidt, M. Radziunas, Chirped photonic crystal for spatially filtered optical feedback to a broadarea laser, Journal of Optics, 20 (2018), pp. 095804/1095804/7, DOI 10.1088/20408986/aada98 .
Abstract
We derive and analyze an efficient model for reinjection of spatially filtered optical feedback from an external resonator to a broad area, edge emitting semiconductor laser diode. Spatial filtering is achieved by a chirped photonic crystal, with variable periodicity along the optical axis and negligible resonant backscattering. The optimal chirp is obtained from a genetic algorithm, which yields solutions that are robust against perturbations. Extensive numerical simulations of the composite system with our optoelectronic solver indicate that spatially filtered reinjection enhances lowerorder transversal optical modes in the laser diode and, consequently, improves the spatial beam quality. 
T. Abbas, H. Ammari, G. Hu, A. Wahab, J.Ch. Ye, Elastic scattering coefficients and enhancement of nearly elastic cloaking, Journal of Elasticity, 128 (2017), pp. 203243, DOI 10.1007/s1065901796247 .
Abstract
The concept of scattering coefficients has played a pivotal role in a broad range of inverse scattering and imaging problems in acoustic and electromagnetic media. In view of their promising applications, we introduce the notion of scattering coefficients of an elastic inclusion in this article. First, we define elastic scattering coefficients and substantiate that they naturally appear in the expansions of elastic scattered field and far field scattering amplitudes corresponding to a plane wave incidence. Then an algorithm is developed and analyzed for extracting the elastic scattering coefficients from multistatic response measurements of the scattered field. Moreover, the estimate of the maximal resolving order is provided in terms of the signaltonoise ratio. The decay rate and symmetry of the elastic scattering coefficients are also discussed. Finally, we design scatteringcoefficientsvanishing structures and elucidate their utility for enhancement of nearly elastic cloaking. 
G. Hu, A. Kirsch, T. Yin, Factorization method in inverse interaction problems with biperiodic interfaces between acoustic and elastic waves, Inverse Problems and Imaging, 10 (2016), pp. 103129.
Abstract
Consider a timeharmonic acoustic plane wave incident onto a doubly periodic (biperiodic) surface from above. The medium above the surface is supposed to be filled with homogeneous compressible inviscid fluid with a constant mass density, whereas the region below is occupied by an isotropic and linearly elastic solid body characterized by the Lamé constants. This paper is concerned with direct (or forward) and inverse fluidsolid interaction (FSI) problems with unbounded biperiodic interfaces between acoustic and elastic waves. We present a variational approach to the forward interaction problem with Lipschitz interfaces. Existence of quasiperiodic solutions in Sobolev spaces is established at arbitrary frequency of incidence, while uniqueness is proved only for small frequencies or for all frequencies excluding a discrete set. Concerning the inverse problem, we show that the factorization method by Kirsch (1998) is applicable to the FSI problem in periodic structures. A computational criterion and a uniqueness result are justified for precisely characterizing the elastic body by utilizing the scattered acoustic near field measured in the fluid. 
Y. Guo, D. Hömberg, G. Hu, J. Li, H. Liu, A time domain sampling method for inverse acoustic scattering problems, Journal of Computational Physics, 314 (2016), pp. 647660.
Abstract
This work concerns the inverse scattering problems of imaging unknown/inaccessible scatterers by transient acoustic nearfield measurements. Based on the analysis of the migration method, we propose efficient and effective sampling schemes for imaging small and extended scatterers from knowledge of timedependent scattered data due to incident impulsive point sources. Though the inverse scattering problems are known to be nonlinear and illposed, the proposed imaging algorithms are totally “direct” involving only integral calculations on the measurement surface. Theoretical justifications are presented and numerical experiments are conducted to demonstrate the effectiveness and robustness of our methods. In particular, the proposed static imaging functionals enhance the performance of the total focusing method (TFM) and the dynamic imaging functionals show analogous behavior to the time reversal inversion but without solving timedependent wave equations. 
G. Hu, A. Rathsfeld, T. Yin, Finite element method to fluidsolid interaction problems with unbounded periodic interfaces, Numerical Methods for Partial Differential Equations. An International Journal, 32 (2016), pp. 535.
Abstract
Consider a timeharmonic acoustic plane wave incident onto a doubly periodic (biperiodic) surface from above. The medium above the surface is supposed to be filled with a homogeneous compressible inviscid fluid of constant mass density, whereas the region below is occupied by an isotropic and linearly elastic solid body characterized by its Lamé constants. This paper is concerned with a variational approach to the fluidsolid interaction problems with unbounded biperiodic Lipschitz interfaces between the domains of the acoustic and elastic waves. The existence of quasiperiodic solutions in Sobolev spaces is established at arbitrary frequency of incidence, while uniqueness is proved only for small frequencies or for all frequencies excluding a discrete set. A finite element scheme coupled with DirichlettoNeumann mappings is proposed. The DirichlettoNeumann mappings are approximated by truncated Rayleigh series expansions, and, finally, numerical tests in 2D are performed. 
G. Hu, H. Liu, Nearly cloaking the elastic wave fields, Journal de Mathématiques Pures et Appliquées, 104 (2015), pp. 10451074.
Abstract
In this work, we develop a general mathematical framework on regularized approximate cloaking of elastic waves governed by the Lamé system via the approach of transformation elastodynamics. Our study is rather comprehensive. We first provide a rigorous justification of the transformation elastodynamics. Based on the blowupapoint construction, elastic material tensors for a perfect cloak are derived and shown to possess singularities. In order to avoid the singular structure, we propose to regularize the blowupapoint construction to be the blowupasmallregion construction. However, it is shown that without incorporating a suitable lossy layer, the regularized construction would fail due to resonant inclusions. In order to defeat the failure of the lossless construction, a properly designed lossy layer is introduced into the regularized cloaking construction . We derive sharp asymptotic estimates in assessing the cloaking performance. The proposed cloaking scheme is capable of nearly cloaking an arbitrary content with a high accuracy. 
H. Gross, S. Heidenreich, M.A. Henn, M. Bär, A. Rathsfeld, Modeling aspects to improve the solution of the inverse problem in scatterometry, Discrete and Continuous Dynamical Systems  Series S, 8 (2015), pp. 497519.

G. Hu, X. Liu, F. Qu, B. Zhang, Variational approach to rough surface scattering problems with Neumann and generalized impedance boundary conditions, Communications in Mathematical Sciences, 13 (2015), pp. 511537.

G. Hu, A. Rathsfeld, Scattering of timeharmonic electromagnetic plane waves by perfectly conducting diffraction gratings, IMA Journal of Applied Mathematics, 80 (2015), pp. 508532.
Abstract
Consider scattering of timeharmonic electromagnetic plane waves by a doubly periodic surface in $R^3$. The medium above the surface is supposed to be homogeneous and isotropic with a constant dielectric coefficient, while below is a perfectly conducting material. This paper is concerned with the existence of quasiperiodic solutions for any frequency of incidence. Based on an equivalent variational formulation established by the mortar technique of Nitsche, we verify the existence of solutions for a broad class of incident waves including plane waves, under the assumption that the grating profile is a Lipschitz biperiodic surface. Our solvability result covers the resonance case where a Rayleigh frequency is allowed. Nonuniqueness examples are also presented in the resonance case and the TE or TM polarization case for classical gratings. 
TH. Arnold, A. Rathsfeld, Reflection of plane waves by rough surfaces in the sense of Born approximation, Mathematical Methods in the Applied Sciences, 37 (2014), pp. 20912111.
Abstract
The topic of the present paper is the reflection of electromagnetic plane waves by rough surfaces, i.e., by smooth and bounded perturbations of planar faces. Moreover, the contrast between the cover material and the substrate beneath the rough surface is supposed to be low. In this case, a modification of Stearns' formula based on Born approximation and Fourier techniques is derived for a special class of surfaces. This class contains the graphs of functions if the interface function is a radially modulated almost periodic function. For the Born formula to converge, a sufficient and almost necessary condition is given. A further technical condition is defined, which guarantees the existence of the corresponding far field of the Born approximation. This far field contains plane waves, farfield terms like those for bounded scatterers, and, additionally, a new type of terms. The derived formulas can be used for the fast numerical computations of far fields and for the statistics of random rough surfaces. 
G. Hu, A. Rathsfeld, Convergence analysis of the FEM coupled with Fouriermode expansion for the electromagnetic scattering by biperiodic structures, Electronic Transactions on Numerical Analysis, 41 (2014), pp. 350375.

G. Schmidt, Conical diffraction by multilayer gratings: A recursive integral equations approach, Applications of Mathematics, 58 (2013), pp. 279307.
Abstract
In this paper we consider an integral equation algorithm to study the scattering of plane waves by multilayer diffraction gratings under oblique incidence. The scattering problem is described by a system of Helmholtz equations with piecewise constant coefficients in $R^2$ coupled by special transmission conditions at the interfaces between different layers. Boundary integral methods lead to a system of singular integral equations, containing at least two equations for each interface. To deal with an arbitrary number of material layers we present the extension of a recursive procedure developed by Maystre for normal incidence, which transforms the problem to a sequence of equations with $2 times 2$ operator matrices on each interface. Necessary and sufficient conditions for the applicability of the algorithm are derived. 
G. Hu, F. Qu, B. Zhang, A linear sampling method for inverse problems of diffraction gratings of mixed type, Mathematical Methods in the Applied Sciences, 35 (2012), pp. 10471066.

G. Hu, Inverse wave scattering by unbounded obstacles: Uniqueness for the twodimensional Helmholtz equation, Applicable Analysis. An International Journal, 91 (2012), pp. 703717.
Abstract
In this paper we present some uniqueness results on inverse wave scattering by unbounded obstacles for the twodimensional Helmholtz equation. We prove that an impenetrable onedimensional rough surface can be uniquely determined by the values of the scattered field taken on a line segment above the surface that correspond to the incident waves generated by a countable number of point sources. For penetrable rough layers in a piecewise constant medium, the refractive indices together with the rough interfaces (on which the TM transmission conditions are imposed) can be uniquely identified using the same measurements and the same incident point source waves. Moreover, a Dirichlet polygonal rough surface can be uniquely determined by a single incident point source wave provided a certain condition is imposed on it. 
L. Goray, G. Schmidt, Analysis of twodimensional photonic band gaps of any rod shape and conductivity using a conicalintegralequation method, Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, 85 (2012), pp. 036701/1036701/12.
Abstract
The conical boundary integral equation method has been proposed to calculate the sensitive optical response of 2D photonic band gaps (PBGs), including dielectric, absorbing, and highconductive rods of various shapes working in any wavelength range. It is possible to determine the diffracted field by computing the scattering matrices separately for any grating boundary profile. The computation of the matrices is based on the solution of a 2 x 2 system of singular integral equations at each interface between two different materials. The advantage of our integral formulation is that the discretization of the integral equations system and the factorization of the discrete matrices, which takes the major computing time, are carried out only once for a boundary. It turned out that a small number of collocation points per boundary combined with a high convergence rate can provide adequate description of the dependence on diffracted energy of very different PBGs illuminated at arbitrary incident and polarization angles. The numerical results presented describe the significant impact of rod shape on diffraction in PBGs supporting polaritonplasmon excitation, particularly in the vicinity of resonances and at high filling ratios. The diffracted energy response calculated vs. array cell geometry parameters was found to vary from a few percent up to a few hundred percent. The influence of other types of anomalies (i.e. waveguide anomalies, cavity modes, FabryPerot and Bragg resonances, Rayleigh orders, etc), conductivity, and polarization states on the optical response has been demonstrated. 
H. Gross, M.A. Henn, S. Heidenreich, A. Rathsfeld, M. Bär, Modeling of line roughness and its impact on the diffraction intensities and the reconstructed critical dimensions in scatterometry, Applied Optics, 51 (2012), pp. 73847394.
Abstract
We investigate the impact of line edge and line width roughness (LER, LWR) on the measured diffraction intensities in angular resolved extreme ultraviolet (EUV) scatterometry for a periodic linespace structure designed for EUV lithography. LER and LWR with typical amplitudes of a few nanometers were previously neglected in the course of the profile reconstruction. The 2D rigorous numerical simulations of the diffraction process for periodic structures are carried out with the finite element method (FEM) providing a numerical solution of the twodimensional Helmholtz equation. To model roughness, multiple calculations are performed for domains with large periods, containing many pairs of line and space with stochastically chosen line and space widths. A systematic decrease of the mean efficiencies for higher diffraction orders along with increasing variances is observed and established for different degrees of roughness. In particular, we obtain simple analytical expressions for the bias in the mean efficiencies and the additional uncertainty contribution stemming from the presence of LER and/or LWR. As a consequence this bias can easily be included into the reconstruction model to provide accurate values for the evaluated profile parameters. We resolve the sensitivity of the reconstruction from this bias by using the LER/LWR perturbed efficiency datasets for multiple reconstructions. If the scattering efficiencies are biascorrected, significant improvements are found in the reconstructed bottom and top widths toward the nominal values. 
M.A. Henn, S. Heidenreich, H. Gross, A. Rathsfeld, F. Scholze, M. Bär, Improved grating reconstruction by determination of line roughness in extreme ultraviolet scatterometry, Optics Letters, 37 (2012), pp. 52295231.

C. Pfüller, M. Ramsteiner, O. Brandt, F. Grosse, A. Rathsfeld, G. Schmidt, L. Geelhaar, H. Riechert, Raman spectroscopy as a probe for the coupling of light into ensembles of subwavelengthsized nanowires, Applied Physics Letters, 101 (2012), pp. 083104/1083104/4.

G. Hu, B. Zhang, The linear sampling method for the inverse electromagnetic scattering by a partially coated biperiodic structure, Mathematical Methods in the Applied Sciences, 34 (2011), pp. 509519.

F. Lanzara, V. Maz'ya, G. Schmidt, On the fast computation of high dimensional volume potentials, Mathematics of Computation, 80 (2011), pp. 887904.

F. Lanzara, V.G. Maz'ya, G. Schmidt, Accurate cubature of volume potentials over highdimensional halfspaces, Journal of Mathematical Sciences (New York), 173 (2011), pp. 683700.

J. Elschner, G. Hu, Inverse scattering of elastic waves by periodic structures: Uniqueness under the third or fourth kind boundary conditions, Methods and Applications of Analysis, 18 (2011), pp. 215244.
Abstract
The inverse scattering of a timeharmonic elastic wave by a twodimensional periodic structure in $R^2$ is investigated. The grating profile is assumed to be a graph given by a piecewise linear function on which the third or fourth kind boundary conditions are satisfied. Via an equivalent variational formulation, existence of quasiperiodic solutions for general Lipschitz grating profiles is proved by applying the Fredholm alternative. However, uniqueness of solution to the direct problem does not hold in general. For the inverse problem, we determine and classify all the unidentifiable grating profiles corresponding to a given incident elastic field, relying on the reflection principle for the Navier equation and the rotational invariance of propagating directions of the total field. Moreover, global uniqueness for the inverse problem is established with a minimal number of incident pressure or shear waves, including the resonance case where a Rayleigh frequency is allowed. The gratings that are unidentifiable by one incident elastic wave provide nonuniqueness examples for appropriately chosen wave number and incident angles. 
J. Elschner, G. Hu, Uniqueness in inverse scattering of elastic waves by threedimensional polyhedral diffraction gratings, Journal of Inverse and IllPosed Problems, 19 (2011), pp. 717768.
Abstract
We consider the inverse elastic scattering problem of determining a threedimensional diffraction grating profile from scattered waves measured above the structure. In general, a grating profile cannot be uniquely determined by a single incoming plane wave. We completely characterize and classify the biperiodic polyhedral structures under the boundary conditions of the third and fourth kinds that cannot be uniquely recovered by only one incident plane wave. Thus we have global uniqueness for a polyhedral grating profile by one incident elastic plane wave if and only if the profile belongs to neither of the unidentifiable classes, which can be explicitly described depending on the incident field and the type of boundary conditions. Our approach is based on the reflection principle for the Navier equation and the reflectional and rotational invariance of the total field. 
J. Elschner, G. Hu, Uniqueness in inverse transmission scattering problems for multilayered obstacles, Inverse Problems and Imaging, 5 (2011), pp. 793813.
Abstract
Assume a timeharmonic electromagnetic wave is scattered by an infinitely long cylindrical conductor surrounded by an unknown piecewise homogenous medium remaining invariant along the cylinder axis. We prove that, in TM mode, the far field patterns for all observation directions at a fixed frequency uniquely determine the unknown surrounding medium as well as the shape of the cylindrical conductor. A similar uniqueness result is obtained for the scattering by multilayered penetrable periodic structures in a piecewise homogenous medium. The periodic interfaces and refractive indices can be uniquely identified from the near field data measured only above (or below) the structure for all quasiperiodic incident waves with a fixed phaseshift. The proofs are based on the singularity of the Green function to a two dimensional elliptic equation with piecewise constant leading coefficients. 
G. Schmidt, B.H. Kleemann, Integral equation methods from grating theory to photonics: An overview and new approaches for conical diffraction, Journal of Modern Optics, 58 (2011), pp. 407423.

G. Schmidt, Integral equations for conical diffraction by coated gratings, Journal of Integral Equations and Applications, 23 (2011), pp. 71112.
Abstract
The paper is devoted to integral formulations for the scattering of plane waves by diffraction gratings under oblique incidence. For the case of coated gratings Maxwell's equations can be reduced to a system of four singular integral equations on the piecewise smooth interfaces between different materials. We study analytic properties of the integral operators for periodic diffraction problems and obtain existence and uniqueness results for solutions of the systems corresponding to electromagnetic fields with locally finite energy. 
G. Hu, F. Qu, B. Zhang, Direct and inverse problems for electromagnetic scattering by a doubly periodic structure with a partially coated dielectric, Mathematical Methods in the Applied Sciences, 33 (2010), pp. 147156.

S. ChandlerWilde, J. Elschner, Variational approach in weighted Sobolev spaces to scattering by unbounded rough surface, SIAM Journal on Mathematical Analysis, 42 (2010), pp. 25542580.

L.I. Goray, G. Schmidt, Solving conical diffraction grating problems with integral equations, Journal of the Optical Society of America A. Optics, Image Science, and Vision, 27 (2010), pp. 585597.
Abstract
Offplane scattering of timeharmonic plane waves by a diffraction grating with arbitrary conductivity and general border profile is considered in a rigorous electromagnetic formulation. The integral equations for conical diffraction were obtained using the boundary integrals of the single and double layer potentials including the tangential derivative of single layer potentials interpreted as singular integrals. We derive an important formula for the calculation of the absorption in conical diffraction. Some rules which are expedient for the numerical implementation of the theory are presented. The efficiencies and polarization angles compared with those obtained by Lifeng Li for transmission and reflection gratings are in a good agreement. The code developed and tested is found to be accurate and efficient for solving offplane diffraction problems including highconductive surfaces, borders with edges, real border profiles, and gratings working at short wavelengths. 
H. Gross, J. Richter, A. Rathsfeld, M. Bär, Investigations on a robust profile model for the reconstruction of 2D periodic absorber lines in scatterometry, Journal of the European Optical Society  Rapid Publications, 5 (2010), pp. 10053/110053/7.

J. Elschner, G. Hu, Global uniqueness in determining polygonal periodic structures with a minimal number of incident plane waves, Inverse Problems. An International Journal on the Theory and Practice of Inverse Problems, Inverse Methods and Computerized Inversion of Data, 26 (2010), pp. 115002/1115002/23.
Abstract
In this paper, we investigate the inverse problem of recovering a twodimensional perfectly reflecting diffraction grating from the scattered waves measured above the structure. Inspired by a novel idea developed by Bao, Zhang and Zou [to appear in Trans. Amer. Math. Soc.], we present a complete characterization of the global uniqueness in determining polygonal periodic structures using a minimal number of incident plane waves. The idea in this paper combines the reflection principle for the Helmholtz equation and the dihedral group theory. We characterize all periodic polygonal structures that cannot be identified by one incident plane wave, including the resonance case where a Rayleigh frequency is allowed. Furthermore, we show that those unidentifiable gratings provide nonuniqueness examples for appropriately chosen wave number and incident angles. We also indicate and fix a gap in the proof of the main theorem of Elschner and Yamamoto [Z. Anal. Anwend., 26 (2007), 165177], and generalize the uniqueness results of that paper. 
H. Gross, A. Rathsfeld, F. Scholze, M. Bär, Profile reconstruction in extreme ultraviolet (EUV) scatterometry: Modeling and uncertainty estimates, Measurement Science and Technology, 20 (2009), pp. 105102/1105102/11.
Abstract
Scatterometry as a nonimaging indirect optical method in wafer metrology is also relevant to lithography masks designed for Extreme Ultraviolet Lithography, where light with wavelengths in the range of 13 nm is applied. The solution of the inverse problem, i.e. the determination of periodic surface structures regarding critical dimensions (CD) and other profile properties from light diffraction patterns, is incomplete without knowledge of the uncertainties associated with the reconstructed parameters. With decreasing feature sizes of lithography masks, increasing demands on metrology techniques and their uncertainties arise. The numerical simulation of the diffraction process for periodic 2D structures can be realized by the finite element solution of the twodimensional Helmholtz equation. For typical EUV masks the ratio period over wave length is so large, that a generalized finite element method has to be used to ensure reliable results with reasonable computational costs. The inverse problem can be formulated as a nonlinear operator equation in Euclidean spaces. The operator maps the sought mask parameters to the efficiencies of diffracted plane wave modes. We employ a GaußNewton type iterative method to solve this operator equation and end up minimizing the deviation of the measured efficiency or phase shift values from the calculated ones. We apply our reconstruction algorithm for the measurement of a typical EUV mask composed of TaN absorber lines of about 80 nm height, a period of 420 nm resp. 720 nm, and with an underlying MoSimultilayer stack of 300 nm thickness. Clearly, the uncertainties of the reconstructed geometric parameters essentially depend on the uncertainties of the input data and can be estimated by various methods. We apply a Monte Carlo procedure and an approximative covariance method to evaluate the reconstruction algorithm. Finally, we analyze the influence of uncertainties in the widths of the multilayer stack by the Monte Carlo method. 
H. Gross, A. Rathsfeld, Sensitivity analysis for indirect measurement in scatterometry and the reconstruction of periodic grating structures, Waves in Random and Complex Media. Propagation, Scattering and Imaging, 18 (2008), pp. 129149.

J. Elschner, M. Yamamoto, Uniqueness in determining polygonal periodic structures, Zeitschrift fur Analysis und ihre Anwendungen. Journal for Analysis and its Applications, 26 (2007), pp. 165177.

H. Gross, R. Model, M. Bär, M. Wurm, B. Bodermann, A. Rathsfeld, Mathematical modelling of indirect measurements in scatterometry, Measurement, 39 (2006), pp. 782794.

A. Rathsfeld, G. Schmidt, B.H. Kleemann, On a fast integral equation method for diffraction gratings, Communications in Computational Physics, 1 (2006), pp. 9841009.

G. Bruckner, J. Elschner, The numerical solution of an inverse periodic transmission problem, Mathematical Methods in the Applied Sciences, 28 (2005), pp. 757778.

J. Elschner, M. Yamamoto, Uniqueness results for an inverse periodic transmission problem, Inverse Problems. An International Journal on the Theory and Practice of Inverse Problems, Inverse Methods and Computerized Inversion of Data, 20 (2004), pp. 18411852.

G. Bao, K. Huang, G. Schmidt, Optimal design of nonlinear diffraction gratings, Journal of Computational Physics, 184 (2003), pp. 106121.

G. Bruckner, J. Elschner, A twostep algorithm for the reconstruction of perfectly reflecting periodic profiles, Inverse Problems. An International Journal on the Theory and Practice of Inverse Problems, Inverse Methods and Computerized Inversion of Data, 19 (2003), pp. 315329.

J. Elschner, G.C. Hsiao, A. Rathsfeld, Grating profile reconstruction based on finite elements and optimization techniques, SIAM Journal on Applied Mathematics, 64 (2003), pp. 525545.

J. Elschner, G. Schmidt, M. Yamamoto, An inverse problem in periodic diffractive optics: Global uniqueness with a single wavenumber, Inverse Problems. An International Journal on the Theory and Practice of Inverse Problems, Inverse Methods and Computerized Inversion of Data, 19 (2003), pp. 779787.

J. Elschner, G. Schmidt, M. Yamamoto, Global uniqueness in determining rectangular periodic structures by scattering data with a single wave number, Journal of Inverse and IllPosed Problems, 11 (2003), pp. 235244.

J. Elschner, G. Schmidt, Conical diffraction by periodic structures: Variation of interfaces and gradient formulas, Mathematische Nachrichten, 252 (2003), pp. 2442.
Contributions to Collected Editions

C. Brée, V. Raab, D. Gailevičius, V. Purlys, J. Montiel, G.G. Werner, K. Staliunas, A. Rathsfeld, U. Bandelow, M. Radziunas, Genetically optimized photonic crystal for spatial filtering of reinjection into broadarea diode lasers, in: 2019 Conference on Lasers and ElectroOptics Europe and European Quantum Electronics Conference, OSA Technical Digest, IEEE, Piscataway, 2019, pp. 11, DOI 10.1109/CLEOEQEC.2019.8871622 .
Abstract
Modern highpower broadarea semiconductor laser diodes (BASLDs) deliver optical output powers of several ten Watts at high electrooptical conversion efficiencies, which makes them highly relevant for numerous industrial, medical and scientific applications. However, lateral multimode behavior in BASLDs due to thermal lensing turns out highly detrimental, as it results in poor focusability and decreased laser beam brightnesss. Approaches to overcome this issue include improved epitaxial layer design, the optimization of evanescent spatial filtering by tailoring the emitter geometry and facet reflectivity, or Fourier spatially filtered reinjection from an external resonator [1]. 
G. Schmidt, Boundary integral methods for periodic scattering problems, in: Around the Research of Vladimir Maz'ya II. Partial Differential Equations, A. Laptev, ed., 12 of International Mathematical Series, Springer Science+Business Media, New York [et al.], 2010, pp. 337363.

H. Gross, F. Scholze, A. Rathsfeld, M. Bär, Evaluation of measurement uncertainties in EUV scatterometry, in: Modeling Aspects in Optical Metrology II, H. Bosse, B. Bodermann, R.M. Silver, eds., 7390 of Proceedings of SPIE, SPIE, 2009, pp. 7390OT/17390OT/11.

H. Gross, A. Rathsfeld, M. Bär, Modelling and uncertainty estimates for numerically reconstructed profiles in scatterometry, in: Advanced Mathematical and Computational Tools in Metrology and Testing VIII, F. Pavese, M. Bär, A.B. Forbes, J.M. Linares, C. Perruchet, N.F. Zhang, eds., 78 of Series on Advances in Mathematics for Applied Sciences, World Scientific, Singapore, 2009, pp. 142147.

M.A. Henn, R. Model, M. Bär, M. Wurm, B. Bodermann, A. Rathsfeld, H. Gross, On numerical reconstructions of lithographic masks in DUV scatterometry, in: Modeling Aspects in Optical Metrology II, H. Bosse, B. Bodermann, R.M. Silver, eds., 7390 of Proceedings of SPIE, SPIE, 2009, pp. 7390OQ/17390OQ/11.

H. Gross, R. Model, A. Rathsfeld, F. Scholze, M. Wurm, B. Bodermann, M. Bär, Modellbildung, Bestimmung der Messunsicherheit und Validierung für diskrete inverse Probleme am Beispiel der Scatterometrie, in: Sensoren und Messsysteme, 14. Fachtagung Ludwigsburg, 11./12. März 2008, 2011 of VDIBerichte, VDI, 2008, pp. 337346.

R. Model, A. Rathsfeld, H. Gross, M. Wurm, B. Bodermann, A scatterometry inverse problem in optical mask metrology, in: 6th International Conference on Inverse Problems in Engineering: Theory and Practice, 1519 June 2008, Dourdan (Paris), France, 135 of J. Phys.: Conf. Ser., Inst. Phys., 2008, pp. 012071/1012071/8.

H. Gross, A. Rathsfeld, F. Scholze, M. Bär, U. Dersch, Optimal sets of measurement data for profile reconstruction in scatterometry, in: Modeling Aspects in Optical Metrology, H. Bosse, B. Bodermann, R.M. Silver, eds., 6617 of Proceedings of SPIE, 2007, pp. 66171B/166171B/12.

M. Wurm, B. Bodermann, F. Scholze, Ch. Laubis, H. Gross, A. Rathsfeld, Untersuchung zur Eignung der EUVScatterometrie zur quantitativen Charakterisierung periodischer Strukturen auf Photolithographiemasken, in: Proc. of the 107th Meeting of DGaO (German Branch of the European Optical Society), June 610, 2006, in Weingarten, DGaOProceedings, 2006, pp. P74/1P74/2.

P. DE Bisschop, A. Erdmann, A. Rathsfeld, Simulation of the effect of a resistsurface bound air bubble on imaging in immersion lithography, in: Optical Microlithography XVIII, B.W. Smith, ed., 5754 of Proceedings of SPIE, 2005, pp. 243253.

G. Bruckner, J. Elschner, M. Yamamoto, An optimization method for the grating profile reconstruction, Proceedings 3rd ISAAC Congress, Berlin, August 20  25, 2001, H.G.W. Begehr, R.P. Gilbert, M.W. Wong, eds., II of Progress in Analysis, World Scientific, New Jersey [u.a.], 2003, pp. 13911404.

J. Elschner, R. Hinder, G. Schmidt, Direct and inverse problems for diffractive structures  Optimization of binary gratings, in: Mathematics  Key Technology for the Future. Joint Projects Between Universities and Industry, W. Jäger, H.J. Krebs, eds., Springer, Berlin [u.a.], 2003, pp. 293304.

G. Schmidt, Electromagnetic scattering by periodic structures (in Russian), Proceedings of the International Conference on Differential and Functional Differential Equations, Moscow, Russian Federation, August 11  17, 2002, 3 of Sovrem. Probl. Mat. Fund. Naprav., 2003, pp. 113128.
Preprints, Reports, Technical Reports

G. Schmidt, BenutzerHandbuch Dipog1.4, Technical Report no. 8, WIAS, Berlin, 2004, DOI 10.20347/WIAS.TECHREPORT.8 .
Abstract
This is the manual of the software package DIPOG, version 1.4, which can be used to simulate and optimize binary and multilevel optical gratings. The algorithms are based on the finiteelement solution of a system of Helmholtz equations, which are equivalent to the timeharmonic electromagnetic field equations, and on gradient methods for solving optimization problems. The package offers several options to postprocess the calculated electromagnetic fields. 
A. Rathsfeld, DIPOG2.0. User Guide. Direct Problems for Optical Gratings over Triangular Grids, Technical Report no. 7, WIAS, Berlin, 2004, DOI 10.20347/WIAS.TECHREPORT.7 .
Abstract
This is the description of how to use the programs FEM, GFEM, FEM_CHECK, GFEM_CHECK, FEM_PLOT, GFEM_PLOT, FEM_FULLINFO, and GFEM_FULLINFO of the package DIPOG2.0. The package is a collection of finite element (FEM) programs to determine the efficiencies of the diffraction of light by a periodic grating structure. It is based on the software package PDELIB and solves the classical case of TE and TM polarization and the case of conical diffraction. The code provides a conventional FEM and a generalized FEM (GFEM). The latter is the variational approach of the conventional FEM combined with a new trial space. We note that the DIPOG2.0 programs require the installation of the previous version DIPOG1.3 or DIPOG1.4, of the grid generator TRIANGLE1.4, and of the equations solver PARDISO. Additionally, some of them need the graphical package openGL (or the MESA emulation of openGL) together with GLTOOLS2.4 or, alternatively, the package GNUPLOT. Examples of data and output files are enclosed. 
A. Rathsfeld, Convergence of the method of rigorous coupledwave analysis for the diffraction by twodimensional periodic surface structures, Preprint no. 3081, WIAS, Berlin, 2023, DOI 10.20347/WIAS.PREPRINT.3081 .
Abstract, PDF (424 kByte)
The scattering matrix algorithm is a popular numerical method to simulate the diffraction of optical waves by periodic surfaces. The computational domain is divided into horizontal slices and, by a domain decomposition method coupling neighbour slices over the common interface via scattering data, a clever recursion is set up to compute an approximate operator, mapping incoming waves into outgoing. Combining this scattering matrix algorithm with numerical schemes inside the slices, methods like rigorous coupled wave analysis and Fourier modal methods were designed. The key for the analysis is the scattering problem over the slices. These are scattering problems with a radiation condition generalized for inhomogeneous cover and substrate materials and were first analyzed in [7]. In contrast to [7], where the scattering matrix algorithm for transverse electric polarization was treated without full discretization (no approximation by truncated Fourier series), we discuss the more challenging case of transverse magnetic polarization and look at the convergence of the fullydiscretized scheme, i.e., at the rigorous coupled wave analysis for a fixed slicing into layers with vertically invariant optical index. 
A. Rathsfeld, On a halfspace radiation condition, Preprint no. 2669, WIAS, Berlin, 2019, DOI 10.20347/WIAS.PREPRINT.2669 .
Abstract, PDF (430 kByte)
For the Dirichlet problem of the Helmholtz equation over the half space or rough surfaces, a radiation condition is needed to guarantee a unique solution, which is physically meaningful. If the Dirichlet data is a general bounded continuous function, then the wellestablished Sommerfeld radiation condition, the angular spectrum representation, and the upward propagating radiation condition do not apply or require restrictions on the data, in order to define the involved integrals. In this paper a new condition based on a representation of the second derivative of the solution is proposed. The twice differentiable halfspace Green's function is integrable and the corresponding radiation condition applies to general bounded functions. The condition is checked for special functions like plane waves and point source solution. Moreover, the Dirichlet problem for the half plane is discussed. Note that such a "continuous" radiation condition is helpful e.g. if finite sections of the roughsurface problem are analyzed. 
G. Schmidt, Scattering of general incident beams by diffraction gratings, Preprint no. 2355, WIAS, Berlin, 2016, DOI 10.20347/WIAS.PREPRINT.2355 .
Abstract, PDF (1036 kByte)
The paper is devoted to the electromagnetic scattering of arbitrary timeharmonic fields by periodic structures. The FloquetFourier transform converts the full space Maxwell problem to a twoparameter family of diffraction problems with quasiperiodic incidence waves, for which conventional grating methods become applicable. The inverse transform is given by integrating with respect to the parameters over a infinite strip in ℝ². For the computation of the scattered fields we propose an algorithm, which extends known adaptive methods for the approximate calculation of multiple integrals. The novel adaptive approach provides autonomously the expansion of the incident field into quasiperiodic waves in order to approximate the scattered fields within a prescribed error tolerance. Some application examples are numerically examined.
Talks, Poster

A. Rathsfeld, Analysis of scattering matrix algorithm for diffraction by periodic structures (online talk), University of Tokyo, Graduate School of Mathematical Sciences, Japan, March 13, 2024.

V. Aksenov, Simulation of Wasserstein gradient flows with lowrank tensor methods for sampling, 10th International Congress on Industrial and Applied Mathematics (ICIAM 2023), Minisymposium 00837 ``Particle Methods for Bayesian Inference'', August 20  25, 2023, Waseda University, Tokyo, Japan, August 24, 2023.

A. Rathsfeld, Analysis of scattering matrix algorithm, 10th International Congress on Industrial and Applied Mathematics (ICIAM 2023), Minisymposium 00297 ``Wave Scattering Problems: Numerical Methods with Applications'', August 20  25, 2023, Waseda University, Tokyo, Japan, August 22, 2023.

A. Rathsfeld, Analysis of scattering matrix algorithm for diffraction by periodic surface structures, Chinese Academy of Sciences, Institute of Computational Mathematics and Scientific/Engineering Computing, Beijing, China, September 4, 2023.

A. Rathsfeld, Planewave scattering by biperiodic gratings and rough surfaces: Radiation condition and farfield model, Nankai University, Department of Scientific and Engineering Computing, Tianjin, China, September 7, 2023.

A. Rathsfeld, Planewave scattering by biperiodic gratings and rough surfaces: Radiation condition and farfield model (online talk), University of Tokyo, Graduate School of Mathematical Sciences, Japan, October 26, 2023.

A. Rathsfeld, Simulation and inverse problems for rough surfaces by numerical methods of periodic grating structures (online talk), University of Tokyo, Graduate School of Mathematical Sciences, Japan, November 29, 2023.

C. Brée, V. Raab, D. Gailevičius, V. Purlys, J. Montiel, G.G. Werner, K. Staliunas, A. Rathsfeld, U. Bandelow, M. Radziunas, Genetically optimized photonic crystal for spatial filtering of reinjection into broadarea diode lasers, CLEO/EuropeEQEC 2019, Munich, June 23  27, 2019.

G. Hu, Direct and inverse problems in elastodynamics, Workshop ``Theory, Numerics and Application of Partial Differential Equations'', December 10, 2016, Chinese Academy of Sciences, Beijing, December 10, 2016.

G. Hu, Inverse medium scattering problems, Workshop on Inverse Problems and their Applications, November 18  20, 2016, Southeast University, Nanjing, China, November 20, 2016.

D.R.M. Renger, Functions of bounded variation with an infinitedimensional codomain, Meeting in Applied Mathematics and Calculus of Variations, September 13  16, 2016, Università di Roma ``La Sapienza'', Dipartimento di Matematica ``Guido Castelnuovo'', Italy, September 16, 2016.

G. Schmidt, Scattering of general incident beams by diffraction gratings, European Optical Society Annual Meeting (EOSAM) 2016, September 26  30, 2016, Berlin, September 28, 2016.

G. Hu, Mathematical problems in timeharmonic wave scattering, Shandong Normal University, Department of Mathematics, Jinan, China, November 11, 2014.

G. Hu, Mathematical problems in timeharmonic wave scattering from bounded and unbounded obstacles, South University of Science and Technology of China (SUSTC), Department of Financial Mathematics and Financial Engineering, Shenzhen, November 3, 2014.

J. Elschner, Direct and inverse problems for diffraction gratings, Symposium ``Light Scattering: Simulation and Inversion'', May 27  28, 2013, Bremen, May 28, 2013.

G. Schmidt, On an integral equation formulation for scattering by biperiodic structures, Analysis of Partial Differential Equations, A Symposium in honour of Prof. Vladimir Maz'ya on the occasion of his 75th birthday, December 16  17, 2013, University of Liverpool, UK, December 17, 2013.

G. Hu, Direct and inverse scattering of elastic waves by diffraction gratings, 6th International Conference ``Inverse Problems, Control and Shape Optimization'' (PICOF '12), April 2  4, 2012, Palaiseau, France, April 4, 2012.

G. Hu, Direct and inverse scattering of elastic waves by diffraction gratings, Workshop 3 ``Wave Propagation and Scattering, Inverse Problems and Applications in Energy and the Environment'', November 21  25, 2011, Johann Radon Institute for Computational and Applied Mathematics (RICAM), Linz, Austria, November 24, 2011.

N. Kleemann, Shape derivatives for conical diffraction by nonsmooth interfaces, Technische Universität Berlin, Institut für Mathematik, January 6, 2011.

N. Kleemann, Shape derivatives for conical diffraction by nonsmooth interfaces, FriedrichSchillerUniversität Jena, Mathematisches Institut, February 11, 2011.

A. Rathsfeld, On Born approximation for the scattering by rough surfaces, 262. PTB Seminar, EUV Metrology, October 27  28, 2011, PhysikalischTechnische Bundesanstalt, Berlin, October 28, 2011.

G. Schmidt, Fast computation of highdimensional volume potentials, Trilateral Workshop on Separation of Variables and Applications (SVA), September 8  10, 2010, La CollesurLoup, France, September 10, 2010.

G. Schmidt, Integral methods for conical diffraction by multiprofile gratings, Annual International Conference ``Days on Diffraction 2010'', June 8  11, 2010, St. Petersburg, Russian Federation, June 9, 2010.

G. Schmidt, On the computation of volume potentials over highdimensional rectangular domains, The First Workshop on Approximate Approximations and Their Applications, December 14  15, 2010, University of Liverpool, UK, December 15, 2010.

A. Rathsfeld, Modelling and algorithms for simulation and reconstruction in scatterometry, Workshop on Scatterometry and Ellipsometry on Structured Surfaces, March 18  19, 2009, PhysikalischTechnische Bundesanstalt, Department ``Imaging and Wave Optics'', Braunschweig, March 18, 2009.

A. Rathsfeld, Numerical aspects of the scatterometric measurement of periodic surface structures, Conference on Applied Inverse Problems 2009, July 20  24, 2009, University of Vienna, Austria, July 21, 2009.

G. Schmidt, Existence and uniqueness of solution for a system of Helmholtz equations, International Conference on Elliptic and Parabolic Equations, November 30  December 4, 2009, WIAS, December 3, 2009.

A. Rathsfeld, Scatterometry: Inverse problems and optimization of measurements, University of Tokyo, Department of Mathematical Sciences, Japan, March 6, 2008.

G. Schmidt, Integral equations for conical diffraction by coated gratings, Annual International Conference ``Days on Diffraction'', June 3  6, 2008, St. Petersburg, Russian Federation, June 3, 2008.

J. Elschner, On uniqueness in inverse scattering by obstacles and diffraction gratings, Conference ``Boundary Elements  Theory and Applications'' (Beta 2007), May 22  24, 2007, Leibniz Universität Hannover, May 22, 2007.

J. Elschner, On uniqueness in inverse scattering with finitely many incident waves, Workshop ``Inverse Problems in Wave Scattering'', March 5  9, 2007, Mathematisches Forschungsinstitut Oberwolfach, March 6, 2007.

A. Rathsfeld, Sensitivity analysis for indirect measurement in scatterometry and the reconstruction of periodic grating structures, 6th International Congress on Industrial and Applied Mathematics (ICIAM 2007), July 16  20, 2007, ETH Zürich, Switzerland, July 20, 2007.

J. Elschner, Inverse problems for diffraction gratings, Waves Meeting, September 21  23, 2006, University of Reading, UK, September 22, 2006.

J. Elschner, Variational approach to scattering by unbounded surfaces, 12th Conference on Mathematics of Finite Elements and Applications (MAFELAP 2006), June 13  16, 2006, Brunel University, Uxbridge, UK, June 15, 2006.

J. Elschner, Variational approach to scattering by unbounded surfaces, Autumn School ``Analysis of Maxwell's Equations'' (Research Training Group GRK 1294 ``Analysis, Simulation and Design of Nanotechnological Processes''), October 17  19, 2006, Universität Karlsruhe, October 18, 2006.

A. Rathsfeld, Inverses Problem, Sensitivitätsanalyse, optimierte Messstrategie, BMBFProjekttreffen ABBILD, PhysikalischTechnische Bundesanstalt, Berlin, November 13, 2006.

A. Rathsfeld, Sensitivity analysis for scatterometry and reconstruction of periodic grating structures, PhysikalischTechnische Bundesanstalt, Berlin, October 26, 2006.

J. Elschner, Inverse Probleme für optische Gitter, PhysikalischTechnische Bundesanstalt, Berlin, April 13, 2005.

J. Elschner, Inverse problems for diffraction gratings, Inverse Scattering Workshop, University of North Carolina, Charlotte, USA, June 3, 2005.

J. Elschner, Inverse problems for diffraction gratings, University of Delaware, Department of Mathematics, Newark, USA, June 9, 2005.

A. Rathsfeld, Finite elements for the rigorous simulation of timeharmonic waves, 3rd IISB Lithography Simulation Workshop, September 16  18, 2005, Pommersfelden, September 16, 2005.

A. Rathsfeld, Integralgleichungsmethode für optische Gitter  Weiterentwicklung der IESMP, Kickoff Meeting of BMBF Project ``NAOMI'', Carl Zeiss AG, Jena, May 31, 2005.

A. Rathsfeld, Local optimization of polygonal gratings for classical and conical diffraction, Conference ``Diffractive Optics 2005'', Warsaw, Poland, September 3  7, 2005.

A. Rathsfeld, Local optimization of polygonal gratings for classical and conical diffraction, WIAS Workshop ``New Trends in Simulation and Control of PDEs'', September 26  28, 2005, Berlin, September 26, 2005.

A. Rathsfeld, Optimierung von optischen Gittern mit tt DiPoG2.1, 2nd Meeting ``Inverses Problem in der Scatterometrie'', PhysikalischTechnische Bundesanstalt, Braunschweig, October 18, 2005.

A. Rathsfeld, Optimization of diffraction gratings with tt DiPoG, sc Matheon MF 1 Workshop ``Optimization Software'', KonradZuseZentrum für Informationstechnik Berlin, June 1, 2005.

G. Schmidt, Simulation und Optimierung periodischer diffraktiver Strukturen mit DiPoG, PhysikalischTechnische Bundesanstalt, Berlin, April 13, 2005.

J. Elschner, Direct and inverse problems for the periodic Helmholtz equation I, University of Tokyo, Department of Mathematical Sciences, Japan, February 12, 2004.

J. Elschner, Direct and inverse problems for the periodic Helmholtz equation II, University of Tokyo, Department of Mathematical Sciences, Japan, February 13, 2004.

J. Elschner, Inverse scattering for diffraction gratings, European Congress on Computational Methods in Applied Sciences and Engineering (ECCOMAS 2004), June 24  28, 2004, Jyväskylä, Finland, June 26, 2004.

J. Elschner, Inverse scattering of plane waves from periodic surfaces, Chemnitzer Minisymposium 2004 zu Inversen Problemen, Universität Chemnitz, September 23, 2004.

J. Elschner, Recent progress in inverse periodic diffraction problems, Workshop ``Mathematical Analyses and Numerical Methods for Applied Inverse Problems'', January 19  21, 2004, University of Tokyo, Japan, January 20, 2004.

A. Rathsfeld, Simulation und Optimierung diffraktiver Strukturen für die Mikrooptik, Seminar des Forschungsschwerpunktes Photonik, Technische Universität Berlin, Optisches Institut, October 22, 2004.

G. Bruckner, J. Elschner, A. Rathsfeld, G. Schmidt, Simulation, optimization and reconstruction of diffractive structures, Conference ``Diffractive Optics 2003'', Oxford, UK, September 17  20, 2003.

J. Elschner, Inverse problems for diffraction gratings: Uniqueness results, Meeting ``Inverse Problems in Wave Scattering and Impedance Tomography'', April 20  25, 2003, Mathematisches Forschungsinstitut Oberwolfach, April 22, 2003.

J. Elschner, Inverse problems for periodic diffractive structures, Meeting ``Functional Analysis and Partial Differential Equations'', June 2  3, 2003, HansurLesse, Belgium, June 3, 2003.

J. Elschner, On the numerical solution of inverse periodic transmission problems, University of Tokyo, Department of Mathematical Sciences, Japan, August 5, 2003.
External Preprints

G. Hu, B. Zhang, The linear sampling method for the inverse electromagnetic scattering by a partially coated biperiodic structure, Preprint no. arXiv:1003.3067, Cornell University Library, arXiv.org, 2010.

J. Elschner, G. Schmidt, M. Yamamoto, An inverse problem in periodic diffractive optics: Global uniqueness with a single wave number, Preprint no. 5, University of Tokyo, Graduate School of Mathematical Sciences, 2003.