Anomalistic behavior in diffraction responses of grating can be easily detected and can indirectly provide information
about the grating parameters such as the grating period, height, duty-cycle and profile. More precisely, the absorption
resonance (Wood's anomaly) which arises from the excitation of a surface plasmon polariton (SPP) in reflective
sub-wavelength diffraction gratings are of interest as well as Rayleigh's anomaly which takes the form of a discontinuity
in the diffraction response and which is the consequence of the excitation of a new propagating mode. In this paper we
describe how these anomalies can be used as a non-destructive metrology tool to estimate the grating parameters by an
IR spectral scatterometry measurement. We briefly describe the theoretical conditions for which SPP are excited. We
investigate the wavelength sensitivity of Wood's anomaly in the zeroth order diffraction response to individual grating
parameter variations at CO2 laser wavelengths. A numerical electromagnetic grating solver software package "Gsolver"
was used for the theoretical modeling. We show that this non-destructive IR spectral scatterometry measurement based
on feature extraction allows us to measure grating parameter variations with nanometer resolution. The measurement
time needed to scan a 4" wafer has been shown to be of the order of a few minutes. This is much faster as compared to
traditional techniques as (deconstructive) SEM inspection or white light interferometry. Furthermore, the extension of
this technique to larger wafers does not impose any difficulties.
Laser induced temperature distributions inside doped semiconductor materials are used to derive laser beam profiles by
means of the thermo-electric Seebeck effect. Thermal diffusion will lead to a discrepancy between the optical intensity
profile of the laser beam and the measured temperature distribution inside the semiconductor. An advanced numerical
4D finite element model describing the laser induced spatial temperature distribution in function of time in a layered
GaAs based structure was developed in Comsol Multiphysics. Non-linearities as the temperature dependence of the
absorption coefficient, the thermal conductivity and the Seebeck coefficient were taken into account. This model was
used to investigate the optical chopper frequency dependence on the spatial thermal cross-talk level and the responsivity
near the illuminated surface of the detector structure. It was shown that the frequency dependent cross-talk level can be
reduced significantly by applying short chopping periods due to the dependence of the thermal diffusion length on the
frequency. The thermal cross-talk is reduced to -21 dB and -38.6 dB for the first and second neighboring pixel
respectively for a lock-in frequency of 140 Hz. Experimental results of the spatial thermal cross-talk level and the
responsivity were compared with simulations and satisfactory agreements between both were achieved. High power CO2
laser profile measurements obtained with our thermo-electric detector and a commercially available Primes detector were
compared.
We present a new modulation concept for medium infrared (8 - 12 μm) wavelengths. The operation principle of the
presented modulator is based on evanescent wave absorption by means of a bulk, single or multiple quantum well
structure. A sub-wavelength grating ensures efficient coupling of the optical field to the absorption medium. Modulation
is then achieved by depletion of this absorption medium. We present an analysis of concept parameters and point out
their respective advantages and disadvantages with respect to the modulation performance. In this context, we
investigated the impact of different absorption media as bulk, single and multiple quantum well structures and found that
single quantum well structures are best suited for modulation purposes. Simulations pointed out that an absolute
modulation depth of the order of 60% can be achieved. We also investigated the impact of the diffraction order on the
modulation performance. Furthermore, some preliminary experimental results on this modulation concept are presented
and compared with simulations.
The theoretical background of Seebeck infrared detectors based on nonlinear free carrier absorption in doped
semiconductors has been presented. The 3D-distribution of the electron and lattice temperatures created by the
absorption of an optical beam with a cylindrical symmetry in layered structures was developed. Five different operation
regimes of the detector are presented, showing that all beams form CW down to picoseconds can be detected. We will
discuss how one can control the detectable power and intensity levels and the cross-talk in multi-pixel arrays by means
of the doping concentration, geometry of the absorption region and pixellation format, the positioning of a heat sink, and
micro-machining techniques. Experimental backing for the model will be given for the pulsed regime and the CW
regime. We also demonstrate operation of the detector in the +1 kW power level.
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