Degenerate two-photon absorption (TPA) is investigated in a 186 nm thick gallium arsenide (GaAs) p-i-n diode
embedded in a resonant metallic nanostructure. The full device consists in the GaAs layer, a gold subwavelength grating
on the illuminated side, and a gold mirror on the opposite side. For TM-polarized light, the structure exhibits a resonance
close to 1.47 μm, with a confined electric field in the intrinsic region, far from the metallic interfaces. A 109 times
increase in photocurrent compared to a non-resonant device is obtained experimentally, while numerical simulations
suggest that both gain in TPA-photocurrent and angular dependence can be further improved. For optimized grating
parameters, a maximum gain of 241 is demonstrated numerically and over incidence angle range of (−30°; +30°). This
structure paves the way towards low-noise infrared detection, using non-degenerate TPA, involving two photons of
vastly different energies in the same process of absorption in a large bandgap semiconductor material.
The multispectral imaging technique consists of imaging a given scene at various wavelengths of
interest, each one containing a different spectral information. By analyzing this spectral content,
the chemical species that are present can be localized on the image and identified by reconstructing
their spectral signature. In this way, following Ebbesen's seminal work in plasmonics
[1], purely metallic or hybrid metallodielectric structures [2, 3] seem to be ideal candidates to
perform spectral filtering due to their extraordinary transmission efficiency [4] and polarization
selectivity. Moreover, their compact feature makes it possible for them to gather in wide arrays
of filters that, once integrated into a cooled infrared camera, can achieve real-time multispectral
imaging [5].
As seen in Figure 1.d. the spectral signature reconstruction of a chemical species strongly
depends on the number of filters and their transmission spectra for the designed matrix. In
order to improve the multispectral camera, a complementary approach consists of changing the
filter design to realize a tunable filter whose spectral shape can be adjusted in real time according
to the imaged scene. We focused our attention on the superposition of subwavelength gratings
which seems to be a structure of great potential for multispectral imaging applications [6, 7].
The B-spline modal method is adapted for the design and analysis of nanostructured devices in conical mounting.
The eigenmodes in each layer are calculated for two specific polarization states, and then combined for the
calculation of the scattering matrices. We take advantage of the sparsity of the generated matrices to decrease
the computation time, and adress the need for fast computation in complex systems. Moreover, we demonstrate
the physical interest of computing the conical response on various classical structures.
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