We present the results of a new model for the simulation of quantum well infrared photodetectors (QWIPs) both in dark conditions and under illumination. This model takes into account the elementary mechanisms involved in the detection process (injection at the contacts, balance between capture and emission in each well) in a self-consistent way. The main feature emerging from the model is the redistribution of the electric field along the structure in order to maintain current conservation. The calculated dark current, electrical noise, responsivity, and detectivity of different QWIP structures are compared with experimental measurements and the agreement is found to be fairly good. This model may be considered as a step toward more powerful simulation tools for QWIPs.

Steady-state and transient responses of a nonintentionally doped GaN photodetector are investigated. The kinetics of the photoresponse demonstrate the existence of deep levels in the gap, acting as recombination centers with an acceptor character. The photoresponse displays two competing processes: a bimolecular recombination, dominating at high optical power range, and a monomolecular recombination involving long response times. The observed persistent photoconductivity and the huge photoconductive gain are due to the small electron capture cross section and a much faster hole capture rate.

InAs quantum dots (QDs) overgrown by a Ga0.85In0.15NxAs1−x (0⩽x⩽0.017) layer have been realized on GaAs substrate by molecular beam epitaxy. When the nitrogen composition increases, the photoluminescence (PL) wavelength redshifts up to 1.52μm. It is shown that PL properties of InAs∕Ga0.85In0.15N0.012As0.988 QDs are improved by thermal annealing. Finally, 1.45μm PL emission with a 38.5meV full width at half maximum is obtained at room temperature.

The use of tunnel junctions (TJs) is a potential solution in blue LEDs to poor p-contacts, replacing it by another n-contact. TJs are even more advantageous for UV emitting structures, which suffer from the considerably low injection efficiency in high Al concentration UV LEDs. In this work we report our work on Ge n-doped GaN and AlGaN TJs grown on top of blue and UV LEDs, respectively, by a hybrid growth. We have achieved state of the art mobility (67cm 2 /V.s) and resistivity (1.7x10 -4 Ω.cm) at a free electron concentration of 5.5x10 20 cm -3 in Ge-doped GaN. With an emission wavelength of 436nm, the GaN TJ slightly increased the optical power of the blue LED. The AlGaN TJs, on the other hand, improved the optical power of the UV LED (304nm) by at least a factor of 3, suggesting the enhancement of the hole injection efficiency by the use of TJs in UV emitting structures.

These metasurfaces shape the wavefront by controlling its propagation with local subwavelength phase discontinuities. [4] From the earliest experiments, associated to the reflection of light on arrays of subwavelength metallic patches, [5][6][7] the concept of metasurfaces has rapidly evolved. It started from metallic metasurfaces working at a single wavelength to get to subwavelength high contrast gratings [8][9][10][11] and even subwavelength antenna arrays of various shapes and materials working over a large bandwidth. [12][13][14][15][16][17][18] Today, a broad operation wavelength range is accessible, going from THz down to the visible. Thanks to their reduced thicknesses and high transmission in the visible, the latest metasurface components could enable the next generation of flat optical devices. Large variety of examples of flat and small area components related to dielectric metasurfaces can be found in literature, showing impressive performances and unexpected effects, such as high transmission lenses with high numerical aperture (NA), polarization controlled properties and multiplexed optical information. [19][20][21][22][23][24][25][26][27][28][29] Until recently, metasurfaces have exclusively been considered as passive devices, i.e., their optical properties such as phase, amplitude and polarization responses were considered as fixed with respect to any change of environmental parameters. Their functionalities can be further expanded to a larger extent by designing tunable metasurfaces. Several attempts to achieve this tunability and switching have already been proposed, see for example the modification of the reflectivity and transmission through metasurfaces fabricated from or on the top of phase change materials such as vanadium dioxide, [30][31][32][33] chalcogenides based on germanium antimony telluride [34][35][36] and liquid crystals. [37][38][39] Going beyond the proof of principle of new functionalities and practically implementing these pioneering passive and active devices necessitates new materials, enabling reduced fabrication cost, increased productivity and even higher optical performances. On the other hand, nanofabrication of metasurfaces usually involves several processing steps such as dry etching that induce unavoidable defects degrading the device optical properties and performance.Gallium nitride, an already widespread semiconductor, is selected for the realization of our components, as it offers A new class of quasi 2D optical components, known as metasurfaces and exhibiting exceptional optical properties have emerged in recent years. The scattering properties of their subwavelength patterns allow molding the wavefront of light in almost any desired manner. While the proof of principle is demonstrated by various approaches, only a handful of low cost and fabrication friendly materials are suitable for practical implementations. To further develop this technology toward broadband application and industrial production, new materials and new fabrication methods are required. In a...

We demonstrate phase-matched second harmonic generation in gallium nitride on silicon microdisks. The microdisks are integrated with side-coupling bus waveguides in a two-dimensional photonic circuit. The second harmonic generation is excited with a continuous wave laser in the telecom band. By fabricating a series of microdisks with diameters varying by steps of 8 nm, we obtain a tuning of the whispering gallery mode resonances for the fundamental and harmonic waves. Phase matching is obtained when both resonances are matched with modes satisfying the conservation of orbital momentum, which leads to a pronounced enhancement of frequency conversion.

GaN based materials are believed to be very stable materials, in particular, under irradiation by high energy photons such as x rays. We have studied x-ray detectors based on GaN Schottky diodes. Vertical Schottky diodes were fabricated based on a 20μm thick undoped GaN layer grown on a conductive GaN substrate. Their photoresponse to near UV light and to x rays was measured. While the response to near UV light was fast and linear as expected, anomalous behaviors were observed under x-ray illumination. The photocurrent increases as the third power of the incident x-ray flux. The photocurrent transient when the x rays is turned on are long and nonexponential (S shape) and strongly differs from the off transient which is fast and exponential. Also, a very strong quenching of the x-ray photoresponse is observed when the detector is simultaneously illuminated with visible light. All of these anomalous behaviors are explained in the frame of a complete model involving traps and tunnel currents. A reasonable quantitative agreement between the model and the experimental data is obtained.

No abstract