HgCdTe compounds can be engineered to fabricate “gapped-at-will” structures. Therefore, 1D, 2D and even 3D massless particles can be observed in topological phase transitions driven by intrinsic and external physical parameters. In this work, we report on our experimental results, obtained by temperature-dependent Terahertz and Mid-Infrared magneto-spectroscopy, of topological phase transitions in HgCdTe-based quantum wells and bulk samples. These transitions are accompanied with the appearance of 2D and 3D massless particles called Dirac and Kane fermions, respectively.
Emission of terahertz (THz) radiations from interdigitated GaN quantum-wells structures under DC-bias has
been measured at room temperature. This measurements has been performed by a 4K Si-Bolometer associated
with a Fourier Transform Spectrometer. Using an analytical model, we have shown that the observed peak
at approximately 3 THz due to 2D ungated plasma-waves oscillations in the quantum well, is emitted by the
metallic contacts of our device acting as antennas.
We report on measurements of radiation transmission in the 0.220-0.325 THz and 0.75-1.1 THz
frequency ranges through GaN quantum wells grown on sapphire substrates at nitrogen and room
temperatures. Significant enhancement of the transmitted beam intensity with applied voltage is
found at nitrogen temperature. This effect is explained by changes in the mobility of two-dimensional
electrons under electric bias. We have clarified which physical mechanism modifies the electron mobility
and we suggest that the effect of voltage-controlled sub-terahertz transmission can be used for
the development of electro-optic modulators operating in the sub-THz frequency range.
III-Nitride semiconductors are promising nonlinear materials for optical wavelength conversion. However second
harmonic generation in bulk GaN is weak because GaN is strongly dispersive. We show that appropriate photonic crystal
patterning in GaN helps to overcome dispersion and provides quasi-phase matching conditions, resulting in substantially
increased conversion efficiency obtained in a flexible manner. Enhancement factors of more than five orders of
magnitude can be achieved. Use of photonic crystals makes it possible to reduce the effective observation volume,
thereby opening new opportunities such as the study of single-molecule dynamics, even in high concentration solutions.
We have demonstrated sharp enhancement of the fluorescence of single molecules immobilized on the surface of a GaN
photonic crysta,l when the molecules are excited via the resonant second harmonic generation process.
In this work we review the most important results concerning the physics and applications of FETs as Terahertz
detectors. We present two experiments showing: i) Terahertz detection based on low cost 130 nm silicon
technology Field Effect Transistors in the sub-THz range (0.2 THz up to 1.1 THz) and ii) first results on
detection by FETs of emission from 3.1 THz Quantum Cascade Lasers.
Two-dimensional (2D) plasma waves in field effect transistors are well known since the pioneer work of Dyakonov
and Shur. The application to terahertz (THz) detection was proven recently both at cryogenic and room temperatures.
Aside from these experiments, we used the interband photoexcitation brought by the difference-frequency
component of a photomixed laser beam to excite very efficiently plasma waves in HEMT channel at room temperature.
Owing to a specific experimental setup avoiding unwanted high-frequency electrical oscillations of the
HEMTs, we obtained the spectral profiles of THz 2D plasma waves resonances of InGaAs HEMTs for many
experimental conditions. The effect of geometrical HEMTs parameters (lengths of the gate and surrounding
regions) as well as biasing conditions (drain and gate voltages) was evaluated on both plasma oscillations frequencies
and amplitudes. Simultaneously, a numerical approach, based on hydrodynamic equations coupled to a
pseudo-2D Poisson solver, was developed that compares well with experiments. Using this unique combination
of experiments and numerical simulations, a comprehensive spectroscopy of plasma waves in HEMTs is thus
obtained. It provides a deeper insight into the physical processes involved in plasma wave excitation and allows
predicting for mixer operation at THz frequency only using the plasma wave nonlinearity. Mixing experiments
are under progress.
The stable two-mode operation of a 4-sections semiconductor laser emitting at 1.55 μm is experimentally demonstrated and analysed. An interpretation of the two-mode regime involving a saturable absorber is theoretically developed and the characteristic parameters of this saturable absorber deduced. This work exhibits the possibility of terahertz wave generation by photomixing using this device.
The stable two-mode operation of a 4-sections semiconductor laser emitting at 1.55 μm is demonstrated and analysed. The two-mode operation only depends on the current feed in the Bragg section. The characterization of the two-mode laser operation exhibits the possibility of terahertz wave generation by photomixing using this device. An interpretation of the two-mode regime involving a saturable absorber is discussed.
Investigations of polarizations effects in second-harmonic generation of a one-dimensional photonic crystal based on gallium nitride were performed for the fundamental beam incident on the surface of the photonic crystal. The angle of incidence, the azimuthal rotation angle of the photonic crystal, the frequency, and the polarization behaviour for strongly enhanced second-harmonic generation agree well with the identified position and polarization of the resonant Bloch modes. Along the direction, giant enhancements of 7500 times in the second-harmonic conversion have been obtained in the one-dimensional photonic crystal by comparison with the unpatterned GaN layer. The combined role of the resonant coupling of the fundamental field and of the second-harmonic field has been observed as the polarization of the fundamental beam is rotated.
The second-harmonic field generated has been measured in reflection from the surface of one-dimensional and two-dimensional photonic crystals etched into a GaN layer. A very large second-harmonic enhancement is observed when simultaneously the incident beam at the fundamental frequency w excites a resonant Bloch mode and the second-harmonic field generated is coupled into a resonant Bloch mode at 2w. A smaller, but still substantially enhanced, second-harmonic generation level was also observed when the fundamental field was coupled into a resonant mode, while the second-harmonic field was not. By using calculated and experimental equifrequency surfaces, it is possible to identify the geometrical configurations that will allow quasi-phase matching to be satisfied - and observed experimentally in the available wavelength tuning range of the laser. The extended transparency window of III-nitride wide-bandgap semiconductors, coupled with large non linearities, is an appealing feature pointing towards the control and manipulation of light in photonic structures.