Graphene plasmonics provides a powerful means to extend the reach of metasurface technology to the terahertz spectral range, with the distinct advantage of active tunability. However, most prior proposals of graphene THz plasmonic devices assume electrical characteristics that are not compatible with sufficiently large samples grown by chemical vapor deposition (CVD). Here we introduce a comprehensive design platform that can overcome this limitation, based on graphene nanoribbons combined with metallic antennas on a vertical cavity. With this configuration, we present a single device structure that can be electrically reconfigured to enable multiple wavefront-shaping functionalities, including tunable beam steering and focusing with variable numerical aperture. Applications of the same platform to THz light emission will also be discussed. These capabilities are promising for a significant impact in multiple THz technologies for sensing, imaging, and wireless communications.
We report the development of terahertz intersubband photodetectors based on GaN/AlGaN quantum wells, covering the frequency range that is fundamentally inaccessible to existing III-V semiconductor devices due to Reststrahlen absorption. Two different approaches have been employed to mitigate the deleterious effects of the intrinsic polarization fields of nitride heterostructures: the use of suitably designed double-step quantum wells, and epitaxial growth on semipolar GaN substrates. Promising results are obtained with both approaches, which could be extended to other device applications as a way to utilize the intrinsic advantages of nitride semiconductors for THz intersubband optoelectronics.
The development of practical light sources based on group-IV semiconductors is a major outstanding goal of
optoelectronics research, as a way to enable the continued integration of electronic and photonic functionalities on a
CMOS compatible platform. However, this goal is severely complicated by the indirect energy bandgap of silicon,
germanium, and related alloys. A possible solution is provided by the ability of biaxial tensile strain in Ge to lower the
conduction-band edge at the direct (Γ) point relative to the L-valley minima, until at a strain of about 1.9% the
fundamental bandgap becomes direct. Here we show that, by virtue of their ultrasmall thicknesses, Ge nanomembranes
under externally applied mechanical stress are capable of accommodating such relatively high strain levels (up to over
2%). With this approach, we have demonstrated strong strain-enhanced Ge photoluminescence accompanied by a large
strain-induced red shift in emission wavelength. A theoretical model of the emission properties of tensilely strained Ge
has also been developed and applied to the measured high-strain luminescence spectra, providing evidence of population
inversion. Finally, mechanically flexible photonic-crystal cavities have been developed on these nanomembranes, and
used to demonstrate particularly large (20×) strain-induced enhancements in radiative efficiency, together with the
observation of luminescence signatures associated with band-edge cavity modes. These results are promising for the
development of group-IV semiconductor lasers for the technologically important short-wave infrared spectral region.
We present a comparison among GaN/AlGaN, GaN/InAlGaN, and GaAs/AlGaAs quantum cascade structures based on a Monte Carlo study of carrier dynamics, to highlight the improvements offered by nitride latticematched structures. We take into account the interactions of electrons with other electrons as well as LO-phonons. The results obtained from the Monte Carlo simulations are used to calculate the population inversion of each structure to determine its temperature dependence. This study shows that the nitride-based structures offer significantly improved high-temperature performance compared to the GaAs device, including the possibility of room-temperature operation. Furthermore, by virtue of its lattice-matched nature, the GaN/InAlGaN materials system can potentially enable the high-quality growth of thick quantum cascade structures without plastic relaxation, as a way to overcome the structural issues that have so far hindered the development of these devices with nitride semiconductors.
Due to their large conduction-band offsets, GaN/AlGaN quantum wells can accommodate intersubband transitions at
record short wavelengths throughout the mid-infrared and into the near-infrared spectral regions. As a result, they are
currently the subject of extensive research efforts aimed at extending the spectral reach and functionality of intersubband
optoelectronic devices. Here we review our recent work in this area, based on GaN/AlGaN quantum-well samples
grown by molecular beam epitaxy on sapphire substrates. In particular, we have investigated the intersubband
absorption properties of a wide range of structures, including isolated and coupled quantum wells. Furthermore, we
have developed a new class of ultrafast all-optical switching devices, based on intersubband cross-absorption saturation
in GaN/AlGaN quantum-well waveguides operating at fiber-optic communication wavelengths. Strong self-phase
modulation of ultrafast optical pulses has also been measured in these waveguides, revealing a large refractive-index
nonlinearity which is related to the same intersubband carrier dynamics. Finally, we have demonstrated optically
pumped intersubband light emission from GaN/AlN quantum wells resonantly excited with a pulsed OPO. The
measured room-temperature output spectra are peaked near 2 μm, which represents a new record for the shortest
intersubband emission wavelength from any quantum-well materials system.
Nitride semiconductor quantum structures feature some unique properties for intersubband device development,
including a record large conduction-band offset that allows extending the operating wavelength to the near-infrared
spectral region, and large optical phonon energies that are advantageous for the development of THz devices. In this
paper we review our recent work aimed at the demonstration of novel intersubband device functionalities using these
materials. In particular, we have developed ultrafast all-optical switching devices operating at fiber-optic
communication wavelengths, based on intersubband cross-absorption saturation in GaN/AlN quantum-well waveguides.
Strong self-phase modulation of ultrafast optical pulses has also been measured in these waveguides, revealing a large
intersubband refractive-index nonlinearity which is also promising for all-optical switching applications. Furthermore,
we have demonstrated optically pumped intersubband light emission from GaN/AlN quantum wells at the record short
wavelength of about 2 μm. Finally, we have used a rigorous Monte Carlo model to show that GaN/AlGaN quantum
wells are promising for the development of THz quantum cascade lasers capable in principle of operation without
cryogenic cooling.
KEYWORDS: Modulation, Quantum cascade lasers, Semiconductor lasers, Analog electronics, Eye, Digital modulation, Telecommunications, Data communications, Temperature metrology, Laser damage threshold
In this paper we present measured modulation responses on quantum-cascade lasers (QCL) up to 10 GHz. The obtained modulation response shows a flat response over the whole frequency range, proving the broadband capabilities of these devices. Even more striking is the absence of a strong resonance peak, which demonstrates the absence of relaxation oscillations, a feature which often limits the high speed performance of standard diode lasers. This property is quite attractive for use of these devices as high-speed data sources, particularly in applications where linearity is important. To address this possible application, the digital modulation properties of these devices were tested in a standard bit-error-rate-measurement at 2.5 GBit/s for cryogenically cooled QCL, showing the suitability of QCLs in digital as well as analog telecommunication application. In addition we present recent data showing bit error measurements and eye diagrams obtained for the pulsed mode operation of QCL near room-temperature.
We present experimental results for an optical free-space high-speed link using direct modulated mid-infrared (8.1 μm) quantum cascade lasers. A stable link was realized over a distance of 200m and QPSK encoded multimedia data were transmitted and received error-free, incorporating several hundred digitally encoded multimedia channels. The reliability of the system against weather influence (fog) was experimentally compared to that of a near-infrared (0.85 μm) link. Under clear weather conditions comparable results were obtained in both links. However, a higher stability of the MIR link was clearly observed under a dense fog situation.
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