We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium
arsenide (GaAs). We used three types of micro-structured GaAs to quasi-phase-match the interaction: optically
contacted, orientation-patterned, and diffusion-bonded GaAs. The room-temperature GaAs was placed in an optical
parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO
signal and idler waves. 250-GHz-bandwidth radiation was generated with frequencies spanning 0.4-3.5 THz. We
measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant
oscillator (DRO) configuration, the efficiency increased by 21 times over the singly resonant oscillator (SRO)
performance with an optical-to-THz efficiency of 10-4 and average THz power of 1 mW.
Zincblende semiconductors (GaAs, GaP) show great potential for quasi-phase-matched (QPM) THz generation because
of their small (20 times less than in lithium niobate) absorption coefficient at terahertz frequencies, small mismatch
between the optical group and THz phase velocities, high thermal conductivity, and decent electro-optical coefficient.
Terahertz-wave generation was demonstrated recently in QPM GaAs, using optical rectification of femtosecond pulses.
Here we report on a new efficient widely tunable (0.5-3.5 THz) source of THz radiation based on quasi-phase-matched
GaAs crystal. The source is based on difference frequency generation inside the cavity of a synchronously pumped near-degenerate
picosecond OPO and takes advantage of resonantly enhanced both the signal and the idler waves. THz average power as high as 1 mW was achieved in a compact setup.
We report a new highly efficient source of frequency-tunable (0.5-3.5 THz) narrow-bandwidth terahertz wave packets with up to 1 mW average power, based on parametric down-conversion in quasi-phase-matched GaAs. Different lasers were employed as a pump source, including femtosecond OPA/DFG system (wavelength range 2-4μm), Tm-fiber femtosecond laser (wavelength ~2μm), and near-degenerate synchronously-pumped picosecond OPO system with extra- and intracavity THz generation. We prove experimentally that the optical-to-terahertz conversion efficiency is fluence-dependent, with the scaling factor being the same for femtosecond (optical rectification) and picosecond (difference frequency generation) pump pulses, with optical-to-terahertz conversion efficiency on the order of 0.1% per μJ.
A new technique utilizing harmonic Fourier spectra created by the non-linear properties of a compact Fourier transform infrared interferometer (FTIR) was proposed and realized to improve the system resolution. The compact standing wave FTIR (SWFTIR) system consists of a partial transparent hetero-junction bipolar phototransistor (HPT) and a free scanning highly reflective mirror. The overall size of the system is less than 5×5×5cm3, and the resolution at 1.5μm is better than 37.5cm-1 at the 5th harmonic spectral component. The SWFTIR array system has theoretical resolution of better than 1cm-1 covering the whole near-infrared region with potential compact portable applications.
We have demonstrated all-epitaxially fabricated orientation-patterned AlGaAs waveguides with reduced waveguide core corrugation for the quasi-phase-matched second harmonic generation (SHG) pumped at 1.55 μm. The attenuation coefficient is measured to be ~4.5 dB/m at 1.55 μm, and ~9.7 dB/cm at 780 nm. The conversion efficiency at continuous wave operation is 43%W-1 with an 8-mm long waveguide.
The dilute-nitride GaInNAs shows great promise in becoming the next choice for long-wavelength (0.9 to 1.6 μm) photodetector applications due to the ability for it to be grown lattice-matched on GaAs substrates. GaAs-based devices have several advantages over InP-based devices, such as substrate cost, convenience of processing, and optoelectronic band parameters. This paper will present results from the first high-quality thick GaInNAs films grown by solid state molecular beam epitaxy with a nitrogen plasma source and the first high efficiency photodetectors which have been fabricated from those materials. GaInNAs films up to 2 microns thick have been grown coherently on GaAs substrates. These films exhibit reasonable photoluminescence intensities at peak wavelengths of 1.22 to 1.13 μm before and after a rapid thermal anneal at a series of temperatures. PIN photodiodes with these thick GaInNAs films in the intrinsic regions show responsivity (better than 0.5 A/W at 1.064 μm), dark current (200 nA at -2 V), and signal-to-noise ratio (greater than 105) approaching those of commercially available InGaAs/InP devices. Furthermore, it will be shown that these devices show significantly lower dark current and higher signal-to-noise ratio than similar metamorphic InGaAs/GaAs structures.
For applications such as fiber optic networks, wavelength conversion, or extracting information from a predetermined channel, are required operations. All-optical systems, based on non-linear optical frequency conversion, offer advantages compared to present systems based on optical-electronic-optical (OEO) conversion. Thanks to the large nonlinear susceptibility of AlGaAs (d14 = 90pm/V) and mature device fabrication technologies, quasi-phasematched non-linear interactions in orientation-patterned AlGaAs waveguides for optical wavelength conversion have already been demonstrated. However, they require long interaction length (~ centimeters) and a complex fabrication process. Moreover, the conversion efficiency remains relatively low, due to losses and poor confinement. We present here the design and fabrication of a very compact (~ tens of microns long) device based on tightly confining waveguides and photonic crystal microcavities. Our device is inherently phase-matched due to the short length and should significantly increase the conversion efficiency due to tight confinement and high cavity-Q value. We characterized the waveguides, measuring the propagation loss by the Fabry-Perot method and by a variant of the cutback method, and both give a consistent loss value (~5 dB/mm for single-mode waveguides and ~3 dB/mm for multimode waveguide). We also characterized the microcavities measuring the transmission spectrum and the cavity-Q value, obtaining Q's as large as 700.
We present the design and fabrication process for an AlGaAs optical frequency conversion device based on tightly confining waveguides and a Photonic Bandgap Crystal Microcavity. We first theoretically analyze the improvement in non-linear conversion efficiency due to a high confinement cavity, compared to traditional QPM waveguides. The theoretical analysis is supported by finite difference frequency and time domain simulations. The theoretical conversion efficiency estimated with these tools is ~4%/mW for a device ~10 μm long. Influence of sidewall roughness on the Q of the cavity is also analyzed. Then, we describe the fabrication process of our device, which involves molecular beam epitaxy, electron beam lithography and plasma etching.
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