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The effect of vacuum preheating on the structure, electric, and optical properties of the thermally oxidized W-doped VO2 films was studied. The results show that the film crystalline orientation and the inhomogeneous distribution of W increase after preheating. It was found that preheating at 700 °C and 800 °C reduces the metal-insulator transition (MIT) hysteresis because of the preferable orientation of the film along the (011) direction. The film preheated at 900 °C exhibits an undoped-like MIT due to a great increase in the inhomogeneous distribution of W. Preheating can affect the apparent activation energy of the cool-state resistances and increase the cool-state IR transmittances. By means of the Lorentz–Drude model, the conductance electron density (n c) and energy band gaps (E g) were obtained and discussed. It was shown that preheating can decrease the cool-state n c and affect the E g, indicating that the electronic band structure is changed by W doping.

This article presents recent results related to development of thick quasi-phase-matched GaP for incorporation in high power sources radiating in the mid-infrared. The focus was on increasing the growth rate and layer thickness of orientation patterned GaP, while ensuring equal vertical deposition rates and rectangular shape of the domain mesas for both opposite crystallographic lattice orientations. Additionally, we present solutions which confine the presence of uncontrollable hillock growth. The experiments were conducted in a hot-wall horizontal quartz reactor using a standard low-pressure hydride vapor phase epitaxial (HVPE) process. GaP was grown on bare, half-patterned (HP) and orientation-patterned (OP) templates fabricated on (100) GaP and (100) misoriented 4° towards (111)B substrates. The domains were oriented either along the [011] or the [011] direction. We compared a sub-lattice inversion MBE assisted process and a wafer fusion bonding technique to create OP templates. When the growth was performed on bare material, the properly chosen growth conditions resulted in reproducible growth of up to 370 μm thick layers with high optical, surface and structural quality, grown at a growth rate of 100 μm/h in one-hour long experiments and 45 μm/h for 8-h long growths. The presence of a core inside the hillocks growing on the layer surface was eliminated. The hillocks were flattened and widened, which allowed often a single hillock to span several domains with alternating opposite crystallographic orientations, when growth was performed on patterned templates. The HP templates were used to determine the optimal substrate and pattern orientations prior to starting growth experiments on OP-templates. As an additional result they revealed that growth could be hillock-free for certain orientations. Growths on OP templates achieved stable growth rates of 50–70 μm/h with domain walls propagating vertically. The growth followed the periodicity of the initial pattern. The maximal thickness achieved to date on OP GaP is about 350 μm.

Quasi-phase-matched GaAs layers up to 600 μm thick have been produced by low-pressure hydride vapor phase epitaxy (HVPE) regrowth on templates fabricated using wafer fusion bonding. We have used this combination of techniques to demonstrate an alternative approach for commercial development of orientation-patterned GaAs (OP-GaAs) for nonlinear optical frequency conversion applications. We report on the characterization of this material and present a comparison with OP-GaAs grown using the conventional all-epitaxial templates. The primary advantages of the wafer fusion method are the avoidance of the molecular beam epitaxy growth of GaAs/Ge/GaAs and the ability to use substrates without misorientation. We have observed that some of the misorientation-related growth defects which affect conventional OP-GaAs growth are not as problematic for regrowth on wafer fusion bonded templates. The overall quality of antiphase domain propagation was similar to conventional OP-GaAs layers. A disadvantage was that growth rates were approximately half the values measured for regrowth on misoriented templates.

Nonlinear optical frequency conversion is an effective technique for generating infrared (IR) and terahertz (THz) wavelengths not readily available from existing laser sources. Birefringent materials such as LiNbO3 are often used to generate wavelengths where gaps exist, but are unsuitable in the mid-IR, far-IR, and THz regions as these materials are often opaque in these regions. As an alternative, GaAs has been employed for frequency conversion in these regions using quasi-phase-matching (QPM) to overcome the material’s lack of birefringence. QPM has been successfully demonstrated in GaAs using fused stacks of thin alternately oriented layers or inverted orientation patterned (OP) grating templates overgrown with thick columnar GaAs layers. Although GaAs has a high nonlinear coefficient d14 = 170 pm/V at 1.064 μm and good thermal conductivity (52 W/m K), it suffers from strong two-photon absorption below 1.7 μm making it inefficient when pumped with a source less than or equal to this wavelength. GaP also has a high nonlinear coefficient d14 = 71 pm/V at 1.064 μm, better thermal conductivity (110 W/m K) and much lower two-photon absorption in the 1 μm region. Therefore, OPGaP is desirable for NLO applications in the mid-IR and THz that use commercially available pump lasers in the 1.06–1.55 μm wavelength range. In this work the fabrication of OPGaP templates suitable for thick columnar hydride vapor phase epitaxial growth of GaP is reported using a commercially viable wafer fusion technique.

This paper describes advances in the development of quasi-phase-matched (QPM) gallium phosphide (GaP) crystals for agile laser sources in the mid-infrared regions between 3–5 and 8–12 μm. In the quest for a nonlinear optical material with the potential to efficiently convert near infrared energy (wavelength ∼1 μm) to a powerful mid-infrared source, we have investigated the growth of GaP by hydride vapor phase epitaxy (HVPE). The process is shown to produce high quality thick layers at rapid growth rates in a low-pressure horizontal reactor. This process was used to grow thick layers on orientation-patterned (OP) templates. The OP-GaP templates were fabricated by lithographic patterning of inverted wafer-fused GaP. HVPE growth on both OP-GaP and OP-GaAs templates was performed, showing that HVPE can successfully replicate the initial template pattern. However, for longer growth duration at given conditions the patterned structure can be lost, with annihilation of every other domain.

Halide vapor phase epitaxial growth of GaP was performed on GaP half- and orientation — patterned templates. This allowed optimization of substrate material, patterning approach and pattern orientation, which resulted in growth of +350 μm thick quasi-phase matched GaP with successful replication of the initial patterns.

This article describes efforts to achieve fast deposition of thick Quasi-Phase-Matched (QPM) GaP structures with high surface and structural quality on oriented patterned (OP) templates in a Hydride Vapor Phase Epitaxial (HVPE) process. These QPM structures will be incorporated in devices for conversion of frequencies from the near infrared to the mid infrared and THz regions, where powerful and tunable sources are in great demand for both military and civilian applications. In contrast with GaAs—the most studied OP QPM material—the two-photon absorption of GaP is predicted to be extremely low, which allows pumping with a number of convenient sources between 1 – 1.7 µm. Unpatterned GaP layers up to 370 µm thick were grown with growth rates up to 93 µm/hr with high reproducibility on bare substrates. The layers demonstrated smooth surface morphology with RMS < 1 nm and high structural quality with FWHM equal to 39 arcsec for layers grown on GaP and 112 arcsec for those grown on GaAs. Growth on OP-GaP templates resulted in 142 µm thick QPM structures deposited at a growth rate of 71 µm/h with good vertical (normal to the layer surface) propagation of the initial pattern. When the growth was performed on OP-GaAs one of the domains showed a trend toward a faceting growth. Further investigations are in progress to equalize the vertical and lateral growth of the two domains, and determine the best orientation of the substrate and pattern in order to achieve structures thick enough for high power nonlinear applications.

A coherent transceiver using a THz quantum cascade (TQCL) laser as the transmitter and an optically pumped molecular laser as the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire high-resolution images of fully illuminated targets, including scale models and concealed objects. Phase stability of the received signal, sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric attenuation at 2.408 THz, the coherence length of the TQCL transmitter will allow coherent imaging over distances up to several hundred meters. Image data obtained with the system is presented.

A coherent transceiver using a THz quantum cascade laser as the transmitter and an optically pumped molecular laser as the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire high-resolution images of fully illuminated targets, including scale models. Phase stability of the received signal, sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric attenuation at 2.408 THz (2.0 dB/m at 40% RH), the coherence length of the QCL transmitter will allow coherent imaging over distances up to several hundred meters. In contrast to non-coherent heterodyne detection, coherent imaging allows signal integration over time intervals considerably longer than the reciprocal of the source, or signal bandwidth, with consequent improvement in the signal-to-noise ratio. Image data obtained with the system will be presented.

The fabrication of thick orientation-patterned GaAs (OP-GaAs) films is reported using a two-step process where an OP-GaAs template with the desired crystal domain pattern was prepared by wafer fusion bonding and then a thick film was grown over the template by low pressure hydride vapor phase epitaxy (HVPE). The OP template was fabricated using molecular beam epitaxy (MBE) followed by thermocompression wafer fusion, substrate removal, and lithographic patterning. On-axis (100) GaAs substrates were utilized for fabricating the template. An approximately 350 μm thick OP-GaAs film was grown on the template at an average rate of ~70 μm/hr by HVPE. The antiphase domain boundaries were observed to propagate vertically and with no defects visible by Nomarski microscopy in stain-etched cross sections. The optical loss at ~2 μm wavelength over an 8 mm long OP-GaAs grating was measured to be no more than that of the semi-insulating GaAs substrate. This template fabrication process can provide more flexibility in arranging the orientation of the crystal domains compared to the Ge growth process and is scalable to quasi-phase-matching (QPM) devices operating from the IR to terahertz frequencies utilizing existing industrial foundries.