This paper reviews our research on photon upconversion devices for wavelengths from 1.5 μm to 0.87 μm. The 1.5 μm is
chosen for its importance for eye-safe active imaging; whereas 0.87 μm corresponds to the bandgap of GaAs which is the
active region of our high efficiency light emitting diode (LED). The basic idea is to integrate a 1.5 μm detector with a 0.87
µm LED, connected in series. The detected photocurrent drives the LED, thereby achieving the upconversion. Various
approaches of integration methods and device designs have been tested. The upconversion approach provides an alternative
to the standard hybrid integration with readout circuits and may be advantageous for some applications.
Detection of both UV and IR radiation is useful for numerous applications such as firefighting and military
sensing. At present, UV and IR dual wavelength band detection requires separate detector elements. Here
results are presented for a GaN/AlGaN single detector element capable of measuring both UV and IR response.
The initial detector used to prove the dualband concept consists of an undoped AlGaN barrier layer between
two highly doped GaN emitter/contact layers. The UV response is due to interband absorption in the AlGaN
barrier region producing electron-hole pairs which are then swept out of the barrier by an applied electric field
and collected at the contacts. The IR response is due to free carrier absorption in the emitters and internal
photoemission over the work function at the emitter barrier interface, followed by collection at the opposite
contact. The UV threshold for the initial detector was 360 nm while the IR response was in the 8-14 micron
range. Optimization of the detector to improve response in both spectral ranges will be discussed. Designs
capable of distinguishing the simultaneously measured UV and IR by using three contacts and separate IR and
UV active regions will be presented. The same approach can be used with other material combinations to cover
additional wavelength ranges, e.g. GaAs/AlGaAs NIR-FIR dual band detectors.
We report on the progress of devices and applications of quantum-well photodetectors (QWIP) for the terahertz (~ 1-10 THz) spectrum region. We discuss device design and show that the device dark current can by effectively reduced by employing wider quantum barriers. We demonstrate several GaAs/AlGaAs QWIPs for different peak wavelength with background limited infrared performance (BLIP). We report experimental results on intersubband absorption spectra, measured using multi-pass waveguide geometry. We show that the experimentally measured intersubband energy levels agree excellently with the theoretical simulations, provided that many-body effects are taken into consideration, including exchange-correlation and depopulation effects. We report the results of QWIP photo-current spectra and detector responsivity. We discuss the high frequency capability of THz-QWIP and present experimental results of device time response measured using microwave rectification technique. We discuss its application in free space terahertz communication in combination with a terahertz quantum cascade laser (QCL). We discuss the terahertz to near infrared (THz-to-NIR) optical upconversion using a monolithic integration of THz GaAs/AlGaAs QWIP and NIR GaAs/AlGaAs LED, and its potential applications in terahertz imaging.
The development of technologies in the terahertz spectrum or the very-far-infrared region has been slow mainly because of lack of convenient detectors and lasers. We report on the design and simulated performance of quantum-well photodetectors for the terahertz (1 - 10 THz). Quantum well, barrier, and doping parameters are optimized in terms of operating temperature, absorption, and detectivity. We also report on our experimental demonstration of GaAs/AlGaAs photodetectors with background limited infrared performance (BLIP). These devices are suited for a variety of applications, especially in conjunction with the newly developed THz quantum cascade lasers. One of such example is THz free space communication.
Many applications are expected in the terahertz spectral region and terahertz technology is viewed as one of the most important ones in the coming decade. We report on the design and simulated performance of quantum-well photodetectors for the terahertz (1 - 10 THz) or the very-far-infrared region. We also report on our experimental demonstration of GaAs/AlGaAs photodetectors with background limited infrared performance (BLIP). The device dark current characteristics were optimized by employing thick barriers to reduce inter-well tunneling. BLIP operations were observed for all samples (three in total) designed for different wavelengths. BLIP temperatures of 17, 13, and 12 K were achieved for peak detection frequencies at 9.7, 5.4, and 3.2 THz, respectively. Furthermore, we discuss areas of improvement to make these detectors a viable technology.
InSb has been intensively studied in decades and widely used for fabricating high-performance devices because of its good chemical stability, low effective mass, high electron and hole mobility, and narrow band gap. The most important device applications for InSb are in thermal image sensing in the mid-infrared (3-5 μm) spectral range. The industry standard for fabricating InSb-based focal plane arrays for thermal imaging is based on indium bump technology to interconnect the InSb array to a Si-based readout integrated circuit chip. This hybridization is a "one-piece-at-a-time" process and thus time-consuming and costly. An alternative approach is to employ a device that up-converts mid-infrared light to a wavelength below 1 μm, which can then efficiently be detected by Si charged coupled devices. We reported herein such a mid-infrared optical up-converter based on InSb using wafer fusion technology. The up-conversion device consists of an InSb p+nn+ photodiode and a GaAs/AlGaAs LED, which were grown separately and wafer-bonded together. Experimental results demonstrated mid-infrared to 0.84 μm up-conversion operation at 77K. The measured LED external efficiency and photodiode responsivity show that an external up-conversion efficiency of 0.093 W/W was obtained. Effects of electrical gain and photon recycling inside this integrated device are discussed.
An InGaAs photodetector array interconnected with a silicon readout IC is the industry standard for 1.2-1.6 μm imaging applications. However, the indium-bump technique it employs for interconnection makes it expensive. An alternative approach is to combine a CCD with a device that upconverts 1.2-1.6 μm radiation to a wavelength
below 1 μm. Here we report the realization of a 1.5 μm to 0.87 μm optical upconversion device using wafer fusion technology. The device consists of an InGaAs/InP PIN photodetector and an AlGaAs/GaAs light emitting diode (LED). Incoming 1.5 μm light is absorbed by the InGaAs photodetector. The resulting photocurrent drives the GaAs LED, which emits at 0.87 μm. The PIN and LED structures are epitaxially grown on an InP and a GaAs substrate, respectively. The two wafers are wafer fused together, the GaAs substrate is removed, and the sample is processed using conventional microfabrication technology. In this paper, we first present the design and fabrication process of the device. We then discuss the approaches to increase device efficiency. We show, both experimentally and theoretically, that the active layer doping affects the LED internal quantum efficiency, especially under low current injection. An optimum doping value is obtained. The LED extraction efficiency is increased using several approaches, including micro-lens and surface scattering. Overall device efficiency is further improved by introducing a gain mechanism into the photodetector. Our results show the potentials of this integrated photodetector-LED device for 1.2-1.6 μm imaging applications.
Imaging devices working in the near infrared (NIR), especially in the so-called eye-safe range, i.e., around 1.5 mm, have become increasingly important in many military and commercial applications; these include night vision, covert surveillance, range finding and semiconductor wafer inspection. We proposed a new approach in which a wafer-fused optical up-converter, combined with a commercially available charged coupled device (CCD), functions as an infrared camera. The optical up-converter converts incoming infrared light into shorter wavelength radiation that can be efficiently detected by the silicon CCD (cutoff wavelength about 1 mm). An optical up-converter with high efficiency at room-temperature is critical for low cost and large-area infrared imaging applications. A prototype 1.5 mm optical up-converter based on wafer fusion technology has been successfully fabricated. The device consists of an InGaAs/InP pin photodetector and a GaAs/AlGaAs light emitting diode. Experimental results show that the end-to-end up-conversion efficiency is 0.0177 W/W at room-temperature, corresponding to an internal quantum up-conversion efficiency of 76%. In this paper, the design, fabrications and characterization of the optical up-conversion devices is presented. Issues related to device optimization, such as improving internal and external up-conversion efficiency, are addressed. Preliminary results demonstrate the room-temperature up-conversion imaging operation of a pixelated wafer-fused device.
Circular grating surface emitting distributed Bragg reflector lasers exhibit extremely small divergence angles and moderately high output power levels which makes them an attractive candidate for free-space interconnects. Computational simulations are used to model the observed results and assist in the design of new functionality such as focusing and beam shaping. Numerical analysis of the coupled mode equations for DBR lasers and free-space propagation of the emitted field have been performed. Surface emission is realized through grating outcoupling provided by a second order grating while feedback is accomplished with an inner annulus consisting of either a first order or second order grating. These gratings are written using electron beam lithography (EBL) and subsequently etched using ECR-RIE. The flexibility of the EBL process enables a variety of different grating designs to be created on a single sample and subsequently compared during device testing.
Our group has investigated a number of finite conjugate aberration compensation systems in conjunction high numerical aperture objectives for multiple-layer optical data storage. The results of our investigation are that a Burch-type objective lens in conjunction with a Galilean telescope is a compact, simple and effective optical system for spherical aberration compensation.