Developing smaller, faster, and more efficient lasers, detectors, and sensors through first-principle design, nanoscale engineering, and prototyping for space communications, computing, lidar ranging, and spectroscopic profiling applications.
Our research and development activities will have a significant impact on the missions of the Exploratory Systems by providing a high-bit-rate transceiver for the space-based optical communication systems, high-performance, multi-spectral IR detectors, and widely-tunable diode lasers with a wider spectral coverage for laser ranging of planetary terrains and for spectroscopic profiling of planetary atmospheres.
Semiconductor Lasers: We pursue a suite of semiconductor laser technologies that combine significant expansion of wavelength range of current technologies (such as Vertical-Cavity Surface-Emitting lasers, VCSELs, and quantum cascade lasers) and exploration of new nanowire nanolaser technology for applications in planetary terrain-ranging and spectroscopic-profiling of planetary atmospheres in the same platform preparatory to conducting aerocapture or direct entry aeromaneuvers. The more traditional diode lasers have advantages of high power and moderate tunability, but suffer from limited wavelength choices. The technology is more mature in near-infrared range but coverage in longer (mid-IR and above) and shorter wavelength (visible to UV) is still in the early stage of development. These wavelength ranges are necessary for planetary ranging and spectroscopic profiling of planetary atmosphere depending on the atmospheric conditions of a given planet. On the other hand, nanowire lasers show strong promise in covering these wavelength ranges. Nanowires as shown in Fig.1 are grown vertically on substrates and have a dimension in the range of 100s of nanometers. One of advantages of such nanolasers is that electrons are confined by air/vacuum. This advantage significantly broadens the material base that can be used for lasing and makes lasing possible beyond the near-IR range. We have in-house capability to grow nanowires of various inorganic semiconductors for UV and visible range lasing. To achieve wide tunability, we explore strong wavelength dependence on nanowire diameter for stage-wise tuning to cover a wider range. Complimentary to this approach, we intend to expand wavelength further to mid- and long-wavelength-IR range by developing quantum cascade lasers. Such a combined approach will allow us to cover most of the wavelength ranges between 300 nanometers to 10 micrometers. Figure 1. Nanowires to be used as image arrays for exploratory missions.
In the area of semiconductor laser based transceiver technology for space-based optical communications, we pursue a novel modulation format to increase data rate based on VCSELs and their arrays. Our current focus is to explore the multimode dynamics of VCSELs to increase data rate from a few GHz to over 60 GHz. In addition, to complement VCSEL technology, we also explore more advanced technology based on nanowire lasers, which are much smaller than VCSELs. The advantages of such lasers include higher efficiency, a potentially simpler fabrication process, and a denser array than VCSEL. These lasers are potentially important for high data-rate interconnects for on/off chip (board) communications, for lab-on-a-chip spectrometers, and for data processing in autonomous and intelligent space vehicles and orbiters. Figure 2. Right: image of 2D nanowire arrays; left: light field in a nanolaser.
In the area of IR detectors , nanowire array can be used to develop a large pixel array where each pixel is made of many single crystal, 5-100 nm diameter semiconductor nanowires (SNWs). While each pixel is electronically accessible, a multi-color focal plane array is created by tuning the spectral response of individual pixels within 0.5 - 5 microns range by controlling the nanowire diameter and/or dopants. The key advantages of the proposed technology are: 1) No need of bulky optical elements for frequency selection and filtering, since such filtering is provided by addressing a nanowire with the appropriate diameter; 2) Multi-spectral imaging is possible on a single chip by having a nanowire diameter distribution across the chip; 3) Improved thermal performance by bandgap tuning via nanowire diameter would allow increased operation temperatures compared to the bulk InSb detectors, which are better suited for the extreme temperature environment of the Moon and Mars.
Optoelectronic devices are critical elements of any space mission system, with application ranging from detectors, sensors, and lasers, to solar cells. For example, semiconductor diode laser-based high data-rate transmitter technologies are critical to communication and computing for Lunar and Mars human and robotic missions. Bi-directional space backbone links between Earth and/or Earth orbit to planetary surfaces and orbits, communications between orbiters, crew vehicles, and between planetary surface explorers all require transmitter technologies that go far beyond current technology. In addition, high-bit rate transmitter arrays are important for on/off chip (board) communication for data processing and computing needs on-board space vehicles and orbiters. In terms of laser sources, many planetary exploratory systems need capabilities to map and range planetary terrain , to detect potential hazards, and to spectroscopically profile planetary atmospheres. Finally, NASA's exploratory missions to the Moon and Mars depend heavily on ultraviolet (UV) and infrared (IR) detectors for spectroscopy and imaging. Applications include chemical sensing for profiling planetary atmospheres, detection of biohazards in space habitats, and the study of chemical composition of the interstellar medium.
In terms of semiconductor laser sources , current technologies give only limited choices of wavelength range, tunability, reliability and power level for space exploration applications. Revolutionary new approaches are needed to broaden the choices for system designers, and to significantly improve the existing technology in terms of power level and wavelength range. While IR detector technology is very mature, past experience with quantum well IR detectors demonstrate that technological innovations can enable new sensors that greatly enhance mission capabilities. In addition, the extreme environment of the Moon requires detectors with better thermal performance. Finally, UV detector development is currently hampered by materials quality and substrate issues.