This Small Business Innovation Research (SBIR) Phase I project will focus on the demonstration of a frequency tunable quantum cascade laser (QCL) operating at terahertz frequencies (1-10 THz) for applications including spectroscopy and non-destructive imaging. This novel source is based on external micropositioning driver for actuated the tuning mechanism that manipulates the QCL's optical waveguide dimensions. This manipulation effectively changes the optical index of the waveguide and the emission frequency of the QCL. The frequency tuning will be actuated electronically, resulting in fast sweeps (>100 Hz) over frequency ranges of >300 GHz. The tunable QCL will be housed in a small footprint cry cooler, and will have output powers greater than 100 µW - an improvement of 3 orders of magnitude over current commercial systems. Phase II improvements will result in larger tunable ranges at center frequencies ranging from 1.5 to 5 THz.
The broader impact/commercial potential of this project will be in detection and characterization of narrow gas lines (MHz) or solid absorption features (GHz) in spectroscopy or spectroscopic imaging, useable for academic, industrial and governmental research. The electrical operation of the MEMS tunable source and the turnkey nature of the cryogenic system will allow non-expert researcher access to the terahertz spectral region. Future uses include incorporation into a swept-source optical coherence tomography system for non-destructive evaluation (NDE). Terahertz frequency radiation is of particular interest for use in NDE because they are able to penetrate materials that are opaque at infrared and visible wavelengths such as structural foams, polymers and paints. Potential applications include the validation of the structural integrity of foams used in the aerospace industry; continuous monitoring of paint processes in the automotive industry; and the validation of pharmaceuticals tablet coatings used in controlled release formulations.
LongWave Photonics accomplished all of the Phase I objectives for the development of a frequency tunable terahertz quantum cascade laser for spectroscopy applications. To achieve this goal a recipe for the permanent and precise attaching of a silicon MEMS structure to a GaAs quantum cascade laser die was achieved. The micron level precision necessary for the aligment of the MEMS and GaAs pieces was achived with an in-house developed die attach machine. Once bonded, the combined MEMS/GaAs device survived repeated thermal cycling to cryogenic temperatures. To actuate the MEMS structure a cryogenically operated motor was demonstrated. The motor survived multiple thermal cyclings to cryogenic temperatures and demonstrated nanometer level positioning accuracy. To control the laser frequency, software was developed to control the position of the motor at cryogenic temperatures. In a novel demosntration, continuous frequency tuning of a terahertz frequency QCL was achieved. This is the broadest demonstration of electronically (software) controlled tuning in the terahertz frequency range to date. To prepare for follow on work, electromagnetic simulations were performed of the QCL structures to optimize for output power, and frequency tuning. Ultimately, it is hope that this tunable laser will be used for spectroscopy of gasses, where the broad tuning range, and the narrow laser linewidth can be used for precise measurements. Immediate applications are expected for remote sensing.
