UNCOOLED MEMS TERAHERTZ MICROSPECTROMETER FOR STANDOFF DET.
Creators: name: University Of California Office Of The President
Publisher: name: University of California Office of the President
publisherIdentifier: https://ror.org/00dmfq477
publisherIdentifierScheme: ROR
Year: 2024

description: We propose the demonstration of a new approach to terahertz radiation (commonly referred to as T-Rays) detection based on the exchange of momentum between radiation and an uncooled microcantilever. Due to recently invented sources, T-Rays (wavelength = 100–1000 microns) are being rapidly exploited because of their unique properties. T-Rays penetrate normal wrapping materials such cloth and paper but interact strongly with explosive materials, chemical and biological agents. They are also important in surveillance, night vision and medical imaging. They are being applied for quality control in food and pharmaceuticals inside packaging. Thus improvement of the sensitivity of detection in the terahertz region is very important. However, the main problem for remote detection using T-Rays is that their spectral absorption signatures, for example of explosive materials, coincides with strong absorption features by atmospheric water vapor. This results in the significant attenuation of the signal through the atmosphere and the standoff detection distance is limited to less than 100 m. As sensitive detectors have to be cooled between 4-20 K, uncooled detectors are very desirable because cooling systems add cost and cause reliability problems that are incompatible with most applications. The uncooled detectors, which are less sensitive than cooled ones, measure the changes of electrical parameters (such as resistance or capacitance) due to the change of an absorber temperature. This limits the sensitivity due to heat exchange with the environment. One of the most promising directions for improvement is related to the emerging technology of high quality factor micro-cavities. These systems use a high quality micro-cavity, one mirror of which is a tunable micro electro-optical and mechanical systems element with a high reflectivity coating. Originally, these systems were developed for scientific applications such as ring-down spectroscopy of single atoms. Recently a high quality factor micro-cavities has been applied for micromirror laser cooling due to the effect of radiation pressure. In these experiments one microcavity mirror was a movable micromirror mounted on a microcantilever which senses the radiation pressure of the laser beam. This optical scheme could cool the microcantilevers to their quantum mechanical ground states. We propose to exploit the radiation pressure on microcantilevers in a microcavity for terahertz detection. The radiation of a heterodyne laser is mixed with the terahertz signal and sent to an optical microcavity, one component of which is a mirror made by coating the top surface of a micro-cantilever. The cantilever oscillations are measured by an interferometer. The significant improvement of sensitivity is due to enhancement of the intracavity field and the non-absorption mechanism of detection. Currently microcantilever-based infrared detectors exploit the thermo-mechanical effect, in which a bilayer microcantilever illuminated by radiation exhibits bending due to the difference in the thermal expansion coefficients of the different layers. This measurement method has limitations related to temperature fluctuations from heat and radiation exchange with the environment. The method proposed here is potentially less sensitive to temperature fluctuations. The proposed method could be used as a high resolution miniature spectrometer. In the heterodyne detection method, the microcantilever senses the beats between the heterodyne field and the spectral components of the signal. The spectral resolution of the proposed spectrometer is defined by the heterodyne source line width. The signal spectrum can be scanned by tuning the heterodyne frequency. This radiation pressure sensor potentially has an order of magnitude higher sensitivity than the best conventional uncooled detectors. The objectives of the proposal include: • Experimental study of the process of a broad-band radiation interacting with micromechanical systems in a high quality factor optical cavity. • We will study both theoretically and experimentally the limiting factors for these detectors including thermal noise, background fluctuations, vibrational noise, etc. We will develop a noise model for the system. • Understanding the dynamics of the system to maintain stable cantilever operation and avoid instabilities and nonlinear hysteresis effects. • Development of amplitude and phase modulated signal processing methods to improve signal to noise.