Gamani Karunasiri

Real-time imaging using a 2.8 THz quantum cascade laser and uncooled infrared microbolometer camera

Real-time imaging in the terahertz (THz) spectral range was achieved using a milliwatt-scale, 2.8 THz quantum cascade laser and an uncooled, 160 x 120 pixel microbolometer camera modified with Picarin optics. Noise equivalent temperature difference of the camera in the 1-5 THz frequency range was estimated to be at least 3 K, confirming the need for external THz illumination when imaging in this frequency regime. Despite the appearance of fringe patterns produced by multiple diffraction effects, single-frame and extended video imaging of obscured objects show high-contrast differentiation between metallic and plastic materials, supporting the viability of this imaging approach for use in future security screening applications.

Bi-material terahertz sensors using metamaterial structures.

In this paper we report on the design, fabrication and characterization of terahertz (THz) bi-material sensors with metamaterial absorbers. MEMS fabrication-friendly SiOx and Al are used to maximize the bimetallic effect and metamaterial absorption at 3.8 THz, the frequency of a quantum cascade laser illumination source. Sensors with different configurations were fabricated and the measured absorption is near 100% and responsivity is around 1.2 deg/μW, which agree well with finite element simulations. The results indicate the potential of using these detectors to fabricate focal plane arrays for real time THz imaging.

Microelectromechanical systems bimaterial terahertz sensor with integrated metamaterial absorber

This Letter describes the fabrication of a microelectromechanical systems (MEMS) bimaterial terahertz (THz) sensor operating at 3.8 THz. The incident THz radiation is absorbed by a metamaterial structure integrated with the bimaterial. The absorber was designed with a resonant frequency matching the quantum cascade laser illumination source while simultaneously providing structural support, desired thermomechanical properties and optical readout access. Measurement showed that the fabricated absorber has nearly 90% absorption at 3.8 THz. A responsivity of 0.1°/μW and a time constant of 14 ms were observed. The use of metamaterial absorbers allows for tuning the sensor response to the desired frequency to achieve high sensitivity for potential THz imaging applications.

Normal incident InGaAs/GaAs multiple quantum well infrared detector using electron intersubband transitions

A normal incident infrared detector has been fabricated using electron intersubband transition in a InGaAs/GaAs quantum well structure. With the light polarized in the plane of the layers (normal incident) a nominally forbidden absorption peak was observed. Such an absorption is most likely a result of spin‐flip intersubband transitions induced by the spin‐orbit coupling. In addition, for the light polarized in the plane of incidence, the usual intersubband absorption due to envelope function transition is observed. The responsivity of 0.2 A/W was obtained for the normal incident infrared on the detector. This work demonstrates the fabrication of high sensitivity quantum well infrared detectors operating in the normal incident mode without fabricating grating structures on the device for focal plane applications.

Strong terahertz absorption using SiO2/Al based metamaterial structures

Metamaterial absorbers with nearly 100% absorption in the terahertz (THz) spectral band have been designed and fabricated using a periodic array of aluminum (Al) squares and an Al ground plane separated by a thin silicon dioxide (SiO2) dielectric film. The entire structure is less than 1.6 mm thick making it suitable for the fabrication of microbolometers or bi-material sensors for THz imaging. Films with different dielectric layer thicknesses exhibited resonant absorption at 4.1, 4.2, and 4.5 THz with strengths of 98%, 95%, and 88%, respectively. The measured absorption spectra are in good agreement with simulations using finite element modeling.

Near- and mid-infrared detection using GaAs∕InxGa1−xAs∕InyGa1−yAs multiple step quantum wells

A dual-band multiple-quantum-well infrared photodetector capable of simultaneously detecting wavelengths near 0.9 μm and 10 μm has been fabricated using GaAs∕InGaAs step quantum wells. The detection of the near (0.82–0.95 μm)- and mid (9–11 μm)-infrared wavelength bands was achieved using interband and intersubband transitions. The measured peak responsivities of the near- and mid-infrared bands were 0.4 A∕W and 1 A∕W, respectively, at 0.8 V bias across the device. The broken symmetry of the step quantum well allows transitions from the ground states of heavy and light holes to the first-excited electron state allowing the photoexcited carriers to be efficiently collected. The estimated values of the detectivities for near- and mid-infrared bands at 40 K and 0.8 V bias are approximately 4.5×109cm(Hz)1∕2∕W and 1.1×1010cm(Hz)1∕2∕W, respectively.

Fabrication of a microelectromechanical directional sound sensor with electronic readout using comb fingers

By emulating the hearing organ of the Ormia ochracea fly, a microelectromechanical sound sensor was fabricated which is able to determine the direction of incident sound despite an overall size much smaller than the wavelength of interest. The sensor consists of two wings that are coupled in the middle and attached to the surrounding substrate by two legs. The design incorporated interdigitated comb fingers on the wings and the substrate which enables electrostatic (capacitive) readout. Measured electrical response showed a strong dependence on the direction of incident sound.

Al/SiOx/Al single and multiband metamaterial absorbers for terahertz sensor applications

Abstract. To increase the sensitivity of uncooled thermal sensors in the terahertz (THz) spectral range (1 to 10 THz), we investigated thin metamaterial layers exhibiting resonant absorption in this region. These metamaterial films are comprised of periodic arrays of aluminum (Al) squares and an Al ground plane separated by a thin silicon-rich silicon oxide (SiOx) dielectric film. These standard MEMS materials are also suitable for fabrication of bi-material and microbolometer thermal sensors. Using SiOx instead of SiO2 reduced the residual stress of the metamaterial film. Finite element simulations were performed to establish the design criteria for very thin films with high absorption and spectral tunability. Single-band structures with varying SiOx thicknesses, square size, and periodicity were fabricated and found to absorb nearly 100% at the designed frequencies between three and eight THz. Multiband absorbing structures were fabricated with two or three distinct peaks or a single-broad absorption band. Experimental results indicate that is possible to design very efficient thin THz absorbing films to match specific applications.

Narrowband terahertz emitters using metamaterial films

In this article we report on metamaterial-based narrowband thermal terahertz (THz) emitters with a bandwidth of about 1 THz. Single band emitters designed to radiate in the 4 to 8 THz range were found to emit as high as 36 W/m(2) when operated at 400 °C. Emission into two well-separated THz bands was also demonstrated by using metamaterial structures featuring more complex unit cells. Imaging of heated emitters using a microbolometer camera fitted with THz optics clearly showed the expected higher emissivity from the metamaterial structure compared to low-emissivity of the surrounding aluminum.

Fabrication of Bi-material MEMS detector arrays for THz imaging

Recently, there has been a significant interest in Terahertz (THz) technology, primarily for its potential applications in detection of concealed objects as well as in medical imaging for non-invasive diagnostics. This region of the spectrum has not been fully utilized due to lack of compact and efficient THz sources and detectors. However, there are several reports recently on real-time THz imaging using uncooled microbolometer camera and quantum cascade laser (QCL) operating as a THz illuminator. The cameras used in these studies are optimized for infrared wavelengths and do not provide optimal sensitivity in the THz spectral range. The fabrication of microbolometer focal plane arrays (FPAs) is relatively complex due to the required monolithic integration of readout electronics with the MEMS pixels. The recent developments in bi-material based infrared FPAs, utilizing optical readout, substantially simplifies the FPA fabrication process by decoupling readout and sensing. In this paper, design and fabrication of a bi-material based FPAs, optimized for the THz wavelengths, as well as design and integration of the readout optical system for real-time imaging will be described.