Avijit received his M.Sc. and B.Sc. degree in Electrical Engineering from Bangladesh University of Engineering and Technology (BUET) in 2016 and 2014 respectively. Prior to joining at OMEMS group, he also worked as a lecturer in BRAC University, Bangladesh.
Avijit’s research focuses on laser induced heat transfer in optical materials, specially in microscale/nanoscale systems at high temperatures. He is also interested in plasmonic bio-sensors and solar photovoltaic system.
Avi defended his thesis in spring 2023. Dr. Das is now at Intel in Portland, Oregon.
Avijit loves travelling and hanging out with new people. He also likes sports i.e., soccer, volleyball, cricket, badminton and tennis.
Thumbs-Up: Road Trip, Hiking, Group hangout, biking, sports and consistent result (at last).
Thumbs-Down: Too much eating, Too much noise and Too much sleep.
In the first project, a theoretical and experimental investigation of photon diffusion is discussed in highly absorbing microscale graphite. A Nd:YAG continuous wave laser is used to heat the graphite samples with thicknesses of 40 \textmum and 100 \textmum. Optical intensities of 10 kW cm-2 and 20 kW cm-2 are used in laser heating. The graphite samples are heated to temperatures of thousands of kelvins within milliseconds, which are recorded by a 2-color, high-speed pyrometer. To compare the observed temperatures, the differential equation of heat conduction is solved across the samples with proper initial and boundary conditions. In addition to lattice vibrations, photon diffusion is incorporated into the analytical model of thermal conductivity for solving the heat equation. The numerical simulations showed close matching between experiment and theory only when including the photon diffusion equations and existing material properties data found in the previously published works with no fitting constants. The results indicate that the commonly overlooked mechanism of photon diffusion dominates the heat transfer of many microscale structures near their evaporation temperatures. In addition, the treatment explains the discrepancies between thermal conductivity measurements and theory that were previously described in the scientifc literature. In the second project, a subwavelength perforated metamaterial absorber is developed for a maximum absorption-to-thermal mass ratio to construct an uncooled thermal infrared (λ∼8-12 \textmum) detector operating at a time constant of ∼7.7 ms, faster than the video frame rates, with a noise equivalent temperature difference (NETD) of 4.5 mKand a detectivity of 3.8\texttimes109cm\textsurdHz/W. The designed metamaterial absorber consists of Ti, SiNx, and Ni nanoscale films with an overall fill factor of ∼28%, where subwavelength interference and Fabry Perot resonance induce an absorption per unit mass of approximately 1.3-27.6 times higher than the previously reported infrared absorbers. We read out the fabricated detector optically via Mach Zehnder interferometer.
An uncooled detector has reached the thermodynamic temperature fluctuation limit, such that 98% of its total noise consisted of phonon and photon fluctuations of the detector body. The device has performed with a detectivity of 3.8\texttimes109 c m H z/W, which is the highest reported for any room temperature device operating in the long-wave infrared (λ∼8-12\textmum). The device has shown a noise-equivalent temperature difference of 4.5 mK and a time constant of 7.4 ms. The detector contains a subwavelength perforated absorber with an absorption-per-unit-thermal mass-per-area of 1.54\texttimes1022 k g -1 m -2, which is approximately 1.6–32.1 times greater than the state-of-the-art absorbers reported for any infrared application. The perforated absorber membrane is mostly open space, and the solid portion consists of Ti, S i N x, and Ni layers with an overall fill factor of ∼28%, where subwavelength interference, cavity coupling, and evanescent field absorption among units induce the high absorption-per-unit-thermal mass-per-area. Readout of the detector occurs via infrared-absorption-induced deformation using a Mach–Zehnder interferometry technique (at λ=633n m), chosen for its long-term compatibility with array reads using a single integrated transceiver.
This paper presents the design, fabrication, and analysis of a subwavelength perforated long-wave infrared (LWIR) broadband absorber, which realizes an average absorption of 92% with a thermal mass 1/3 – 1/24 times smaller than the state-of-the-art infrared absorbers.
The absorption-to-mass ratio of the infrared arrays is enhanced to ∼1.33-7.33 times larger than the previously reported structures by incorporating two design characteristics: first, the coupling of evanescent fields in the air gaps around pixels to create effectively larger pixel sizes and, second, the use of guided-mode resonance (GMR) within the subwavelength metal–dielectric gratings. The bilayer Ti-Si3N4 gratings achieve broadband long-wave infrared (LWIR, λ∼8-12\textmum) absorption by the combined effects of free carrier absorption by the thin Ti films and vibrational phonon absorption by the thick Si3N4 films. In the presence of GMR, this broadband absorption can be enormously enhanced even with low fill factor subwavelength grating cells. Further, the spacing and design of the cells can be modified to form a pixel array structure that couples the light falling in the air gaps via evanescent field coupling. Calculations are performed using the finite difference time domain technique. Excellent broadband absorption is observed for the optimized arrays, yielding maximum absorption of 90% across the LWIR and an average absorption per unit mass (absorption/mass) per pixel of 3.45\texttimes1013kg-1.
High quality factor (Q) photonic devices in the room temperature thermal infrared region, corresponding to deeper long-wave infrared with wavelengths beyond 9 microns, have been demonstrated for the first time. Whispering gallery mode diamond microresonators were fabricated using single crystal diamond substrates and oxygen-based inductively coupled plasma (ICP) reactive ion etching (RIE) at high angles. The spectral characteristics of the devices were probed at room temperature using a tunable quantum cascade laser that was free space-coupled into the resonators. Light was extracted via an arsenic selenide (As2Se3) chalcogenide infrared fiber and directed to a cryogenically cooled mercury cadmium telluride (HgCdTe) detector. The quality factors were tested in multiple microresonators across a wide spectral range from 9 to 9.7 microns with similar performance. One example resonance (of many comparables) was found to reach 3648 at 9.601 µm. Fourier analysis of the many resonances of each device showed free spectral ranges slightly greater than 40 GHz, matching theoretical expectations for the microresonator diameter and the overlap of the whispering gallery mode with the diamond.
Germanium is one of the most commonly used materials in the longwave infrared (λ∼8-12\textmum), but ironically, its absorption coefficient is poorly known in this range. An infrared photothermal common-path interferometry system with a tunable quantum cascade pump laser is used to measure the absorption coefficient of >99.999% pure undoped germanium as a function of wavelengths between 9 and 11 \textmum, varying between about 0.15 and 0.45 cm^-1 over this range.
Radiation diffusion was investigated as a heat transfer mechanism in highly absorbing microscale graphite. A 50 μm thick graphite sheet was heated by a 1064 nm Nd:YAG continuous wave (CW) laser with optical intensities of 10 kW/cm2 and 20 kW/cm2. Extremely high temperatures (i.e., ≥2000 K) were achieved on the graphite sheet within miliseconds, which were measured by a two-color pyrometer. The recorded temperatures were later compared with numerical solutions of differential heat conduction equation. A close match was found between numerical and experimental results only when radiation diffusion was incorporated in the thermal conductivity along with the lattice vibration.
This paper presents a theoretical and experimental investigation of photon diffusion in highly absorbing microscale graphite. A Nd:YAG continuous wave laser is used to heat the graphite samples with thicknesses of 40 μm and 100 μm. Optical intensities of 10 kW cm-2 and 20 kW cm-2 are used in the laser heating. The graphite samples are heated to temperatures of thousands of kelvins within milliseconds, which are recorded by a 2-color, high speed pyrometer. To compare the observed temperatures, differential equation of heat conduction is solved across the samples with proper initial and boundary conditions. In addition to lattice vibrations, photon diffusion is incorporated in the analytical model of thermal conductivity for solving the heat equation. The numerical simulations showed close matching between experiment and theory only when including the photon diffusion equations and existing material properties data found in the previously published works with no fitting constants. The results indicate that the commonly-overlooked mechanism of photon diffusion dominates the heat transfer of many microscale structures near their evaporation temperatures. In addition, the treatment explains the discrepancies between thermal conductivity measurements and theory that were previously described in the scientific literature.