CMOS (Complementary Metal Oxide Semiconductor) transmitter and receiver circuits for rotational spectroscopy are demonstrated. The IC’s implemented in 65-nm CMOS consist of a 208-252 GHz transmitter and a 225-280 GHz receiver. Use of CMOS electronics can reduce the cost of electronics for rotational spectrometer application from over $50k to less than $1k. The receiver (RX) includes an on-chip antenna for air-to-chip interface, a 2nd order sub-harmonic down-conversion mixer, a low noise IF amplifier and an amplitude detector. The transmitter (TX) includes an on-chip antenna for chip-to-air interface, Fractional-N synthesizer with a frequency step less than 1 kHz with a built-in frequency shift keying circuit as well as a frequency up-converter to generate the signal at the RF. The integrated circuits were assembled into a rotational spectrometer and utilized to detect numerous gases including Ethanol and Acetone in human breath. It is the first ever demonstration of spectroscopy on pure gases as well as breath using CMOS circuits, and this work paves the way toward a more compact, affordable and efficient rotational spectroscopy system.
Historically, spectroscopy has been a cumbersome endeavor due to the relatively large sizes (3ft – 100ft in length) of
modern spectroscopy systems. Taking advantage of the photoacoustic effect would allow for much smaller absorption
chambers since the photoacoustic (PA) effect is independent of the absorption path length. In order to detect the
photoacoustic waves being generated, a photoacoustic microphone would be required. This paper reports on the
fabrication efforts taken in order to create microelectromechanical systems (MEMS) cantilevers for the purpose of
sensing photoacoustic waves generated via terahertz (THz) radiation passing through a gaseous sample. The cantilevers
are first modeled through the use of the finite element modeling software, CoventorWare®. The cantilevers fabricated
with bulk micromachining processes and are 7x2x0.010mm on a silicon-on-insulator (SOI) wafer which acts as the
physical structure of the cantilever. The devices are released by etching through the wafer’s backside and etching
through the buried oxide with hydrofluoric acid. The cantilevers are placed in a test chamber and their vibration and
deflection are measured via a Michelson type interferometer that reflects a laser off a gold tip evaporated onto the tip of
the cantilever. The test chamber is machined from stainless steel and housed in a THz testing environment at Wright
State University. Fabricated devices have decreased residual stress and larger radii of curvatures by approximately 10X.
Single resonance chemical remote sensing, such as Fourier-transform infrared spectroscopy, has limited recognition
specificity because of atmospheric pressure broadening. Active interrogation techniques promise much greater
chemical recognition that can overcome the limits imposed by atmospheric pressure broadening. Here we introduce
infrared - terahertz (IR/THz) double resonance spectroscopy as an active means of chemical remote sensing that
retains recognition specificity through rare, molecule-unique coincidences between IR molecular absorption and a
line-tunable CO2 excitation laser. The laser-induced double resonance is observed as a modulated THz spectrum
monitored by a THz transceiver. As an example, our analysis indicates that a 1 ppm cloud of CH3F 100 m thick can
be detected at distances up to 1 km using this technique.
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