(a) Photo of the three cascaded frequency multipliers showing the 0.69 × 0.69 mm integrated diagonal feed horn in the 2.7 THz output stage. (b) Photo of the multipliers with two GaAs amplifiers followed by a larger GaN amplifier. A significantly larger power combined GaAs amplifier was used for the measurements described.
Split block mounting of the 2.7 THz balanced tripler device. Beam leads are used to hold the 3 μm thick device in place and provide the ground contact for the diodes in the balanced antiparallel configuration. The integral waveguide probes can be seen projecting into the small output waveguide at the left and much larger input waveguide in the center.
Calibrated output power spectrum of two THz multiplier sources at room temperature in a pure nitrogen atmosphere. Sources #1 and #2 are two different multipliers of the same design, demonstrating that the performance is repeatable.
Top panel is the complete sequence of tone burst modulated line measurements made on HD broadened by parahydrogen at 18 K. The modulation and HD pressure were fixed while the parahydrogen pressure was varied resulting in total pressures between 0.56 Torr (strongest line) and 7 Torr (weakest line). The bottom panel is a blow up of the higher pressure measurements from the top panel allowing the noise level to be observed. The apparent loss of line strength is due to the broader lines becoming less efficiently modulated in the convolution method used to retrieve the line parameters (Ref. 41). The zero pressure rest frequency was derived from the complete series of similar measurements including the self-broadening, normal hydrogen broadening, helium broadening, and the data shown here (Ref. 42).
Room temperature heterodyne spectrum of water in natural abundance in a 10 cm long absorption path at 2 mTorr of pressure showing 16O, 17O, 18O, and HDO transitions. The 100 GHz of frequency space was sampled in 200 kHz steps resulting in approximately half million data points. The 53,3–52,4 transition is optically thick. The signal-to-noise ratio approaches 105 demonstrating that high sensitivity broadband spectroscopy without any cryogenics is possible above 2 THz.
The Doppler limited absorption profile of the 41,4–30,3 ortho water line recorded at the same 10 kHz per point as the observed Lamb dip shown in the inset. The other inset shows the hyperfine structure of the transition. The half-width and half-height of the dip is ∼75 kHz and is broadened by some residual 60 Hz in the frequency source. Room temperature heterodyne detection was utilized.
The top trace is the amplitude modulated power spectrum in arbitrary units through the 2.3 m absorption cell. The bottom trace is the second derivative tone burst modulated spectrum of methanol vapor. The strongest lines have optical depths of ∼10. The usable source bandwidth extends 10 GHz below and 30 GHz above the frequency range shown.
Enlargement of a portion of Fig. 6 showing the turning point of the E-state K = −8 vt = 2 to K = −9 vt = 1 Q (ΔJ = 0) of methanol near 2.549 THz (bottom trace second derivative) with scale on the left compared with an instrument limited Fourier Transform spectrum in (top trace in absorption) with scale on the right recorded in a 2.7 meter path (Ref. 48). The noise in the bottom trace is about 5 counts peak-to-peak, but the spectrum is oversampled so that it can be smoothed to 1 count without loss of signal. The standing waves are 10–50 counts peak-to-peak. The J = 28 Q-branch line at 2 550 262 MHz was used to stabilize a QCL (Ref. 49).
Doppler limited spectrum of methanol recorded near 2.5191 GHz (narrower line) compared to a quantum cascade laser recording (broader line) of the same frequency window scaled to approximately the same peak height. The quantum cascade laser was locked to a free running methanol laser and recorded at higher pressure to measure the pressure broadening (Ref. 50). The asymmetry in the QCL line shape is due to the strong power slope of the QCL. The strong central feature, the vt = 2-1 202−213 E state line, is measured to have a frequency of 2.519109985(50) THz. The standing wave between the source and detector is apparent, but is small even in comparison to vt = 1, 117 +/− to 124 +/−A state line at 2.518914556(100) THz on the left-hand edge of the spectrum. The line denoted with a U is measured to have a frequency of 2.519148974(150) THz.
Measured and calculated CO frequencies.
Observed and calculated frequencies of CO isotopologues measured in natural abundance.
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