Measurement of 24-h continuous human CH4 release in a whole room indirect calorimeter.

We describe the technology and validation of a new whole room indirect calorimeter (WRIC) methodology to quantify volume of methane (VCH4) released from the human body over 24 h concurrently with the assessment of energy expenditure and substrate utilization. The new system extends the assessment of energy metabolism by adding CH4, a downstream product of microbiome fermentation that could contribute to energy balance. Our new system consists of an established WRIC combined with the addition of off-axis integrated-cavity output spectroscopy (OA-ICOS) to measure CH4 concentration ([CH4]). Development, validation, and reliability of the system included environmental experiments to measure the stability of the atmospheric [CH4], infusing CH4 into the WRIC and human cross-validation studies comparing [CH4] quantified by OA-ICOS and mid-infrared dual-comb spectroscopy (MIR DCS).Our infusion data indicated that the system measured 24-h [CH4] and VCH4 with high sensitivity, reliability, and validity. Cross-validation studies showed good agreement between OA-ICOS and MIR DCS technologies (r = 0.979, P < 0.0001). Human data revealed 24-h VCH4 was highly variable between subjects and within/between days. Finally, our method to quantify VCH4 released by breath or colon suggested that over 50% of the CH4 was eliminated through the breath. The method allows, for the first time, measurement of 24-h VCH4 (in kcal) and therefore the measurement of the proportion of human energy intake fermented to CH4 by the gut microbiome and released via breath or from the intestine; also, it allows us to track the effects of dietary, probiotic, bacterial, and fecal microbiota transplantation on VCH4.NEW & NOTEWORTHY This is the first time that continuous assessment of CH4 is reported in parallel with measurements of O2 consumption and CO2 production inside a whole room indirect calorimeter in humans and over 24 h. We provide a detailed description of the whole system and its parts. We carried out studies of reliability and validity of the whole system and its parts. CH4 is released in humans during daily activities.

Two-octave-wide (3-12 µm) subharmonic produced in a minimally dispersive optical parametric oscillator cavity.

We report a subharmonic (frequency-divide-by-2) optical parametric oscillator (OPO) with a continuous wavelength span of 3 to 12 µm (-37dB level) that covers most of the molecular rovibrational "signature" region. The key to obtaining such a wide spectral span is the use of an OPO with a minimal dispersion-through the choice of intracavity elements, the use of all gold-coated mirrors, and a special "injector" mirror. The system delivers up to 245 mW of the average power with the conversion efficiency exceeding 20% from a 2.35 µm Kerr-lens mode-locked pump laser.

Broadband high-resolution molecular spectroscopy with interleaved mid-infrared frequency combs.

Traditionally, there has been a trade-off in spectroscopic measurements between high resolution, broadband coverage, and acquisition time. Originally envisioned for precision spectroscopy of the hydrogen atom in the ultraviolet, optical frequency combs are now commonly used for probing molecular ro-vibrational transitions throughout broad spectral bands in the mid-infrared providing superior resolution, speed, and the capability of referencing to the primary frequency standards. Here we demonstrate the acquisition of 2.5 million spectral data points over the continuous wavelength range of 3.17-5.13 µm (frequency span 1200 cm-1, sampling point spacing 13-21 MHz), via interleaving comb-tooth-resolved spectra acquired with a highly-coherent broadband dual-frequency-comb system based on optical subharmonic generation. With the original comb-line spacing of 115 MHz, overlaying eight spectra with gradually shifted comb lines we fully resolve the amplitude and phase spectra of molecules with narrow Doppler lines, such as carbon disulfide (CS2) and its three isotopologues.

Coherence properties of a 2.6-7.5 µm frequency comb produced as a subharmonic of a Tm-fiber laser.

We study the temporal coherence of an ultrabroadband frequency comb produced in a degenerate GaAs optical parametric oscillator (OPO) pumped by a stabilized Tm-fiber comb, by observing multiheterodyne beats in the RF domain. We infer that in such a regime the OPO automatically produces a stable frequency comb that is phase and frequency locked to the pump. By varying intracavity dispersion, we achieve a comb spanning 2.6-7.5 µm at a -20 dB level. Low pump threshold (down to 7 mW), high average power (up to 73 mW), broad spectral coverage, flat spectrum, and high coherence make this comb a source suitable for various applications, foremost dual-comb molecular spectroscopy.

Midinfrared frequency comb from self-stable degenerate GaAs optical parametric oscillator.

We pump a degenerate frequency-divide-by-two optical parametric oscillator (OPO) based on orientation-patterned GaAs with a stable Tm frequency comb at 2 micrometer wavelength and measure the OPO comb offset frequency and linewidth. We show frequency division by two with sub-Hz relative linewidth of the comb teeth. The OPO thermally self-stabilizes and oscillates for nearly an hour without any active control.

Cr:ZnS laser-pumped subharmonic GaAs optical parametric oscillator with the spectrum spanning 3.6-5.6 µm.

Using a subharmonic optical parametric oscillator (OPO) based on orientation-patterned GaAs, we produced a broadband instantaneous output that spans 3.6-5.6 µm at 50-dB level (4.4-5.2 µm at 3 dB level), has 110 mW of average power, and is suitable for producing wideband-frequency combs in the mid-infrared. The OPO was synchronously pumped by a compact Kerr-lens mode-locked femtosecond Cr:ZnS oscillator with the central wavelength 2.38 µm and pulse repetition frequency 175 MHz.

Grating tunable 4-14 µm GaAs optical parametric oscillator pumped at 3 µm.

We demonstrate a broadly and continuously tunable optical parametric oscillator (OPO) based on orientation-patterned GaAs (OP-GaAs) operating at 2 kHz repetition rate. With the choice of the pump wavelength near λ = 3 µm, we were able to achieve tunable output in the whole range of 4-14.2 µm with a linewidth between 2 and 6 cm(-1), using a single OP-GaAs structure with a domain reversal period of 150 µm. The OPO output was tuned using (i) an intracavity diffraction grating, and (ii) fine adjustment of the pump wavelength near 3 µm. In certain portions of the spectrum this system potentially allows fast (sub-millisecond scale) wavelength tuning over > 2500 nm by fast steering the diffraction grating at a fixed pump wavelength.

Measurements of the group delay and the group delay dispersion with resonance scanning interferometer.

We developed a method for group delay and group delay dispersion measurements, based on location of interference resonance peaks. Such resonance peaks can be observed in transmittance or in reflectance when two mirrors are placed parallel to each other and separated by a thin air spacer. By using a novel approach, based on simultaneous processing of the data acquired for different spacer distances we obtained reliable results with high resolution. Measurements were performed both in transmittance and reflectance layouts depending on the reflectivity of the mirror to be measured. The developed method allows dispersion measurements of ultraviolet mirrors and ultra-broadband mirrors spanning more than one optical octave to be performed.

Mid-IR frequency comb source spanning 4.4-5.4 µm based on subharmonic GaAs optical parametric oscillator.

Broadband mid-IR output suitable for producing 1000-nm-wide frequency combs centered at 4.9 µm was achieved in a degenerate subharmonic optical parametric oscillator (OPO) based on 500-µm-long Brewster-angled orientation-patterned GaAs crystal. The OPO was synchronously pumped at 182 MHz repetition rate by 100 fs pulses from a Cr²âº:ZnSe laser with the central wavelength of 2.45 µm and the average power of 100 mW.

Optical terahertz wave generation in a planar GaAs waveguide.

We report generation of terahertz (THz) radiation in a planar 61-microm-thick GaAs waveguide with a TM0 propagation mode, achieved by phase-matched difference frequency mixing. The THz output was centered near 2 THz and had 1 microW average power. As a pump source we utilized both the signal and the idler outputs of a near-degenerate type II synchronously pumped optical parametric oscillator operating near 2 microm with the average powers of 250 and 750 mW, correspondingly.

GaAs optical parametric oscillator with circularly polarized and depolarized pump.

We demonstrate an optical parametric oscillator (OPO) based on GaAs pumped with linearly polarized and circularly polarized light and show that the relative OPO thresholds agree with theoretical expectations. For the circularly polarized pump, the threshold was as low as for the [111]-linearly polarized pump case. The pump was also passed through a Lyot depolarizer to produce pseudo-depolarized light, and the OPO threshold in this case was only 22% higher than that for [001]-linearly polarized pump.

Intracavity terahertz-wave generation in a synchronously pumped optical parametric oscillator using quasi-phase-matched GaAs.

We generated 1 mW of average output power at 2.8 THz (bandwidth of approximately 300 GHz) in a diffraction-limited beam by placing a 6-mm-long quasi-phase-matched GaAs crystal inside the cavity of a synchronously pumped optical parametric oscillator (OPO). The OPO used type-II-phase-matched periodically poled lithium niobate as a gain medium and was pumped by a mode-locked laser at 1064 nm, with a 7 ps pulse duration, 50 MHz repetition rate, and 10 W average output power. The terahertz radiation was generated by difference frequency mixing between the signal and idler waves of the near-degenerate doubly resonant OPO.

Multiphoton absorption and nonlinear refraction of GaAs in the mid-infrared.

We report the wavelength dependencies of the two- and three-photon absorption coefficients of undoped GaAs in the spectral range 1.3-3.5 microm, as well as nonlinear refractive index n2 in the range 1.7-3.25 microm. The data were obtained by using the single-beam Z-scan method with 100-fs-long optical pulses. Anisotropy of the three-photon absorption coefficient was observed and found to be consistent with the crystal symmetry of GaAs.

Optical parametric generation of a mid-infrared continuum in orientation-patterned GaAs.

We have generated an ultrabroad mid-infrared continuum by using single-pass optical parametric generation (OPG) in orientation-patterned GaAs (OP-GaAs). The spectrum spans more than an octave, from 4.5 to 10.7 microm, measured 20 dB down from the peak. The 17.5 mm long, 0.5 mm thick, all-epitaxially-grown OP-GaAs sample with a 166.6-microm quasi-phase-matching period was pumped with 3.1-3.3 microm wavelength, 1 ps pulses up to 2 microJ in energy. The OPG threshold was observed at 55 nJ pump energy with the pump polarized along the [111] crystal direction. The slope efficiency near threshold was 51%, and the external conversion efficiency was as high as 15%.

High-power source of THz radiation based on orientation-patterned GaAs pumped by a fiber laser.

We demonstrate a new source of frequency-tunable THz wave packets based on parametric down-conversion process in orientation-patterned GaAs (OP-GaAs) that produces muW-level THz average powers at the repetition rate of 100 MHz. The OP-GaAs crystal is pumped by a compact all-fiber femtosecond laser operating at the wavelength of 2 mum. Such combination of fiber laser and OP-GaAs technologies promises a practical source of THz radiation which should be suitable for many applications including imaging and spectroscopy.

Optical parametric oscillation in quasi-phase-matched GaAs.

We demonstrate an optical parametric oscillator (OPO) based on GaAs. The OPO utilizes an all-epitaxially-grown orientation-patterned GaAs crystal that is 0.5 mm thick, 5 mm wide, and 11 mm long, with a domain reversal period of 61.2 microm. Tuning either the near-IR pump wavelength between 1.8 and 2 microm or the temperature of the GaAs crystal allows the mid-IR output to be tuned between 2.28 and 9.14 microm, which is limited only by the spectral range of the OPO mirrors. The pump threshold of the singly resonant OPO is 16 microJ for the 6-ns pump pulses, and the photon conversion slope efficiency reaches 54%. We also show experimentally the possibility of pump-polarization-independent frequency conversion in GaAs.

Broadly tunable noncritically phase-matched ZnGeP2 optical parametric oscillator with a 2-microJ pump threshold.

We report a high-repetition-rate (1-10-kHz) optical parametric oscillator (OPO) based on noncritically phase-matched ZnGeP2 (ZGP). The pump source was an OPO based on periodically pole lithium niobate that was pumped in turn by a Q-switched diode-pumped 1-microm Nd:YAG laser. The ZGP OPO yielded continuously tunable output from 3.7 to 10.2 microm by tuning of the pump wavelength from 2.3 to 3.7 microm. At the optimal pump focusing, the minimum ZGP OPO threshold achieved was 2 microJ, which is to our knowledge the lowest ever reported for a singly resonant OPO. The output energy in the 6-8-microm range was > 20 microJ, and the quantum efficiency of converting 1-microm radiation to the mid IR exceeded 10%.

Measurement of the nonlinear coefficient of orientation-patterned GaAs and demonstration of highly efficient second-harmonic generation.

Quasi-phase-matched (QPM) GaAs structures, 0.5 mm thick, 10 mm long, and with 61-mum grating periods, were grown by a combination of molecular-beam epitaxy and hydride vapor phase epitaxy. These were characterized by use of mid-IR second-harmonic generation (SHG) with a ZnGeP(2) (ZGP) optical parametric oscillator as a pump source. The SHG efficiencies of QPM GaAs and QPM LiNbO(3) were directly compared, and a ratio of nonlinear coefficients d(14)(GaAs)/d(33) (LiNbO(3))=5.01+/-0.3 was found at 4.1-mum fundamental wavelength. For input pulse energies as low as 50muJ and approximately 60-ns pulse duration, an internal SHG conversion efficiency of 33% was measured in QPM GaAs.

Laser-induced quantum coherence in a semiconductor quantum well.

The phenomenon of electromagnetically induced quantum coherence is demonstrated between three confined electron subband levels in a quantum well which are almost equally spaced in energy. Applying a strong coupling field, two-photon resonant with the 1-3 intersubband transition, produces a pronounced narrow transparency feature in the 1-2 absorption line. This result can be understood in terms of all three states being simultaneously driven into "phase-locked" quantum coherence by a single coupling field. We describe the effect theoretically with a density matrix method and an adapted linear response theory.

ZnGeP2 optical parametric oscillator with 3.8-12.4-mum tunability.

A ZnGeP>(2) (ZGP) optical parametric oscillator (OPO) with wide mid-IR tunability has been demonstrated. The singly resonant angle-tuned ZGP OPO was pumped by 100-ns erbium laser pulses at lambda =2.93mum and yielded output that was continuously tunable from 3.8 to 12.4 mum (type I phase matching) and from 4 to 10 mum (type II phase matching). An OPO pump threshold was less than 1 mJ in the whole 4-12 mum range of the output, and the quantum conversion efficiency reached 35%. An OPO linewidth was typically a few wave numbers; however, with a single intracavity etalon (uncoated Si plate) in a type II OPO it was narrowed to <0.5cm(-1). We demonstrate the sensitive detection of N(2)O gas with the narrow-linewidth OPO.