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Experimental setup for dual-comb photoacoustic spectroscopy. Two frequency combs (lower left) are combined on a beam-splitter and filtered. The combined beam is focused onto a sample in a water bath (upper left). A focused ultrasound transducer measures the photoacoustic pressure waves emitted by the sample when it absorbs light. The transducer output is amplified and filtered before being digitized with a field-programmable gate array (FPGA). Approximately 6% of the combined beam is picked off by a calcium-fluoride wedge (center) and used as a reference signal. Another 6% of the beam is used for triggering the data acquisition system (right-center). Additional details are discussed in the methods section.
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Spectrally resolved photoacoustic imaging is promising for label-free imaging in optically scattering materials. However, this technique often requires acquisition of a separate image at each wavelength of interest. This reduces imaging speeds and causes errors if the sample changes in time between images acquired at different wavelengths. We demon...
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... setup. Figure 2 shows the experimental setup for DCPAS. We use two self-referenced erbium fiber frequency combs at 160 MHz repetition rate. ...
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... addition to considering these arguments, we also evaluated the potential of DCPAS by directly comparing DCPAS with a PAS system using a tunable pulsed optical parametric oscillator (OPO). As shown in Supplementary Fig. 1 and Supplementary Fig. 2, the current DCPAS implementation achieves a similar SNR to that obtained with pulsed PAS with 20 nJ pulse energies after accounting for wavelength multiplexing. Existing fibercoupled OPO-based PAS systems can provide up to 20× more pulse energy and a correspondingly higher SNR. ...
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... results and cannot imply endorsement by the National Institute of Standards and Technology.). Each of these samples is laminated onto the top of a N-BK7 wedge by gently applying pressure with a pair of tweezers. The wedge is sealed against an opening in the bottom of a water bath so that the sample is immersed in the water (location (v) in Fig. 2) and oriented toward the transducer. The distance from the sample to the transducer is equal to the transducer focal length of approximately 2 cm. We also measured the optical absorption spectra of the PDMS and paraffin in transmission-mode with a NIR spectrophotometer (Perkin-Elmer Lambda 1050 equipped with a 3-detector module [The ...
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... a 1550 nm ± 5 nm bandpass filter and then amplified to ≈400 mW with a triple-pumped erbium-doped fiber amplifier. After amplification, the light from each comb is spectrally broadened in ≈3 cm of highly nonlinear fiber with a dispersion of ≈2.2 ps nm −1 km −1 . This yields ≈60 mW of light from each comb spanning 158 THz to 187 THz (location i in Fig. 2). This 158 THz to 187 THz light from each comb is combined on a beam-splitter, transmitted through a 187 THz low-pass filter and a linear polarizer, and coupled into singlemode PM1550 fiber (location ii in Fig. 2) for transport to the PA setup. The light is then launched into free space by a f = 50.8 mm reflective collimator. About 88% ...
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... fiber with a dispersion of ≈2.2 ps nm −1 km −1 . This yields ≈60 mW of light from each comb spanning 158 THz to 187 THz (location i in Fig. 2). This 158 THz to 187 THz light from each comb is combined on a beam-splitter, transmitted through a 187 THz low-pass filter and a linear polarizer, and coupled into singlemode PM1550 fiber (location ii in Fig. 2) for transport to the PA setup. The light is then launched into free space by a f = 50.8 mm reflective collimator. About 88% of the light is transmitted through a CaF 2 wedge and ≈6% is reflected by both the front and back faces. The back reflection is coupled into single-mode fiber and detected by an extended InGaAs photodiode ...
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... for transport to the PA setup. The light is then launched into free space by a f = 50.8 mm reflective collimator. About 88% of the light is transmitted through a CaF 2 wedge and ≈6% is reflected by both the front and back faces. The back reflection is coupled into single-mode fiber and detected by an extended InGaAs photodiode (location (iii) in Fig. 2) to record the optical excitation spectrum. The front reflection is coupled into a single-mode fiber delay line, for ≈100 μs of delay. Then it is detected by a standard InGaAs photodiode to provide a trigger for the FPGA data acquisition ...
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... the ultrasound transducer, the PA signal is amplified by ≈80 dB and filtered with a 15 MHz low-pass filter before it is input to the data acquisition system, as shown in Fig. 2. The optical excitation signal is filtered with a 50 MHz low-pass filter after the photodetector and before the data acquisition system. The PA signal and the optical excitation signal are digitized on separate channels of the data acquisition system, and the resulting time-domain signals are interferograms, as shown in Fig. 3a, b. For ...
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... PA signal and the optical excitation signal are digitized on separate channels of the data acquisition system, and the resulting time-domain signals are interferograms, as shown in Fig. 3a, b. For the SNR calculations used in Fig. 5, the time-domain SNR is defined as the peak height of the interferogram divided by the standard deviation of the noise outside of the interferogram centerburst (see Supplementary Fig. 2 and Supplementary Note 1). As the SNR is low for the weakly absorbing polymer samples, multiple interferograms are coherently averaged in real-time as is common in DCS 34,45 . ...
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... residual phase noise from the interferograms before co-adding them for long-term averaging 34,45 . To measure slow phase drifts, the phase correction firmware requires a stronger signal than is present in a single PA interferogram. We resolve this issue by adding the optical DCS voltage to the DCPAS voltage as a delayed trigger signal as shown in Fig. ...
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... Photoacoustic spectroscopy (PAS) is an analytical technique used for detecting and quantifying trace gases in various samples [1][2][3][4] . This technique capitalizes on the photoacoustic effect, which occurs when a sample absorbs light energy and then emits acoustic waves due to the rapid thermal expansion caused by the absorbed energy [5][6][7][8] . ...
... Through the utilization of this scheme, our CE-PAS sensor ensures robust and reliable performance over extended periods, offering substantial advantages for continuous monitoring applications. [28] 0.7 166 450 @NG 2.52×10 -10 NG PAS 1.654 CH4 [29] 10 NA 50 @700s 2.77×10 -10 ±8% (1.1 h) 1.683 CHCl3 [30] 24.5 NA 38 @180s 1.24×10 -9 ±2% (1.4 h) 1.575 H2S [31] 200 NA 11 @1000s 1.1×10 -9 NG This work 1.531 C2H2 9.5 100 1.2 @400s 1.37×10 -11 ±1.16% (4 h) ±2% (15 day) 1 : MDL with different integration time, NA: not applicable, NG: not given. ...
We present a high sensitivity and long-term stability cavity-enhanced photoacoustic spectroscopy (CE-PAS) system with optical cavity and acoustic frequency dual-locking scheme for trace acetylene (C2H2) detection. The first mechanism involves locking the optical wavelength to the cavity resonance by comparing the phase-sensitive component of the reflected light with a reference signal in a feedback loop. The second locking mechanism controls the gas proportion inside the photoacoustic cell to lock the acoustic frequency, suppressing the drift caused by environmental temperature variation. By minimizing amplitude fluctuations through dual-locking, the sensor achieves improved stability with reduced fluctuations from ±10.12% to ±1.16%. The linear responsivity and excellent linearity of the sensor are demonstrated over a concentration variation spanning four orders of magnitude. Experimental results showcase a minimum detection limit of 1.2 parts-per-billion (ppb) at an integration time of 400 s, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 1.37×10-11 cm-1 ·W· Hz-1/2 for C2H2 detection. Long-term stability is within ±2% over a 15-day period. The combination of high sensitivity and long-term stability make this CE-PAS sensor suitable for a wide range of applications in environmental monitoring, industrial process control, and gas leak detection.
... A sound transducer such as a microphone can then be employed, together with gas molecules, to form a dual-comb detector based on optical-to-acoustic-to-electric energy conversion. The advantage of this photoacoustic method is its wavelength independence, therefore making it applicable in dual-comb measurements 24,25 ranging from the ultraviolet (UV) to the mid-infrared (MIR), even to the terahertz (THz), without detector switching. However, similar to the limitations imposed by a single photoreceiver in conventional DCS, the use of a wideband microphone imposes restrictions on the dynamic range and the ultimate spectral resolution, due to the simultaneous sampling of all photoacoustic signals and the time window of the Fourier transform. ...
The extension of dual-comb spectroscopy (DCS) to all wavelengths of light along with its ability to provide ultra-large dynamic range and ultra-high spectral resolution, renders it extremely useful for a diverse array of applications in physics, chemistry, atmospheric science, space science, as well as medical applications. In this work, we report on an innovative technique of quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy (QEMR-PAS), in which the beat frequency response from a dual comb is frequency down-converted into the audio frequency domain. In this way, gas molecules act as an optical-acoustic converter through the photoacoustic effect, generating heterodyne sound waves. Unlike conventional DCS, where the light wave is detected by a wavelength-dependent photoreceiver, QEMR-PAS employs a quartz tuning fork (QTF) as a high-Q sound transducer and works in conjunction with a phase-sensitive detector to extract the resonant sound component from the multiple heterodyne acoustic tones, resulting in a straightforward and low-cost hardware configuration. This novel QEMR-PAS technique enables wavelength-independent DCS detection for gas sensing, providing an unprecedented dynamic range of 63 dB, a remarkable spectral resolution of 43 MHz (or ~0.3 pm), and a prominent noise equivalent absorption of 5.99 × 10-6 cm-1·Hz-1/2.
... More than 10 times faster acquisition with mid-IR comb-based PAS was obtained by replacing the conventional FTS with phase-controlled FTS, albeit at the expense of reduced performance [41]. Various fast and sensitive methods of comb-based PAS have been developed using the dual-comb approach, but they were all implemented in the near-IR spectra range around 1.5 µm [42][43][44][45][46]. ...
We present the first mid-infrared optical frequency comb spectrometer employing an absorption cell based on self-fabricated, all-silica antiresonant hollow-core fiber (ARHCF). The spectrometer is capable of measuring sub-mL sample volumes with 26 m interaction length and noise equivalent absorption sensitivity of 8.3 × 10−8 cm−1 Hz−1/2 per spectral element in the range of 2900 cm⁻¹ to 3100 cm⁻¹. Compared to a commercially available multipass cell, the ARHCF offers a similar interaction length in a 1000 times lower gas sample volume and a 2.8 dB lower transmission loss, resulting in better absorption sensitivity. The broad transmission windows of ARHCFs, in combination with a tunable optical frequency comb, make them ideal for multispecies detection, while the prospect of measuring samples in small volumes makes them a competitive technique to photoacoustic spectroscopy along with the robustness and prospect of coiling the ARHCFs open doors for miniaturization and out-of-laboratory applications.
... The absorption spectrum needs to be extracted from the large background signal, which is not a trivial task especially for weak absorbance. Compared to direct absorption measurements, DCS can also be performed by taking advantage of other spectroscopic techniques such as photoacoustic and photothermal detection [23][24][25][26] . These indirect absorption measurements enable the background-free detection of molecular spectra, where only the comb lines absorbed by the gas medium can generate the photoacoustic/thermal multiheterodyne beatnotes. ...
... Sadiek et al. reported the first PAS using a frequency comb and implemented a Fourier transform spectrometer (FTS) to modulate the intensity of the frequency comb 29 . To eliminate the mechanical parts in FTS, photoacoustic DCS has been recently demonstrated for measuring gaseous acetylene (C 2 H 2 ) 23 and polymer films 24 . In these proof-of-concept experiments, the detection sensitivity still needs to be improved, i.e., a minimum detection limit (MDL) of 10 ppm was achieved for C 2 H 2 detection at a recording time of 1000 s 23 . ...
Photoacoustic dual-comb spectroscopy (DCS), converting spectral information in the optical frequency domain to the audio frequency domain via multi-heterodyne beating, enables background-free spectral measurements with high resolution and broad bandwidth. However, the detection sensitivity remains limited due to the low power of individual comb lines and the lack of broadband acoustic resonators. Here, we develop cavity-enhanced photoacoustic DCS, which overcomes these limitations by using a high-finesse optical cavity for the power amplification of dual-frequency combs and a broadband acoustic resonator with a flat-top frequency response. We demonstrate high-resolution spectroscopic measurements of trace amounts of C 2 H 2 , NH 3 and CO in the entire telecommunications C-band. The method shows a minimum detection limit of 0.6 ppb C 2 H 2 at the measurement time of 100 s, corresponding to the noise equivalent absorption coefficient of 7 × 10 −10 cm ⁻¹ . The proposed cavity-enhanced photoacoustic DCS may open new avenues for ultrasensitive, high-resolution, and multi-species gas detection with widespread applications.
... It can diversify the applications of a single DCS platform to optimally detect atmospheric trace gases, large molecules, liquids, and solids, for ARTICLE pubs.aip.org/aip/app example, in photo-acoustic measurements 32,33 or muscle tissue measurements where broad spectral coverage and a fast update rate are needed while coarse spectral resolution is adequate. 34 We show that a free-form dual-comb spectrometer using programmable combs can break free from the traditional preset and fixed repetition rates of the combs used in traditional DCS. ...
We explore the advantages of a free-form dual-comb spectroscopy (DCS) platform based on time-programmable frequency combs for real-time, penalty-free apodized scanning. In traditional DCS, the fundamental spectral point spacing, which equals the comb repetition rate, can be excessively fine for many applications. While fine point spacing is not itself problematic, it comes with the penalty of excess acquisition time. Post-processing apodization (windowing) can be applied to tailor the resolution to the sample, but only with a deadtime penalty proportional to the degree of apodization. The excess acquisition time remains. With free-form DCS, this deadtime is avoided by programming a real-time apodization pattern that dynamically reverses the pulse periods between the dual frequency combs. In this way, one can tailor the spectrometer’s resolution and update rate to different applications without penalty. We show the operation of a free-form DCS system where the spectral resolution is varied from the intrinsic fine 160 MHz up to 822 GHz by applying tailored real-time apodization. Because there is no deadtime penalty, the spectral signal-to-noise ratio increases linearly with resolution by 5000× over this range, as opposed to the square root increase observed for post-processing apodization in traditional DCS. We explore the flexibility to change resolution and update rate to perform hyperspectral imaging at slow camera frame rates, where the penalty-free apodization allows for optimal use of each frame. We obtain dual-comb hyperspectral movies at a 20 Hz spectrum update rate with broad optical spectral coverage of over 10 THz.
... Also, the narrow acoustic bandwidths of the cavity-enhanced photoacoustic (PA) systems (e.g. 1 Hz in ref. 15) limit their overall performances (such as acquisition speed and spectral width). Recently, comb-enabled multiplexed or broadband PAS [16][17][18][19][20] and photothermal spectroscopy 21 have been explored yet with sensitivities limited to sub-ppm (parts per million) levels or above, barely sufficient for trace gas detection. Hence, a novel strategy that improves the sensitivity within a wide acoustic bandwidth for real-time multiplexed PA sensing (that potentially works at any wavelength) is highly demanded. ...
... An ultrasound comb signal is produced by the dual-comb light via the photoacoustic effect and is then detected by the probe light through the optomechanical coupling. the dual-comb lines and the molecules causes a series of superimposed PA waves [18][19][20] , which vibrate the nanomechanical membrane immersed in the sample molecules, as shown in the middle panel in Fig. 1a. The displacement (dz m ) of the membrane is monitored at an ultrahigh sensitivity by recording the phase variation (dφ) of the reflected probe field from the MIM system (the lower panel of Fig. 1a). ...
... Accordingly, we obtain an NNEA of 1.71 × 10 −11 cm −1 ·W·Hz −1/2 (Methods). The results manifest nearly two orders of magnitude sensitivity improvement compared to the existing comb-enabled PAS [16][17][18][19][20] and photothermal systems 21 (see Table 1). Also, the DCOS signals depend linearly on the gas concentrations and the excitation powers (Fig. 4d), favoring quantitative gas analysis. ...
Optical cavities are essential for enhancing the sensitivity of molecular absorption spectroscopy, which finds widespread high-sensitivity gas sensing applications. However, the use of high-finesse cavities confines the wavelength range of operation and prevents broader applications. Here, we take a different approach to ultrasensitive molecular spectroscopy, namely dual-comb optomechanical spectroscopy (DCOS), by integrating the high-resolution multiplexing capabilities of dual-comb spectroscopy with cavity optomechanics through photoacoustic coupling. By exciting the molecules photoacoustically with dual-frequency combs and sensing the molecular-vibration-induced ultrasound waves with a cavity-coupled mechanical resonator, we measure high-resolution broadband ( > 2 THz) overtone spectra for acetylene gas and obtain a normalized noise equivalent absorption coefficient of 1.71 × 10⁻¹¹ cm⁻¹·W·Hz−1/2 with 30 GHz simultaneous spectral bandwidth. Importantly, the optomechanical resonator allows broadband dual-comb excitation. Our approach not only enriches the practical applications of the emerging cavity optomechanics technology but also offers intriguing possibilities for multi-species trace gas detection.
... Hz in Ref. 15) limit their overall performances. Recently, comb-enabled multiplexed or broadband PAS [16][17][18][19][20] and photothermal spectroscopy 21 have been explored yet with sensitivities limited to sub-ppm (parts per million) levels or above, barely sufficient for trace detection. Hence, a novel strategy that improves the sensitivity within a wide acoustic bandwidth for real-time multiplexed PA sensing (that potentially works at any wavelength) is highly demanding. ...
... Accordingly, we obtain a normalized equivalent noise absorption (NNEA) coefficient down to 1.71×10 -11 cm -1 ·W·Hz -1/2 (Methods). The results manifest orders-of-magnitude sensitivity improvement comparing to the existing comb-enabled PAS [16][17][18][19][20] and photothermal systems 21 (see Supplementary Table 1). Also, the DCOS signals depend linearly on the gas concentrations and the excitation powers ( Fig. 4d), favoring quantitative gas analysis. ...
... The component is intensity modulated at a distinguished radio frequency (RF), i.e., the beat frequency of the nth paired comb lines, as Suppose fOF(n) matches a molecularro-vibrational transition and ∆ 0 and ∆ are considerably small; in that case, the nth component will be absorbed by the molecules and yield a PA wave at fRF(n) (due to the intensity modulation). Consequently, the interaction between many of the dual-comb lines and the molecules causes a series of superimposed PA waves[18][19][20] , which vibrate the nanomechanical membrane immersed in the sample molecules, as shown in the middle panel inFig. 1a. ...
Optical cavities are essential for enhancing the sensitivity of molecular absorption spectroscopy, which finds widespread high-sensitivity gas sensing applications. However, the use of high-finesse cavities confines the wavelength range of operation and prevents broader applications. Here, we take a different approach to ultrasensitive molecular spectroscopy, namely dual-comb optomechanical spectroscopy (DCOS), by integrating the high-resolution multiplexing capabilities of dual-comb spectroscopy with cavity optomechanics through photoacoustic coupling. By exciting the molecules photoacoustically with dual-frequency combs and sensing the molecular-vibration-induced ultrasound waves with a cavity-coupled mechanical resonator, we obtain high-resolution broadband (>2 THz) overtone spectra for acetylene gas. The detection limit down to 15 parts per trillion is achieved without the restriction on the comb wavelengths. Our innovative approach not only enriches the practical applications of the emerging cavity optomechanics technology but also offers intriguing possibilities for multi-species trace gas detection.
... We take a different approach and utilize the speed benefit of PC-FTS in cantilever-enhanced photoacoustic spectroscopy (CEPAS). Photoacoustic spectroscopy (PAS) is a highly sensitive spectroscopic technique that is widely used in the analysis of condensed matter [7][8][9][10][11][12], aerosols [13][14][15], and molecular gases [16][17][18][19]. Here, we mainly focus on trace gas analysis, where PAS has been proven to be one of the most sensitive optical techniques available [20][21][22]. ...
We demonstrate a 13-fold speed improvement in broadband cantilever-enhanced photoacoustic spectroscopy (CEPAS) by combining it with phase-controlled Fourier-transform spectroscopy (PC-FTS) instead of traditional Fourier-transform infrared spectroscopy (FTIR). PC-FTS is a modification of FTIR and capable of fundamentally faster interferogram acquisitions. The speed-improvement is beneficial for CEPAS, which is an especially sensitive version of the background-free photoacoustic spectroscopy technique. We used the PC-FTS-CEPAS technique to measure the absorption spectrum of methane in the mid-infrared region (3.3–3.5 µm) with an optical frequency comb as the light source.
... As a kind of Fourier-transform spectrometer, the dual-comb spectroscopy (DCS) uses two coherent frequency combs with a slightly offset repetition rate to perform ultrahigh-resolution, high-sensitivity broadband spectroscopy [40,41]. The remarkable capabilities that enhance the competitiveness of the DCS are applications such as detection and analysis of greenhouse gases [42][43][44][45], spectral light detection and ranging [46][47][48][49], nonlinear spectroscopy [50,51], medical laboratory science [52], imaging [53,54], and zero-background photothermal/photoacoustic detection [55][56][57]. ...
Dual-comb spectroscopy as an emerging tool for spectral analysis has been investigated in a wide range of applications, including absorption spectroscopy, light detection and ranging, and nonlinear spectral imaging. Two mutually coherent combs facilitate high-precision, high-resolution, and broadband spectroscopy. Recently, dual combs generated from a single cavity have become compelling options for dual-comb spectroscopy, enabling huge simplification to measuring systems. Here, we review the progress of single-cavity dual comb lasers in recent years and summarize the distinctive advantages of single-cavity dual combs. First, the principles of optical frequency comb and dual-comb spectroscopy are introduced in time and frequency domains. Then, the implementation techniques and typical applications of single-cavity dual comb lasers are discussed, including directional multiplexing, wavelength multiplexing, polarization multiplexing, and space multiplexing. Finally, an outlook on the development of single-cavity dual combs is presented.
... Steven T. Cundiff et al. creatively proposed tricomb spectroscopy (TCS) to break through the limitation of resolution and acquisition speed in the measurement of multidimensional coherent spectroscopy (MDCS), which provided a comb-resolution multidimensional coherent spectrum under 1 s with decoupling homogeneous and non-homogeneous linewidths 33 . In addition, more spectroscopy techniques, such as photo-acoustic spectroscopy 34,35 , spectroscopic ellipsometry 36 and laser-induced breakdown spectroscopy 37 , have achieved broadband wavelength coverage, high resolution and high acquisition speed by combining with DCS, which has revolutionized precision spectroscopy. ...
... The 2.4-cm −1 resolution (~72 GHz) via apodization in our paper is acceptable for the spectral features of the liquid/solid-phase system. However, it would be ideal if the repetition rate f r of OFC was equal to the desired resolution, and the measurement time could be reduced by more than five orders of magnitude from 1 hour to millisecond level 35 . With the development of the high-repetition-rate sources used in DCS 50,51 , the millisecond-level measurement time of weak COA in liquid/solid-phase system can be achieved. ...
Optical activity (OA) spectroscopy is a powerful tool to characterize molecular chirality, explore the stereo-specific structure and study the solution-state conformation of biomolecules, which is widely utilized in the fields of molecular chirality, pharmaceutics and analytical chemistry. Due to the considerably weak effect, OA spectral analysis has high demands on measurement speed and sensitivity, especially for organic biomolecules. Moreover, gas-phase OA measurements require higher resolution to resolve Doppler-limited profiles. Here, we show the unmatched potential of dual-comb spectroscopy (DCS) in magnetic optical activity spectroscopy (MOAS) of gas-phase molecules with the resolution of hundred-MHz level and the high-speed measurement of sub-millisecond level. As a demonstration, we achieved the rapid, high-precision and high-resolution MOAS measurement of the nitrogen dioxide υ1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\upsilon }_{1}$$\end{document}+υ3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\upsilon }_{3}$$\end{document} band and the nitric oxide overtone band, which can be used to analyze fine structure of molecules. Besides, the preliminary demonstration of liquid-phase chiroptical activity (as weak as 10⁻⁵) has been achieved with several seconds of sampling time, which could become a routine approach enabling ultrafast dynamics analysis of chiral structural conformations.
