WO2026019451A1

WO2026019451A1 - Two-dimensional spectrometer for trace gas detection of complex mixtures

Info

Publication number: WO2026019451A1
Application number: PCT/US2025/014104
Authority: WO
Inventors: Melanie A.R. Reber
Original assignee: University of Georgia; University of Georgia Research Foundation Inc
Current assignee: University of Georgia; University of Georgia Research Foundation Inc
Priority date: 2024-02-02
Filing date: 2025-01-31
Publication date: 2026-01-22
Anticipated expiration: 2026-08-02

Abstract

Described are systems and methods for generating two-dimensional spectra of liquid or gas samples. In examples, the disclosed systems and methods use frequency comb lasers for the pump and probe lasers, such as generated using fiber laser systems or electro-optic modulated laser systems, and employ enhancement cavities for the pump and probe lasers to increase laser intensity for interaction with the liquid or gaseous sample for generation of signals for use in generating two-dimensional spectra. In examples, heterodyne detection of generated signals using a detection laser is also employed.

Description

TWO-DIMENSIONAL SPECTROMETER FOR TRACE GAS DETECTION OF COMPLEX MIXTURES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present international application claims priority to and the benefit of U.S. Provisional Patent Application No. 63/549,010, filed on February 2, 2024, and entitled "TWO- DIMENSIONAL SPECTROMETER FOR TRACE GAS DETECTION OF COMPLEX MIXTURES," the entire contents of which are hereby incorporated by reference in their entirety for all purposes.

FIELD

[0002] This invention is in the field of spectroscopy. This invention relates generally to a spectroscopic instrument for obtaining two-dimensional spectra of liquid or gas samples and associated methods of obtaining two-dimensional spectra of liquid or gas samples.

BACKGROUND

[0003] Various commercial instruments exist of obtaining absorption spectra of gases or liquids, such as ultraviolet-visible (UV-vis) absorption spectrometers or infrared (IR) absorption spectrometers. These instruments, however, are generally suited to only determining the fraction of incident radiation absorbed by gas or liquid samples and are incapable of generating two- dimensional spectra of liquid or gas samples.

SUMMARY

[0004] Described herein are systems and methods for generating two-dimensional spectra of liquid or gas samples, including for identifying individual components in complex mixtures. In examples, the disclosed systems and methods use frequency comb lasers for the pump and probe lasers, such as generated using fiber laser systems or electro-optic modulated laser systems, and employ enhancement cavities for the pump and probe lasers to increase laser intensity for interaction with the liquid or gaseous sample for generation of signals for use in generating two- dimensional spectra. Use of high-resolution detection, as provided by the disclosed two- dimensional spectrometers, can allow for precise identification and differentiation of spectral features of different components of a complex mixture. In addition, the disclosed two-dimensional spectrometers allow for time-resolved detection, further enabling differentiation of components of mixtures as well as for identifying time-dependent behavior of individual species present in a sample and/or, in some cases, interactions between components in a sample. [0005] In a first aspect, two-dimensional spectrometers are provided. An example two- dimensional spectrometer of this aspect comprises a first pulsed laser system for generating pump pulses and probe pulses, a sample positioner for positioning a sample to interact with the pump pulses and the probe pulses and generate signal pulses, a second pulsed laser system for generating detection pulses, and a detector or detection system arranged to receive the signal pulses and the detection pulses and generate a two-dimensional spectrum for the sample. In examples, the two- dimensional spectrum for the sample is a function of excitation frequency (e.g., pump pulse frequency) and detection frequency (e.g., signal emission frequency).

[0006] A plurality of individual species may be present in the sample and the two-dimensional spectrum may include individual and/or differentiable spectral features for a plurality of species present in the sample. Optionally, the two-dimensional spectrum for the sample comprises a plurality of two-dimensional time-resolved spectra based on a time delay between the pump pulses and the probe pulses.

[0007] In examples, multiple pump pulses can be used. For example, the pump pulses may comprise first pump pulses and second pump pulses, with the first pump pulses and the second pump pulses separated in time by a time delay. In some examples, the time delay between pump pulses is referred to as a pump-pump time delay or ti. In examples, a two-dimensional spectrometer may comprise an adjustable delay stage positioned to provide an adjustable time delay between the first pump pulses and the second pump pulses. Advantageously, use of multiple pump pulses with an adjustable time delay between them can allow for signals to be obtained over a range of time delays, permitting use of Fourier transforms over the time delay to reference the pump laser in frequency space.

[0008] In examples, the probe pulses are separated in time from the pump pulses by a time delay. In some examples, the time delay between pump pulses and probe pulses is referred to as a pump-probe time delay or t2. In examples, a two-dimensional spectrometer may comprise an adjustable delay stage positioned to provide an adjustable time delay between the pump pulses and the probe pulses.

[0009] In examples, the first pulsed laser system exhibits a first repetition rate, the second pulsed laser system exhibits a second repetition rate, and the first repetition rate and the second repetition rate are different. Without limitation, the first repetition rate and the second repetition rate are independently selected from the range of 60 MHz to 90 MHz. Optionally, the first repetition rate is within 10% of the second repetition rate or wherein a difference between the first repetition rate and the second repetition rate is less than 10% of the second repetition rate. [0010] Any suitable laser system can be used for the first pulsed laser system and the second pulsed laser system. In some examples, the first pulsed laser system or the second pulsed laser system independently comprise frequency comb laser systems. Optionally, the first pulsed laser system and the second pulsed laser system comprise infrared laser sources. Optionally, the pump pulses and the probe pulses exhibit central wavelengths of from 285 nm to 10000 nm. Optionally, the first pulsed laser system or the second pulsed laser system comprises or is optically coupled to a nonlinear optical mixer. In examples, a nonlinear optical mixer is a second harmonic generation system, a difference frequency generation system, an optical parametric oscillation system, or an optical parametric amplifier system. Optionally, the pump pulses and probe pulses comprise ultrafast laser pulses. Optionally, the pump pulses and the probe pulses exhibit pulse durations of from 40 fs to 150 fs. Optionally, the detection pulses exhibit pulse durations of from 30 fs to 500 ps. In some examples, the first pulsed laser system and the second pulsed laser system independently comprise pulsed laser systems selected from the group consisting of a fiber laser system or an electro-optic modulated laser system. Optionally, a fiber laser system comprises a Yb-doped fiber laser system. Optionally, a Yb-doped fiber laser system comprises a combination of a Yb-doped single mode fiber amplifier and a grating compressor. Optionally, a Yb-doped fiber laser system comprises a high-powered chirped-pulse amplifier with a Yb-doped photonic crystal fiber, a fiber stretcher, and a grating compressor. Optionally, an electro-optic modulated laser system comprises a continuous wave laser source modulated using a series of electro-optic modulators. Optionally, a continuous wave laser source comprises a neodymium-doped yttrium aluminum garnet laser, a quantum-cascade laser, or a diode laser.

[0011] In some examples, a two-dimensional spectrometer of this aspect comprises an enhancement cavity arranged to receive the pump pulses and generate enhanced pump pulses. In examples, the sample interacts with the enhanced pump pulses and the probe pulses to generate signal pulses. In some examples, a two-dimensional spectrometer of this aspect comprises an enhancement cavity arranged to receive the probe pulses and generate enhanced probe pulses. In examples, the sample interacts with the pump pulses and the enhanced probe pulses to generate signal pulses. In some examples, the sample positioner directs the sample into the enhancement cavity or cavities. Optionally, the sample positioner directs the sample into the enhancement cavity or cavities without use of optical elements separating the sample from the enhancement cavity or cavities. In some examples, the enhanced pump pulses exhibit increased phase stability compared to the pump pulses and/or increased intensity compared to the pump pulses. In some examples, the enhanced probe pulses exhibit increased phase stability compared to the probe pulses and/or increased intensity compared to the probe pulses. [0012] The two-dimensional spectrometers described are useful for generating two dimensional spectra of fluid samples. In some examples, the sample positioner comprises a flowing gas inlet, a flowing gas jet, a gas supersonic beam, a flowing liquid inlet, a flowing liquid jet, a flowing microjet liquid sheet, a gas cell coated with an antireflection coating, or a gas cell with Brewster's angle windows. Optionally, the sample comprises a gas sample or a liquid sample. Optionally, the sample comprises one or a mixture of atomic or molecular gases. Optionally, the sample comprises a pure liquid, a mixture of liquids, or a liquid with suspended solids or dissolved gases therein.

[0013] Various detection schemes can be used for the two-dimensional spectrometers described herein. In some examples, a heterodyne detection scheme is used. Optionally, the detector comprises a dual-comb spectrometer for generating the two-dimensional spectrum for the sample using detected signal pulses and detection pulses. For example, the detector comprises or is coupled to a computer or signal processor programmed to perform a Fourier transform of a signal generated by an optical detector arranged to detect the signal pulses and the detection pulses.

[0014] In another aspect, methods for obtaining two-dimensional spectra are described. An example method of this aspect comprises generating pump pulses and probe pulses using a first pulsed laser system, positioning a sample to interact with the pump pulses and the probe pulses to generate signal pulses, generating detection pulses using a second pulsed laser system, directing the signal pulses and the detection pulses to a detector to generate a detection signal, and generating a two-dimensional spectrum for the sample using the detection signal. In examples, the first pulsed laser system or the second pulsed laser system independently comprise frequency comb laser systems. In examples, the two-dimensional spectrum for the sample is a function of excitation frequency and detection frequency.

[0015] In examples, the pump pulses comprise first pump pulses and second pump pulses, and wherein the first pump pulses and the second pump pulses are separated in time by a time delay. In some examples, a method of this aspect comprises adjusting a time delay between the first pump pulses and the second pump pulses. In examples, the pump pulses wherein the probe pulses are separated in time from the pump pulses by a time delay. In some examples, a method of this aspect comprises adjusting a time delay between the pump pulses and the probe pulses. In examples, the first pulsed laser system exhibits a first repetition rate, the second pulsed laser system exhibits a second repetition rate, and the first repetition rate and the second repetition rate are different. Optionally, the first repetition rate and the second repetition rate are independently selected from the range of 60 MHz to 90 MHz. Optionally, the first repetition rate is within 10% of the second repetition rate or wherein a difference between the first repetition rate and the second repetition rate is less than 10% of the second repetition rate. Optionally, the first pulsed laser system and the second pulsed laser system comprise infrared laser systems. In examples, the pump pulses, the probe pulses, and the detection pulses exhibit central wavelengths of from 300 nm to 8000 nm. Optionally, the first pulsed laser system or the second pulsed laser system comprises or is optically coupled to a nonlinear optical mixer. In examples, the nonlinear optical mixer is selected from a second harmonic generation system, a difference frequency generation system, and optical parametric oscillation system, or an optical parametric amplifier system.

[0016] In examples, wherein the pump pulses, the probe pulses, and the detection pulses exhibit pulse widths of from 40 fs to 150 fs. Optionally, the detection pulses exhibit pulse widths of from 30 fs to 500 ns.

[0017] In examples, the first pulsed laser system and the second pulsed laser system independently comprise pulsed laser systems selected from the group consisting of a fiber laser system or an electro-optic modulated laser system. Optionally, a fiber laser system comprises a Yb-doped fiber laser system. Optionally, a Yb-doped fiber laser system comprises a combination of a Yb-doped single mode fiber amplifier and a grating compressor. Optionally, a Yb-doped fiber laser system comprises a high-powered chirped-pulse amplifier with a Yb-doped photonic crystal fiber, a fiber stretcher, and a grating compressor. Optionally, an electro-optic modulated laser system comprises a continuous wave laser source modulated using a series of electro-optic modulator. Optionally, a continuous wave laser source comprises a neodymium-doped yttrium aluminum garnet laser, a quantum-cascade laser, or a diode laser.

[0018] A plurality of individual species may be present in the sample and the two-dimensional spectrum may include individual and/or differentiable spectral features for a plurality of species present in the sample. Optionally, the two-dimensional spectrum for the sample comprises a plurality of two-dimensional time-resolved spectra based on a time delay between the pump pulses and the probe pulses. In some examples, methods of this aspect can compare measured spectral features for a sample including one or a plurality of species with known spectral features to allow for precise and affirmative identification of the species in sample. In some cases, the time- resolved spectra can be used for aid or use in affirmative identification of species in the sample.

[0019] In some examples, a method of this aspect comprises directing the pump pulses to an enhancement cavity arranged to receive the pump pulses and generate enhanced pump pulses. In some examples, a method of this aspect comprises directing the probe pulses to an enhancement cavity arranged to receive the probe pulses and generate enhanced probe pulses. For example, the sample may interact with the enhanced pump pulses and/or the enhanced probe pulses to generate signal pulses. In examples, a sample positioner directs the sample into the enhancement cavity. For example, the sample may be directed into the enhancement cavity without use of optical elements separating the sample from the enhancement cavity. In examples, the sample is positioned to interact with the pump pulses and the probe pulses using a flowing gas inlet, a flowing gas jet, a gas supersonic beam, a flowing liquid inlet, a flowing liquid jet, a flowing microjet liquid sheet, a gas cell coated with an antireflection coating, or a gas cell with Brewster's angle windows. Optionally, the sample comprises a gas sample or a liquid sample. Optionally, the sample comprises one or a mixture of atomic or molecular gases. Optionally, the sample comprises a pure liquid, a mixture of liquids, or a liquid with suspended solids or dissolved gases therein.

[0020] In some examples, detection of the signal pulses and generation of a two-dimensional spectrum for the sample may comprise or employ heterodyne detection, such as there the signal pulses and detection pulses are simultaneously detected, such as using a dual-comb spectrometric technique, for example, involving generating the two-dimensional spectrum for the sample comprises by performing a Fourier transform of the detected signal pulses and detection pulses.

[0021] Without wishing to be bound by any particular theory, there can be discussion herein of beliefs or understandings of underlying principles relating to the invention. It is recognized that regardless of the ultimate correctness of any mechanistic explanation or hypothesis, an embodiment of the invention can nonetheless be operative and useful.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 provides a schematic illustration of an example two-dimensional spectrometer.

[0023] FIG. 2 provides a schematic illustration of an example laser system, such as for use with a two-dimensional spectrometer.

[0024] FIG. 3 provides a schematic illustration of an example interaction region of a two- dimensional spectrometer.

[0025] FIG. 4 provides a diagram of an example method for operating a two-dimensional spectrometer.

DETAILED DESCRIPTION

[0026] The present disclosure provides systems and methods for generating two-dimensional spectra of liquid or gas samples and, in some examples, provides a bench-top instrument for measuring two-dimensional spectra of liquid or gas samples. In examples, the disclosed systems, methods, and instruments can use frequency comb lasers for the pump and probe laser sources, such as generated using fiber laser systems or electro-optic modulated laser systems and can use heterodyne detection schemes. Advantageously, the disclosed configurations allow for robust and stable operation and detecting signals across a wide range of frequencies. Moreover, the disclosed configurations can use very high repetition rate laser systems, such as in excess of 50 MHz, and can optionally employ external enhancement cavities. The disclosed systems, methods, and instruments can incorporate a gas or liquid inlet, optionally allowing for directly probing a gas or liquid sample in an enhancement cavity. These aspects can allow for quickly generating large signals in a short amount of time and improving the measured signals or signal to noise ratio to a suitable level. Together, the disclosed features of the systems, methods, and instruments described can allow for quickly obtaining two-dimensional spectra of materials in a liquid or gas sample, providing an analytical tool that can allow for not only determining two-dimensional spectra, but provide for a sensitive technique for discriminating and identifying components of a mixture, as each individual component in the mixture can exhibit a unique two-dimensional spectra that can be reliably differentiated from the two-dimensional spectra of other components in the mixture.

[0027] In general the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The following definitions are provided to clarify their specific use in the context of the invention.

[0028] "Pump pulse" or "pump" refers to a laser pulse used as a primary excitation of a sample. Pump pulses can exhibit a frequency bandwidth and a time domain envelope. In some examples, pump pulses are generated as a frequency comb having a short duration and a broad frequency span (e.g., a broad energy span or a broad wavelength span). The short time duration can allow for precise time delays to be introduced between pump pulses and/or between a pump pulse and probe pulse and the broad frequency span can allow for exciting the sample using a large frequency range. For example, the use of frequency combs as pump pulses can simultaneously excite a sample across a range of frequencies contained within the comb.

[0029] "Probe pulse" or "probe" refers to a laser pulse used as a secondary excitation of a sample, upon which the sample emits a signal. Probe pulses can exhibit a frequency bandwidth and a time domain envelope. In some examples, probe pulses are generated as a frequency comb having a short time duration and a broad frequency span (e.g., a broad energy span or a broad wavelength span). The short time duration can allow for precise time delays to be introduced between a probe pulse and a preceding pump pulse and the broad frequency span can allow for probing the sample using a large frequency range. For example, the use of frequency combs as probe pulses can simultaneously probe a sample across a range of frequencies contained within the comb.

[0030] "Frequency comb" refers to a broadband laser source including a plurality of equally spaced frequency lines in the frequency domain. In some examples, a frequency comb can correspond to an ultrashort laser pulse, such as with a pulse width in the time domain of less than or about 1 ps. Frequency combs can be generated with central frequencies on the order of about 500 cm’¹ (about 15 THz or a wavelength of about 20000 nm) to central frequencies on the order of about 35000 cm’¹ (about 1050 THz or a wavelength of about 285 nm). In some examples, a single frequency comb may include individual lines ranging from about 50% below the central frequency to about 50% above the central frequency, such as from about 500 cm’¹ (about 15 THz) to about 1500 cm’¹ (about 45 THz) with a central frequency of about 1000 cm’¹ (about 30 THz) or from about 10000 cm’¹ (about 300 THz) to about 30000 cm’¹ (about 900 THz) with a central frequency of about 20000 cm’¹ (about 600 THz). In some examples, the spacing of lines (mode spacing or repetition rate) in the frequency comb can be a function of the pulse repetition rate, with example pulse repetition rates ranging from about 60 MHz to about 90 MHz.

[0031] "Dual-comb spectrometer" refers to a system for performing spectroscopic measurements using a pair of frequency combs with different repetition rates and a single detector, allowing for quickly obtaining a high-resolution spectrum across a large spectral range. In a dual comb spectrometer, a target signal is encoded on a first frequency comb that is combined with a reference comb and recovered using heterodyne detection, where the combined signal is detected by a detector and that combined signal is processed using a Fourier transform to recover features of the target signal.

[0032] " Two-dimensional spectrum" refers to a measure of the emission signal of a sample upon interacting with a plurality of laser pulses, including at least one pump pulse and a probe pulse, a with the emission characterized based upon the excitation frequency and the detection frequency. The collection of a two-dimensional spectrum can include scanning the time delay between pump pulses and performing a Fourier transform over the time delay between pump pulses. Two- dimensional spectra can be further characterized based upon the time delay between the pump pulse and the probe pulse, and can provide dynamic characteristics of the sample upon and after the excitation by the pump pulse(s), with individual two-dimensional spectra commonly obtained at different pump-probe time delays to show different spectral features at different time delays. The emission signal can be expressed in terms of electric field units, for example, though such a configuration is not limiting and other ways of expressing the emission signal can be implemented by the skilled artisan. Using frequency combs as the pump and probes for obtaining a two- dimensional spectrum can be useful, as the frequency combs can include light across a variety of different frequencies allowing for exciting and probing the sample across a range of frequencies simultaneously, which can reduce the complexity and effort needed to obtain a spectrum of the sample across a range of frequencies as compared to using narrowband pump and probe pulses.

[0033] "Sample positioner" refers to a device or an arrangement of device elements to introduce a sample, such as a gas or liquid sample, to a specific position in space in order for the sample to interact with laser pulses or frequency combs generated by a laser system. In some examples, a sample positioner includes a cell, such as a gas cell, to position a sample within the cell into position for interaction with the laser pulses or frequency combs from the laser system. Example cells can include optical coatings, such as antireflection coatings, or particular geometries, such as Brewster's angle geometries, to minimize reflective losses of the laser pulses or frequency combs passing through the cell. In some examples, a sample positioner includes one or more nozzles for directing a flowing jet of a fluid, such as to provide a constant stream of the fluid with a stable and/or flat geometry. In some examples, a sample positioner comprises a microfluidic device, allowing for generation of streams of fluid using small sample volumes. In some examples, the use of jets of flowing fluids can allow for directing the sample into the optical cavity of a spectrometer without requiring or using optical components (e.g., a cell, cuvette, optical windows, etc.) separating the fluid from the optical cavity.

[0034] FIG. 1 provides a schematic illustration of an example two-dimensional spectrometer 100. Two-dimensional spectrometer 100 includes a source laser system 105, which may comprise a laser system or a plurality of different laser systems and various components for generating pump pulses 110 and probe pulses 115. Example laser systems useful for generating pump pulses 110 and probe pulses 115 are described in further detail herein, but may include a fiber laser system or an electro-optic modulated laser system, for example. Pump pulses 110 and probe pulses 115 are schematically depicted as ultrashort laser pulses, such as exhibiting a sinusoidal electric field with a Gaussian envelope. Pump pulses 110 and probe pulses 115 may further correspond to frequency combs.

[0035] Pump pulses 110 and probe pulses 115 are directed to an interaction region 120, where the pulses interact with a sample 125 for generation of a signal 130. Signal 130 may correspond to a short, pulsed signal, representing emission generated by the sample upon interaction with the pump pulses 110 and probe pulses 115. Sample 125 is introduced into the interaction region 120 two-dimensional spectrometer 100 using a sample positioner 135, schematically depicted in FIG. 1 as an arrow. Samples useful with two-dimensional spectrometer 100 include liquid samples and gas samples.

[0036] Two-dimensional spectrometer 100 also includes a detection laser system 140, which may be a pulsed laser system, generating detection pulses 145. Such a configuration is not intended to be limiting, and any suitable detection laser system 140 may be used. Detection pulses 140 are combined with signal 130 using beam combiner 150 and the combined signal 155 is directed to a heterodyne detector 160 for detection. A signal processor 170 is used to analyze the detected signal for generation of a two-dimensional spectra 175 of the sample 130. In some examples, the detection laser system 140 may have a similar or substantially identical configuration to source laser system 105 except that the repetition frequency of detection pulses 145 is different from the repetition frequency of pump pulses 110 and probe pulses 115. In other examples, the source laser system 105 and detection laser system 140 are different, such as of a fundamentally different type or form (e.g., using different source lasers, frequencies, gain media, geometries, etc.). In one example, source laser system 105 employs a Nd:YAG pumped fiber laser while detection laser system employs a continuous wave laser coupled to one or more electro-optic modulators.

[0037] Two-dimensional spectrometer 100 can include other components besides those illustrated here, and the illustrated components of two-dimensional spectrometer 100 can also include other suitable elements. For example, various optical elements, such as lenses, windows, mirrors, prisms, beam splitters, beam combiners, filters, waveplates, waveguides, crystals, couplers, light sources, laser sources, optical modulators, or the like, may be included in two- dimensional spectrometer 100, source laser system 105, interaction region 120, or detection laser system 140 in any suitable arrangement for generating pump pulses 110, probe pulses 115, and detection pulses 140, for interacting pump pulses 110 and probe pulses 115 with sample 125 to generate signal 130, or for combining signal 130 with detection pulses 140 to generate combined signal 155. Further, two-dimensional spectrometer 100, source laser system 105, interaction region 120, or detection laser system 140 may include other components besides optical components. In some examples, two-dimensional spectrometer 100 may include mechanical elements (e.g., a translation stage, stepper motor, etc.) for adjusting a time delay between pump pulses 110, for adjusting a time delay between pump pulses 110 and probe pulses 115, and/or for adjusting a relative timing of signal 130 and detection pulses 150 to ensure they suitably overlap in time for detection by heterodyne detector 160. [0038] Signal processor 170 or another controller or processing device can be used to control the source laser system 105, the detection laser system 140, or other components of two-dimensional spectrometer 100. In some examples, signal processor 170 or another controller or processing device is used to control or adjust timing of pump pulses 110, probe pulses 115, and detection pulses 145, such as to control various delays between pulses. In examples, the time delay between pump pulses 110 is controlled to allow scanning the time delay across a range of times to allow for signal processor 170 to perform a Fourier transform over the time delay between pump pulses 110 for generation of two-dimensional spectrum 175.

[0039] In the example two-dimensional spectrum 175 shown in FIG. 1, signal intensity is shown in electric field units with contour lines. For purposes of illustration, different fill patterns represent different electric field intensities and dotted contours represent negative electric field intensities, as depicted in the legend shown at the top of two-dimensional spectrum 175. The circular feature at the highest excitation frequency and highest detection frequency (top right) in the two-dimensional spectrum 175 shows the highest total positive intensity and 6 contour lines, while the feature at the highest excitation frequency and lowest detection frequency (bottom right) of the two-dimensional spectrum 175 shows a negative intensity with two contour lines. The two- dimensional spectrum 175 may correspond to a spectrum obtained at a particular time delay between pump pulses 110 and probe pulses 115, but other two-dimensional spectra may be obtained at other time delays between pump pulses 110 and probe pulses 115. In examples, the time delay between pump pulses 110 and probe pulses 115 is controlled to allow scanning the time delay across a range of pump-probe delay times to allow for signal processor 170 to generate a two-dimensional spectrum 175 for each of the pump-probe delay times.

[0040] FIG. 2 provides a schematic illustration of an example laser system 200, which may be the same as or different from source laser system 105 and/or detection laser system 140 shown in FIG. 1 . Laser system 200 includes a laser source 205, an amplifier 210, a compressor 215, a beam splitters 220, a delay stage 225, a beam splitter 235, a delay stage 240, and a beam combiner 235. As illustrated, laser system 200 is configured to generate pump pulses 230 and probe pulses 250. Laser system 200 can be used to similarly generate detection pulses, though not all components depicted in FIG. 2 would be needed for all instances of such a system. For example, beam splitters 220 and 235, delay stage 240, and beam combiner 245 could be excluded in a laser system 200 used to generate detection pulses.

[0041] In examples, laser source 205 may comprise any suitable laser, such as a fiber laser (e.g., a Yb-fiber oscillator). Other suitable laser sources can be used. In some examples, laser source 205 may comprise a narrow-linewidth continuous wave laser (e.g., a diode laser, a quantumcascade laser, a neodymium-doped yttrium aluminum garnet (Nd: YAG) laser, or the like) coupled to one or more electro-optic modulators (EOM) to generate a pulsed, frequency comb laser in a configuration referred to herein as an "EOM laser." Laser source 205 may include various optical elements, such as lenses, windows, mirrors, prisms, filters, waveplates, waveguides, crystals, couplers, light sources, gain media, optical modulators, or the like.

[0042] The laser light output from laser source 205 is shown as directed to amplifier 210. In some examples, however, the laser light output from laser source 205 is used without external amplification, in which case amplifier 210 is not present. Similarly, compressor 215 may be optional and not used in all configurations. In examples, any suitable amplification system or configuration can be used, such as a fiber amplifier (e.g., Yb:doped single mode fibers, Yb:doped photonic crystal fibers, etc.), a chirped-pulse amplifier, etc. In some examples, amplifier 210 is paired with compressor 215, such as for purposes of offsetting stretching that occurs with certain amplifiers (e.g., chirped-pulse amplifiers).

[0043] As depicted in FIG. 2, laser light from laser source 205, after optional amplification at amplifier 210 and compression at compressor 215 is directed to beam splitter 220. Part of the light from beam splitter 220 passes to delay stage 225 for use as probe pulses 230. Part of the light from beam splitter 220 passes to another beam splitter 235, which directs some light to delay stage 240, and some light to beam combiner 245. At beam combiner 245, the light from beam splitter 235 and delay stage 245 are recombined for use as pump pulses 250.

[0044] With respect to timing of the output of probe pulses 230 and pump pulses 250, for a single pulse train, two pump pulses 250 are generated, with a time delay between them, followed after a second time delay by a single probe pulse 230. In examples, delay stage 225 and delay stage 240 can be independently controlled so as to allow for control over the delay between pump pulses 250 and for control over the delay between pump pulses 250 and probe pulses 230. As noted above, the time delay between pump pulses 250 can be systematically varied to allow for collection of a two-dimensional spectrum in frequency space by performing a Fourier transform over the time delay between pump pulses 230.

[0045] Laser system 200 may include additional components other than those illustrated in FIG. 2, depending on the particular configuration. In some examples, various optical elements, such as lenses, windows, mirrors, prisms, beam splitters, beam combiners, filters, waveplates, waveguides, crystals, couplers, light sources, laser sources, optical modulators, or the like. For example, some laser systems may comprise components for frequency modification, such as using nonlinear broadening, harmonic generation, difference frequency generation, optical parametric amplification, optical parametric oscillation, or the like.

[0046] FIG. 3 provides a schematic illustration of an example interaction region 300 of a two- dimensional spectrometer. Interaction region 300 includes a sample positioner 305 for directing a sample 310 to interact with pump pulses 315 and probe pulses 320 directed into interaction region 300. As illustrated in FIG. 3, interaction region 300 can comprise optical components to admit pump pulses 315 and probe pulses 320. For example, optical elements 325, 330, 335, and 340 are illustrated. In some examples, optical elements 325, 330, 335, and 340 can be windows, lenses, prisms or other optical components. In examples, the sample can include a single species for which a two-dimensional spectrum or spectra are to be obtained. In other examples, the sample can include multiple different species for which two-dimensional spectra are to be obtained. Optionally, the two-dimensional spectra can be used to determine the number of different species within the sample. Optionally, the two-dimensional spectra can be used to positively and/or affirmatively establish an identify of the different species within the sample.

[0047] In some configurations, optical elements 325, 330, 335, and/or 340 are used for creation of enhancement cavities for increasing intensity of the pump pulses 315 and/or probe pulses 320 for interaction with the sample 310. The use of enhancement cavities also provides phase stability for the pump pulses 315 and/or probe pulses 315. As one example, optical elements 325 and 330 may comprise reflective surfaces (e.g., mirrors) configured for cavity enhancement of the pump pulses 315. As another, optical elements 335 and 340 may comprise reflective surfaces (e.g., mirrors) configured for cavity enhancement of the probe pulses 320. Although not illustrated in FIG. 3, the source laser system and enhancement cavities may include additional component beyond those illustrated in order to achieve stabilization and enhancement, such as various locking electronics and optical components, such as for a Pound-Drever-Hall locking configuration (e.g., mirrors, phase modulators, windows, piezoelectric elements, etalons, waveplates, photodetectors, filters, frequency generators, phase shifters, mixers, or the like). Furthermore, other enhancement geometries beyond those depicted in FIG. 3 can be used, such as a bow-tie configuration.

[0048] Sample positioner 305 may comprise any suitable components for directing sample 310 into the interaction region 310 for interaction with pump pulses 315 and probe pulses 320. In some examples, sample positioner 305 comprises a nozzle, such as used for generating a stream or jet of fluid of sample 310 that passes through the paths of the pump pulses 315 and probe pulses 320. In some examples, sample positioner 305 comprises a gas nozzle or a liquid nozzle. Example liquid nozzles may be similar in configuration to those used in a dye laser for generating a continuous stream of liquid containing dye molecules. In some configurations, microfluidic nozzles are used for generating a stable and continuous flow of the sample 310. Use of microfluidic nozzles may be beneficial for reducing the fluid volume of sample 310 needed for introduction into the interaction region 300. For example, microfluidic nozzles may allow for minimizing the amount of sample 310 needed for obtaining one or multiple two-dimensional spectra since a microfluidic nozzle can provide a jet of sample 310 using small volumes of sample 310 (e.g., 5 ml or less).

[0049] Upon interaction with the pump pulses 325 and probe pulses 320, sample 310 can emit signal pulses 345. Signal pulses 345 can be combined with detection pulses 350 at a beam splitter and then the combined pulses can be detected by a detector 360. For some examples, the combined pulses can exhibit a beat frequency in the radio frequency domain.

[0050] FIG. 4 provides a diagram of an example method 400 for obtaining a two-dimensional spectrum. Some, any, or all of the operations of method 400 described below can be performed using two-dimensional spectrometers described herein, including two-dimensional spectrometer 100 of FIG. 1. The operations of method 400 may be performed in any suitable order, and method 400 may include more or fewer operations than those depicted in FIG. 4.

[0051] The method 400 can begin at block 402 by generating pump pulses and probe pulses using a first pulsed laser system. The first pulsed laser system may be an example of source laser system(s) 105 of FIG. 1 or laser system 200 of FIG. 2. The first pulsed laser system may be an infrared laser system. The first pulsed laser system can include a nonlinear optical mixer. The nonlinear optical mixer can be selected from a second harmonic generation system, a difference frequency generation system, and optical parametric oscillation system, or an optical parametric amplifier system

[0052] At block 404, a sample can be positioned to interact with the pump pulses and the probe pulses to generate signal pulses. Samples can be introduced into an interaction region of the first pulsed laser system using a sample positioner. In some examples, the sample can be a liquid or a gas. In some examples, the sample can include a plurality of different species. In some examples, the sample can include one or a mixture of atomic or molecular gases, a pure liquid, a mixture of liquids, or a liquid with suspended solids or dissolved gases.

[0053] In some examples, the pump pulses can include first pump pulses and second pump pulses separated in time by a time delay. In some examples, the pump pulses and the probe pulses can be separated by a time delay. These time delays can be adjusted by the two dimensional spectrometer. In some examples, the sample can be positioned to interact with the pump pulses and the probe pulses using a flowing gas inlet, a flowing gas jet, a gas supersonic beam, a flowing liquid inlet, a flowing liquid jet, a flowing microjet liquid sheet, a gas cell coated with an antireflection coating, or a gas cell with Brewster's angle windows.

[0054] At block 406, a second pulsed laser system can generate detection pulses. The second pulsed laser system can be an example of detection laser system 140 of FIG. 1 or laser system 200 of FIG. 2. The second pulsed laser system can be an infrared laser system. The second pulsed laser system can include a nonlinear optical mixer. The nonlinear optical mixer can be selected from a second harmonic generation system, a difference frequency generation system, and optical parametric oscillation system, or an optical parametric amplifier system

[0055] At block 408, the signal pulses and the detection pulses can be directed to a detector to generate a detection signal. The detector can include a dual-comb spectrometer for generating the detection signal using the signal pulses and detection pulses

[0056] At block 410, a two-dimensional spectrum for the sample can be generated using the detection signal. Generating the two-dimensional spectrum for the sample can include performing a Fourier transform of the detection signal. The two-dimensional spectrum for the sample can include individual and/or differentiable spectral features for a plurality of species present in the sample. In some examples, the two-dimensional spectrum for the sample can include a plurality of two-dimensional time-resolved spectra based on a time delay between the pump pulses and the probe pulses. The two-dimensional spectrum for the sample can be a function of excitation frequency and detection frequency.

[0057] In some examples, the first pulsed laser system can exhibit a first repetition rate and the second pulsed laser system can exhibit a second repetition rate. In some examples, the first repetition rate and the second repetition rate can be different. In some examples, the first repetition rate and the second repetition rate are independently selected from the range of 60 MHz to 90 MHz. In some examples, the first repetition rate is within 10% of the second repetition rate. In some examples, a difference between the first repetition rate and the second repetition rate is less than 10% of the second repetition rate.

[0058] In some examples, the pump pulses, the probe pulses, and the detection pulses can exhibit central wavelengths of from 300 nm to 8000 nm. The pump pulses, the probe pulses, and the detection pulses can exhibit pulse durations of from 40 fs to 150 fs. In some examples, the detection pulses can exhibit pulse durations of from 30 fs to 500 ns. [0059] In some examples, the first pulsed laser system and the second pulsed laser system can independently include pulsed laser systems selected from the group consisting of a fiber laser system or an electro-optic modulated laser system. The fiber laser system can include a Yb-doped fiber laser system. The electro-optic modulated laser system can include a continuous wave laser source modulated using a series of electro-optic modulator. The Yb-doped fiber laser system can include a combination of a Yb-doped single mode fiber amplifier and a grating compressor, or wherein the Yb-doped fiber laser system comprises a high-powered chirped-pulse amplifier with a Yb-doped photonic crystal fiber, a fiber stretcher, and a grating compressor. The continuous wave laser source can include a neodymium-doped yttrium aluminum garnet laser, a quantum-cascade laser, or a diode laser. In some examples, the first pulsed laser system or the second pulsed laser system can independently include frequency comb laser systems.

[0060] In some examples, the pump pulses can be directed to an enhancement cavity arranged to receive the pump pulses and generate enhanced pump pulses. In these examples, the sample can interact with the enhanced pump pulses and the probe pulses to generate signal pulses. A sample positioner can direct the sample into the enhancement cavity. In some examples, the sample can be directed into the enhancement cavity without use of optical elements separating the sample from the enhancement cavity. The enhanced pump pulses can exhibit increased phase stability compared to the pump pulses and/or increased intensity compared to the pump pulses.

STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS [0061] All references throughout this application, for example patent documents, including issued or granted patents or equivalents and patent application publications, and non-patent literature documents or other source material are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference.

[0062] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art, in some cases as of their filing date, and it is intended that this information can be employed herein, if needed, to exclude (for example, to disclaim) specific embodiments that are in the prior art.

[0063] When a group of substituents is disclosed herein, it is understood that all individual members of those groups and all subgroups and classes that can be formed using the substituents are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. As used herein, "and/or" means that one, all, or any combination of items in a list separated by "and/or" are included in the list; for example " 1, 2 and/or 3" is equivalent to " 1, 2, 3, 1 and 2, 1 and 3, 2 and 3, or 1, 2, and 3".

[0064] Every formulation or combination of components described or exemplified can be used to practice the invention, unless otherwise stated. Specific names of materials are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same material differently. It will be appreciated that methods, device elements, starting materials, and synthetic methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such methods, device elements, starting materials, and synthetic methods are intended to be included in this invention. Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure.

[0065] As used herein, "comprising" is synonymous with "including," "containing," or "characterized by," and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, "consisting of excludes any element, step, or ingredient not specified in the claim element. As used herein, "consisting essentially of does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term "comprising", particularly in a description of components of a composition, in a description of a method, or in a description of elements of a device, is understood to encompass those compositions, methods, or devices consisting essentially of and consisting of the recited components or elements, optionally in addition to other components or elements. The invention illustratively described herein suitably may be practiced in the absence of any element, elements, limitation, or limitations which is not specifically disclosed herein.

[0066] The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A two-dimensional spectrometer, comprising: a first pulsed laser system for generating pump pulses and probe pulses; and a sample positioner for positioning a sample to interact with the pump pulses and the probe pulses and generate signal pulses; a second pulsed laser system for generating detection pulses; and a detector arranged to receive the signal pulses and the detection pulses and generate a two-dimensional spectrum for the sample.

2. The two-dimensional spectrometer of claim 1, wherein the sample includes a plurality of different species.

3. The two-dimensional spectrometer of claim 1, wherein the two-dimensional spectrum for the sample includes individual and/or differentiable spectral features for a plurality of species present in the sample.

4. The two-dimensional spectrometer of claim 1, wherein the two-dimensional spectrum for the sample comprises a plurality of two-dimensional time-resolved spectra based on a time delay between the pump pulses and the probe pulses.

5. The two-dimensional spectrometer of claim 1, wherein the pump pulses comprise first pump pulses and second pump pulses, and wherein the first pump pulses and the second pump pulses are separated in time by a time delay.

6. The two-dimensional spectrometer of claim 5, further comprising an adjustable delay stage positioned to provide an adjustable time delay between the first pump pulses and the second pump pulses.

7. The two-dimensional spectrometer of claim 1, wherein the probe pulses are separated in time from the pump pulses by a time delay.

8. The two-dimensional spectrometer of claim 1, further comprising an adjustable delay stage positioned to provide an adjustable time delay between the pump pulses and the probe pulses.

9. The two-dimensional spectrometer of claim 1, wherein the first pulsed laser system exhibits a first repetition rate, wherein the second pulsed laser system exhibits a second repetition rate, and wherein the first repetition rate and the second repetition rate are different.

10. The two-dimensional spectrometer of claim 9, wherein the first repetition rate and the second repetition rate are independently selected from the range of 60 MHz to 90 MHz.

11. The two-dimensional spectrometer of claim 9, wherein the first repetition rate is within 10% of the second repetition rate or wherein a difference between the first repetition rate and the second repetition rate is less than 10% of the second repetition rate.

12. The two-dimensional spectrometer of claim 1, wherein the first pulsed laser system and the second pulsed laser system comprise infrared laser sources.

13. The two-dimensional spectrometer of claim 1, wherein the pump pulses and the probe pulses exhibit central wavelengths of from 285 nm to 10000 nm.

14. The two-dimensional spectrometer of claim 1, wherein the first pulsed laser system or the second pulsed laser system comprises a nonlinear optical mixer.

15. The two-dimensional spectrometer of claim 14, wherein the nonlinear optical mixer is selected from a second harmonic generation system, a difference frequency generation system, an optical parametric oscillation system, or an optical parametric amplifier system.

16. The two-dimensional spectrometer of claim 1, wherein the pump pulses and the probe pulses exhibit pulse durations of from 40 fs to 150 fs.

17. The two-dimensional spectrometer of claim 1, wherein the detection pulses exhibit pulse durations of from 30 fs to 500 ps.

18. The two-dimensional spectrometer of claim 1, wherein the first pulsed laser system and the second pulsed laser system independently comprise pulsed laser systems selected from the group consisting of a fiber laser system or an electro-optic modulated laser system.

19. The two-dimensional spectrometer of claim 18, wherein the fiber laser system comprises a Yb-doped fiber laser system or wherein the electro-optic modulated laser system comprises a continuous wave laser source modulated using a series of electro-optic modulators.

20. The two-dimensional spectrometer of claim 19, wherein the Yb-doped fiber laser system comprises a combination of a Yb-doped single mode fiber amplifier and a grating compressor, or wherein the Yb-doped fiber laser system comprises a high-powered chirped-pulse amplifier with a Yb-doped photonic crystal fiber, a fiber stretcher, and a grating compressor.

21. The two-dimensional spectrometer of claim 19, wherein the continuous wave laser source comprises a neodymium-doped yttrium aluminum garnet laser, a quantumcascade laser, or a diode laser.

22. The two-dimensional spectrometer of claim 1, wherein the first pulsed laser system or the second pulsed laser system independently comprise frequency comb laser systems.

23. The two-dimensional spectrometer of claim 1, further comprising an enhancement cavity arranged to receive the pump pulses and generate enhanced pump pulses, wherein the sample interacts with the enhanced pump pulses and the probe pulses to generate signal pulses.

24. The two-dimensional spectrometer of claim 23, wherein the sample positioner directs the sample into the enhancement cavity.

25. The two-dimensional spectrometer of claim 23, wherein the sample positioner directs the sample into the enhancement cavity without use of optical elements separating the sample from the enhancement cavity.

26. The two-dimensional spectrometer of claim 23, wherein the enhanced pump pulses exhibit increased phase stability compared to the pump pulses and/or increased intensity compared to the pump pulses.

27. The two-dimensional spectrometer of claim 1, wherein the sample positioner comprises a flowing gas inlet, a flowing gas jet, a gas supersonic beam, a flowing liquid inlet, a flowing liquid jet, a flowing microjet liquid sheet, a gas cell coated with an antireflection coating, or a gas cell with Brewster's angle windows.

28. The two-dimensional spectrometer of claim 1, wherein the sample comprises a gas sample or a liquid sample.

29. The two-dimensional spectrometer of claim 1, wherein the sample comprises one or a mixture of atomic or molecular gases.

30. The two-dimensional spectrometer of claim 1, wherein the sample comprises a pure liquid, a mixture of liquids, or a liquid with suspended solids or dissolved gases therein.

31. The two-dimensional spectrometer of claim 1, wherein the two-dimensional spectrum for the sample is a function of excitation frequency and detection frequency.

32. The two-dimensional spectrometer of claim 1, wherein the detector comprises a dual-comb spectrometer for generating the two-dimensional spectrum for the sample using detected signal pulses and detection pulses.

33. The two-dimensional spectrometer of claim 1, wherein the detector comprises or is coupled to a computer or signal processor programmed to perform a Fourier transform of a signal generated by an optical detector arranged to detect the signal pulses and the detection pulses.

34. A method of obtaining a two-dimensional spectrum, the method comprising: generating pump pulses and probe pulses using a first pulsed laser system; positioning a sample to interact with the pump pulses and the probe pulses to generate signal pulses; generating detection pulses using a second pulsed laser system; directing the signal pulses and the detection pulses to a detector to generate a detection signal; and generating a two-dimensional spectrum for the sample using the detection signal.

35. The method of claim 34, wherein the sample includes a plurality of different species.

36. The method of claim 34, wherein the two-dimensional spectrum for the sample includes individual and/or differentiable spectral features for a plurality of species present in the sample.

37. The method of claim 34, wherein the two-dimensional spectrum for the sample comprises a plurality of two-dimensional time-resolved spectra based on a time delay between the pump pulses and the probe pulses.

38. The method of claim 34, wherein the pump pulses comprise first pump pulses and second pump pulses, and wherein the first pump pulses and the second pump pulses are separated in time by a time delay.

39. The method of claim 38, further comprising adjusting a time delay between the first pump pulses and the second pump pulses.

40. The method of claim 34, wherein the pump pulses wherein the probe pulses are separated in time from the pump pulses by a time delay.

41. The method of claim 34, further comprising adjusting a time delay between the pump pulses and the probe pulses.

42. The method of claim 34, wherein the first pulsed laser system exhibits a first repetition rate, wherein the second pulsed laser system exhibits a second repetition rate, and wherein the first repetition rate and the second repetition rate are different.

43. The method of claim 42, wherein the first repetition rate and the second repetition rate are independently selected from the range of 60 MHz to 90 MHz.

44. The method of claim 42, wherein the first repetition rate is within 10% of the second repetition rate or wherein a difference between the first repetition rate and the second repetition rate is less than 10% of the second repetition rate.

45. The method of claim 34, wherein the first pulsed laser system and the second pulsed laser system comprise infrared laser systems.

46. The method of claim 34, wherein the pump pulses, the probe pulses, and the detection pulses exhibit central wavelengths of from 300 nm to 8000 nm.

47. The method of claim 34, wherein the first pulsed laser system or the second pulsed laser system comprises a nonlinear optical mixer.

48. The method of claim 47, wherein the nonlinear optical mixer is selected from a second harmonic generation system, a difference frequency generation system, and optical parametric oscillation system, or an optical parametric amplifier system.

49. The method of claim 34, wherein the pump pulses, the probe pulses, and the detection pulses exhibit pulse durations of from 40 fs to 150 fs.

50. The method of claim 34, wherein the detection pulses exhibit pulse durations of from 30 fs to 500 ns.

51. The method of claim 34, wherein the first pulsed laser system and the second pulsed laser system independently comprise pulsed laser systems selected from the group consisting of a fiber laser system or an electro-optic modulated laser system.

52. The method of claim 51, wherein the fiber laser system comprises a Yb- doped fiber laser system or wherein the electro-optic modulated laser system comprises a continuous wave laser source modulated using a series of electro-optic modulator.

53. The method of claim 51, wherein the Yb-doped fiber laser system comprises a combination of a Yb-doped single mode fiber amplifier and a grating compressor, or wherein the Yb-doped fiber laser system comprises a high-powered chirped-pulse amplifier with a Yb-doped photonic crystal fiber, a fiber stretcher, and a grating compressor.

54. The method of claim 51, wherein the continuous wave laser source comprises a neodymium-doped yttrium aluminum garnet laser, a quantum-cascade laser, or a diode laser.

55. The method of claim 34, wherein the first pulsed laser system or the second pulsed laser system independently comprise frequency comb laser systems.

56. The method of claim 34, further comprising: directing the pump pulses to an enhancement cavity arranged to receive the pump pulses and generate enhanced pump pulses, wherein the sample interacts with the enhanced pump pulses and the probe pulses to generate signal pulses.

57. The method of claim 56, wherein a sample positioner directs the sample into the enhancement cavity.

58. The method of claim 56, wherein the sample is directed into the enhancement cavity without use of optical elements separating the sample from the enhancement cavity.

59. The method of claim 56, wherein the enhanced pump pulses exhibit increased phase stability compared to the pump pulses and/or increased intensity compared to the pump pulses.

60. The method of claim 34, wherein the sample is positioned to interact with the pump pulses and the probe pulses using a flowing gas inlet, a flowing gas jet, a gas supersonic beam, a flowing liquid inlet, a flowing liquid jet, a flowing microjet liquid sheet, a gas cell coated with an antireflection coating, or a gas cell with Brewster's angle windows.

61. The method of claim 34, wherein the sample comprises a gas sample or a liquid sample.

62. The method of claim 34, wherein the sample comprises one or a mixture of atomic or molecular gases.

63. The method of claim 34, wherein the sample comprises a pure liquid, a mixture of liquids, or a liquid with suspended solids or dissolved gases therein.

64. The method of claim 34, wherein the two-dimensional spectrum for the sample is a function of excitation frequency and detection frequency.

65. The method of claim 34, wherein the detector comprises a dual-comb spectrometer for generating the detection signal using the signal pulses and detection pulses.

66. The method of claim 34, wherein generating the two-dimensional spectrum for the sample comprises perform a Fourier transform of the detection signal.