We have developed a sub-Doppler resolution spectrometer. A difference frequency generation source, which consists of a pump source of a Nd:YAG laser, a signal source of an extended-cavity laser diode, and a waveguide-type PPLN, covers from 87 to 93 THz (2900 to 3100 cm⁻¹). An enhanced-cavity absorption cell remarkably improves the sensitivity of Lamb dips. An optical frequency comb controls the central frequency of the source with an uncertainty of a few kilohertz. Because the idler frequency is swept based on absolute frequency through the comb, recorded spectra can be repeatedly accumulated without any frequency drift. We have applied the spectrometer to resolve the hyperfine structure of the fundamental band of HCl with a spectral resolution of about 250 kHz. To reduce the transit-time broadening, a novel enhanced-cavity absorption cell coupled with an idler wave of 1.9-mm beam radius at the beam waist has been introduced. The A₁-A₂ splitting of the ν₁ and ν₄ bands of CH₃D is resolved for a few tens low-J transitions with the Lamb-dip linewidth of 60 to 100 kHz. Very recently, the source linewidth has reduced to 3 kHz using a linewidth transfer technique from the Nd:YAG laser to the extended-cavity laser diode through a novel optical frequency comb with a fast servo control. When methane sample is cooled with liquid-nitrogen, and the beam radius is expanded to 3 mm, the observed Lamb dip is 20 kHz wide without any enhanced-cavity absorption cell.

Sub-Doppler resolution spectroscopy of the fundamental bands of H³⁵Cl and H³⁷Cl has been carried out from 87 to 90 THz using a comb-referenced difference-frequency-generation (DFG) spectrometer. While the frequencies of the pump and signal waves are locked to that of the individual nearest comb mode, the repetition rate of the comb is varied for sweeping the idler frequency. Therefore, the relative uncertainty of the frequency scale is 10^–11, and the spectral resolution remains about 250 kHz even when the spectrum is accumulated for a long time. The hyperfine structures caused by chlorine nucleus are resolved for the R(0) to R(4) transitions. The figure depicts wavelength-modulation spectrum of the R(0) transition of H³⁵Cl. Three Lamb dips correspond to the F= 0, 1, and –1 components left to right, and the others with arrows are cross-over resonances which are useful for determining the weak F=–1 component frequencies for the R(1) to R(3) transitions. We have determined 49 and 44 transition frequencies of H³⁵Cl and H³⁷Cl with an uncertainty of 10 kHz. Six molecular constants of the vibrational excited state for each isotopomer are determined. They reproduce the determined frequencies with a standard deviation of about 10 kHz. r0pt Figure

Sub-Doppler resolution spectroscopy of CH₃D has been carried out for the ν₁ and ν₄ fundamental bands using a comb-referenced difference-frequency generation spectrometer. Thirty transitions from the low-J" and K" = 3 levels are observed with a resolution of 60 to 100 kHz, and the A₁-A₂ splitting is resolved for twenty-three of the thirty transitions. Most of them are overlapped in Doppler broadening and resolved for the first time, as far as we know. The absolute transition frequencies are determined with a typical uncertainty of 4 kHz. The A₁-A₂ splitting constant of the K" = 3 levels is yielded as 2h_3,v=0 = (1.5641 ±0.0026) Hz for the ground vibrational state. Those of the K′ = 3 levels for the v₁ = 1 states and of the (K′ = 2, l = −1) and (K′ = 4, l = 1) levels for the v₄ = 1 state are also determined including the J′-dependent terms.

In high-resolution molecular spectroscopy, the precise measure of the optical frequency is crucial to evaluate minute shifts and splittings of the energy levels. On the other hand, in such spectroscopy, thousands of spectral lines distributed over several wavenumbers have to be measured by a continuously scanning cw laser. Therefore, the continuously changing optical frequency of the scanning laser has to be determined with enough precision. To satisfy these contradictory requirements, we have been developed two types of high-resolution spectroscopic systems employing an optical frequency comb. One of the systems employs RF band-pass filters to generate equally spaced frequency markers for optical frequency calibration, and is appropriate for wide wavelength-range measurement with relatively high scanning rate.^a In the other system, the beat frequency between the optical frequency comb and the scanning laser is controlled by an acousto-optic frequency shifter. This system is suitable for more precise measurement, and enables detailed analyses of frequency characteristics of scanning laser.^b In the present study, we observe Doppler-free two-photon absorption spectra of A¹B_1u (v₄ = 1) ← X¹A_g (v = 0) transition of naphthalene around 298 nm. The spectral lines are rotationally resolved and the resolution is about 100 kHz. For ^qQ transition, the rotational lines are assigned, and molecular constants in the excited state are determined. In addition, we analyze the origin of the measured linewidth and Coriolis interactions between energy levels. To determine molecular constants more precisely, we proceed to measure and analyze spectra of other transitions, such as ^sS transitions. ^a A. Nishiyama, D. Ishikawa, and M. Misono, J. Opt. Soc. Am. B 30, 2107 (2013). ^b A. Nishiyama, A. Matsuba, and M. Misono, Opt. Lett. 39, 4923 (2014).

Fourier transform spectrometers (FTS) based on optical frequency combs (OFC) allow detection of broadband molecular spectra with high signal-to-noise ratios within acquisition times orders of magnitude shorter than traditional FTIRs based on thermal sourcesMandon, J., G. Guelachvili, and N. Picque, Nat. Phot., 2009. 3(2): p. 99-102. Due to the pulsed nature of OFCs the interferogram consists of a series of bursts rather than a single burst at zero optical path difference (OPD). The comb mode structure can be resolved by acquiring multiple bursts, in both mechanical FTS systemsZeitouny, M., et al., Ann. Phys., 2013. 525(6): p. 437-442.nd dual-comb spectroscopyZolot, A.M., et al., Opt. Lett., 2012. 37(4): p. 638-640. However, in all existing demonstrations the resolution was ultimately limited either by the maximum available OPD between the interferometer arms or by the total acquisition time enabled by the storage memory. We present a method that provides spectral resolution exceeding the limit set by the maximum OPD using an interferogram containing only a single burst. The method allows measurements of absorption lines narrower than the OPD-limited resolution without any influence of the instrumental lineshape function. We demonstrate this by measuring undistorted CO₂ and CO absorption lines with linewidth narrower than the OPD-limited resolution using OFC-based mechanical FTS in the near- and mid-infrared wavelength ranges. The near-infrared system is based on an Er:fiber femtosecond laser locked to a high finesse cavity, while the mid-infrared system is based on a Tm:fiber-laser-pumped optical parametric oscillator coupled to a multi-pass cell. We show that the method allows acquisition of high-resolution molecular spectra with interferometer length orders of magnitude shorter than traditional FTIR.

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Mandon, J., G. Guelachvili, and N. Picque, Nat. Phot., 2009. 3(2): p. 99-102.. Zeitouny, M., et al., Ann. Phys., 2013. 525(6): p. 437-442.a Zolot, A.M., et al., Opt. Lett., 2012. 37(4): p. 638-640..

We will present a metrological application of our recently developed Optical Feedback Frequency Stabilized - Cavity Ring Down Spectrometer (OFFS-CRDS). This instrument, which ideally fits with an optical frequency comb for absolute frequency calibration, relies on the robust lock of a steady cavity ring down resonator against a highly stable, radiofrequency tuned optical source. At 1.6 μm, over 7 nm, we demonstrate Lamb dip spectroscopy of CO₂ with line frequency retrieval at the kHz level, a dynamic in excess of 700,000 on the absorption scale and a detectivity of 4x10⁻¹³cm⁻¹Hz^−1/2. Such an instrument nicely meets the requirements for the most demanding spectroscopy spanning from accurate isotopic ratio determination and very precise lineshape recordings to Boltzmann constant redefinition.

Ultrafast spectroscopy on gas phase systems is typically restricted to techniques involving photoionization, whereas solution phase experiments utilize the detection of light. At Stony Brook, we are developing new techniques for performing femtosecond time-resolved spectroscopy using frequency combs and high-finesse optical resonators. A large detection sensitivity enhancement over traditional methods enables the extension of all-optical ultrafast spectroscopies, such as broad-band transient absorption spectroscopy (TAS) and 2D spectroscopy, to dilute gas phase samples produced in molecular beams. Here, gas phase data can be directly compared to solution phase data. Initial demonstration experiments are focusing on the photodissociation of iodine in small neutral argon clusters, where cluster size strongly influences the effects solvent-caging and geminate recombination. I will discuss these initial results, our high power home-built Yb:fiber laser systems, and also extensions of the methods to the mid-IR to study the vibrational dynamics of hydrogen bonded clusters.

We present noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS), a recently developed technique for sensitive, broadband, and high resolution spectroscopyA. Khodabakhsh, C. Abd Alrahman, and A. Foltynowicz, Opt. Lett. 39, 5034-5037 (2014). In NICE-OFCS an optical frequency comb (OFC) is locked to a high finesse cavity and phase-modulated at a frequency precisely equal to (a multiple of) the cavity free spectral range. Since each comb line and sideband is transmitted through a separate cavity mode in exactly the same way, any residual frequency noise on the OFC relative to the cavity affects each component in an identical manner. The transmitted intensity contains a beat signal at the modulation frequency that is immune to frequency-to-amplitude noise conversion by the cavity, in a way similar to continuous wave noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS)J. Ye, L. S. Ma, and J. L. Hall, J. Opt. Soc. Am. B 15, 6-15 (1998). The light transmitted through the cavity is detected with a fast-scanning Fourier-transform spectrometer (FTS) and the NICE-OFCS signal is obtained by fast Fourier transform of the synchronously demodulated interferogram. Our NICE-OFCS system is based on an Er:fiber femtosecond laser locked to a cavity with a finesse of  ∼ 9000 and a fast-scanning FTS equipped with a high-bandwidth commercial detector. We measured NICE-OFCS signals from the 3ν₁+ν₃ overtone band of CO₂ around 1.57 μm and achieved absorption sensitivity 6.4×10⁻¹¹cm⁻¹ Hz^−1/2 per spectral element, corresponding to a minimum detectable CO₂ concentration of 25 ppb after 330 s integration timeA. Khodabakhsh, A. C. Johansson, and A. Foltynowicz, Appl. Phys. B (2015) doi:10.1007/s00340-015-6010-7. We will describe the principles of the technique and its technical implementation, and discuss the spectral lineshapes of the NICE-OFCS signals.

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A. Khodabakhsh, C. Abd Alrahman, and A. Foltynowicz, Opt. Lett. 39, 5034-5037 (2014).. J. Ye, L. S. Ma, and J. L. Hall, J. Opt. Soc. Am. B 15, 6-15 (1998).. A. Khodabakhsh, A. C. Johansson, and A. Foltynowicz, Appl. Phys. B (2015) doi:10.1007/s00340-015-6010-7..

We present a new approach to Cavity Enhanced - Direct Frequency Comb Spectroscopy where the full emission bandwidth of a Titanium:Sapphire laser is exploited at GHz resolution. The technique is based on a low-resolution Vernier filtering obtained with an appreciable -actively stabilized- mismatch between the cavity Free Spectral Range and the laser repetition rate, using a diffraction grating and a split-photodiode Rutkowski et al, Opt. Lett., 39(23)2014 This particular approach provides an immunity to frequency-amplitude noise conversion, reaching an absorption baseline noise in the 10⁻⁹ cm⁻¹ range with a cavity finesse of only 3000. Spectra covering 1800 cm⁻¹ ( ∼  55 THz) are acquired in recording times of about 1 second, providing an absorption figure of merit of a few 10⁻¹¹ cm⁻¹/√{Hz}. Initially tested with ambient air, we report progress in using the Vernier frequency comb method with a discharge source of small radicals.

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Rutkowski et al, Opt. Lett., 39(23)2014.

We report the generation and detection of a decade-spanning TeraHertz (THz) frequency comb (0.15-2.4 THz) using two Ti:Sapphire femtosecond laser oscillators and ASynchronous OPtical Sampling THz Time-Domain Spectroscopy (ASOPS-THz-TDS). The measured linewidth of the comb at 1.5 THz is 3 kHz over a 60 second acquisition. With time-domain detection of the comb, we measure three transitions of water vapor at 10 mTorr between 1-2 THz with an average Doppler-limited fractional uncertainty of 5.9×10⁻⁸. Significant improvements in bandwidth, resolution, and sensitivity are possible with existing technologies and will enable future studies of jet-cooled hydrogen-bonded clusters.

High-resolution ASynchronous OPtical Sampling (ASOPS) is a technique that substantially improves the combined frequency resolution and bandwidth of ASOPS based TeraHertz Time-Domain Spectroscopy (THz-TDS) systems. We employ two mode-locked femtosecond Ti:Sapphire oscillators with repetition frequencies of 80 MHz operating at a fixed repetition frequency offset of 100 Hz. This offset lock is maintained by a Phase-Locked Loop (PLL) operating at the 60th harmonic of the repetition rate of the Ti:Sapphire oscillators. Their respective time delay is scanned across 12.5 ns requiring a scan time of 10 ms, supporting a time delay resolution of up to 15.6 fs. ASOPS-THz-TDS enables high-resolution spectroscopy that is impossible for a THz-TDS system employing a mechanical delay stage. We measure a timing jitter of 1.36 fs for the system using an air-gap etalon and an optical cross-correlator. We report a Root-Mean-Square deviation of 20.7 MHz and a mean deviation of 14.4 MHz for water absorption lines from 0.5 to 2.7. High-resolution ASOPS-THz-TDS enables high resolution spectroscopy of both gas-phase and condensed-phase samples across a decade of THz bandwidth.

Raman spectroscopy is an important spectroscopic technique used in chemistry to provide a fingerprint by which molecules can be identified. It helps us to observe vibration- rotation, and other low-frequency modes in a system. Dual comb Raman spectroscopy allows measuring a wide bandwidth with high resolution in microseconds. The stimulate Raman spectroscopy had been performed in early days where the nonlinear conversion efficiency depended on laser peak power. Hence we propose an approach for rapidly resolving the Raman spectroscopy of CF₄ molecule by two Ti:sapphire comb lasers. Our progress on this proposal will be presented in the conference. First, we have realized a compact dual Ti:sapphire comb laser system T.-W. Liu, C.-M. Wu, Y.–C. Hsu and W.-Y. Cheng, Appl. Phys. B 117, 699 (2014)here the dual Ti:sapphire laser system possesses the specification of 1 GHz repetition rate. In our dual comb system, 1 GHz repetition rate, 100 kHz ∆f_rep and 2.4 THz optical filter are chosen according to the demands of our future works on spectroscopy. Therefore, the maximum mode number within free spectral range is 5*10³, and the widest range of dual-comb based spectra in that each spectrum could be uniquely identified is 5 THz. The actual bandwidth is determined by the employed optical filter and is set to be 2.4 THz here, so that the corresponding data acquisition time is 10 μs. Secondly, since the identification of the tremendous spectral lines of CF₄ molecule relies on a stable reference and a reliable data-retrieving system, we propose a first-step experiment on atomic system where the direct 6S-8S 822-nm two-photon absorption P. Fendel, S. D. Bergeson, Th. Udem, and T. W. Hänsch, Opt. Lett. 32, 701 (2007)nd 8S-6P_3/2 (794 nm) enhanced stimulate Raman would be realized directly by using Ti:sapphire laser. We have successfully performed direct comb laser two-photon spectroscopy for both with and without middle-level enhanced. For the level enhanced two-photon spectrum, our experimental setup achieves Doppler-free spectrum and a record narrow linewidth (1 MHz).

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T.-W. Liu, C.-M. Wu, Y.–C. Hsu and W.-Y. Cheng, Appl. Phys. B 117, 699 (2014)w P. Fendel, S. D. Bergeson, Th. Udem, and T. W. Hänsch, Opt. Lett. 32, 701 (2007)a