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By reducing both the internal and translational temperature of any species down to a few kelvins, the buffer-gas-cooling (BGC) technique has the potential to dramatically improve the quality of ro-vibrational molecular spectra, thus offering unique opportunities for transition frequency measurements with unprecedented accuracy. However, the difficulty in integrating metrological-grade spectroscopic tools into bulky cryogenic equipment has hitherto prevented from approaching the kHz level even in the best cases. Here, we overcome this drawback by an original opto-mechanical scheme which, effectively coupling a Lamb-dip saturated-absorption cavity ring-down spectrometer to a BGC source, allows us to determine the absolute frequency of the acetylene (ν1+ν3) R(1)e transition at 6561.0941 cm-1 with a fractional uncertainty as low as 6∙10-12. By improving the previous record with buffer-gas-cooled molecules by one order of magnitude, our approach paves the way for a number of ultra-precise low-temperature spectroscopic studies, aimed at both fundamental Physics tests and optimized laser cooling strategies.

We present an original opto-mechanical scheme which, effectively coupling a Lamb-dip saturated-absorption cavity ring-down spectrometer to a buffer-gas-cooling (BGC) source, allows us to determine the absolute frequency of the acetylene (ν 1 + ν 3) R(1)e transition at 6561.0941 cm−1 with an overall (statistical + systematic) uncertainty as low as 1.2 kHz. By improving the previous record with buffer-gas-cooled molecules by one order of magnitude, our achievement opens the door to new kind of ultra-precise low-temperature spectroscopic studies.

The high-detection-sensitivity saturated-absorption cavity ring-down (SCAR) technique is extended to Lamb-dip spectroscopy of rovibrational molecular transitions in the near-infrared region. Frequency-comb-referenced sub-Doppler saturation measurements, performed on the acetylene ( ν 1 + ν 3 + ν 4 ← ν 4 ) R(14)e line at 6562 cm − 1 , are analyzed by a SCAR global line profile fitting routine, based on a specially developed theoretical model. Compared to a conventional cavity ring-down evaluation, our approach yields dip profiles with a linewidth freed from saturation broadening effects, reduced by 40%, and a signal-to-noise ratio increased by 90%. Ultimately, an overall (statistical and systematic) fractional uncertainty as low as 7 × 10 − 12 is achieved for the absolute line-center frequency. At the same time, our method is also able to accurately infer the linear (non-saturated) behavior of the gas absorption, providing Lamb-dip-based line strength measurements with a relative uncertainty of 0.5%.

We review the recent developments in precision ro-vibrational spectroscopy of buffer-gas-cooled neutral molecules, obtained using infrared frequency combs either as direct probe sources or as ultra-accurate optical rulers. In particular, we show how coherent broadband spectroscopy of complex molecules especially benefits from drastic simplification of the spectra brought about by cooling of internal temperatures. Moreover, cooling the translational motion allows longer light-molecule interaction times and hence reduced transit-time broadening effects, crucial for high-precision spectroscopy on simple molecules. In this respect, we report on the progress of absolute frequency metrology experiments with buffer-gas-cooled molecules, focusing on the advanced technologies that led to record measurements with acetylene. Finally, we briefly discuss the prospects for further improving the ultimate accuracy of the spectroscopic frequency measurement.

We realize a phase-stabilised optical fiber backbone that connects the Italian National Metrology Institute with two radio telescopes over a 600 km baseline. This allows referencing of Very Long Baseline Interferometry (VLBI) facilities with the best atomic frequency standards available today and the implementation of a common-clock architecture, which we are now using to assess VLBI ultimate performances.

We realized a coherent fiber link, 1739 km long, to connect a facility of the Italian Space Agency devoted to Space Geodesy. We describe here the motivations, the architecture of the link, the Time and Frequency station at the remote end and the first results.

Quantum cascade lasers (QCLs) are becoming a key tool in several advanced experiments in the field of precision molecular spectroscopy and absolute frequency metrology. In view of this, a thorough control of their emission properties must be achieved, including a narrow linewidth as well as a high frequency-stability combined with referencing to a primary standard. Here, we give a review on recent developments and next perspectives in this scope, with particular regard to the use of QCLs in fundamental physics spectroscopic searches.

We demonstrate continuous-wave cavity ring-down spectroscopy of a partially hydrodynamic molecular beam emerging from a buffer-gas-cooling source. Specifically, the (nu1+nu3) vibrational overtone band of acetylene (12C2H2) around 1.5 micron is accessed using a narrow-linewidth diode laser stabilized against a GPS-disciplined rubidium clock via an optical frequency comb synthesizer. As an example, the absolute frequency of the R(1) component is measured witha fractional accuracy of 1x10-9. Our approach represents the first step towards the extension of more sophisticated cavity-enhanced interrogation schemes, like two-photon excitation, to buffer-gas-cooled molecular beams. In this respect, a frequency-comb-referenced absolute calibration of the 13C12CH2 spectrum around 6497 cm-1 is also reported, preliminary to the implementation of a Doppler-free two-color interrogation scheme.References: L. santamaria, V. Di sarno, P. De Natale, M. De Rosa, M. Inguscio, S. Mosca, I. Ricciardi, D. Calonico, F. Levi, P. Maddaloni, Phys. Chem. Chem. Phys. 18, 16715 (2016).

We demonstrate continuous-wave cavity ring-down spectroscopy of a partially hydrodynamic molecular beam emerging from a buffer-gas-cooling source. Specifically, the ( v1 + v(3)) vibrational overtone band of acetylene (C2H2) around 1.5 mu m is accessed using a narrow-linewidth diode laser stabilized against a GPS-disciplined rubidium clock via an optical frequency comb synthesizer. As an example, the absolute frequency of the R(1) component is measured with a fractional accuracy of 1 X 10(-9). Our approach represents the first step towards the extension of more sophisticated cavity-enhanced interrogation schemes, including saturated absorption cavity ring-down or two-photon excitation, to buffer-gascooled molecular beams.

Modulation transfer spectroscopy is used to demonstrate absolute frequency stabilization of an 8.6-µm-wavelength quantum cascade laser against a sub-Doppler absorption of the C H F 3 molecule. The obtained spectral emission properties are thoroughly characterized through a self-referenced optical frequency comb, stabilized against either a GPS-disciplined Rb clock or a 1.54-µm Er-fiber laser locked to a high-finesse ultra-low-expansion optical cavity. Fractional long-term stability and accuracy at a level of 4 × 10 − 12 (at 100 s) and 3 × 10 − 10 , respectively, are demonstrated, along with an emission linewidth as narrow as 10 kHz for observation times of 0.1 s.

We report on a precision spectroscopic study of room-temperature H12C13CH in the 6463–6520 cm − 1 range, using a continuous-wave diode laser phase-locked to a near-infrared optical frequency comb synthesizer stabilized, in turn, against a GPS-disciplined rubidium clock. By tuning the comb repetition frequency, several ro-vibrational transitions in the P and R branches of the 2ν3 overtone band are recorded. Accuracy levels as low as 100 kHz, 5 · 10 − 10 in fractional terms, are obtained in the determination of absolute line-center frequencies. In addition, line intensities are measured for the first time, with a best fractional uncertainty of 1%, and used to retrieve the purely vibrational transition dipole moment in conjunction with the first-order Herman–Wallis factor. Finally, the pressure broadening coefficients γself are also derived for the studied transitions.

We report on metrological-grade saturated absorption spectroscopy of CHF3 at 8.63 μm based on CW distributed-feedback QCL laser and a mid-IR self-frequency referenced optical comb. The experimental setup for the saturation spectroscopy of CHF3 is shown in Fig. 1. A CW DFB-QCL with a maximum output power of ∼40 mW tunable in the wavelength range from 8.55 to 8.65 μm, in coincidence with the us (asymmetric FCF bending) vibrational band of CHF3, is used to probe the gas sample. The QCL output beam is focused at the input of a 30-dB optical isolator to a diameter of 1.6∼mm, by using antireflection-coated aspheric lens mounted onto the laser housing. Then the laser beam passing through a 20-cm plano-convex lens and a 50% beam splitter is coupled to a 25-cm-long stainless steel cell containing the CHF3 gas (98% purity). The transmitted beam first passes through a second 50% beam splitter to detect the single-pass CHF3 absorptions and then is back-reflected into the gas cell to implement the double pass configuration (collinear pump and probe saturated absorption technique). The probe beam is detected exploiting the reflection on the first beam splitter by a four-stage electric cooled MCT detector (10∼MHz bandwidth). By modulating the QCL driving current at a frequency _/m=100 kHz also a wavelength modulation method is implemented to retrieved the first derivative signal by means of a first-harmonic coherent demodulation. To absolute calibrate the QCL emission frequency a difference frequency generation optical frequency comb, covering the 8–14 μm spectral region, is used [1]. The mid-IR comb is properly tuned to a central wavelength of 8.6 μm (average output power of 4∼mW in an optical bandwidth of 0.8 μm) and its repetition frequency is stabilized against a RF synthesizer locked to a GPS-disciplined Rb frequency standard (Allan deviation σ(τ)=10−11τ−1/2 and fractional frequency accuracy of 10−13, respectively, where τ is the integration time).

We report on absolute measurements of saturated-absorption line-center frequencies of room-temperature trifluoromethane using a quantum cascade laser at 8.6 μm and the frequency modulation spectroscopy method. Absolute calibration of the laser frequency is obtained by direct comparison with a mid-infrared optical frequency comb synthesizer referenced to a radio-frequency Rb standard. Several sub-Doppler transitions falling in the υsub5/subvibrational band are investigated at around 1158.9 cmsup-1/supwith a fractional frequency precision of 8.6·10sup-12/supat 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6·10sup-11/supis mainly limited by the reproducibility of the frequency measurements.

We report the design of an experiment that aims to constrain, over a-few-year timescale, the fractional temporal variation of the proton-to-electron mass ratio, beta = m(p)/m(e), at a level of 10(-15)/yr by means of a spectroscopic frequency measurement on a beam of cold CF3H molecules. This is extracted from a buffer-gas-cooling source and then collimated by means of an electrostatic hexapole lens. Employed in a two-photon Ramsey-fringes interrogation scheme, the probe source is based on a mid-infrared quantum cascade laser, phase-locked to a specially-developed optical frequency comb that is ultimately referenced to the Cs primary standard via an optical fiber link. (C) 2014 Elsevier Inc. All rights reserved.

We report a high-precision spectroscopic study of room-temperature trifluoromethane around 8.6 mu m, using a CW quantum cascade laser phase-locked to a mid-infrared optical frequency comb. This latter is generated by a nonlinear down-conversion process starting from a dual-branch Er: fiber laser and is stabilized against a GPS-disciplined rubidium clock. By tuning the comb repetition frequency, several transitions falling in the nu(5) vibrational band are recorded with a frequency resolution of 20 kHz. Due to the very dense spectra, a special multiple-line fitting code, involving a Voigt profile, is developed for data analysis. The combination of the adopted experimental approach and survey procedure leads to fractional accuracy levels in the determination of line center frequencies, down to 2 x 10(-10). Line intensity factors, pressure broadening, and shifting parameters are also provided. (C) 2015 AIP Publishing LLC.

We report on our preliminary results in the realization and characterization of a porous silicon (PSi) resonant mirror (RM) for optical biosensing. We have numerically and experimentally studied the coupling between the electromagnetic field, totally reflected at the base of a high refractive index prism, and the optical modes of a PSi waveguide. This configuration is very sensitive to changes in the refractive index and/or in thickness of the sensor surface. Due to the high specific area of the PSi waveguide, very low DNA concentrations can be detected confirming that the RM could be a very sensitive and label-free optical biosensor.