Pasquale Maddaloni

Frequency-comb-referenced singly-resonant OPO for sub-Doppler spectroscopy

We present a widely-tunable, singly-resonant optical parametric oscillator, emitting more than 1 W between 2.7 and 4.2 μm, which is phase locked to a self-referenced frequency comb. Both pump and signal frequencies are directly phase-locked to the frequency comb of a NIR-emitting fs mode-locked fibre laser, linked, in turn, to the caesium primary standard. We estimate for the idler frequency a fractional Allan deviation of ∼ 3 × 10⁻¹²τ⁻½ between 1 and 200 s. To test the spectroscopic performance of the OPO, we carried out saturation spectroscopy of several transitions belonging to the ν1 rovibrational band of CH₃I, resolving their electronic quadrupole hyperfine structure, estimating a linewidth better than 200 kHz FWHM for the idler, and determining the absolute frequency of the hyperfine components with a 50-kHz-uncertainty.

Frequency comb generation in quadratic nonlinear media

We experimentally demonstrate and theoretically explain the onset of optical frequency combs in a simple cavity-enhanced second-harmonic-generation system, exploiting second-order nonlinear interactions. Two combs are simultaneously generated around the fundamental pump frequency, with a spectral bandwidth up to about 10 nm, and its second harmonic. We observe different regimes of generation, depending on the phase-matching condition for second-harmonic-generation. Moreover, we develop an elemental model which provides a deep physical insight into the observed dynamics. Despite the different underlying physical mechanism, the proposed model is remarkably similar to the description of third-order effects in microresonators, revealing a potential variety of new effects to be explored and laying the groundwork for a novel class of highly efficient and versatile frequency comb synthesizers based on second-order nonlinear materials.

SPECIAL issue: 22nd colloquium on high-resolution molecular spectroscopy HRMS Dijon 2011 (Part 2) A narrow-linewidth optical parametric oscillator for mid-infrared high-resolution spectroscopy

We present a narrow-linewidth, singly-resonant cw optical parametric oscillator, emitting more than 1 W in the 2.7–4.2 µm range. The OPO is pumped by a narrow linewidth (40 kHz) fibre-laser system and the signal frequency is locked to a high-finesse Fabry–Pérot cavity in order to increase the spectral resolution, thus obtaining a residual linewidth of 70 kHz for the signal. We tested the spectral performance of our OPO on several transitions in the ν1 rovibrational band of CH3I, measuring line intensities and showing sub-Doppler dip detection.

Off-axis integrated-cavity-output spectroscopy for trace-gas concentration measurements: modeling and performance

We present a detailed study of the properties of off-axis-aligned high-finesse optical cavities under output time integration for spectroscopic applications. The dependence of the characteristics of the sample absorption spectra on a number of experimental parameters is investigated by developing a general model, where a Voigt absorption line shape is included into the cavity-transfer function. In this way, we address the issue of the measurement of trace amounts of gases in ambient air and single out the optimum conditions to give accurate estimates of the gas mixing ratio by the extraction of the integrated absorbance of the spectra. Quantitative results by the model are displayed for the detection of natural-abundance methane at 3 μm as a function of the air-sample total pressure. Finally, we compare the model predictions with the results of a previous experiment, evidencing their consistency.

Axion dark matter detection by laser spectroscopy of ultracold molecular oxygen: a proposal

Generalizing Sikivies approach (Sikivie 2014 Phys. Rev. Lett.113 201301), according to which dark matter axions may induce transitions between Zeeman states in an atomic system, an experiment based on a molecular gas is proposed for a search in the axion mass range between 1.4 and 1.9 meV. In particular, a 16O2 sample is brought to the temperature of 280 mK via buffer-gas cooling and then subjected to an external magnetic field; axion-driven transitions are eventually detected by resonance-enhanced multi-photon ionization spectroscopy.

Thickness measurement of thin transparent plates with a broad-band wavelength scanning interferometer

A novel broad-band telecom laser source is used to make a lateral-shear scanning-wavelength interferometer for measuring the thickness of thin plates. We show that the wide tunability range allows us to detect samples down to tens of microns with a relative uncertainty of less than 0.5%. A comparable accuracy in the thickness characterization of double-layer structures is also demonstrated. In turn, the wide tunability range needs the dispersion law of the materials to be taken into account in the model for correct thickness evaluation, although simultaneous measurement of dispersion and thickness are in principle possible with this technique.

Direct generation of optical frequency combs in χ(2) nonlinear cavities

Abstract Quadratic nonlinear processes are currently exploited for frequency comb transfer and extension from the visible and near infrared regions to other spectral ranges where direct comb generation cannot be accomplished. However, frequency comb generation has been directly observed in continuously pumped quadratic nonlinear crystals placed inside an optical cavity. At the same time, an introductory theoretical description of the phenomenon has been provided, showing a remarkable analogy with the dynamics of third-order Kerr microresonators. Here, we give an overview of our recent work on χ(2) frequency comb generation. Furthermore, we generalize the preliminary three-wave spectral model to a many-mode comb and present a stability analysis of different cavity field regimes. Although our work is a very early stage, it lays the groundwork for a novel class of highly efficient and versatile frequency comb synthesizers based on second-order nonlinear materials.

Frequency-comb-assisted precision laser spectroscopy of CHF3 around 8.6 μm

We report a high-precision spectroscopic study of room-temperature trifluoromethane around 8.6 μm, 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 υ5 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 × 10(-10). Line intensity factors, pressure broadening, and shifting parameters are also provided.

Sub-kilohertz linewidth narrowing of a mid-infrared optical parametric oscillator idler frequency by direct cavity stabilization

We stabilize the idler frequency of a singly resonant optical parametric oscillator directly to the resonance of a mid-infrared Fabry-Perot reference cavity. This is accomplished by the Pound-Drever-Hall locking scheme, controlling either the pump laser or the resonant signal frequency. A residual relative frequency noise power spectral density below 10(3)  Hz(2)/Hz is reached on average, with a Gaussian linewidth of 920 Hz over 100 ms, which reveals the potential for reaching spectral purity down to the hertz level by locking the optical parametric oscillator against a mid-infrared cavity with state-of-the-art superior performance.

Axion dark matter detection by laser induced fluorescence in rare-earth doped materials

We present a detection scheme to search for QCD axion dark matter, that is based on a direct interaction between axions and electrons explicitly predicted by DFSZ axion models. The local axion dark matter field shall drive transitions between Zeeman-split atomic levels separated by the axion rest mass energy mac . Axion-related excitations are then detected with an upconversion scheme involving a pump laser that converts the absorbed axion energy (∼ hundreds of μeV) to visible or infrared photons, where single photon detection is an established technique. The proposed scheme involves rare-earth ions doped into solid-state crystalline materials, and the optical transitions take place between energy levels of 4f electron configuration. Beyond discussing theoretical aspects and requirements to achieve a cosmologically relevant sensitivity, especially in terms of spectroscopic material properties, we experimentally investigate backgrounds due to the pump laser at temperatures in the range 1.9−4.2 K. Our results rule out excitation of the upper Zeeman component of the ground state by laser-related heating effects, and are of some help in optimizing activated material parameters to suppress the multiphonon-assisted Stokes fluorescence. Valid PACS numbers may be entered using the \pacs{#1} command.We present a detection scheme to search for QCD axion dark matter, that is based on a direct interaction between axions and electrons explicitly predicted by DFSZ axion models. The local axion dark matter field shall drive transitions between Zeeman-split atomic levels separated by the axion rest mass energy mac2. Axion-related excitations are then detected with an upconversion scheme involving a pump laser that converts the absorbed axion energy (~hundreds of μeV) to visible or infrared photons, where single photon detection is an established technique. The proposed scheme involves rare-earth ions doped into solid-state crystalline materials, and the optical transitions take place between energy levels of 4fN electron configuration. Beyond discussing theoretical aspects and requirements to achieve a cosmologically relevant sensitivity, especially in terms of spectroscopic material properties, we experimentally investigate backgrounds due to the pump laser at temperatures in the range 1.9 − 4.2 K. Our results rule out excitation of the upper Zeeman component of the ground state by laser-related heating effects, and are of some help in optimizing activated material parameters to suppress the multiphonon-assisted Stokes fluorescence.