A self-operating broadband spectrometer on a droplet.

Small-scale Fourier transform spectrometers are rapidly revolutionizing infrared spectro-chemical analysis, enabling on-site and remote sensing applications that were hardly imaginable just few years ago. While most devices reported to date rely on advanced photonic integration technologies, here we demonstrate a miniaturization strategy which harnesses unforced mechanisms, such as the evaporation of a liquid droplet on a partially reflective substrate. Based on this principle, we describe a self-operating optofluidic spectrometer and the analysis method to retrieve consistent spectral information in spite of the intrinsically non-reproducible droplet formation and evaporation dynamics. We experimentally realize the device on the tip of an optical fiber and demonstrate quantitative measurements of gas absorption with a 2.6 nm resolution, in a 100 s acquisition time, over the 250 nm span allowed by our setup's components. A direct comparison with a commercial optical analyzer clearly points out that a simple evaporating droplet can be an efficient small-scale, inexpensive spectrometer, competitive with the most advanced integrated photonic devices.

Modulation Instability Induced Frequency Comb Generation in a Continuously Pumped Optical Parametric Oscillator.

Continuously pumped passive nonlinear cavities can be harnessed for the creation of novel optical frequency combs. While most research has focused on third-order "Kerr" nonlinear interactions, recent studies have shown that frequency comb formation can also occur via second-order nonlinear effects. Here, we report on the formation of quadratic combs in optical parametric oscillator (OPO) configurations. Specifically, we demonstrate that optical frequency combs can be generated in the parametric region around half of the pump frequency in a continuously driven OPO. We also model the OPO dynamics through a single time-domain mean-field equation, identifying previously unknown dynamical regimes, induced by modulation instabilities, which lead to comb formation. Numerical simulation results are in good agreement with experimentally observed spectra. Moreover, the analysis of the coherence properties of the simulated spectra shows the existence of correlated and phase-locked combs. Our results reveal previously unnoticed dynamics of an apparently well assessed optical system, and can lead to a new class of frequency comb sources that may stimulate novel applications by enabling straightforward access to elusive spectral regions, such as the midinfrared.

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.

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.

Absolute measurement of the S(0) and S(1) lines in the electric quadrupole fundamental band of D(2) around 3µm.

The electric quadrupole fundamental (v=1←0) band of molecular deuterium around 3 µm is accessed by cavity ring-down spectroscopy using a difference-frequency-generation source linked to the Cs-clock primary standard via an optical frequency comb synthesizer. An absolute determination of the line position and strength is reported for the first two transitions (J=2←0 and J=3←1) of the S branch. An accuracy of 6×10(-8) is achieved for the line-center frequencies, which improves by a factor 20 previous experimental results [A. R. W. McKellar and T. Oka, Can. J. Phys. 56, 1315 (1978)]. The line strength values, measured with 1% accuracy, are used to retrieve the quadrupole moment matrix elements which are found in good agreement with previous theoretical calculations [A. Birnbaum and J. D. Poll, J. Atmos. Sci. 26, 943 (1969); J. L. Hunt, J. D. Poll, and L. Wolniewicz, Can. J. Phys. 62, 1719 (1984)].

Non-collinear quasi phase matching and annular profiles in difference frequency generation with focused Gaussian beams.

We present and experimentally test a simple model for difference frequency generation (DFG) in periodically-poled crystals with gaussian pumping beams. Focusing of input beams originates several non-collinear quasi-phase-matching configurations of the interacting wavevectors, which contribute to the idler output field. In this picture, we accurately describe a number of effects, such as the occurrence of annular idler intensity profiles and the asymmetric trend of DFG power vs temperature. Finally, we quantitatively test the model by means of an indirect measurement of the crystal poling period.

Absolute frequency measurement of molecular transitions by a direct link to a comb generated around 3-microm.

A 3-microm continuous-wave difference-frequency source is directly referenced to a mid-infrared optical frequency comb synthesizer by measuring their beat-note signal by a fast HgCdTe detector. Absolute frequency metrology of molecular vibration spectra is demonstrated by locking the 3-microm coherent radiation to the nearest comb tooth and tuning the comb mode spacing across the Doppler-broadened absorption profile of a CH(4) ro-vibrational transition.

Combining a difference-frequency source with an off-axis high-finesse cavity for trace-gas monitoring around 3 microm.

High-sensitivity spectroscopy of methane around 3 microm was carried out by means of a 5.5-mW cw difference-frequency generator in conjunction with a high finesse cavity in off-axis alignment. By cavity-output integration a minimum detectable absorption coefficient of 5.7*10-9 cm-1Hz-1/2 was achieved, which compares well with results already reported in the literature. Detection of methane in natural abundance was also performed in ambient air, for different values of total pressure, allowing direct concentration measurements via evaluation of the integrated absorbance of the spectra. In particular, at atmospheric pressure, a minimum detectable concentration of 850 parts per trillion by volume (pptv)*Hz-1/2 was demonstrated.

Collective excitations of a trapped Bose-Einstein condensate in the presence of a 1D optical lattice.

We study low-lying collective modes of an elongated 87Rb condensate produced in a 3D magnetic harmonic trap with the addition of a 1D periodic potential which is provided by a laser standing wave along the axial direction. While the transverse breathing mode remains unperturbed, quadrupole and dipole oscillations along the optical lattice are strongly modified. Precise measurements of the collective mode frequencies at different heights of the optical barriers provide a stringent test of the theoretical model recently introduced [M. Krämer, Phys. Rev. Lett. 88, 180404 (2002)]].

Expansion of a coherent array of Bose-Einstein condensates.

We investigate the properties of a coherent array containing about 200 Bose-Einstein condensates produced in a far detuned 1D optical lattice. The density profile of the gas, imaged after releasing the trap, provides information about the coherence of the ground-state wave function. The measured atomic distribution is characterized by interference peaks. The time evolution of the peaks, their relative population, as well as the radial size of the expanding cloud are in good agreement with the predictions of theory. The 2D nature of the trapped condensates and the conditions required to observe the effects of coherence are also discussed.

Time-domain atom interferometry across the threshold for Bose-Einstein condensation.

We have performed time-domain interferometry experiments with matter waves trapped in a harmonic potential above and below the Bose-Einstein phase transition, by means of the method of separated oscillating fields, with a variable time delay T. We observe the oscillations of the population between two internal Zeeman states versus the delay T to be rapidly depleted both below and slightly above Bose-Einstein condensation. We give a quantitative explanation in terms of the phase evolution due to the entanglement between the internal and external degrees of freedom.

Josephson junction arrays with Bose-Einstein condensates.

We report on the direct observation of an oscillating atomic current in a one-dimensional array of Josephson junctions realized with an atomic Bose-Einstein condensate. The array is created by a laser standing wave, with the condensates trapped in the valleys of the periodic potential and weakly coupled by the interwell barriers. The coherence of multiple tunneling between adjacent wells is continuously probed by atomic interference. The square of the small-amplitude oscillation frequency is proportional to the microscopic tunneling rate of each condensate through the barriers and provides a direct measurement of the Josephson critical current as a function of the intermediate barrier heights. Our superfluid array may allow investigation of phenomena so far inaccessible to superconducting Josephson junctions and lays a bridge between the condensate dynamics and the physics of discrete nonlinear media.

Spatial interference of coherent atomic waves by manipulation of the internal quantum state.

A trapped >(87)Rb Bose-Einstein condensate is initially put into a superposition of two internal states. Under the effect of gravity and by means of a second transition, we prepare two vertically displaced condensates in the same internal state. These constitute two coherent sources of matter waves with adjustable spatial separation. Fringe patterns, observed after free expansion, are associated with the interplay between internal and external degrees of freedom and substantially agree with those for a double-slit experiment.

Collective oscillations of two colliding bose-einstein condensates

Two 87Rb condensates ( F = 2, m(f) = 2, and m(f) = 1) are produced in highly displaced harmonic traps and the collective dynamical behavior is investigated. The mutual interaction between the two condensates is evidenced in the center-of-mass oscillations as a frequency shift of 6.4(3)%. Calculations based on a mean-field theory well describe the observed effects of periodical collisions both on the center-of-mass motion and on the shape oscillations.