Saturated-absorption spectroscopy: eliminating crossover resonances by use of copropagating beams

We demonstrate a new technique for saturated-absorption spectroscopy by use of copropagating beams that does not have the problem of crossover resonances. The pump beam is locked to a transition, and its absorption signal is monitored while the probe beam is scanned. As the probe comes into resonance with another transition, the pump absorption is reduced and the signal shows a Doppler-free dip. We use this technique to measure hyperfine intervals in the D2 line of 85Rb with a precision of 70 kHz and to resolve hyperfine levels in the D2 line of 39K that are less than 10 MHz apart.     

Saturated absorption spectroscopy of acetylene molecules with an optical nanofiber

We performed saturated absorption spectroscopy of acetylene (C?H?) ν? + ν? band transitions with an optical nanofiber (ONF). Owing to high-intensity light around the ONF, we observed a Lamb dip at relatively low-power laser (~10 mW) without a cavity. Our results showed that the simple ONF spectrometer is advantageous for performing saturation absorption spectroscopy and serves as a practical low-cost wavelength reference in the optical fiber communication band.     

Saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride molecules

We report an experimental investigation on the Doppler-free saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride (BaF) molecules in a 4 K cryogenic cell. The obtained spectra with a resolution of 19 MHz, much smaller than previously observed in absorption spectroscopy, clearly resolve the hyperfine transitions. Moreover, we use these high-resolution spectra to fit the hyperfine splittings of excited A(v = 0) state and find the hyperfine splitting of the laser-cooling-relevant A²Π_1/2(v = 0, J = 1/2,+) state is about 18 MHz, much higher than the previous theoretically predicted value. This provides important missing information for laser cooling of BaF molecules.     

Saturated absorption Stark spectroscopy of CH3OH with CO2 lasers

The vibration-torsion-rotation spectrum of CH₃OH has been studied by saturated absorption spectroscopy in applied electric fields up to 20 kV/cm, using ¹²C¹⁶O₂ and ¹²C¹⁶O₂ lasers. Frequency offsets for 129 absorption lines were measured relative to the frequency of a fluorescence stabilized reference laser with an accuracy of ±0.5 MHz for 75 lines inside the ±40 MHz tuning range and 2–5% for lines located up to 1.2 GHz outside the tuning range, but Stark tuned into resonance. For 36 lines we obtained fully-resolved Stark spectra, allowing determination of both lower and upper state K. Using this information in combination with information available from optically-pumped FIR laser emission, a total of 44 lines were completely assigned as transitions involving torsional n = 0, 1, 2 and 3 states of the CO stretch. Certain states belonging to n = 1 displayed an anomalous Stark effect which is at present not understood.     

Saturated absorption spectroscopy of acetylene gas inside large-core photonic bandgap fiber

Saturated absorption spectroscopy is performed on the acetylene nu(1) + nu(3) band near 1532 nm inside photonic bandgap fibers of small (approximately 10 microm) and large (approximately 20 microm) core diameters. The observed linewidths are narrower in the 20 microm fiber and vary from 20 to 40 MHz depending on pressure and power. Variations in the background light transmission, attributed by others to surface modes, are significantly reduced in the 20 microm fiber. The optimum signal for use as a frequency reference in a 0.8 m long, 20 microm diameter fiber is found to occur at about 0.5 torr for 30 mW of pump power. The saturation power is found by modeling the propagation and attenuation of light inside the fiber.     

A new type of resonances in saturated absorption spectroscopy of 3-level systems

We report the experimental observation of new resonances in saturated absorption spectra of a J = 1 to J = 0 transition of Ne atoms in a static magnetic field. These resonances, which are distinct from the well-known Zeeman and cross-over resonances, result from the modification of stimulated Raman processes by the simultaneous resonant saturation of an optical transition. The light-shifts of the various resonances are also studied.     

Multipole resonances in open-shell nuclei: Connection with direct-reaction spectroscopy

Probability distributions for the excitation of the 1ħω multipole resonances E1, M6, and M2 in the sd-shell nuclei ¹⁸O, ²²Ne, ²⁴Mg, ²⁶Mg, ²⁷Al, ²⁸Si, ³²S, and ⁴⁰Ca were obtained by means of the spectroscopy of direct pickup reactions within the particle-core coupling (PCC) version of the multiparticle shell model. The deviation of the ground state of a nucleus featuring A nucleons from the case of closed shells or subshells manifests itself in a broad range of the energy distribution of holes among states of (A − 1) nuclei. This energy spread is the main source of the fragmentation of multipole resonances in open-shell nuclei. A comparison of PCC theoretical results with experimental data confirms the applicability of the approach used over a broad range of momentum transfers and proves the existence of connections between direct and resonance processes.     

Photo-induced phase transitions probed by X-ray absorption spectroscopy: Fe(II) spin crossover complex

Photo-induced phase transitions in spin-crossover complex [Fe(II)(2-pic)₃]Cl₂EtOH were investigated by X-ray absorption spectroscopy (XAS) technique. In situ XAS results showed that the metal-ligand (Fe–N) bond distance expands by ca. 0.20 Å upon photo-induced (S=0→S=2) spin conversion. We find that nearly octahedral (O_h) symmetry of FeN₆ coordination remains in the photo-induced high spin state. The next-nearest neighbor Fe–C radial distribution shows a subtle change due to local distortions of ligand molecules. Inhomogeneous distribution of strains may disturb cooperative photo-induced phase transitions removing centers of inversion symmetry.     

Saturated absorption linewidth: Influence of the source frequency noise

The effect of the source FM noise on the lineshape in sub-Doppler saturation spectroscopy is examined both theoretically and experimentally. We show that the line profile is simply given by the convolution of the homogeneous profile with an effective spectrum, depending on the relative values of the sample relaxation time and of the source frequency correlation time but not departing significantly from the source field power spectrum in the usual cases. This result is well supported by a millimeter wave experiment involving a nearly perfect source, frequency modulated by a Gaussian noise.     

Optical absorption resonances of trapped particles

We have previously shown the possibility of the existence of a one-photon resonance with homogeneous width in the center of an absorption line of trapped particles. The results of a detailed investigation of this resonance are given in the first part of the present paper. The second part is devoted to a two-photon resonance in a standing-wave field: the time of coherent interaction with the field is increased for trapped particles, which permits the elimination of transit resonance broadening.     

Mid-infrared high-resolution absorption spectroscopy by use of a semimonolithic entangled-cavity optical parametric oscillator

By recording low-pressure absorption lines of N2O around 3.9 microm, we fully qualify a pulsed entangled-cavity doubly resonant optical parametric oscillator as a power tool for high-resolution spectroscopy. This compact source runs at a high repetition rate (>10 kHz) with a low threshold of oscillation (<8 microJ), is mode-hop-free tunable over 5 cm(-1), and displays single-frequency Fourier-transformed-limited operation (linewidth <0.005 cm(-1)). A high potential for nonlinear spectroscopy is also expected given the high peak power (70 W) and the good quality (M2 < 2) of the output beam.     

高灵敏度光谱测量技术:腔振铃吸收光谱

腔振铃激光吸收光谱技术是近年快速发展的一项新颖的光谱技术,它不仅检测灵敏度高,而且结构简单,不需要高昂的光谱设备,特别适合于测量弱吸收物质,包括气体、固体、液体等稳态粒子和金属化合物、自由基、团簇等瞬态粒子.本文在介绍其基本原理的基础上,介绍了使用脉冲激光与连续激光光源的2种技术方案.     

Oscillator strengths for the shape resonances in the N K absorption spectrum of NaNO3 measured with the use of synchrotron radiation

Spectral dependences of the total absorption cross sections near the N K(ls) ionization threshold have been measured in transmission for the first time for solid-phase sodium nitrate NaNO₃ using synchrotron radiation of the Russian-German soft x-ray beamline at the electron storage ring BESSY-II. On the basis of these measurements, which were performed with correct consideration of the background and instrumental distortions, partial N K absorption cross sections and the oscillator strengths for the π and σ shape resonances were determined for NaNO₃.     

Optical-feedback cavity-enhanced absorption spectroscopy with a quantum cascade laser

Optical-feedback cavity-enhanced absorption spectroscopy is demonstrated in the mid-IR by using a quantum cascade laser (emitting at 4.46 μm). The laser linewidth reduction and frequency locking by selective optical feedback from the resonant cavity field turns out to be particularly advantageous in this spectral range: It allows strong cavity transmission, which compensates for low light sensitivity, especially when using room-temperature detectors. We obtain a noise equivalent absorption coefficient of 3 × 10(-9)/cm for 1 s averaging of spectra composed by 100 independent points. At 4.46 μm, this yields a detection limit of 35 parts in 10(12) by volume for N(2)O at 50 mbar, corresponding to 4 × 10(7) molecules/cm(3), or still to 1 fmol in the sample volume.     

Cavity-enhanced absorption spectroscopy with a rapidly swept diode laser

Cavity-enhanced absorption spectroscopy is explained in terms of the transmission function of a rapidly swept interferometer, and the integrated transmission is shown to be proportional to the cavity ringdown time. The technique is demonstrated on the b¹Σ_g ⁺-X³Σ_g ^-  (1,0) band in molecular oxygen at 687 nm using a tunable diode laser and a relative-ly high-Q optical cavity (finesse ≈4000). A detection limit of 3×10^-8 cm^-1 s^1/2 is achieved for a 0.8 cm^-1 scanning range. Received: 24 June 2002 / Revised version: 5 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: peverall@physchem.ox.ac.uk     

Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser

We report what we believe are the first spectroscopic measurements to be made with a room-temperature quantum-cascade distributed-feedback laser. Using wavelength modulation spectroscopy, we detected N(2)O and CH(4) in the chemical fingerprint wavelength range near 8microm . The noise equivalent absorbance for our measurement was 5 parts in 10(5), limited by excess amplitude modulation on the laser output, which corresponds to a 1-Hz bandwidth detection limit of 250 parts N(2)O in 10(9) parts N(2) in a 1-m path length.     

环行染料激光器用于内腔吸收光谱的研究

本文报告了利用新型连续波环行染料激光器,即使行波工作于超过阈值两倍以上,仍在大气中获得了近10~5的内腔吸收灵敏放大;并首次利用单频扫描方法,在不降低灵敏放大的同时,得到了10~(-2)的光谱分辨率。实验结果与经改进后的Brunner-Paul理论分析一致。     

Narrow resonances of two-photon absorption of super-narrow pulses in a gas

First cw laser oscillation with thresholds below 1 mW was observed for various B¹П_u → X¹ ∑ _g ⁺ transitions of diatomic molecular sodium excited by different argon laser lines in the range of 454–488 nm. For pump powers of 0.5 W output powers up to 3 mW and single-pass gain up to 0.1 cm⁻¹ were obtained. Some properties of the heat pipe laser system are discussed.     

Saturated absorption by HF gas within a HF laser cavity

The power of a HF laser versus cavity frequency is measured in the presence of saturable HF gas. A resonant power maximum is observed which is relevant to laser frequency stabilization and molecular line broadening measurement.     

Saturated absorption experiments using a free running CW dye laser

The hyperfine structure of some new iodine lines has been observed over the tuning range of Rhodamine 6G (5800–6200 Å), using a free running, single mode cw dye laser. The resolution obtained in this experiment allowed to calculate the hyperfine constants of the excited state (³π₀⁺) of iodine with an accuracy of 2% for the quadrupole coupling constant and 50% for the magnetic one.