Build-up Cavity Research Articles

Fast-Beam Laser Spectroscopy of Helium-like Silicon

Doppler-tuned laser spectroscopy has been applied to a foil-stripped 1 MeV/u silicon ion beam to measure the 1s2s 1 S 0-1S2P 3 P 1 intercombination interval in 28Si12+. To obtain sufficient transition probability the Si12+ beam was merged co-linearly with over 2 kW of continuous-wave laser light at 1319 nm inside an ultra-high finesse build-up cavity. The measurement differentiates between current theories for the n = 2 levels of helium-like ions.

High-resolution spectroscopy of the /sup 39/K transitions at 770 nm and /sup 13/C/sub 2/H/sub 2/ saturated lines by a solid-state laser at 1.54 μm: toward an accurate frequency standard in the optical communication band

A novel and compact Er-Yb:glass laser was developed with single-frequency output power >10 mW in a wavelength range from 1531 to 1547 nm. The laser output was efficiently frequency doubled at 770.1 nm in a single-pass periodically poled lithium niobate waveguide and a second harmonic power in excess of 15 /spl mu/W was achieved, with an internal conversion efficiency of /spl sim/220% W/sup -1/. This frequency-doubled beam was used for high-resolution saturated spectroscopy of the /sup 39/K 4S/sub 1/2 /-4P/sub 1/2 / absorption line. The best resolved crossover peak showed 16% line contrast and 10-MHz linewidth. In a second experiment, working at the fundamental frequency, saturation spectroscopy of /sup 13/C/sub 2/H/sub 2/ was achieved using a low-pressure gas cell inside a build-up cavity. The measured absorption dip has 3% line contrast and 1.2 MHz linewidth. Both the atomic and molecular saturated lines are very well suited for frequency locking in order to establish accurate frequency standards in the optical communication band.

Low-power Lamb-dip spectroscopy of very weak CO_2 transitions near 425 µm

We report saturated-absorption spectra recorded by use of 5 muW of infrared radiation coupled into a build-up cavity. Single-pass generation of difference-frequency radiation tunable near 4.25 mum in a periodically poled LiNbO(3) crystal was used. Lamb dips of weak transitions of the fundamental rovibrational band of CO(2) were observed, for what is believed to be the first time, up to the J=82 level. Application of these results to the extension of frequency-comb-based metrology in the infrared is discussed.

Measurement of the 1s2s 1S0-1s2p 3P1 intercombination interval in helium-like silicon.

Using Doppler-tuned fast-beam laser spectroscopy the 1s2s 1S0-1s2p 3P1 intercombination interval in 28Si12+ has been measured to be 7230.5(2) cm(-1). The experiment made use of a single-frequency Nd:YAG (1.319 microm) laser and a high-finesse optical buildup cavity. The result provides a precision test of modern relativistic and QED atomic theory.

Efficient and stable operation of an Ar+-pumped continuous-wave ring laser from 505–560 nm using a coumarin laser dye

A coumarin laser dye has been found to produce efficiencies and lifetimes comparable to those of the rhodamines. Coumarin 521 (coumarin 334) produces up to 400 mW of single frequency light (frequency width <1 MHz) at 520 nm in a ring laser when excited with 2.4 W of 457.9 nm light from an Ar+ ion laser. Operation with a 488 nm pump shows slightly lower efficiency. Dye lifetime to 75% full power has exceeded 1000 W h over a period of several months. Methanol and ethylene glycol are the only solvents required. This dye operates in a portion of the tuning range of a continuous-wave ring dye laser which is otherwise extremely difficult to access using an argon ion laser as the pump source. Used in conjunction with an external buildup cavity, 8–12 mW of 258 nm light is reliably produced for weeks at a time.

State selective vibrational (2ν3) activation of the chemisorption of methane on Pt (111)

The state-selective dissociative sticking coefficient on Pt(111) surfaces for CH4 in the 2ν3 J=1,2 ro-vibrational levels has been measured using thermal energy atomic scattering. Continuous wave laser excitation of a molecular beam of CH4 seeded in He with a 1.5 μm color center laser, tunable around 6000 cm−1 and coupled to the beam by means of a resonant build-up cavity, allows pumping of up to 11% of the molecules to the excited ro-vibrational state. The laser/molecular beam combination allows precise control over the translational energy as well as the vibrational state of the methane that impinges on the clean Pt(111) surface. The intensity of the specular reflection of the incident helium beam is used to quantitatively monitor the coverage of chemisorbed methane on the platinum surface as a function of time (exposure). The sticking coefficient (S0) of CH4 with 5.4 kJ/mol normal translational energy is found to increase from 6.2×10−6 to 1.8×10−4 upon 2ν3 excitation (the overtone of the asymmetric stretch of methane). This represents a ∼30 fold enhancement in reactivity of the methane with the Pt(111) surface upon vibrational excitation. We also measured the changes of S0 obtained by varying the nozzle temperature and methane concentration over a tenfold range of energy. We find that 72 kJ/mol of vibrational energy in the excited CH4 is approximately equivalent to at least 30 kJ/mol of normal translational energy. This corresponds to a utilization efficiency of the 2ν3 vibrational energy of greater than 40%. In the only other measurement of this kind published in the literature, [L. B. F. Juurlink, P. R. McCabe, R. R. Smith, C. L. DiCologero, and A. L. Utz, Phys. Rev Lett. 83, 868 (1999)] for the fundamental (v=1) excitation of ν3 of CH4 on Ni(100) surfaces, a comparable value for the vibrational energy utilization efficiency was found (59%). Further work is necessary to determine if this result is general and if and how it may change by changing the vibrational mode excited by the laser.

Stark field induced perturbations in the ν2+3ν3 vibrational overtone band of acetylene

The ν2+3ν3 vibrational overtone of acetylene was recorded using an optothermal molecular beam spectrometer. The sample excitation region was surrounded by a build-up cavity and by electrodes which allowed the observation of Stark splittings of the spectral transitions. An analysis of the J dependence of the splittings determined that the (01300)0 is mixed by the Stark field with the IR (infrared) dark state (11200)0, and that the energy of the ν1+ν2+2ν3 transition is 11 611.585±0.018 cm−1. The spectra also showed transitions to highly excited bending levels: These transitions were observable because they borrowed intensity from the more intense ν2+3ν3 stretching overtone as they were tuned into coincidence. The bending states were found to have extremely high polarizabilities which may account for previously reported collision dynamics.

First overtone helium nanodroplet isolation spectroscopy of molecules bearing the acetylenic CH chromophore

High-resolution helium nanodroplet isolation spectra of the first overtone (2ν1) of the acetylenic stretch of several substituted acetylenes (RC≡C–H) at T=0.38 K, have been observed for the first time. A tunable 1.5 μm laser is coupled, using a power buildup cavity, to a beam of He droplets seeded with the molecule to be studied. Absorption spectra are recorded by monitoring the beam depletion as a function of laser frequency with a thermal detector. The spectra of hydrogen cyanide (HCN), monodeuteroacetylene (DCCH), cyanoacetylene (NCCCH), propyne (CH3CCH), trifluoropropyne (CF3CCH), 3,3-dimethylbutyne ((CH3)3CCCH), and trimethylsilylacetylene ((CH3)3SiCCH) have been recorded. Due to the superfluid nature of the droplet, rotational resolution is achieved despite the presence of some solvent-induced broadening. The spectroscopic constants have been extracted by means of spectral simulations. The resulting rotational constants are smaller than for the bare molecule by a factor which depends on the molecule nonsphericity and its gas-phase moment of inertia. The linewidths are found to be at least twice as large as those of the corresponding fundamental (ν1) transitions observed in a helium droplet by Nauta et al. [Faraday Discuss. Chem. Soc. 113, 261 (1999) and references therein]. The helium-induced spectral shifts are found to be very small, but cannot be easily rationalized.

Experimental demonstration of negative dispersion without absorption

We present the results of an experiment proving in principle that negative (anomalous) dispersive transparent media can be realized. The Ca-transition 4s 2 1S 0 → 4s4p 1P 1 at λ=423 nm was chosen to investigate the absorption and index of refraction spectra of a two-level system strongly driven at resonance. For the simultaneous high precision measurement of loss and phase shift imposed on a probe field during the interaction with a chopped Ca-atomic beam we used a novel spectrometer which is based on a phase modulated Mach–Zehnder interferometer. So far, a dispersion of ∂ ω n 0=−(3.1±1.0)×10 −10/MHz accompanied by residual absorption of α 0=(0.125±0.040)%/cm has been achieved. These values are compared to the requirements which have to be achieved for a certain application of negative dispersion without absorption, that is for a broadband high build-up cavity (white-light cavity).

Laser power build-up cavity for high-resolution laser spectroscopy

The design, construction, and operation of an optical build-up cavity (BUC) designed primarily for use in molecular beam laser spectroscopy where signal is proportional to laser power, is presented. The design is such that the cavity has no internal adjustment controls, needing only alignment with respect to the laser and molecular beams. When used with a single-mode titanium:sapphire laser, a circulating cw power of 900 W was achieved. The BUC increased the signal to noise of the experiment by a factor of 390 relative to a single crossing of laser and molecular beam without degrading resolution. Performance of the BUC is limited by heating of its mirrors by the high power densities incident upon them (∼3 MW cm−2).

Deceleration of a calcium atomic beam with a frequency-doubled diode laser

A calcium atomic beam has been decelerated by a single extended-cavity diode laser, frequency doubled to 423 nm. A potassium niobate crystal is placed in an external power buildup cavity, and the second-harmonic laser beam, counterpropagating with the atomic beam, is tuned into resonance with the strong 1S0-1P1 transition of calcium. For input power of 78 mW at 846 nm, we generate 22 mW at 423 nm after correction for the reflectivity of our cavity output coupler. To keep the atoms always in resonance during the deceleration process, the Zeeman tuning technique was used.

Internally-coupled build-up cavity for frequency doubling of cw lasers

Replacing the input coupler in a ring build-up cavity with a broadband beamsplitter optimizes frequency doubling of cw laser radiation by allowing for careful control of input coupling by varying the input angle. We describe this simple and novel modification and discuss several other applications.

Molecular Beam Infrared Spectrum of Nitromethane in the Region of the First C−H Stretching Overtone

The infrared spectrum of the first C−H stretching overtone of nitromethane (CH3NO2) was measured in the wavenumber region 6047−6055 cm-1 with a resolution of 5 MHz using a molecular beam laser spectrometer equipped with bolometric optothermal detection and a power build-up cavity. The assignment of the spectrum was made using ground-state combination differences and an asymmetric rotor energy level simulation program. The following parameters were obtained for the m = 0 upper state: ν0 = 6050.1378(24) cm-1, A = 0.55399(84) cm-1, B = 0.2936(15) cm-1, C = 0.2567(13) cm-1. The intramolecular vibrational energy redistribution (IVR) rate of the C−H stretch was calculated from the spectrum. The IVR rate of the different JKaKc levels in the spectrum ranged from 170 to 300 ps, comparable to the rate measured for methyl C−H stretching overtones in molecules of similar size (CH3SiH3 and CH3CD3).

Chromophore dependence of intramolecular vibrational relaxation: Si–H stretch second overtone versus C–H stretch first overtone in methylsilane

The intramolecular vibrational relaxation (IVR) of an excited Si–H stretch (second overtone) and C–H stretch (first overtone) in methylsilane has been examined by eigenstate resolved infrared spectroscopy. The experiment probes a molecular beam produced in a supersonic expansion, excited by a laser in a power buildup cavity, and detected by a liquid helium cooled silicon bolometer. The Si–H stretch [local mode (3,0,0), both A and E combinations] is compared with the nearly isoenergetic C–H stretch [predominantly the 2ν70 band]. With the calculated density of states almost unchanged, the two modes exhibit very different IVR behavior, which is quantified in terms of the lifetime of the bright states and the coupling between the bright states and the dark states.

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 ?m

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS2. IR frequency tuning between 3076 and 3183 cm−1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)−1/2.

Recent progress in tunable nonlinear optical devices for infrared spectroscopy

A laser source based on difference-frequency generation (DFG) in AgGaS 2 has been developed for high resolution IR kinetic spectroscopy (IRKS) of free radicals. Continuously tunable radiation has been generated between 4 and 9 μm (2500–1100 cm −1) by mixing two cw single-mode dye/Ti:Al 2O 3 lasers in a 90° type I phasematching configuration. Spectra of the free radicals HOCO, DOCO and HCCN were obtained by using this source. Concurrently, the suitability of III–V single-mode cw diode lasers for DFG of tunable IR radiation has been explored by mixing two laser diodes (808 and 690 nm) in AgGaS 2 (IR power generated ≈ 3 nW). Techniques to increase the IR DFG power level such as the use of high-power optical semiconductor amplifiers or an external build-up cavity for the nonlinear crystal have been investigated. As much as 50 μW of cw IR radiation, tunable near 4.3 μm have been generated by mixing a high-power tapered semiconductor amplifier at 858 nm and a Ti:Al 2O 3 laser in AgGaS 2. Recent progress in generating cw IR radiation near 3.2 μm by mixing an extended-cavity diode laser near 795 nm and a compact diode-pumped Nd:YAG laser in AgGaS 2 with an external enhancement cavity is also described.

Sensitive detection of laser-ablation plumes in an optical build-up cavity

Cavity-enhanced detection is used to monitor the dynamics of minute vapor plumes produced by focusing a pulsed laser beam onto a surface. The time evolution of the various processes, namely vaporization, acoustic wave generation and photothermal effect range from nsec to msec.

Doubly-resonant second-harmonic generation in β-barium-borate

We describe a versatile tunable continuous wave ultraviolet source for spectroscopy. An angle-tuned β-barium-borate frequency doubler is placed inside a doubly resonant build-up cavity. By compensating for double refraction, this scheme provides efficient second harmonic generation in a pure TEM 00 mode.