Chikako Ishibashi

Highly Sensitive Cavity-Enhanced Sub-Doppler Spectroscopy of a Molecular Overtone Band with a 1.66 µm Tunable Diode Laser

Sensitive sub-Doppler resolution molecular spectroscopy developed by Hall and co-workers [J. Opt. Soc. Am. B 15 (1998) 6] has been applied to the 1.66 µm region using a widely tunable external-cavity diode laser. We have recorded saturated absorption spectra of the 2ν3 vibrational overtone band of methane with a sensitivity of 9.5×10-11 cm-1 at a time constant of 1.25 s and a spectral linewidth of 320 kHz (full width at half maximun; FWHM) over the tunable range of 1.63 to 1.68 µm.

Absolute frequency measurement of the saturated absorption lines of methane in the 1.66 μm region

Abstract We have determined the absolute frequencies of the R(0) and Q(1) transitions of the 2 ν 3 band of 12 CH 4 in the 1.66-μm region to be 180.345 065 08(37) and 180.021 253 10(61) THz. The numbers in parentheses are the standard deviations in units of the last digit of the quoted frequencies. We have used the 1.54-μm saturated absorption line of 13 C 2 H 2 as a frequency reference and an optical frequency comb generator for bridging the frequency difference of 14 THz between the 1.66- and 1.54-μm radiations.

Coriolis‐dependent Stark effect of the 2ν3 band of methane observed by saturated absorption spectroscopy

We studied the vibration‐induced dipole moment of methane by observing Stark‐modulation spectra of the 2ν3 band centered at 1.66 μm. The spectral purity of an external cavity semiconductor laser and radiation density in a Fabry–Perot cavity absorption cell are sufficiently high to record sub‐Doppler saturated absorption lines with a spectral resolution of 0.8 MHz. First‐order Stark shifts of the E‐type symmetry components in the P(2), Q(2), Q(4), Q(5), R(2), R(4), and R(5) transitions were measured, and the vibration‐induced dipole moments for the v3=2 state were determined. Their values clearly demonstrate the Coriolis‐induced dipole term, yielding [5.79(5)+0.440(18)×(2L−3)] mD for the JR level, where L is defined to be −(R+1), 0, and R for the R=J−1, J, and J+1 levels, R and J are rotational and total angular momentum quantum numbers, and numbers in parentheses are 90% confidence intervals in unit of the last digit.

Precise frequency-difference measurement between the 1.66-µm transitions of methane

We determined 66 frequency differences (FDs) between rovibrational lines of methane at the 1.66-mum region. Following the technique developed by Nakagawa et al.[Opt.Lett.20, 410 (1995)], we measured the FDs as the optical beat frequency between two external-cavity diode lasers locked at 1-MHz-wide saturated absorption dips of the methane lines. Even though the methane lines often overlap in Doppler-limited resolution, the spectrometer that we use resolves them and determines their FDs with better than 40-kHz precision. This fact demonstrates that the methane lines are promising candidates for frequency reference and that this technique has great potential for high-resolution molecular spectroscopy.

Infrared radio-frequency double-resonance spectroscopy of molecular vibrational-overtone bands using a Fabry–Perot cavity-absorption cell

Infrared (ir) radio-frequency (rf) double-resonance spectroscopy was carried out on the vibrational-overtone band of methyl iodide molecules (CH3I). An optical Fabry-Perot cavity was employed as an absorption cell to record saturated ir spectral lines even by a small-power extended-cavity diode laser with a high sensitivity and a wide tunability in the presence of a rf field. These features allowed investigation of molecules strongly coupled with monochromatic and bichromatic rf fields, or dressed molecules, at various rf power levels and detuning frequencies for a variety of ir and rf transitions. At appropriate energy-level schemes, quantum-interference effects were observed. All resultant spectra showed good agreement with dressed-state theoretical calculations, indicating that the present spectrometer is valid for precise investigation of dressed molecules.

Sub-Doppler resolution molecular spectroscopy in the 1.66-μm region

We constructed a widely tunable spectrometer with sub-Doppler resolution and high sensitivity in the 1.66-micrometer region using a Fabry-Perot cavity as an absorption cell and a low- power extended-cavity laser diode as a light source. The light electric field is enhanced at the antinodes of the standing wave in the cavity cell, which enabled observation of saturated absorption spectra of the molecular overtone bands even though their transition dipole moments are small. The spectrometer sensitivity was drastically enhanced using a frequency modulation technique. The attained sensitivity allowed us to reduce sample gas pressure, optical power, and modulation amplitude, which resulted in a resolution of 320 kHz. We applied the spectrometer to precise frequency measurements of the 2(nu) 3-band transitions of methane, and determined 66 frequency differences between them and two absolute frequencies with an accuracy of 40 and 600 kHz, respectively. We also recorded hyperfine-resolved spectrum of the 2(nu) 4 band of methyl iodide, which gave unambiguous assignments.

Near-infrared laser spectrometer with sub-Doppler resolution, high sensitivity, and wide tunability-a case study in the 1.65-/spl mu/m region of methyl iodine spectrum

We demonstrate the potential capability of the cavity-enhanced spectrometer for molecular spectroscopy. We take the 1.65-/spl mu/m spectrum of methyl iodine (CH/sub 3/I) as a sample for the following reasons. First, the global spectrum with a Doppler-limited resolution has been recorded, and has not been analyzed yet. Second, there are a lot of absorption lines extending in a wide wavelength range, and the hyperfine structures and Q-branch lines are overlapped in the Doppler-limited resolution spectrum and can be resolved by the present spectrometer.