High Sensitivity Frequency Modulation Spectroscopy and the Path to Single Molecule Detection.

Advances in optical spectroscopy rely on improved detection techniques that reduce noise to fundamental detection limits. When properly applied, frequency modulation laser spectroscopy (FM spectroscopy or FMS) approaches the quantum limit for detection. Having demonstrated that double modulation FMS could achieve such sensitivity in absorption spectroscopy, researchers raced to the goal of measuring the absorption of an individual molecule.

Subsecond grating growth in a photorefractive polymer.

We survey the dynamics of the photorefractive effect in a methyl methacrylate copolymer with the nonlinear chromophore p-nitroaniline in a pendant side group doped with a charge-transport agent, diethylaminobenzaldehyde diphenylhydrazone, a material that represents a new class of photorefractive polymer. The grating growth times are several orders of magnitude smaller than that for the previous epoxy-based photorefractive polymers and fall below 1 s at the highest intensities used. Grating competition and revelation effects suggest that charge carriers other than photogenerated holes are mobile.

Generation of microwaves by mixing two optical frequencies in a nonlinear crystal: a novel approach to high-bandwidth optical mixers.

Optical mixing of waves separated in frequency by many gigahertz can be accomplished by allowing the waves to mix in a nonlinear medium to generate microwave difference frequencies. This basis for high-bandwidth optical mixers is demonstrated by mixing optical frequencies approximately 4 GHz apart of a pulsed dye-laser beam at approximately 660 nm in a LiTaO(3) crystal to produce readily detectable microwave power at approximately 4 GHz.

Frequency-modulation-polarization spectroscopy.

We have combined frequency-modulation and polarization spectroscopy to detect anisotropic spectral features with high sensitivity. The many line shapes possible with this technique and the potential for laser noise suppression were demonstrated by using saturation holes in iodine vapor.

Reading and writing of photochemical holes using GaAlAs-diode lasers.

A current-tuned GaAlAs-diode laser is utilized both to burn and to detect narrow photochemical holes in the inhomogeneously broadened 833-nm zero-phonon line of the R' color center in LiF. Applications for reading and writing data into frequency-domain optical memories based on photochemical hole burning are discussed.

Pulsed frequency-modulation spectroscopy at 3302 A.

We report the first known demonstration of frequency-modulation spectroscopy outside the visible spectral region. Specifically, we report the use of second-harmonic generation on a frequency-modulated visible dye-laser beam to produce a frequency-modulated ultraviolet beam that is used to observe the Na 3S ? 4P absorption at 3302 A.

Continuous-wave Doppler-free two-photon frequency-modulation spectroscopy in Rb vapor.

Frequency-modulation spectroscopy was employed successfully to measure the Doppler-free absorption and dispersion signals originating from several two-photon transitions in (85)Rb and (87)Rb. The two-photon cross section of the hyperfine transition 5S(1/2) (F = 2) ? 5D(5/2) (F = 4) in (87)Rb was determined to be sigma((2)) asymptotically equal to 4 x 10(-20) cm(4)/W. The transition isotope shift (TIS) of the transition 5S(1/2) ? 5D(5/2) between (87)Rb and (85)Rb was found to be Deltanu(TIS) = +166 +/- 2 MHz.

Two-photon holographic recording with continuous-wave lasers in the 750-1100-nm range.

Two-photon holographic recording with cw laser sources in the 750-1100-nm spectral range is accomplished using recording materials consisting of an alpha-diketone dissolved in a poly-alpha-cyanoacrylate host. Gating of the holographic sensitivity by irradiation with incoherent UV radiation is demonstrated. Hologram diffraction efficiencies as large as 10% are achieved.

Frequency-modulation spectroscopy with a pulsed dye laser.

We report the use of frequency-modulation spectroscopy with a Nd:YAG-pumped dye laser of 5-nsec pulse duration. In spite of the fact that the dye laser is not transform limited and has 30% peak-to-peak intensity fluctuations, we are easily able to measure absorptions as small as 2%.

Pulsed frequency-modulation spectroscopy as a means for fast absorption measurements.

Absorption measurements are accomplished by utilizing short pulses of frequency-modulated (FM) light. The absorption is measured by detecting the heterodyne beat signal that occurs when the FM spectrum is distorted by the absorption feature of interest. By using a single short laser pulse it is demonstrated that the beat signal can build up far above the noise level within 1 microsec.

Two-photon holography with continuous-wave lasers.

Two-photon holography with continuous-wave laser sources is accomplished by using carbazole dissolved in a polymethyl methacrylate host matrix as a recording medium. Gating of the holographic sensitivity for 4880-A radiation by irradiation with 3336-A radiation is demonstrated. It is shown that two-photon holographic recording can be accomplished with a wide variety of molecules characterized by four-level spectroscopic systems with cascade excited metastable intermediate levels.

Effects of magnetic fields on four-wave mixing processes in atomic vapors.

We have observed the effects of magnetic fields on four-wave mixing processes in atomic vapors. We have studied coherent vacuum-ultraviolet generation in Sr vapor via the process omega(1) + omega(2) + omega(3) ? omega(4) in fields of up to 50 kG. Magnetic tuning of the resonances of the third-order nonlinear susceptibility X((3)) was used to modify the dependence of the sum-frequency output power on the generated wavelength.