Intensity-Resolved Above Threshold Ionization of Xenon with Short Laser Pulses

We present intensity-resolved above threshold ionization (ATI) spectra of xenon using an intensity scanning and deconvolution technique. Experimental data were obtained with laser pulses of 58 fs and central wavelength of 800 nm from a chirped-pulse amplifier. Applying a deconvolution algorithm, we obtained spectra that have higher contrast and are in excellent agreement with characteristic 2 $U_p$ and 10 $U_p$ cutoff energies contrary to that found for raw data. The retrieved electron ionization probability is consistent with the presence of a second electron from double ionization. This recovered ionization probability is confirmed with a calculation based on the PPT tunneling ionization model [Perelomov, Popov, and Terent'ev, Sov. Phys. JETP 23, 924 (1966)]. Thus, the measurements of photoelectron yields and the proposed deconvolution technique allowed retrieval of more accurate spectroscopic information from the ATI spectra and ionization probability features that are usually concealed by volume averaging.Comment: 21 pages, 7 figure

High-power mid-infrared frequency comb source based on a femtosecond Er:fiber oscillator

We report on a high-power mid-infrared frequency comb source based on a femtosecond Er:fiber oscillator with a stabilized repetition rate at 250 MHz. The mid-infrared frequency comb is produced through difference frequency generation in a periodically poled MgO-doped lithium niobate crystal. The output power is about 120 mW with a pulse duration of about 80 fs, and spectrum coverage from 2.9 to 3.6 um. The coherence properties of the produced high-power broadband mid-infrared frequency comb are maintained, which was verified by heterodyne measurements. As the first application, the spectrum of a ~200 ppm methane-air mixture in a short 20 cm glass cell at ambient atmospheric pressure and temperature was measured.Comment: 3 pages, 5 figure

FREQUENCY COMB VERNIER SPECTROSCOPY OF METHANE IN THE MID-IR WITH TEMPORAL RETRIEVAL OF COMB LINES

We develop a spectroscopic method combining the broadband spectral coverage and frequency resolution of a frequency comb with the optical path enhancement of a resonant optical cavity. The method requires only one frequency comb laser. We present measurements of the methane absorption spectrum performed in ambient air in the mid-IR from 3$\mu$m to 4$\mu$m using a comb derived from difference frequency generation. The resonant cavity provides the dual purpose of optical path enhancement along with Vernier filtering of the comb modes, allowing individual comb modes to be resolved with a grating. As the resonant cavity length is scanned, the transmitted comb lines in the whole spectral range of the cavity reflectivity are continuously recorded with a mid-IR camera, yielding an absorption spectrum in seconds. We show how this method can be realized with fewer moving parts and with broader spectral coverage than similar techniques reported in the literature, along with showing how the technique can be modified to suit different mid-IR detectors. This work was supported by Robert A. Welch Foundation, grant No. A1546, the Qatar Foundation, grant NPRP 8-735-1-154

ACETONE AND METHANE DETECTION WITH WAVELENGTH MODULATION SPECTROSCOPY IN THE NEAR- AND MID-IR

A high sensitivity sensor, combining a multipass cell and wavelength modulation spectroscopy in the near-IR spectral region (~1.651$\mu$m) was designed and implemented for trace gas detection. The sensor uses a DFB laser and software lock-in detection, realized with a LabVIEW code. The high sensitivity was achieved by combining the multipass cell having a long effective absorption length of 290 meters, the wavelength modulation spectroscopy, and noise suppression by using a dual beam scheme. The developed spectroscopic technique demonstrates an improved sensitivity for methane in ambient air and a relatively short detection time compared to previously reported sensors. The average methane concentration measured in ambient air was 2.01ppm with a relative error of $\pm$2.5\%. With Allan deviation analysis, it was found that the methane detection limit of 1.2ppb was achieved in 650s. A modification of this scheme for acetone detection with a mid-IR distributed feedback interband cascade laser with the center wavelength around 3.367$\mu$m was also developed, achieving the detection limit was 0.58 ppm with 1s and down to 0.12 ppm with 60s signal averaging. This work was supported by Robert A. Welch Foundation, grant No. A1546, the Qatar Foundation, grant NPRP 8-735-1-154

Surface plasmon resonance study of the actin-myosin sarcomeric complex and tubulin dimers

Biosensors based on the principle of surface plasmon resonance (SPR) detection were used to measure biomolecular interactions in sarcomeres and changes of the dielectric constant of tubulin samples with varying concentration. At SPR, photons of laser light efficiently excite surface plasmons propagating along a metal (gold) film. This resonance manifests itself as a sharp minimum in the reflection of the incident laser light and occurs at a characteristic angle. The dependence of the SPR angle on the dielectric permittivity of the sample medium adjacent to the gold film allows the monitoring of molecular interactions at the surface. We present results of measurements of cross-bridge attachment/detachment within intact mouse heart muscle sarcomeres and measurements on bovine tubulin molecules pertinent to cytoskeletal signal transduction models.Comment: Submitted to Journal of Modern Optics *Corresponding author: Andreas Mershin ([email protected]

Real-time dual frequency comb spectroscopy in the near infrared

We use two femtosecond Erbium-doped fiber lasers with slightly different repetition rates to perform a modern type of Fourier transform spectroscopy without moving parts. The measurements are done in real time, and it takes less than 50 microseconds. We work with somewhat different spectral outputs from two Erbium-doped fiber lasers and employ spectral filtering based on a 2f-2f grating setup to select the common spectral region of interest, thereby increasing the signal-to-noise ratio. The interferogram is taken with a 20 cm long gas cell, containing a mixture of acetylene and air at atmospheric pressure, and is fast-Fourier-transformed to obtain the broadband spectral fingerprint of the gas.Comment: 4 pages, 4 figure

Spontaneous Raman spectra of dipicolinic acid in microcrystalline form

Abstract. Dipicolinic acid (DPA) is an important component of bacterial spores. The Raman spectrum of DPA in the form of compacted powder was measured in reflection at room temperature with excitation by a nanosecond laser at 532 nm. The spectrum presents a set of characteristic frequency bands in the region 700-3090 cm À1 that were identified with characteristic vibrational modes of the DPA molecule