Low Repetition Rate Laser Research Articles

High-speed stereo DPIV measurement of wakes of two bat species flying freely in a wind tunnel

Previous studies on wake flow visualization of live animals using DPIV have typically used low repetition rate lasers and 2D imaging. Repetition rates of around 10 Hz allow ~1 image per wingbeat in small birds and bats, and even fewer in insects. To accumulate data representing an entire wingbeat therefore requires the stitching-together of images captured from different wingbeats, and at different locations along the wing span for 3D-construction of wake topologies. A 200 Hz stereo DPIV system has recently been installed in the Lund University wind tunnel facility and the high-frame rate can be used to calculate all three velocity components in a cube, whose third dimension is constructed using the Taylor hypothesis. We studied two bat species differing in body size, Glossophaga soricina and Leptonycteris curasoa. Both species shed a tip vortex during the downstroke that was present well into the upstroke, and a vortex of opposite sign to the tip vortex was shed from the wing root. At the transition between upstroke/downstroke, a vortex loop was shed from each wing, inducing an upwash. Vorticity iso-surfaces confirmed the overall wake topology derived in a previous study. The measured dimensionless circulation, Γ/Uc, which is proportional to a wing section lift coefficient, suggests that unsteady phenomena play a role in the aerodynamics of both species.

Molecular Perspective of Antigen-mediated Mast Cell Signaling

Antigen-mediated cross-linking of the high affinity receptor for IgE (Fc epsilon RI), in the plasma membrane of mast cells, is the first step in the allergic immune response. This event triggers the phosphorylation of specific tyrosines in the cytoplasmic segments of the beta and gamma subunits of Fc epsilon RI by the Src tyrosine kinase Lyn, which is anchored to the inner leaflet of the plasma membrane. Lyn-induced phosphorylation of Fc epsilon RI occurs in a cholesterol-dependent manner, leading to the hypothesis that cholesterol-rich domains, or "lipid rafts," may act as functional platforms for IgE receptor signaling. Testing this hypothesis under physiological conditions remains challenging because of the notion that these functional domains are likely transient and much smaller than the diffraction limit of optical microscopy. Here we use ultrafast fluorescence dynamics to investigate the correlation between nanostructural changes in the plasma membrane (labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine (diI-C18)) and IgE-Fc epsilon RI cross-linking in adherent RBL mast cells stimulated with multivalent antigen. Time-dependent two-photon fluorescence lifetime imaging microscopy of diI-C18 shows changes in lifetime that agree with the kinetics of stimulated tyrosine phosphorylation of Fc epsilon RI, the first identifiable biochemical step of the allergic response, under the same conditions. In addition, two-photon fluorescence lifetime imaging microscopy of Alexa Fluor 488-labeled IgE indicates that Förster resonance energy transfer occurs with diI-C18 in the plasma membrane. Our live cell studies provide direct evidence for the association of IgE-Fc epsilon RI with specialized cholesterol-rich domains within approximately 4-nm proximity and with an energy transfer efficiency of 0.22 +/- 0.01 at maximal association during IgE receptor signaling.

Single-shot two-dimensional infrared spectroscopy

Multidimensional infrared spectroscopy is a robust tool for studying the structural dynamics of molecules. In particular, twodimensional infrared (2DIR) spectroscopy can reveal vibrational coupling among the internal modes of molecules, uncovering the transient structure of complex systems. While spectroscopically very powerful, current experimental techniques are time consuming to perform, requiring ~10(6) laser shots for a single 2DIR spectrum. In this work, we demonstrate a new technique that can acquire a full 2DIR correlation spectrum using a single ultrafast laser pulse. This apparatus will allow 2DIR spectroscopy to be extended to systems that were unattainable with previous technology, including, irreversible chemical reactions, rapid flow experiments, or with low repetition rate laser systems.

Pulsed laser deposition with a high average power free electron laser: Benefits of subpicosecond pulses with high repetition rate

We have conducted experiments exploring pulsed laser deposition of thin films using the high average power Thomas Jefferson National Accelerator Facility Free Electron Laser. The combination of parameters of this laser, including subpicosecond pulses, high average power, high repetition rate, and tunability, makes it a unique tool for the study of the effects of laser characteristics on thin-film quality. When compared to ablation and deposition with an ultrafast, high energy per pulse, low repetition rate laser (amplified Ti:sapphire), we find that the lower energy per pulse with high repetition rate of the free electron laser leads to very different plasma emission and produces films with high quality with the potential of very high deposition rates. This is demonstrated in the optical spectroscopy of plasma emission from Ti and the growth of Ni80Fe20 thin films.

White-light Continuum Generation for Measurements of Carrier-Envelope Phase Changes of Femtosecond Laser Pulses.

We demonstrated measurement of carrier-envelope phase (CEP) changes of amplified laser pulses by utilizing spectral interferometry between the white-light continuum and its second harmonic. The white-light continuum was generated by self-phase modulation (SPM) in a gas-filled hollow fiber. This method enables us to measure the CEP changes of low-repetition rate laser pulses on a shot-to-shot basis. The CEP-change of the amplified laser pulses exhibits random distribution that is mainly due to the CEP change fluctuation of the seed pulses from a carrier-envelope-offset (CEO) uncontrolled oscillator. We also demonstrated the CEO measurement of a mode-locked Ti:Sapphire oscillator. We utilized the self-referencing method, which measures the temporal beat frequency between the white-light continuum and its second harmonic. The white-light continuum was generated by the SPM in a micro-structure fiber. The temporal evolution of the beat signal showed CEO fluctuation of the uncontrolled oscillator.

Photodetachment photoelectron spectroscopy of multiply charged anions using electrospray ionization

A magnetic-bottle time-of-flight (TOF) photoelectron spectrometer, coupled with an electrospray ionization source, has been developed for the investigation of multiply charged anions in the gas phase. Anions formed in the electrospray source are guided by a radio-frequency quadrupole ion guide into a quadrupole ion trap, where the ions are accumulated. A unique feature of this apparatus involves the coupling of a TOF mass spectrometer to the ion trap with perpendicular ion extraction. The ion trap significantly improves the duty cycle of the experiments and allows photodetachment experiments to be performed with low repetition-rate lasers (10–20 Hz). This novel combination makes the photodetachment photoelectron spectroscopy studies of multiply charged anions possible for the first time. Furthermore, the perpendicular extraction of ions, pulsed out of the ion trap, to the TOF mass spectrometer allows the ion energies to be conveniently referenced to ground, simplifying the configuration of the TOF mass spectrometer and the subsequent magnetic-bottle TOF photoelectron spectrometer. The mass resolution (M/ΔM) achieved is about 800 for smaller ions. The magnetic-bottle photoelectron spectrometer resolution is about 11 meV full width at half maximum for 0.5 eV photoelectrons with an overall resolution of ΔE/E∼2%. The detailed design, construction, and operation of the new apparatus are presented.

A photoswitched small TEA CO2 laser

A small photoswitched TEA CO2 laser has been developed. This small aperture, low repetition rate laser can produce 69 mJ optical pulses from a discharge volume of 12.5cm3, with an energy efficiency of 11.1%. This energy efficiency is the highest so far reported with such lasers. Corona UV sources driven by a simple inexpensive solid state exciter are used to trigger the discharge. Also demonstrated is the wider input energy loading and enhanced discharge stability provided by overvolting the laser head.

A solid-state switched small TEA CO2 laser

A corona pre-ionized, solid-state switched transverse, electric, atmospheric CO2 laser has been developed. This small aperture, low repetition rate laser can produce 114 mJ optical pulses from a discharge volume of 15.6 cm3, with an energy efficiency of 12.7%. The device is driven by a simple inexpensive solid-state exciter, which uses a series stack of thyristors, to provide an output efficiency similar to that produced by a conventional thyratron circuit.

Lifetimes and quenching of B 2Σ+ PO by atmospheric gases

Pulsed laser excited fluorescence in the B 2Σ+ ← X 2π system of gas phase PO was used to measure the lifetime for v′=0 of the B state. Rotationally resolved measurements for a few selected J′ levels, at Ar or He carrier gas pressures of ∼2 Torr, reveal no dependence of the lifetime on the rotational level excited. Earlier measurements of relative fluorescence intensities in the v′=0 vibrational progression were reinterpreted to extract the dependence of the electronic transition moment on internuclear distance. Using this transition moment, no lifetime dependence on rotational level is to be expected, even at low pressures. Rate constants for quenching of the B state PO by N2, O2, CO2, and H2O, and upper limits thereof for He and Ar are reported. O2 was found to react with ground state PO. A crude measurement of the rate constant was performed. The result is compared to two other known measurements. The rate constant is in excellent agreement with the previous measurement, but in poor agreement with that of a concurrent study. Further work is required to determine the reason for the discrepancy. A cathode ray–vidicon type waveform digitizer was found to lend itself readily to the lifetime measurements, wherein a low repetition rate laser was used. Unfortunately, it was discovered that small nonlinearities (∼1% of full scale) in the digitizer response have a marked effect on the fits to the exponential lifetime decays and, especially, the quenching rate constants. We believe this problem is not commonly recognized. A simple method for calibration of this type of digitizer and removal of these effects is presented.

Spectrometer with electronic readout for CO2lasers

The spectrometer uses an array of 50 pyroelectric detectors etched from a PVF2 sheet in the precise positions for the various lines in the P and R branches of the CO2 spectrum. The detectors are accessed in sequence to give a bar chart display of the spectrum. Compared to the conventional spectrum analyser (using a phosphor screen for CW or a graphite screen for pulsed lasers) the instrument gives a substantial advantage in sensitivity with a noise equivalent power of 100 mu W for CW or 200 nJ for pulsed. The dynamic range is 3000:1, enabling the detection of weak satellite lines, while a digital store allows monitoring of each shot of an inconsistent but low repetition rate laser.

Stabilization, attenuation and shaping of picosecond laser pulses by two-photon absorption in dyes

The attenuation of ultrashort laser pulses by two-photon absorption is theoretically described when a dye is used as the absorbing medium. It is shown that two-photon absorption also yields an intensity stabilization of low repetition rate laser emissions without a significant loss of power. Consequently this technique is proposed for pulse attenuation and stabilization near the damage threshold of second harmonic generator crystals, and its performance is theoretically compared with other attenuation methods.

The capabilities of high repetition rate laser radars for measurement of atmospheric properties above 30km

Abstract A smaller receiver, low energy per pulse, high repetition rate laser radar is shown to yield performance comparable to that of a large receiver, high energy per pulse, low repetition rate laser radar for the measurement of atmospheric properties above 30 km.