Quantitative measurement of timing and phase dynamics in a mode-locked laser

We present results of an experimental study of the timing and phase dynamics in a mode-locked Ti:sapphire laser. By measuring the response of two widely spaced comb lines to a sinusoidal modulation of the pump power, we determine quantitatively the response of both the central pulse time and the phase. Because of the distinct response of the pulse energy, central frequency, and gain to the modulation, we are able to distinguish their contributions to the timing and phase dynamics.     

Effects of lipopolysaccharide-induced inflammation on expression of growth-associated genes by corticospinal neurons

Background  

Inflammation around cell bodies of primary sensory neurons and retinal ganglion cells enhances expression of neuronal growth-associated genes and stimulates axonal regeneration. We have asked if inflammation would have similar effects on corticospinal neurons, which normally show little response to spinal cord injury. Lipopolysaccharide (LPS) was applied onto the pial surface of the motor cortex of adult rats with or without concomitant injury of the corticospinal tract at C4. Inflammation around corticospinal tract cell bodies in the motor cortex was assessed by immunohistochemistry for OX42 (a microglia and macrophage marker). Expression of growth-associated genes c-jun, ATF3, SCG10 and GAP-43 was investigated by immunohistochemistry or in situ hybridisation.     

Optical nonlinearity in Ar and N2 near the ionization threshold

We directly measure the nonlinear optical response in argon and nitrogen in a thin gas target to laser intensities near the ionization threshold. No instantaneous negative nonlinear refractive index is observed, nor is saturation, in contrast with a previous measurement [Opt. Express 17, 13429 (2009)] and calculations [Phys. Rev. Lett. 106, 183902 (2011)]. In addition, we are able to cleanly separate the instantaneous and rotational components of the nonlinear response in nitrogen. In both Ar and N2, the peak instantaneous index response scales linearly with the laser intensity until the point of ionization, whereupon the response turns abruptly negative and ～constant, consistent with plasma generation.     

Quantum interference control of photocurrent injection in Er-doped GaAs

We measure two-color quantum interference control of photocurrent injection in erbium-doped GaAs. The signal size is the same order of magnitude as from a low-temperature grown GaAs sample, and much larger than in a semi-insulating GaAs sample. Thus erbium-doped GaAs could be useful for fabrication of monolithic optical carrier-envelope phase detectors. We also describe a prism-based two-color interferometer useful for minimizing stray light in quantum interference control measurements.     

Beta-delayed proton emitter kr

The nucleide ⁷³Kr has been identified by on-line mass separation as a precursor of β-delayed proton emission. The proton branch is (6.8 ±1.2) × 10⁻³ proton/decay. The protons populate the ground state and also the first excited 2⁺ state at 866 keV in ⁷²Se with a relative intensity of (35±9) %. The value of Q_EC−B_p, where B_p is the proton separation energy for the nucleus ⁷³Br, is found to be 4.85 ±0.30 MeV based on the fraction of proton events preceded by positron decay.     

Impulsive stimulated Raman scattering: comparison between phase-sensitive and spectrally filtered techniques

Impulsive stimulated Raman scattering is used to generate and control coherent phonons and other low-frequency modes. In transparent materials, pump-probe experiments are usually performed by spectrally resolving the probe beam and measuring the spectral shift as a function of pump-probe time delay. By measuring the optical phase of the probe pulse as a function of time delay, we find that the phonon signal can be increased by a factor alpha(omegadelta)(-1), where omega is the phonon frequency and delta is the pulse duration.     

Effect of a plasma grating on pump-probe experiments near the ionization threshold in gases

Calculations are performed of the phase shift caused by the spatial modulation in the plasma density due to interference between a strong pump pulse and a weak probe pulse. It is suggested that a recent experiment [Opt. Express 17, 13429 (2009)] observed an effective birefringence from this plasma grating rather than from the higher-order Kerr effect.     

Contactless photoconductive terahertz generation

We describe a pulsed terahertz (THz) emitter that uses a rapidly oscillating, high-voltage bias across electrodes insulated from a photoconductor. Because no carriers are injected from the electrodes, trap-enhanced electric fields do not form. The resulting uniform field allows excitation with a large laser spot, lowering the carrier density for a given pulse energy and increasing the efficiency of THz generation. Compared to a dc bias, less susceptibility to damage is observed.