High-flux energetic protons whose maximum energies are up to 4MeV are generated by an intense femtosecond titanium:sapphire laser pulse interacting with 7.5, 12.5, and 25μm thick polyimide tape targets. Laser pulse with an energy of 1.7J and with a duration of 34fs is focused with an f/3.4 parabolic mirror giving an intensity of 3×1019Wcm−2. The main pulse to amplified spontaneous emission (ASE) intensity contrast ratio is 2.5×107. The conversion efficiency from the laser energy into the proton kinetic energies is achieved to be ∼3%, which is comparable to or even higher than those achieved in the previous works; using nanometer-thick targets, in combination with the short-pulse lasers that have almost the same pulse width and the intensity but different main pulse to ASE intensity contrast of ∼1010 [Neely et al., Appl. Phys. Lett. 89, 021502 (2006)], in which the authors claim that the main mechanism is target normal sheath acceleration; or using the 7.5μm thick polyimide target, in combination with the short-pulse laser, which has almost the same pulse width and the intensity, but the main pulse to ASE intensity contrast ratio was controlled to be 2.5×105 [Yogo et al., Phys. Rev. E 77, 016401 (2008)], in which the authors claim the efficient acceleration by the mechanism of the underdense plasma model. The contrast ratio of the present experiment is in between these two experiments. The possible mechanism of this regime is discussed.

No abstract

No abstract

No abstract

An ion spectrometer, composed of a time-of-flight spectrometer (TOFS) and a Thomson parabola spectrometer (TPS), has been developed to measure energy spectra and to analyze species of laser-driven ions. Two spectrometers can be operated simultaneously, thereby facilitate to compare the independently measured data and to combine advantages of each spectrometer. Real-time and shot-to-shot characterizations have been possible with the TOFS, and species of ions can be analyzed with the TPS. The two spectrometers show very good agreement of maximum proton energy even for a single laser shot. The composite ion spectrometer can provide two complementary spectra measured by TOFS with a large solid angle and TPS with a small one for the same ion source, which are useful to estimate precise total ion number and to investigate fine structure of energy spectrum at high energy depending on the detection position and solid angle. Advantage and comparison to other online measurement system, such as the TPS equipped with microchannel plate, are discussed in terms of overlay of ion species, high-repetition rate operation, detection solid angle, and detector characteristics of imaging plate.

Characteristics of electron beams produced by the laser wakefield acceleration are presented. The dependence of the electron beam parameters on the laser focal spot size is investigated. The experimental result shows the generation of quasimonoenergetic electron beam although the laser spot size was several times larger than the plasma wavelength. Stable electron beam generation at large laser spots was owing to the stable laser propagation in plasma channels. At a small laser spot, the beam quality is poor and this is attributed to the the filamentation instability of the laser beam.

We report the fabrication and performance of a lensed photonic crystal fiber (PCF) designed as a compact but effective side-viewing optical imaging probe. The lensed-PCF probe was implemented in a single body without using any other fibers or additional optics. The beam expansion region and a focusing ball lens, necessary for a focuser, were simultaneously formed along a small piece of PCF by applying arc discharges. The side-viewing ability was provided by polishing the ball lens with a femtosecond laser to form a total internal reflection surface. The working distance and the transverse resolution of the fabricated single-body lensed-PCF were experimentally measured to be 570 and 6.8 microm, respectively. With the proposed lensed-PCF probe, optical coherence tomography images of an in vitro biological sample were successfully obtained.

All-optical single and multiple wavelength conversion and tuning by the cascaded sum- and difference frequency generation (cSFG/DFG) have been demonstrated in a temperature gradient controlled periodically poled Ti:LiNbO3 (Ti:PPLN) channel waveguide. Up to 4 channels of wavelength division multiplexed (WDM) signals which have 100 GHz channel spacing were simultaneously wavelength converted at a 16.8 degrees C temperature difference between both end faces in a Ti:PPLN waveguide. The 3 dB signal conversion bandwidth was measured to be as broad as 48 nm at single channel conversion. The maximum wavelength conversion efficiency and optical signal to noise ratio of wavelength converted channel were approximately -16 dB and -20 dB at a total pump power level of 810 mW.