Tae Jun Yu

Generation of high-contrast, 30 fs, 1.5 PW laser pulses from chirped-pulse amplification Ti:sapphire laser

A high-contrast, 30 fs, 1.5 PW Ti:sapphire laser has been developed for research on high field physics. The maximum output energy of 60.2 J was obtained from a booster amplifier pumped by four frenquency-doubled Nd:glass laser systems. Parasitic lasing was suppressed by index matching fluid with absorption dye and the careful manipulation of the time delay between the seed and pump pulses. After compression, the measured pulse duration was 30.2 ± 1.8 fs, and the output energy was 44.5 J, yielding a peak power of about 1.5 PW. A saturable absorber and two ultrafast Pockels cells were installed in the front-end system for the minimization of the amplified spontaneous emission (ASE) and pre-pulse intensity. An adaptive optics system was implemented for obtaining the near diffraction-limited focal spot.

0.1 Hz 1.0 PW Ti:sapphire laser

We report on the generation of 1.0 PW, 30 fs laser pulses at a 0.1 Hz repetition rate from a chirped-pulse amplification Ti:sapphire laser system. The energy of the laser pulses is amplified up to 47 J in a final three-pass booster amplifier having 96 J pump energy. To the best of our knowledge, this is the first petawatt Ti:sapphire laser system at a 0.1 Hz repetition rate. The shot-to-shot energy fluctuation of the laser pulses is as low as 0.53% in rms value, and the laser pulses have homogeneous flattop spatial beam profiles.

Transition of proton energy scaling using an ultrathin target irradiated by linearly polarized femtosecond laser pulses.

Particle acceleration using ultraintense, ultrashort laser pulses is one of the most attractive topics in relativistic laser-plasma research. We report proton/ion acceleration in the intensity range of 5x1019 W/cm2 to 3.3x1020 W/cm2 by irradiating linearly polarized, 30-fs, 1-PW laser pulses on 10- to 100-nm-thick polymer targets. The proton energy scaling with respect to the intensity and target thickness was examined. The experiments demonstrated, for the first time with linearly polarized light, a transition from the target normal sheath acceleration to radiation pressure acceleration and showed a maximum proton energy of 45 MeV when a 10-nm-thick target was irradiated by a laser intensity of 3.3x1020 W/cm2. The experimental results were further supported by two- and three-dimensional particle-in-cell simulations. Based on the deduced proton energy scaling, proton beams having an energy of ~ 200 MeV should be feasible at a laser intensity of 1.5x1021 W/cm2.

Novel method for carrier-envelope-phase stabilization of femtosecond laser pulses

The carrier-envelope phase (CEP) of femtosecond pulses from a mode-locked Ti:sapphire laser is stabilized using a novel method operating in the time domain. This is a direct and relatively simple method for stabilizing the CEP of femtosecond laser pulses, compared to the conventional method based on phase-locked loop. Using this method, we have directly locked the pulse-to-pulse CEP slip to zero with an in-loop phase jitter of 0.05 rad. Out-of-loop measurement has revealed a comparable phase jitter of 0.1 rad, verifying a good performance of this locking technique. The capability of electrical CEP modulation is also demonstrated.

Efficient production of a collimated MeV proton beam from a polyimide target driven by an intense femtosecond laser pulse

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 ...

Broadening of the second-harmonic phase-matching bandwidth in a temperature-gradient-controlled periodically poled Ti:LiNbO3 channel waveguide.

We have demonstrated broadening of the phase-matching bandwidth in a periodically poled Ti:LiNbO3 (Ti:PPLN) channel waveguide Lambda = 16.6 microm) by using a temperature-gradient-control technique. With this technique, we have achieved a second-harmonic phase-matching bandwidth of more than 13 nm in a 74-mm-long Ti:PPLN waveguide, which has a 0.21-nm phase-matching bandwidth at a uniform temperature.

Precise and long-term stabilization of the carrier-envelope phase of femtosecond laser pulses using an enhanced direct locking technique

We demonstrate a long-term operation with reduced phase noise in the carrier-envelope-phase (CEP) stabilization process by employing a double feedback loop and an improved signal detection in the direct locking technique [Opt. Express 13, 2969 (2005)]. A homodyne balanced detection method is employed for efficiently suppressing the dc noise in the f-2f beat signal, which is converted into the CEP noise in the direct locking loop working at around zero carrier-envelope offset frequency (f(ceo)). In order to enhance the long-term stability, we have used the double feedback scheme that modulates both the oscillator pump power for a fast control and the intracavity-prism insertion depth for a slow and high-dynamic-range control. As a result, the in-loop phase jitter is reduced from 50 mrad of the previous result to 29 mrad, corresponding to 13 as in time scale, and the long-term stable operation is achieved for more than 12 hours.

Ion spectrometer composed of time-of-flight and Thomson parabola spectrometers for simultaneous characterization of laser-driven ions.

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.

Measurement of the group-delay dispersion of femtosecond optics using white-light interferometry

A compact and practical white-light cross-correlator suitable for fast evaluation of femtosecond optics, in terms of group-delay dispersion, was developed. A 40-W tungsten-halogen lamp was used as a white-light source and the detector selection was made so as to have adequate spectral sensitivity from 600 to 1050 nm, peaked at 800 nm. Group-delay dispersion was obtained, with femtosecond time resolution, from the Fourier transform of the cross-correlation interferogram. The dispersion characteristics of a borosilicate glass plate, broadband femtosecond mirrors, broadband chirped mirrors, and output couplers of different reflectivity have been determined in the wavelength range of 650 to 1050 nm.

Tunable Q-switched erbium-doped fiber laser based on digital micro-mirror array

We propose and demonstrate a tunable Q-switched erbium doped fiber laser with a digitally controlled micro-mirror array device. The tunable and pulsed output of the laser was achieved by the pixelated spatial modulation of the micro-mirror array. The wavelength tuning from 1530 nm to 1555 nm was shown with wavelength selectivity of ~0.1 nm and the pulsed operation was accomplished with 130 Hz repetition rate.