Jae-hwan Lee

Generation of 1.5 cycle 0.3 TW laser pulses using a hollow-fiber pulse compressor

Pulse compression in a differentially pumped neon-filled hollow fiber was used to generate high-power few-cycle laser pulses. The pulse compression process was optimized by adjusting gas pressure and laser chirp to produce the shortest laser pulses. Precise dispersion control enabled the generation of laser pulses with duration of 3.7 fs and energy of 1.2 mJ. This corresponds to an output of 1.5 cycle, 0.3 TW pulses at a 1 kHz repetition rate using positively chirped 33 fs laser pulses.

Laser chirp effect on femtosecond laser filamentation generated for pulse compression.

The influence of laser chirp on the formation of femtosecond laser filamentation in Ar was investigated for the generation of few-cycle high-power laser pulses. The condition for the formation of a single filament has been carefully examined using 28-fs laser pulses with energy over 3 mJ. The filament formation and output spectrum changed very sensitively to the initial laser chirp and gas pressure. Much larger spectral broadening was obtained with positively chirped pulses, compared to the case of negatively chirped pulses that generated much longer filament, and compressed pulses of 5.5 fs with energy of 0.5 mJ were obtained from the filamentation of positively chirped 30-fs laser pulses in a single Ar cell.

Attosecond chirp compensation over broadband high-order harmonics to generate near transform-limited 63 as pulses

By generating broadband high-harmonic pulses from neon and compensating for attosecond chirp by the material dispersion of argon, the generation of near transform-limited 63 as pulses was achieved. The spectral phase analysis showed that, without proper compensation, the attosecond chirp of the broadband harmonics caused splitting of attosecond high-harmonic pulses in addition to pulse broadening. Although it was attained only within a limited spectral range, the attosecond chirp compensation was successful in bringing out pulse compression over broad harmonics, which signifies the effectiveness of the attosecond chirp compensation by material dispersion.

Long-term carrier-envelope-phase stabilization of a femtosecond laser by the direct locking method

We have developed a practical solution to implement the direct locking method for the carrier-envelope phase (CEP) stabilization of femtosecond laser pulses and achieved 24-hour CEP stabilization without realignment of any optical components. The direct locking method realizes the CEP stabilization in the time domain by directly quenching the beat signal from an f-to-2f interferometer and, thereby, locking every pulse to a same CEP. We have accomplished the long-term CEP stabilization using commercially available standard feedback electronics, and maintained the CEP stabilization with low jitter without using any frequency-analyzing components, greatly facilitating the accessibility of the CEP stabilization.

Soft x-ray microscope constructed with a PMMA phase-reversal zone plate

A soft x-ray microscope based on a phase-reversal zone plate was constructed and tested using high harmonic radiation as a coherent light source. The 61st harmonic centered at 13.3 nm was optimized in spectral sharpness and intensity by controlling the incident laser energy and chirp. A phase-reversal zone plate made of polymethyl methacrylate more than doubled the first-order efficiency. The nano patterns, imaged on an x-ray CCD with a magnification of 650, showed that the measured resolution of the microscope was better than 100 nm.

Long-term maintenance of the carrier-envelope phase coherence of a femtosecond laser

The long-term carrier-envelope phase (CEP) coherence of a femtosecond laser with same pulse-to-pulse CEP value, obtained using the direct locking method, is demonstrated by employing a quasi-common-path interferometer (QPI). For the evaluation of the CEP stability, the phase noise properties of a femtosecond laser with the CEP stabilized using a QPI are compared with those obtained using a Mach-Zehnder f-2f interferometer, for which the phase power spectral density and the Allan deviation were calculated from the beat signals of the interferometers. With the improved CEP stability, the long-term CEP coherent signal with an accumulated phase noise well below 1 radian can be maintained for more than 56 hours, i.e., the CEP coherence is preserved without a phase cycle slip for more than 1.6 × 10(13) pulses at a repetition rate of 80 MHz. The relative stability is also estimated to be approximately 1.4 × 10(-22) at a central wavelength of 790 nm.

Table-top soft x-ray microscope adopting a PMMA phase-reversal zone plate

A table-top soft x-ray microscope, adopting a phase-reversal zone plate (PRZP) made of PMMA and high harmonic x-ray source, was constructed. The PMMA PRZP showed enhanced imaging efficiency compared to a metallic Fresnel zone plate.

Comparison of RABITT and FROG measurements in the temporal reconstruction of attosecond pulse trains

Attosecond high-harmonic pulses obtained from Ar were characterized by the two methods — RABITT and FROG CRAB. The comparison of the two results revealed the capabilities of the two methods well.

Carrier-envelope-phase-dependent above-threshold ionization of xenon observed with multi-cycle laser pulses

Carrier-envelope-phase (CEP)-dependent modulation was measured in above-threshold ionization of xenon driven with 30-fs laser pulses. We showed the dependence from the asymmetry map obtained using a velocity map imaging spectrometer, up to 17 eV in photoelectron energy. The dependence appeared to be linear with a slope of one photon energy increase per CEP change of 2π and did not rely on the sign or the amount of laser chirp. Our results indicated the existence of the quantum interference between different multiphoton ionization paths.

Highly efficient soft X-ray microscope using a PMMA phase-reversal zone plate

Compact soft X-ray microscopes, using high harmonics as a light source, were constructed. A phase-reversal zone plate fabricated with polymethyl methacrylate was adopted to improve the focusing efficiency. Microscope images at 13 nm were acquired with sub-100 nm resolution, and the exposure time was reduced by a half, compared to that of the one using a gold Fresnel zone plate. A single-shot microscope image could be obtained using a strong harmonic at 22 nm generated in the two-color laser field consisting of the fundamental and the second harmonic of a femtosecond laser.