Verdet constant dispersion of magnesium fluoride for deep-ultraviolet and vacuum-ultraviolet Faraday rotators.

The Verdet constant dispersion in magnesium fluoride (MgF2) crystals was evaluated over a wavelength range of 190-300 nm. The Verdet constant was found to be 38.7 rad/(T·m) at a wavelength of 193 nm. These results were fitted using the diamagnetic dispersion model and the classical Becquerel formula. The fitted results can be used for the designing of suitable Faraday rotators at various wavelengths. These results indicate the possibility of using MgF2 as Faraday rotators not only in deep-ultraviolet regions, but also in vacuum-ultraviolet regions owing to its large bandgap.

Single-shot multispectral birefringence mapping by supercontinuum vector beams.

We demonstrated a single-shot, multispectral birefringence mapping by use of a supercontinuum (SC) vector beam. The vector beam, which was generated by a pair of axially symmetric wave plates, leads to angular-variant polarization modulation to divide birefringence properties of a sample substrate into Fourier space. This strategy allows multispectral birefringence mapping from a single-shot image captured by a multispectral imaging detector. For SC vector beam analysis, we also compensated the retardance error of the axially symmetric wave plate in the superbroadband spectrum. Resolutions of retardance and azimuthal angle were 0.4° and 0.2°, respectively, and the spatial resolution was 60 µm. Those results are expected to provide us a single-shot, multispectral birefringence mapping with high spatial resolution as compared with using a scanning laser microscope. Our proposal has extendibility to develop high-speed, high-resolution birefringence imaging spectroscopy.

Detection of birefringence singularity by supercontinuum vector beam.

We demonstrated detection of birefringence singularity on the space-variant retarder with an inhomogeneous birefringence distribution by supercontinuum vector beam. The birefringence measurement by supercontinuum vector beam analysis provides kinematics of a singularity point on the space-variant retarder. We conducted numerical calculations and experiments for proof of principle. The calculated results were characterized by relative positions with (x0,y0) between the singularity point and the vector beam. In the experiments, we measured the retardance and the azimuthal angle from intensity profile on a single-shot image captured at wavelengths of λ=450, 550, and 650 nm. The retardances at λ=450nm and 550 nm were changed from Δ=112∘ to 131° and from Δ=120∘ to 152° when the x0 displacement of the space-variant retarder moved from 0 to 350 µm. The measured retardance corresponded with the calculated results in the function of the position of birefringence singularity.

Watt-level 193 nm source generation based on compact collinear cascaded sum frequency mixing configuration.

A compact, dual-stage, collinear-cascaded sum-frequency mixing configuration is presented for generating 193 nm sources. Due to the less-introduced, deep-ultraviolet optical components, the system is less prone to damage. In our proof-of-concept experiments, a 1030 nm laser and a 1553 nm laser synchronized to each other were used as drivers and an average power of ~0.7 W was obtained. For comparison, the noncollinear configuration gave an average power of 0.77 W. The difference of 0.07 W is attributed to the spatial walk-off inside the cesium lithium borate (CLBO) crystal, confirmed by indirect visualization. A possible way to overcome the small gap of 0.07 W is proposed for future work.

Picosecond green and deep ultraviolet pulses generated by a high-power 100 kHz thin-disk laser.

We report on the generation of the second (515 nm) and fourth (257.5 nm) harmonics from a 100 kHz diode-pumped solid-state laser operating at a wavelength of 1030 nm which uses one Yb:YAG thin disk in the regenerative amplifier and delivers 60 W of the average output power in pulses of 4 ps duration. Thirty-five W in green light and 6 W in deep ultraviolet (DUV) were achieved. The sensitivity of the second harmonic generation efficiency toward the lithium triborate crystal temperature is demonstrated in experiment. The overall conversion efficiency from NIR to DUV of 10% was achieved. The ß-barium borate and cesium lithium borate crystals were used as green to DUV convertors and compared regarding the efficiency and spectral bandwidths. The achieved output power is unique for DUV picosecond pulses.

Optimization of beam quality and optical-to-optical efficiency of Yb:YAG thin-disk regenerative amplifier by pulsed pumping.

We demonstrate an optimization method of beam quality and optical-to-optical (O-O) efficiency by using pulsed pumping. By changing the pulse duration and the peak intensity of pump pulse at the repetition rate of 1 kHz, the beam quality and O-O efficiency of the Yb:YAG thin-disk regenerative amplifier can be improved. We applied this method to the regenerative amplifier under the pumping wavelength of both 940 and 969 nm, and found that the method was effective in both pumping wavelengths. Although a Yb:YAG thin disk soldered on a copper tungsten heat sink, which has poor thermal properties compared with a thin disk mounted on a diamond substrate, was applied as a gain media, we obtained 45 mJ output with 19.3% O-O efficiency and nearly diffraction-limited beam.

Suppression of nonlinear phonon relaxation in Yb:YAG thin disk via zero phonon line pumping.

A quantitative comparison of conventional absorption line (940 nm) pumping and zero phonon line (ZPL) (969 nm) pumping of a Yb:YAG thin disk laser is reported. Characteristics of an output beam profile, surface temperature, and deformation of a thin disk under the different pump wavelengths are evaluated. We found that a nonlinear phonon relaxation (NPR) of the excited state in Yb:YAG, which induces nonlinear temperature rise and large aspheric deformation, did not appear in the case of a ZPL pumped Yb:YAG thin disk. This means that the advantage of ZPL pumping is not only the reduction of quantum defect but also the suppression of NPR. The latter effect is more important for high power lasers.

Characteristics of extreme ultraviolet emission from mid-infrared laser-produced rare-earth Gd plasmas.

We characterize extreme ultraviolet (EUV) emission from mid-infrared (mid-IR) laser-produced plasmas (LPPs) of the rare-earth element Gd. The energy conversion efficiency (CE) and the spectral purity in the mid-IR LPPs at λL = 10.6 µm were higher than for solid-state LPPs at λL = 1.06 µm, because the plasma produced is optically thin due to the lower critical density, resulting in a CE of 0.7%. The peak wavelength remained fixed at 6.76 nm for all laser intensities studied. Plasma parameters at a mid-IR laser intensity of 1.3×10(11) W/cm(2) was also evaluated by use of the hydrodynamic simulation code to produce the EUV emission at 6.76 nm.

Fine-pitched microgratings encoded by interference of UV femtosecond laser pulses.

Fine-pitched microgratings are encoded on fused silica surfaces by a two-beam laser interference technique employing UV femtosecond pulses from the third harmonics of a Ti:sapphire laser. A pump and prove method utilizing a laser-induced optical Kerr effect or transient optical absorption change has been developed to achieve the time coincidence of the two pulses. Use of the UV pulses makes it possible to narrow the grating pitches to an opening as small as 290 nm, and the groove width of the gratings is of nanoscale size. The present technique provides a novel opportunity for the fabrication of periodic nanoscale structures in various materials.

Oxygen exchange at the internal surface of amorphous SiO2 studied by photoluminescence of isotopically labeled oxygen molecules.

The exchange between lattice and interstitial oxygen species in an oxide was studied by the 16O-18O isotope shift of the a1Deltag(v=0)-->X3Sigmag-(v=1) infrared photoluminescence band of the oxygen molecules (O2) incorporated into the interstitial voids of amorphous SiO2 (a-SiO2) by thermal annealing in 18O2 gas. A large site to site variation of the oxygen exchange rate, originating from structural disorder of a-SiO2, is found. The average exchange rate has an activation energy of approximately 2 eV, which is much larger than that for the diffusion of interstitial O2 (approximately 0.8-1.2 eV). The average exchange-free diffusion length of interstitial O2 exceeds approximately 1 microm below 900 degrees C, providing definite evidence that oxygen diffuses as interstitial molecules in a-SiO2.