Dinesh Patel

Demonstration of a 100 Hz repetition rate gain-saturated diode-pumped table-top soft x-ray laser

We demonstrate the operation of a gain-saturated table-top soft x-ray laser at 100 Hz repetition rate. The laser generates an average power of 0.15 mW at λ=18.9  nm, the highest laser power reported to date from a sub-20-nm wavelength compact source. Picosecond laser pulses of 1.5 μJ energy were produced at λ=18.9  nm by amplification in a Mo plasma created by tailoring the temporal intensity profile of single pump pulses with 1 J energy produced by a diode-pumped chirped pulse amplification Yb:YAG laser. Lasing was also obtained in the 13.9 nm line of Ni-like Ag. These results increase by an order of magnitude the repetition rate of plasma-based soft x-ray lasers opening the path to milliwatt average power table-top lasers at sub-20 nm wavelengths.

Hall effect measurement in the diamond anvil high‐pressure cell

Hall effect measurements have been carried out in the diamond anvil cell to pressures of 6 GPa. A plasma sprayed coating has been used to insulate the electrical leads from the gasket, similar to the configuration employed by Tozer and King [Rev. Sci. Instrum. 56, 260 (1985)], but a van der Pauw geometry was used to make the Hall and mobility measurements. Application of the technique to GaAs at 300 K is described, for which the Γ‐X conduction band crossover is observed at 4.0 GPa, in good agreement with recent photoluminescence measurements.

High laser-resistant multilayer mirrors by nodular defect planarization [Invited]

Substrate defect planarization has been shown to increase the laser resistance of 1053 nm mirror coatings to greater than 100  J/cm2, an increase of 20-fold, when tested with 10 ns laser pulses. Substrate surface particles that are overcoated with optical interference mirror coatings become nodular defects, which behave as microlenses intensifying light into the defect structure. By a discrete process of angle-dependent ion etching and unidirectional ion-beam deposition, substrate defects can be reduced in cross-sectional area by over 90%.

Experimental Evidence of the Impact of Nitrogen on Carrier Capture and Escape Times in InGaAsN/GaAs Single Quantum Well

We report our experimental results and theoretical analysis on carrier escape time in In0.4Ga0.6As1 -yNy/GaAs (y = 0; 0.005) ridge waveguide single-quantum-well (QW) lasers with N-contents of 0% and 0.5%. The experiments were carried out by using novel time-resolved two-color pump-probe transmission measurements. Our results show a significant decrease of carrier escape time with nitrogen incorporation in the InGaAsN QW, which agrees well with the values obtained from the theoretical calculation based on thermally activated hole leakage. The measurement results provide experimental supports for hole leakage theory.

Strain-induced modifications of the band structure of In/sub x/Ga/sub 1-x/P-In/sub 0.5/Al/sub 0.5/P multiple quantum wells

The effect of strain on the band structure of In/sub x/Ga/sub 1-x/P-In/sub 0.5/Al/sub 0.5/P multiple quantum wells (MQWs) has been investigated from high-pressure and low-temperature photoluminescence measurements. The biaxial strain in the wells was varied between +0.6% compressive to -0.85% tensile strain by changing the well composition x from 0.57 to 0.37. Strain increases the valence band offsets in either tensile or compressively strained structures. Whereas relatively insensitive to tensile strain, the valence band offsets showed a strong dependence on the magnitude of the compressive strain. Good agreement is found between the measured valence band offsets and those predicted by the model solid theory, except for the largest compressively strained MQWs, for which the model calculations underestimate the measured valence band offset. Strain and the associated variations in composition also modified the separation among the well states associated with /spl Gamma//sub 1c/, L/sub 1c/, and X/sub 1c/. From these results, the bandgaps of each conduction band extrema were calculated in In/sub x/Ga/sub 1-x/P for 0.37

Laser damage in dielectric films: What we know and what we don't

Damage mechanisms in thin films are reviewed from femtosecond pulse to CW laser illumination. Special emphasis is given to the role of native and laser induced defects, recent successes and the need for better diagnostic tools.

Influence of process conditions on the optical properties of HfO2/SiO2 coatings for high-power laser coatings

We investigate the variations that occur with changes in the number of layers and with the use of the assist beam main and assist beam energy on the morphology of HfO2/SiO2 quarter wave stacks deposited by dual ion beam sputtering. We show how the addition of sequential HfO2/SiO2 bilayers, up to eight, affects the surface roughness and micro-crystallinity of the top HfO2 layer. We also show that use of the assist source significantly smooths the surface while simultaneously reducing microcrystallinity. The HfO2/SiO2 structures are very robust and can withstand fluences in excess of 3 J/cm2 generated by 1ps pulses from a chirped amplified Ti:Sapphire laser.

1 Joule, 100 Hz Repetition Rate, Picosecond CPA Laser for Driving High Average Power Soft X-Ray Lasers

A diode-pumped cryogenic Yb:YAG CPA laser that produces 1J, 5ps pulses allowed for the first time the uninterrupted generation of 1.8×105 sub-20nm wavelength laser pulses with microjoule energy at 100Hz repetition rate on a table-top.

Advances in ion beam sputtered Sc2O3 for optical interference coatings

Scandium oxide is an attractive candidate for the engineering of interference coatings, although not widely explored. This paper describes the ion beam sputtering of Sc2O3. It is shown that the structural properties of the material are affected by the deposition conditions. Laser damage in different regimes of pulsewidths is investigated. These results show that the 1-on-1 laser damage fluence, in both the thermal and deterministic regimes, varies with deposition conditions but this is not the case for S-on-1, indicating that laser-induced defects are important.

Electrical contact to semiconducting samples for use in the diamond‐anvil cell

Techniques are described for preparing electrical contacts to samples for measurements in the diamond‐anvil high‐pressure cell. Typical dimensions are 150×150×30 μm. Bulk, epitaxial, and multiple‐quantum well (MQW) structures on GaAs and InP wafers have been successfully contacted. The techniques are also suitable for other experiments than those at high pressure, where miniaturization is important.