Conceptual setup of the laser system.
Scheme of the Nd:YAG rod amplifier and wavelength conversion sections of the laser. After amplification at constant section in the rods, the beam is reduced in size by the telescope T3 (magnification M = 0.5) to increase the conversion efficiency in a LBO crystal (dimensions: 4 × 4 × 15 mm3). An harmonic separator (SA) reflects the 532 nm light to pump an OPO cavity. The mirror M19 transmits the idler beam and reflects the signal wavelength to the cryostat outside the clean room. Photodiodes PD1 and PD2 monitor the macropulse shape, respectively, before and after second harmonic generation.
(Dots) Autocorrelation measurement of the laser pulse width after the OPO stage. The continuous line is the best fit obtained with a squared hyperbolic secant function, corresponding to a pulse width of 7.2 ps.
Scheme of the electronic feedback used to stabilize the master oscillator repetition frequency. A fast photodiode monitors the oscillator output, which is compared to a reference microwave generator inside the frequency synthesizer ADF4108. Its output controls the high voltage fed to a piezoelectric transducer (PZT) which adjusts the length of the cavity.
Tuning of the repetition rate around the central frequency. The maximum change that can be obtained is 1.6 MHz.
A 15-h measurement of the master oscillator frequency stability obtained with an Agilent spectrum analyzer ESA E 4405-B.
Plot of the AOM driver function, for two different values of current delivered to the diodes in the pre-amplifier stage. The function is stepwise with a 10 ns discretization time. The lower curve, corresponding to 103 A, allows to extract a greater macropulse energy.
Oscilloscope trace of the train of laser pulses emerging from the OPO cavity at about 800 nm, taken with a 50 ps rise time photodiode (Hamamatsu C4258) placed after the engineered diffuser (see Sec. IV). The horizontal scale is 50 ns/division. The limited sampling of the oscilloscope does not allow to reproduce accurately each single 12 ps pulse.
Laser beam profile: (left) before entering the Nd:YAG rods; (right) after passing through the two rods; diffraction patters are kept within acceptable limits.
Optical parametric oscillator output for two different pump energies: (left) 20 mJ macro-pulse total energy; (right) 40 mJ macro-pulse total energy.
Optical scheme to uniformly illuminate the semiconductor. The laser beam is focused onto the surface of the GaAs disk through an engineered diffuser. The square inset shows the power density at the semiconductor surface, recorded with a CCD camera set at the position of the GaAs disk. The circle defines the area to be illuminated.
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