Bottom: b Time-resolved hole-burning set-up Either a CW single-f

Bottom: b Time-resolved hole-burning set-up. Either a CW single-frequency temperature- and current-controlled (T- and I-control) diode laser, or a titanium:sapphire laser, or a dye laser (see the above panel, a) was used. OI optical isolator, AOM/D acousto-optic modulator and driver, A diaphragm, Amp amplifier, P&D GEN pulse- RepSox in vivo and delay generator, WF GEN waveform synthesiser, ⊕ summing amplifier, DIG SCOPE digital oscilloscope,

PIA peripheral interface adapter (Adapted from Creemers and Völker 2000) The holes are either probed in fluorescence excitation at 90° to the direction of excitation or in transmission through the sample, with the same laser but with the power attenuated by a factor of 10–103. The intensity of the probe pulse is reduced with a neutral density filter. The fluorescence GSK458 or transmission signal of the hole is detected with a cooled photomultiplier (PM) and subsequently amplified with an electrometer. The signals are digitized and averaged point by point 1,000 times with a computer (PC) and the pulse scheme of Fig. 2 is used only once and not cycled through (see below). The experiments are controlled with a PC (Creemers and Völker 2000; Völker 1989a, b). Experimental set-up for time-resolved hole burning To perform time-resolved hole-burning experiments (see Fig. 3b), various types of CW single-frequency lasers are used, in combination with acousto-optic

modulators (AOMs), to create the pulse sequence described in Fig. 2. The choice of the laser depends on the absorption wavelength of the sample and the time scale of the experiment (Creemers and Völker 2000; Creemers et al. 1997; Den Hartog et al. 1998a, 1999a, b; Koedijk et al. 1996; Störkel et al. 1998; Wannemacher et

Protirelin al. 1993). For delay times t d, shorter than a few 100 ms and down to microseconds, we use current- and temperature-controlled single-mode diode lasers. The type of diode laser depends on the wavelength needed. The main advantage of these semiconductor lasers is that their frequency can be scanned very fast, up to ~10 GHz/μs, by sweeping the current through the diode. A disadvantage is their restricted wavelength region (5–10 nm, tunable by changing the temperature of the laser). The bandwidth of these diode lasers is ~3 MHz (Den Hartog et al. 1999b). For delay times t d longer than ~100 ms, either a CW single-frequency titanium:sapphire (bandwidth ~0.5 MHz) or a dye laser (bandwidth ~1 MHz) is used. The frequency of these lasers can be scanned continuously over 30 GHz with a maximum scan speed limited to ~100 MHz/ms by piezoelectric-driven mirrors. This speed is about 104–105 times slower than that of diode lasers (Creemers and Völker 2000; Den Hartog et al. 1999b). Burning power densities (Pt/A) between ~50 nW/cm2 and 20 mW/cm2, with burning times t b ranging from 1 μs to ~100 s, are generally used. The delay time t d between burning and probing the holes varies from ~1 μs to ~24 h.

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