Laser Interference Experiments

The schematic setup for the Laser Interference Experiments is shown in Fig. 5. The primary laser beam is split into two or more beams to interfere with each other on the sample surface. By the combination of a polarizer and a half-wave plate, it is possible to adjust the intensity impinging on the sample. The intensity can also be adjusted by a mechanical shutter to choose the number of pulses. By using a two-lens consisting telescope, one can change the diameter of the laser beam and therefore... 

The 3d ab initio simulations [4] for Na3 are based, in a similar way, on three ab initio potential-energy surfaces for Na3(X), Na3(B), and Na3(X), with 3d ab initio dipole coupling between Na3(X) and Na3(B) evaluated by V. Bonacic-Koutecky et al. [5] plus Condon-type coupling between Na3(B) and Na3(X). Additional potential-energy surfaces interfere at the conical intersections of the pseudo-Jahn-Teller distorted Na3(B) state (see Ref. 6), but we have tested carefully [4] that these interferences are negligible in the frequency domains of the experimental femtosecond/picosecond laser pulse experiments [7] as well as in the continuous-wave experiments [8]. 

The most important absorptive short-pass filter is the BG 39, which has a blocking factor of more than 10 between 800 nm and 1,000 nm. It is used to bloek the NIR laser in experiments with two-photon excitation. Even blocking filters of the interference type for two-photon microscopy sometimes use a BG 39 substrate. 

The spectral width Aty can be further increased by focusing the laser pulses into a special optical fiber, which consists of a photonic crystal (Fig. 9.88) where by self-phase modulation the spectrum is considerably broadened and extends over one decade (e.g., from 1064 nm to 532 nm) (Fig. 9.89). This corresponds to a frequency span of 300 THz [1327] It was found by interference experiments, that the coherence properties were preserved in this broadened spectmm, i.e. the nonlinear processes in the optical fiber did not destroy the coherence of the original frequency comb. 

In order to apply the laser interference structuring method, the configuration of laser beams that produce desired interference pattern and hence energy distribution on the surface of the sample has to be calculated prior to the experiment. Calculating the configuration of electromagnetic waves that reproduce a desired interference pattern is an inverse problem and its solution is not known in general. 

Ultrafast TRCD has also been measured in chemical systems by incoriDorating a PEM into the probe beam optics of a picosecond laser pump-probe absorjDtion apparatus [35]. The PEM resonant frequency is very low (1 kHz) in these experiments, compared with the characteristic frequencies of ultrafast processes and so does not interfere with the detection of ultrafast CD changes. 

Problems arise if a light pulse of finite duration is used. Here, different frequencies of the wave packet are excited at different times as the laser pulse passes, and thus begin to move on the upper surface at different times, with resulting interference. In such situations, for example, simulations of femtochemistry experiments, a realistic simulation must include the light field explicitely [1]. 

In a typical experiment, the laser is focused at some fixed position zm and the refractive phase is varied by changing the optical density of the tuning medium. The product signal displays interference of the form of Eq. (4). If two product channels, A and B, are observed simultaneously, their modulation curves display a phase lag, which contains contributions from both the channel phase,... 

A tunable diode laser system and optics (Laser Photonics, L5736) was used to monitor the species release rates during an experiment. It consists of a liquid-nitrogen cooled diode emitter. For CO absorption measurements the laser emits a beam tuned to a wavenumber of 2082 cm4. This line is chosen as it exhibits strong absorption and is not subject to interference from other species likely to be present in the pyrolysate. 

To observe the transient spatiotemporal structure of the WP interference, we have performed the fs pump-fs probe experiment [37], The sample gas was prepared by molecular jet expansion of the mixture of iodine vapor and Ar buffer gas into a vacuum chamber. A continuous gas jet is preferable when we use a high-repetition-rate laser system. The estimated vibrational temperature was 170K[37]. 

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