tailieunhanh - Advances in Lasers and Electro Optics Part 2

Tham khảo tài liệu 'advances in lasers and electro optics part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Nonlinear Optical Absorption of Organic Molecules for Applications in Optical Devices 37 technique with pulse train Misoguti et al. 1999 can also be employed allowing the investigation of the time evolution of nonlinear processes. The excitation source is a frequency-doubled Q-switched and mode-locked Nd YAG laser delivering pulses at 532 nm and 100 ps. Each pulse train contains about 20 pulses separated by 13 ns at a 10 Hz repetition rate. This low repetition rate is generally used to avoid cumulative thermal nonlinearities. The beam is focused onto a quartz cell yielding diameters of tens of pm at the focal plane. A photodetector placed in the far field coupled with a digital oscilloscope and a computer are used to acquire the pulse train signal. Each peak height is proportional to the corresponding pulse fluence once the detection system has a rise time slower than the 100 ps pulse duration. By measuring the beam waist and the pulse train average power one can find out the pulse fluency. The intensity can be determined by carrying out Z-scan measurements with CS2. When the sample is located at the focus the pulse train signal is acquired. Then this signal is normalized to the one obtained when the sample is far from the focus yielding the normalized transmittance as a function of pulse number. All optical measurements were carried out with the sample placed in a quartz cuvette. Figure 1 schematically shows the experimental setup. A 532nm Full pulse single envelope pulse Fig. 1. Experimental setup of the Z-scan technique with pulse trains used to characterize the material s nonlinear response in the pico- and nanosecond regime. Z-scan technique in the femtosecond regime The nonlinear optical absorption of organic molecules in the femtoseconds regime in a large spectral range may be carried out by means of two methodologies a Single wavelength Z-scan technique and b White-Light Continuum Z-scan technique described in more details as follows. a Single .