Z-scan technique with high repetition rate femtosecond laser for nonlinear optical properties measurement

Nonlinear optical properties are crucial in determining the behavior of the optical material under the intense laser beam. Z-scan method is a simple technique to measure nonlinear refraction and absorption with high degree of sensitivity and noncomplex arrangement setup. Ultrafast laser technology benefits the Z-scan method by providing high nonlinearity condition within the material. Nevertheless, most of today’s ultrafast lasers are designed with high repetition rate (HRR) pulse trains which is known able to trigger thermal lensing effect. The lensing formation in Z-scan measurement corrupts the pure nonlinear response thus exhibits invalid results. Thermal lensing effect by the HRR laser is originated by the accumulated heat across the pulse trains due to the significantly long thermal diffusion time of the material in comparison to the very short HRR pulse separation. Optical chopper deployment in Z-scan measurement limits the material time exposure to the HRR beam and eliminates the lensing effect with the optimized chopper frequency. However, the optimization comes with a fixed chopper duty cycle, typically equal opening and blanking period. Therefore, applying duty cycle variation on top of the chopper frequency would reveal a new working range for the Z-scan to obtain accurate measurements without the thermal lensing influence. This research reports the Z-scan measurement with 780 nm HRR femtosecond beam on the nonlinear material AC-39. The experiment is performed with the adoption of the chopper frequency and duty cycle variation to minimize the thermal lensing effect by the precisely control the exposure time on AC-39. The modulated HRR beam Z-scan is carried out over the modulation frequency and duty cycle variation by evaluating the change of the peak and valley transmittance. For a 50% fixed duty cycle, 500 Hz of optimized minimum chopper frequency is achieved. The minimum chopper frequency is reduced further by adopting 10 and 25% duty cycle. It is deduced that this optimization is obtained by keeping the HRR beam time exposure on AC-39 material well below its thermal diffusivity time, thus seizes the thermal lensing build up formation before the next duty cycle. The minimum chopper frequency is correctly predicted by associating the chopper duty cycle factor, F with the material thermal diffusivity time, tc. Additional frequency optimization is achieved by considering the stable peak-valley transmittance difference over the frequency variations. The technique of finding the optimized minimum chopper frequency leads to the thermal diffusivity determination of the sample material for the unknown thermal diffusivity time. The optimization opens up for a potential of low operational frequency and offers a simple and straightforward solution and implementation in Z-scan technique of nonlinear optical properties measurement with high repetition rate femtosecond laser.

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