Intensity-dependent speckle contrast of second harmonic light in a nonlinear disordered medium

Area of Science:

• Optics and Photonics
• Biomedical Imaging
• Nonlinear Optics

Background:

• Laser speckle contrast quantifies light scattering in diffuse media, vital for biomedical imaging.
• Nonlinear harmonic generation is increasingly relevant with ultra-intense lasers.
• Simultaneous measurement of speckle contrast at fundamental and harmonic wavelengths is unexplored.

Purpose of the Study:

• To experimentally characterize intensity-dependent local speckle contrast at fundamental (1064 nm) and second-harmonic (532 nm) wavelengths.
• To investigate the probability distributions of local speckle contrast for both wavelengths.
• To explore the applicability of existing speckle contrast models to harmonic wavelengths.

Main Methods:

• Experimental setup using pulsed infrared (IR) laser excitation on a potassium dihydrogen phosphate diffuse sample.
• Simultaneous measurement of local speckle contrast (LSC) at 1064 nm (IR) and 532 nm (SHG).
• Analysis of LSC probability distributions and fitting to global speckle contrast models.

Main Results:

• Second-harmonic light generation and scattering produced speckle at both wavelengths.
• Radiation pressure reduced IR speckle contrast with increasing input power, consistent with theory.
• LSC probability distributions for both IR and SHG followed log-normal distributions with distinct parameters.
• Parameter evolution with box-size was fitted to global models, suggesting model extension to SHG.

Conclusions:

• Demonstrated simultaneous measurement of speckle contrast at fundamental and second-harmonic wavelengths.
• Observed intensity-dependent speckle behavior and log-normal distributions for both wavelengths.
• Findings suggest potential for extending global speckle contrast models to harmonic light, aiding nonlinear optical imaging applications.