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This report is on experiments and theory on the process of optically stimulated electron population density redistribution in Si-substituted yttrium-iron garnet single crystals at 77 K. It was determined that a photo-induced uniaxial anisotropy field arose in the YIG:Si sample in response to illumination by quasi-linearly polarized laser (λ = 808 nm) leading to redistribution of Fe2+ ions among the nonequivalent octahedral sites. The photo-induced field was measured by variation of ferromagnetic resonance (FMR) frequencies in the X-band. The measured FMR frequency shift demonstrated a pronounced dependence on the polarization vector orientation with respect to crystallographic axes, in accordance with the theory discussed here. The frequency shift dependence on light intensity (for optimal polarization orientation) was found to be nearly linear, at least within the output intensity range of the optical source. The maximum frequency shift was −130 MHz for 75 mW applied optical power. A similar phenomenon was also observed at room temperature but was attributed to the sample heating by the incident light. The results presented here demonstrate the potential of the phenomenon for application in the development of ferrite signal processing devices with dual tuning by both magnetic field and optical irradiation.
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Volume averaging effect in nonlinear processes of focused laser fields
We report theoretical derivations and experimental results on the volume averaging effect of nonlinear processes in focused laser fields. This effect is considered detrimental in revealing the intensity dependence of a nonlinear process, caused by the intensity variation across the sampled volume of a focused laser. Following the treatment in the literature, we prove that if the signal dependence can be expressed as a simple power function of the laser intensity and if the detection region encompasses effectively the whole volume, volume average does not affect the final conclusion on the derived exponent. However, to reveal the detailed saturation effect of a multi-photon process, intensity selective scans involving spatial filters and displacement of the laser focus (z-scan) are required. Moreover, to fully capture the dependence of the signal on the variation of the laser intensity, the degree of spatial discrimination and the corresponding range of z-scan need to be modeled carefully. Limitations in the dynamic range of the detector or the laser power, however, can thwart the desired scan range, resulting in erroneous fitting exponents. Using our nanosecond laser with a non-ideal Gaussian beam profile, based on multiphoton ionization of argon atoms from a collimated molecular beam and from ambient argon gas, we report experimental measurements of the beam waist and Rayleigh range, and compare the experimental intensity dependence of Ar+ with theoretical values. Agreements between theory and experiment are remarkable.
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- Award ID(s):
- 1838522
- PAR ID:
- 10283294
- Date Published:
- Journal Name:
- Journal of chemical physics
- ISSN:
- 1089-7690
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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