Dr. Haojie Ji Profile

Dr. Haojie Ji

at UC Davis

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Proceedings Article | 20 February 2020 Presentation + Paper

1064 nm Raman microscopy using a multifocal excitation pattern

Haojie Ji, Marcos Oliveira, Che-Wei Chang, James Chan

Proceedings Volume 11251, 1125104 (2020) https://doi.org/10.1117/12.2546771

KEYWORDS: Raman spectroscopy, Signal to noise ratio, Microscopy, Silicon, Indium gallium arsenide, Raman scattering, Image processing

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Raman microscopy is well recognized as a nondestructive, label-free biomedical imaging method that provides abundant chemical information of the specimen. Excitation wavelengths in deep near-infrared (e.g., 1064 nm) are used in certain situations, such as when analyzing photosensitive/photolabile specimens to suppress the strong fluorescence and to avoid photodamage. However, the speed and quality of 1064 nm Raman imaging suffers from the low scattering efficiency at this long excitation wavelength and the high noise level of InGaAs detectors. In this study, we investigated a multifocal patterned approach for 1064 nm Raman imaging. A 2-D Hadamard-coded multifocal array generated with X-Y scanning galvomirrors is used to excite and collect multiple Raman spectra simultaneously. The individual spectrum at each focus is retrieved and reconstructed from the superimposed spectra of the multifocal patterns. We demonstrate that the multifocal approach improves both the signal-to-noise ratio (SNR) and the imaging speed of Raman microscopy. Compared to the traditional point scan, at optimal detector conditions, the multifocal approach can be two-times faster for achieving the same image quality and SNR, or provides spectra with three-times higher SNR while applying the same energy dose at the focus. Such improvements of imaging speed and SNR increase up to one or two orders of magnitude under higher noise conditions, such as higher readout rate and higher detector temperatures. The multifocal approach presents advantages for certain imaging situations, such as when heating related damage limits the excitation energy dose that can be applied to the sample.

Proceedings Article | 11 September 2019 Paper

Nonequilibrum phase transition in the light-actuated self-assembly of nanoparticles: effects of surfaces and stochastic forces

Haojie Ji, Robert Collison, Daniel Thomas, Luat Vuong

Proceedings Volume 11083, 1108325 (2019) https://doi.org/10.1117/12.2528987

KEYWORDS: Nanoparticles, Polarization, Plasmons, Particles, Motion models, Metals, Plasmonics

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When we illuminate gold nanofluids over indium-tin-oxide (ITO)-coated substrates, nanoparticle chains selfassemble via optical binding forces. We speculate that charge transfer between gold and ITO pins nanoparticles to the substrate and reduces the lateral Brownian motion as they attach to the substrate. We correspondingly model the self-assembly with additional stochastic or random forces. Simulations show a nonequilibrium-phase transition: when the stochastic force is small, nanoparticle chains align perpendicular to the light polarization and nanoparticles settle at shallow but stable nodes; when the stochastic force is large, however, the nanoparticle chains align parallel to the light polarization and nanoparticles settle at saddlepoints where the optical binding force is largely zero. Since the presence and strength of Brownian forces influence which state is formed, we reconsider the role that surfaces have—not only in relation to charge transfer but also heat transfer.

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