Sub 100 nm resolution confocal focus-engineered coherent anti-Stokes Raman scattering microscopy under non-degenerate pumping condition. (November 2022)

• Record Type: Journal Article
• Authors: Lee, Gwanjin; Jyothsna, Konkada Manattayil; Lim, Heejin; Park, Jonghoo; Lee, JaeDong; Raghunathan, Varun; Kim, Hyunmin
• Is Part Of: Optics and lasers in engineering. Volume 158(2022)
• Publication Date: 2022-11
• Subjects: Sub-diffraction-limit nonlinear optical microscopy -- Coherent anti-Stokes Raman spectroscopy -- Nanoparticles; Lasers in engineering -- Periodicals; Optical measurements -- Periodicals; Optics -- Periodicals; Lasers en ingénierie -- Périodiques; Mesures optiques -- Périodiques; Optique -- Périodiques; 621.36605
• DOI: 10.1016/j.optlaseng.2022.107142
• Languages: English
• ISSNs: 0143-8166

Abstract

Highlights: Focus-engineering using concentric 0-pi-phase modulated beam of the wavefront of the incident beam has already demonstrated by our previous works experimentally with respect to 2 beam, degenerately pumped CARS systems to improve optical resolution. Here, we utilize the same technique in the context of the 3-beam non-degenerately pumped CARS. This results in an inherent-improvement of optical resolution which is utilized to image ZnO nanoparticles at sub 100 nm optical resolution. A critical dimensional comparison between SEM (in a charging-effect-free condition) and CARS was achieved by wrapping the dispersed nanoparticles with single layer graphenes. We correct the chromatic aberration of the objective lens at the low powered probe beam (at 405 and 455 nm wavelength) using a precise z-directional focal movement induced using a liquid lens. This technique enabled us to overlap the focal volumes of the pump, Stokes and probe beams to ensure efficient generation of the non-degenerate CARS signal. Further, unwanted focal sidelobes of the focus-engineered nonlinear focal volume is successfully removed using a pinhole in a de-scanning condition which results in additional ∼10% improvement in optical resolution besides the side lobe removal. Abstract: For the development of microscopic tools that can resolve non-fluorescent samples beyond the diffraction limit, we propose focus-engineered non-degenerate pumped coherent anti-Stokes Raman scattering (CARS) using spatial light modulator (SLM)-based phase shaping, liquid lens focus control, and confocal detection. Non-degenerate pumped CARS (ND-CARS) with frequency-doubled probe pulses resulted in approximately 75% improvement in resolution compared to that of degenerate CARS. Focal adjustment using the liquid lens facilitated the accurate overlapping of three beams. The circular π-phase modulation at the center of the probe-beam wavefront demarcated the net CARS focal volume into a sub 100 nm-scale core and surrounding side lobes. The confocal geometry detection setup successfully removed the side lobes, allowing optical imaging of 81 nm-sized zinc oxide particles at 87 nm, and edge-to-edge resolution was determined to be 103 nm.