Label-Free Biomedical Imaging Using High-Speed Lock-In Pixel Sensor for Stimulated Raman Scattering

Ultrahigh-speed multiplex coherent anti-Stokes Raman scattering microspectroscopy using scanning elliptical focal spot

We present a beam-scanning multiplex coherent anti-Stokes Raman scattering (CARS) microspectroscopy system using parallel excitation and parallel detection schemes based on an elliptical focal spot, which enables highly efficient signal acquisition even for short exposures. The elliptical focal spot was used to simultaneously observe the CARS signals of an enlarged region and reduce the peak irradiance. The developed system realized an acquisition rate of 34 139 spectra/s and enabled ultrahigh-speed acquisition of a vibrational spectroscopic image, covering the fingerprint region of 930-1 830 cm^-1 with 256_(x) × 256_(y) × 512_(spectrum) pixels in 1.92 s or with 128_(x) × 128_(y) × 256_(spectrum) pixels in 0.54 s. We demonstrated ultrahigh-speed hyperspectral imaging of a mixture of polymer beads in liquid linoleic acid and living adipocytes using the developed system. All of the present demonstrations were performed with a low-peak irradiance excitation of ∼19 GW/cm², which has been reported in previous studies to cause less photodamage to living cells. The label-free and ultrahigh-speed identification and visualization of various molecules made possible by the present system will accelerate the development of practical live-cell investigation.

Dual-focus stimulated Raman scattering microscopy: a concept for multi-focus scaling

High-speed imaging is of the utmost importance for video-rate live cell investigations or to study extended sample areas at sufficient spatial resolution within reasonable time scales. Improving the speed of single-focus stimulated Raman scattering (SRS) microscopy is ultimately restricted by the sample's damage threshold and the shot noise of the demodulated laser source. To overcome this limitation, we present a dual-focus SRS approach modulating the pump laser for each focus at a distinct frequency. The corresponding probe beams are detected each by a photodiode and demodulated individually by two separate lock-in units to avoid inter-focal cross-talk. Two laterally or axially displaced images as well as hyperspectral SRS images can be obtained simultaneously within the field of view of the objective lens. The modular implementation presented here can be extended to multiple foci by using multi-channel acousto-optics modulators in combination with multi-channel lock-in amplifiers.

Probe of Alcohol Structures in the Gas and Liquid States Using C⁻H Stretching Raman Spectroscopy

Vibrational spectroscopy is a powerful tool for probing molecular structures and dynamics since it offers a unique fingerprint that allows molecular identification. One of important aspects of applying vibrational spectroscopy is to develop the probes that can characterize the related properties of molecules such as the conformation and intermolecular interaction. Many examples of vibrational probes have appeared in the literature, including the azide group (⁻N₃), amide group (⁻CONH₂), nitrile groups (⁻CN), hydroxyl group (⁻OH), ⁻CH group and so on. Among these probes, the ⁻CH group is an excellent one since it is ubiquitous in organic and biological molecules and the C⁻H stretching vibrational spectrum is extraordinarily sensitive to the local molecular environment. However, one challenge encountered in the application of C⁻H probes arises from the difficulty in the accurate assignment due to spectral congestion in the C⁻H stretching region. In this paper, recent advances in the complete assignment of C⁻H stretching spectra of aliphatic alcohols and the utility of C⁻H vibration as a probe of the conformation and weak intermolecular interaction are outlined. These results fully demonstrated the potential of the ⁻CH chemical group as a molecular probe.