Research

Stimulated Raman scattering microscopy

Have you looked through an optical microscope to observe biological samples? Since biological samples are typically transparent, we have to stain them with dye or label them with fluorescent proteins/molecules. These technologies are quite important in optical microscopy, but are time-consuming. Rapid observation is demanded especially in medical imaging, where doctors in surgery try to find the boundary between healthy tissue and tumor as quickly and precisely as possible.

We are developing stimulated Raman scattering (SRS) microscopy, which allows rapid observation of biological samples without staining/labeling. SRS microscopy utilizes leading-edge laser technologies to detect vibrations of biomolecules as a source of image contrast. To detect molecular vibrations, SRS microscopy exploits SRS effect, which is an interaction between two-color laser pulses and molecular vibrations. When the difference in optical frequencies between the two-color laser pulses matches the molecular vibrational frequency, the high-frequency (short-wavelength) light is attenuated and the low-frequency (long-wavelength) light is amplified through the SRS effect.

Figure 1 shows a schematic of SRS microscopy [1-3]. We use two-color laser pulses at optical frequencies of ω1 and ω2, one of which is intensity-modulated in time. Then these pulses are combined and focused on the sample. When SRS occurs, the modulation is transferred to the other color. The transferred modulation is proportional to the density of molecules of interest. Then the transferred modulation is detected with a photodiode followed by a lock-in amplifier. Images are taken by scanning the laser beam or the sample position.

SRS microscopy was independently developed by Freudiger, Min and Xie in Harvard University [1], Nandakumar and Volkmer in Stuttgart University [2], and Dake, Ozeki and Itoh in Osaka University [3]. Since then, various applications of SRS microscopy have been explored in the world. We recently demonstrated high-speed multicolor tissue imaging based on the combination of video-rate SRS microscopy and wavelength-tunable optical pulse generation [4].

We are currently working on laser technologies for realizing the next level of imaging capability, image analysis for identifying biomolecules, and biomedical applications. For recent achievements, check out our publications.

Fig. 1. Schematic of stimulated Raman scattering microscopy.

IM: intensity modulator, OB: objective lens.

[1] C. W. Freudiger et al., Science 322, 1857 (2008).

[2] P. Nandakumar et al., N. J. Phys. 11, 033026 (2009).

[3] Y. Ozeki et al., Opt. Express 17, 3651 (2009).

[4] Y. Ozeki et al., Nature Photon. 6, 845 (2012).

Research topics

  • High-speed multicolor Raman microscopy
  • Wavelength-tunable pulse source for spectral imaging
  • Raman spectral analysis
  • Practical fiber lasers
  • Laser synchronization