Raman spectroscopy and microscopic Raman imaging with applications of volume holographic optical elements.

Abstract

Holographic optical elements are crucial for the development of compact and efficient Raman spectroscopic instrumentation. Holographic optics allow direct coupling of a single-stage spectrograph to a Raman microprobe. The high transmission efficiency and narrow rejection-band characteristics of the holographic beam splitter provide efficient use of low power lasers, while allowing acquisition of low-frequency Raman spectra. The holographic notch filter and holographic beamsplitter are essential tools for microscopic Raman imaging. Holographic transmission gratings used in an axial transmissive spectrograph provide high efficiency and low f/n operation. When coupled to a Raman microprobe, this single stage spectrograph can deliver converter-limited Raman signal in seconds. The capability of a matched-pair of holographic transmission gratings was explored as a filter device for Raman imaging. Without optimization, transmission efficiencies of 70% were observed throughout the visible light region and band passes of about 300 cm$\sp{-1}$. This filter device allowed microscopic imaging of samples using isolated Raman bands or fluorescence signal. The echo in transformed images from the Hadamard transform Raman imaging microscope were found to originate from diffraction of light from the Hadamard mask and thermal instability of the optical system. Instabilities of the on-board CCD detector amplifier was shown to be a source of flicker noise. The depth of field characteristics of the Hadamard transform Raman microscope were experimentally determined. A universal illumination method was demonstrated for filter-based Raman imaging. The effects of coherent illumination and quasi-incoherent illumination on microscopic Raman image resolution is experimentally demonstrated. The illumination methods developed in this work improved the quality of our Raman imaging and were crucial to the success of digital confocal Raman imaging. Applications were developed which demonstrate the benefits of the digital confocal Raman microscopy. Digital confocal image restoration techniques and extended field visualization of 3D Raman images were demonstrated.