Development of a New Optical Wavelength Rejection Filter: Demonstration of its Utility in Raman Spectroscopy

A new tunable optical filter has been developed which rejects a narrow wavelength interval (<7.5 nm) in the near-UV, visible, or near-IR spectral region and allows adjacent wavelengths to pass (T > 90%). This filter will be useful in optics, in spectroscopy, and for laser applications. The active element of the filter is a crystalline colloidal array of polystyrene spheres. The rejected wavelengths are Bragg diffracted from this ordered array. For a particular sphere concentration and scattering from a particular set of lattice planes, tunability can be achieved by the altering of the angle between the filter and incident light beam. Bragg diffraction and light rejection of these filters are monitored by transmission measurements. The utility of this filter for spectroscopic measurements is demonstrated for Raman spectroscopy. Raman measurements are shown for polypropylene, a highly scattering material with numerous low-frequency modes. The filter selectively attenuates the elastically scattered light and allows the low frequency peaks to be observed. Use of this wavelength rejection filter to reject the Rayleigh scattered light simplifies the instrumentation, decreases the cost, and increases the sensitivity of Raman spectral measurements. This filter also has the potential to replace dispersive elements such as gratings and prisms in a variety of spectroscopic and optical applications.