Spatial Filtering of a Diode Laser Beam for Confocal Raman Microscopy

Abstract

With the development of single-longitudinal mode diode lasers, there has been an increase in using these sources for Raman spectroscopy. This is largely due to the cost-effectiveness of diode lasers, which offer savings not only in initial capital cost, but also electrical, cooling, and replacement costs over time, when compared with ion lasers. The use of diode-lasers in confocal Raman microscopy has remained a challenge, however, due to poor transverse beam quality. In this work, we present the design and implementation of a simple spatial filter capable of adapting a single-mode diode laser source to confocal Raman microscopy, yielding comparable spatial resolution as a gas-ion laser beam for profiling and optical-trapping applications. For profiling applications, spatial filtering improved x,y resolution of the beam by a factor 10, which in turn increased optical-trapping forces by ~90 times and yielded sevenfold greater Raman scattering signal intensity from an optically trapped phospholipid vesicle.

Vibrational Spectroscopy for the Determination of Ionizable Group Content in Ionomer Materials

“…The confocal aperture was defined using the entrance slit of the monochromator in the horizontal dimension and by binning three rows of pixels on the CCD camera (78 µm) in the vertical dimension. 32 The tightly focused excitation beam (radius at 1/e 2 = 0.6 µm) 31 together with the collection aperture in the image plane define a confocal probe volume within the membranes of ∼5 fL where 90% detection efficiency was within a depth along the z -direction of ± 0.6 µm. 33 Raman spectra were measured with 30 s exposure periods and calculated from the average of 20 exposures.…”

“…The confocal Raman microscope has been described in detail previously. 31 Raman scattering was excited at 638 nm. A 100Â, 1.4 N.A.…”

Vibrational Spectroscopic Monitoring of the Gelation Transition in Nafion Ionomer Dispersions

“…oil immersion objective (Nikon, Plan APO VC) mounted on an inverted fluorescence microscope frame (Nikon Eclipse TE-200). 19 Samples were analyzed as individual droplets. An aliquot of aqueous dispersion (40 mL) was dispensed onto a glass microscope coverslip (No.…”

“…The expanded diode-laser beam radiation was directed through the microscope objective, immersion oil and coverslip and was brought to a focus within the aqueous droplet about 5 mm above (z-dimension) the coverslip-aqueous dispersion boundary. Sampling at z & 5 mm ensured the confocal probe volume 20,21 ($5 fL at 90% detection efficiency) 19 was fully localized within the sample. Reported spectra represent the average of 20 acquisitions each collected with a 30 s integration time.…”

“…A second factor that has potential to affect the Raman measurement is the small size of the confocal probe volume ($5 fL) [19][20][21] relative to the droplet. Since the droplet changes shape and can develop concentration gradients during drying, 43,44,46 Raman spectral features, particularly at early drying times, can be affected by inner fluid convective flow and chemical heterogeneity.…”