Abstract
Quantitative phase spectroscopy is presented as a novel method of measuring the wavelength-dependent refractive index of microscopic volumes. Light from a broadband source is filtered to an ~5 nm bandwidth and rapidly tuned across the visible spectrum in 1 nm increments by an acousto-optic tunable filter (AOTF). Quantitative phase images of semitransparent samples are recovered at each wavelength using off-axis interferometry and are processed to recover relative and absolute dispersion measurements. We demonstrate the utility of this approach by (i) spectrally averaging phase images to reduce coherent noise, (ii) measuring absorptive and dispersive features in microspheres, and (iii) quantifying bulk hemoglobin concentrations by absolute refractive index measurements. Considerations of using low coherence illumination and the extension of spectral techniques in quantitative phase measurements are discussed.
Highlights
Quantitative phase microscopy (QPM) has developed into an effective tool for measuring spatial and temporal properties of semitransparent samples, especially in vitro cells and microfluidic systems [1]
While swept source lasers and some tunable filters can have extremely narrow output lines, this paper focuses on the specific application of an acousto-optic tunable filter with a supercontinuum source
We have presented quantitative phase spectroscopy (QPS) as a novel method of investigating refractive index features with high spatial and spectral resolution
Summary
Quantitative phase microscopy (QPM) has developed into an effective tool for measuring spatial and temporal properties of semitransparent samples, especially in vitro cells and microfluidic systems [1]. Park et al demonstrated the ability to measure pure solutions of bovine serum albumin and hemoglobin using a white light source and seven color filters with various center wavelengths [2]. They further showed that it was possible using three specific wavelengths to measure spatial distributions of hemoglobin within individual blood cells [2]. Multi-wavelength illumination has been employed to aid phase unwrapping [4] and to decouple refractive index from cell thickness [5] All of these methods exploit dispersive effects in QPM at small numbers of discrete spectral points
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