Partial-wave microscopic spectroscopy detects subwavelength refractive index fluctuations: an application to cancer diagnosis

Abstract

Existing optical imaging techniques offer us powerful tools to directly visualize the cellular structure at the microscale; however, their capability of nanoscale sensitivity is restricted by the diffractionlimited resolution. We show that mesoscopic light transport theory analysis of the spectra of partial waves propagating within a weakly disordered medium such as biological cells (i.e. partialwave spectroscopy, PWS) quantifies refractive index fluctuations at subdiffractional length scales. We validate this nanoscale sensitivity of PWS using rigorous Finitedifference time-domain (FDTD) simulations and experiments with nanostructured models. We also demonstrate the potential of this technique to detect nanoscale alterations in cells from patients with pancreatic cancer who are otherwise classified as normal by conventional microscopic histopathology.Spectroscopy of elastic scattering is commonly used to probe tissue morphology [1]. However, the sensitivity of light scattering signal to refractive index fluctuations is significantly reduced when the size of the scattering structures falls below the wavelength (~500 nm). Recently, there has been significant interest in understanding biological systems at the nanoscale, which requires measurement of sub-wavelength refractive index fluctuations. According to mesoscopic light transport theory [2][3][4], for an object that is weakly disordered and weakly scattering, it is indeed possible to probe refractive index fluctuations of any length scale including those well below the wavelength [3,4] if one analyzes a signal generated by the multiple interference of 1D-propagating waves reflected from the refractive index fluctuations within the object. The enhanced sensitivity of 1D-propagating waves to sub-wavelength correlation lengths of refractive index fluctuations l c (i.e., l c < wavelength λ) can be understood from the following consideration: while in 3D the scattering coefficient ~(l c / λ) 3 , and, thus, contribution from small length scales is weighted down as , for 1D waves the scattering coefficient is ~ (l c / λ) [5]. The 1D-propagating waves are one of many subset of waves (herein called 1D-partial waves) propagating within a scattering particle. Recently, we reported an optical system [6] capable of isolating 1D-partial waves from different parts of a homogeneous scattering particle. Here we show that the backscattering signal formed by these 1D-partial waves can be detected for heterogeneous objects, such as biological cells, and be used to probe the sub-wavelength refractive index fluctuations.A detailed description of the PWS instrument is given elsewhere [6]. In brief, a broadband light with spatial coherence length <1 μm is focused onto the sample by a low-numerical aperture (NA) objective (Edmund Optics, NA of objective = 0.4, NA of illumination = 0.2, NA of collection = 0.4). The illumination beam diameter (~120 μm) is much larger than biological cells (~8 μm) and is well collimated within a cell located in the waist of the beam. The resulti...

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“…Many studies have been performed on the scattering of light from cells, both theoretical [3][4][5] and experimental [6][7][8][9][10][11]. Additionally, some studies utilize advanced far-field optical techniques to measure refractive index variations at subwavelength resolution [12,13]. These studies focus on the effects of the internal morphology of the cells and variations in refractive indices on light scattering.…”