Brillouin spectroscopy probes the mechanical properties of material by measuring the optical frequency shift induced by photon-phonon scattering interactions. In traditional configurations, Brillouin spectrometers measure only one point of the sample at a time. This results in long acquisition times for mechanical imaging of large areas. In this work, we demonstrate a parallel detection configuration where the Brillouin shift of hundreds of points in a line can be measured simultaneously. In mm-sized samples, this novel configuration effectively shortens the acquisition time of two-dimensional Brillouin imaging from hours to tens of seconds, thus making it a powerful technology for label-free mechanical characterization of tissue and biomaterials.

Brillouin microscopy has recently emerged as a powerful technique to characterize the mechanical properties of biological tissue, cell, and biomaterials. However, the potential of Brillouin microscopy is currently limited to transparent samples, because Brillouin spectrometers do not have sufficient spectral extinction to reject the predominant non-Brillouin scattered light of turbid media. To overcome this issue, we combined a multi-pass Fabry-Perot interferometer with a two-stage virtually imaged phased array spectrometer. The Fabry-Perot etalon acts as an ultra-narrow band-pass filter for Brillouin light with high spectral extinction and low loss. We report background-free Brillouin spectra from Intralipid solutions and up to 100 μm deep within chicken muscle tissue.

We developed a microscopy technique that can measure the local refractive index without sampling the optical phase delay of the electromagnetic radiation. To do this, we designed and experimentally demonstrated a setup with two co-localized Brillouin scattering interactions that couple to a common acoustic phonon axis; in this scenario, the ratio of Brillouin frequency shifts depends on the refractive index, but not on any other mechanical and/or optical properties of the sample. Integrating the spectral measurement within a confocal microscope, the refractive index is mapped at micron-scale three-dimensional resolution. As the refractive index is probed in epi-detection and without assumptions on the geometrical dimensions of the sample, this method may prove useful to characterize biological cells and tissues.When light propagates inside a material, the phase of the electromagnetic wave is sensitive to the optical path, which fundamentally couples the geometrical path and the local index of refraction. Thus, methods to map the refractive index of a material (e.g. phase contrast microscopy [1,2], digital holography microscopy [3,4], optical coherence tomography [5,6], etc. [7-10]) are intrinsically indirect as they rely on the knowledge, assumption, or measurement of the spatial dimensions of the sample. Even when optical path delay and thickness of the sample are well characterized, we can only obtain the average refractive index along the beam propagation axis. This fundamental issue has important practical ramifications as it prevents performing spatiallyresolved measurements of the index of refraction. Mapping the distribution and variations of the local refractive index is potentially crucial to analyze mass density behavior in cell biology [11][12][13][14], cancer pathogenesis [13,15,16], and corneal or lens refraction [17][18][19].Several techniques in the past years have emerged to circumvent the coupling of geometrical path and refractive index in 3D samples using a tomographic approach, i.e. performing multiple measurements from different angles to reconstruct the internal refractive index distribution [20][21][22][23][24]. Tomographic phase microscopy enabled spatiallyresolved measurements of refractive index for the first time. However, as the individual measurements are still based on optical path delay, known geometrical boundary conditions and/or reference refractive index values are needed as well as access to the sample from at least two sides. In addition, even under these conditions, the measurements are subject to artifacts due to phase wrapping when phase variations inside the sample are not smooth [1,2,13].Here we present a novel microscopy technique that probes the refractive index of materials relying on photon-phonon interactions, not optical path delays, and thus decouples optical from geometrical path. The technology is based on measuring two inelastic Brillouin scattering interactions in confocal configuration, probing the same acoustic phonon axis so that the refractive index is the ...

Two-stage cross axis VIPA spectrometers have been widely used in Brillouin microscopy since they provide single shot spectral measurements at high throughput and extinction. However, this spectrometer configuration presents challenges such as size, cost and alignment difficulties between the two cascaded etalons. Here, we present a cross-axis VIPA spectrometer that implements a single etalon, using a light recirculation architecture to achieve the multistage configuration.

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