Brillouin Microscopy allows for the characterization of tissues and cells by measuring the frequency shift of light when they interact with phonons in material. Due to the low scattering cross section and higher powers posing a risk for photodamage, fast acquisition times remain a challenge. To alleviate these issues, parallelizing the signal collection by collecting an entire image allows for 2-dimensional parallelization. We demonstrate a spectrometer based on induced dichroism in atomic vapors, with an improved pumping scheme to our previous design, for use with light sheet microscopy, and demonstrate its ability to capture an entire 2D Brillouin image.
Brillouin Microscopy allows for the characterization of tissues and cells by measuring the frequency shift of light when they interact with phonons in material. Due to the low scattering cross section and higher powers posing a risk for photodamage, fast acquisition times remain a challenge. To alleviate these issues, parallelizing the signal collection by collecting an entire image allows for 2-dimensional parallelization. We demonstrate a spectrometer based on induced dichroism in atomic vapors, with an improved pumping scheme to our previous design, for use with light sheet microscopy, and demonstrate its ability to capture an entire 2D Brillouin image.
Brillouin microscopy is an emerging label-free imaging technique used to assess local viscoelastic properties. Quantum-enhanced stimulated Brillouin scattering is demonstrated using low power continuous-wave lasers at 795 nm. A signal-to-noise ratio enhancement of 3.4 dB is reported by using two-mode intensity-difference squeezed light generated with the four-wave mixing process in atomic rubidium vapor. The low optical power and the excitation wavelengths in the water transparency window have the potential to provide a powerful bio-imaging technique for probing mechanical properties of biological samples prone to phototoxicity and thermal effects. The performance enhancement affordable through the use of quantum light may pave the way for significantly improved sensitivity that cannot be achieved classically. The proposed method for utilizing squeezed light for enhanced stimulated Brillouin scattering can be easily adapted for both spectroscopic and imaging applications in biology.
There are many methods to achieve optical gain for laser applications; such methods have been used to make improved spectroscopy and microscopy apparatus. In particular, gain from acoustic and atomic interactions has attained much interest, as is evident in the literature. In this work, we report the results of several experiments involving nonlinear Brillouin scattering and three-level gain in rubidium vapor. \n\nBrillouin scattering, both spontaneous and stimulated, in principle result from the scattering of light off of acoustic waves in a medium. While both spontaneous and stimulated Brillouin scattering have been applied to spectroscopy of various samples, from condensed matter to gases, only spontaneous Brillouin scattering has been used for microscopy purposes. We report our result from several proof-of-principle experiments in which stimulated Brillouin scattering and impulsive stimulated Brillouin scattering were applied to microscopic purposes for the first time. We show the advantages these methods afford over spontaneous Brillouin scattering and make a statistical comparison between them. \n\nThere are also many ways to achieve gain in atomic vapors. Using a setup similar to the stimulated Brillouin scattering setup we performed gain in a three-level Λ system in warm rubidium vapor. To our knowledge, this is the first time this three-level analog of Mollow’s two-level system with Doppler broadening has been experimentally investigated. Our results show about 0.12% gain in the probe beam. \n\nThis work, and extensions of it, should provide additional methods of performing spectroscopy and microscopy in various materials from atoms to bulk matter.
View Video Presentation: https://doi.org/10.2514/6.2022-3365.vid In this work, we demonstrate a new high throughput, high resolution spectroscopic technique based on dispersion in cesium atomic vapor and use it to image the Rayleigh-Brillouin spectrum from air. This is accomplished by utilizing a prism shaped cesium vapor cell and taking advantage of the steep refractive index in the vicinity of atomic resonances of the cesium D2 hyperfine lines at 852.113 nm. A laser beam from a tunable, continuous, and narrow linewidth Ti:sapphire laser was focused in ambient air and the spectrally dispersed scattering was imaged onto a camera after passing through a 80 mm long cesium prism vapor cell. The strong dispersion enables the measurement of the characteristic thermal and acoustic broadening of the Rayleigh-Brillouin scattering. Mie scattering from particles in the air is also captured and can be separated from molecular Rayleigh-Brillouin scattering by tuning close to the cesium resonance. The trade-off between spectral resolution by dispersion and loss of signal by cesium absorption is analyzed by tuning the laser. This technique shows the feasibility to measure the thermodynamic properties of a gas by reconstruction of the Rayleigh-Brillouin spectrum and could be applicable for advanced LIDAR measurements of atmospheric properties.
Dynamic control of the speed of a light signal, based on stimulated Brillouin scattering in optical fibers, was theoretically studied and also experimentally demonstrated as the core object of this thesis. To date, slow light based on stimulated Brillouin scattering has shown the unmatched flexibility to offer an efficient timing tool for the development of all-optical future router. Nevertheless, the seeming perfect Brillouin slow light suffered from three major obstacles: naturally narrow signal bandwidth, strong change of signal amplitude, and significant signal distortion. The essential contribution of this work has been mostly dedicated to resolve all those impairments so as to make Brillouin slow light a completely operating all-optical delay line for practical applications. Actually, high capability of tailoring the spectral distribution of the effective Brillouin resonance makes possible to resolve partially or completely all those problems. First of all, a broadband spectral window was passively obtained in between two Brillouin gain/loss resonances by simply appending two segments of fibers showing different Brillouin frequency shifts. The global Brillouin gain of the concatenated fibers manifests a gain/loss doublet resonance showing a broad window in between gain/loss peaks. In practice, this configuration has a crucial advantage that it removes the need of the pump modulation, generally used to create a polychromatic pump source. Therefore, a broadband Brillouin slow and fast light was simple realized with a reduced distortion. Secondly, the signal amplification or attenuation associated to the signal delay was completely compensated by superposing Brillouin gain and loss resonances with identical depth but different width. As a result, the Brillouin gain led to effectively a spectral hole in the center of the broadband absorption and opened a transparent window while the sharp change in the refractive index was preserve. This way it makes possible to realize zero-gain Brillouin slow light. This configuration was also exploited to produce Brillouin fast light with a total absence of signal loss, simply by swapping the spectral position of the two pumps. At last, a signal was continuously delayed through a Brillouin fiber delay line without any distortion. Due to the strong induced dispersion, pulse broadening is a major difficulty in all slow light systems and it is impossible to compensate such broadening using a linear system. Therefore, a conventional Brillouin slow light system was combined with a nonlinear optical fiber loop mirror that gives a nonlinear quadratic transmission. Using this configuration, the inevitable pulse broadening was completely compensated at the output of the loop and a signal was delayed up to one symbol without any distortion. Brillouin slow light systems were further studied in the spectral domain. For a given Brillouin resonance the spectrum of a light pulse was optimized to better match the Brillouin bandwidth. When the time envelope of a pulse was properly shaped, it was clearly observed that the spectral width of the pulse became minimized while preserving the pulse duration. This way the maximum time delay through Brillouin slow light could be enhanced for a fixed pulse width. Brillouin fast light was even realized in total absence of any pump source, which is a plain requirement for the generation of Brillouin slow or fast light. This self-generated delay line, key contribution of this thesis, relies on both spontaneous and stimulated Brillouin scattering in optical fibers. In this implementation, a light signal was strongly boosted above the so-called Brillouin threshold, so that most power of the signal was transferred to a backward propagating wave, namely the generation of amplified spontaneous Stokes wave. Since the center frequency of the intense Stokes wave is below the signal frequency by exactly Brillouin shift of the fiber used, this wave led to a Brillouin loss band centered at the signal frequency. Consequently, the signal experienced fast light propagation and the propagation velocity of the signal was self-controlled, simply by varying the signal input power. This technique has many practical advantages such as its high simplicity of the configuration and an invariant signal power in the output of this delay line. Additionally, this system self-adapts the signal bandwidth as the spectrum of the amplified Stokes wave matches the spectral distribution of the signal. An alternative method to generate all-optical delay line was proposed instead of slow light. This scheme makes use of the combination of wavelength conversion and group velocity dispersion. This type of delay line was mainly aimed at improving the storage capability of delaying element. The wavelength of a signal was simply and efficiently converted at a desired wavelength using cross gain modulation in semiconductor optical amplifiers. Then the converted signal was delivered to a high dispersive medium and arrived at the end of the medium with relative time delay due to the group velocity dispersion. A fractional delay of 140 was continuously produced through this delay line for a signal with a duration of 100 ps, preserving signal bandwidth and wavelength. The effect of slow light on linear interactions between light and matter was experimentally investigated to clarify the current scientific argument regarding slow light-enhanced Beer-Lambert absorption. It was predicted that real slowing of the light group velocity could enhance the molecular linear absorptions so as to improve the sensitivity of this type of sensing. However, the experimental results unambiguously show that material slow light (slow light in traveling wave media) does not enhance the Beer-Lambert absorption.
Nonlinear optical processes are used in biological microscopy to surpass the diffraction limit on resolution, image deeper into brain tissues, and identify biomolecules without exogenous labels. These techniques typically require high optical intensities to increase the strength of the nonlinear interactions, which can perturb native biochemistry and damage or kill living samples. Stimulated Raman scattering (SRS) microscopy visualizes the spatial distribution of molecules using a nonlinear interaction between light and chemically specific molecular vibrations. However, the detection of biomolecules at low concentrations is limited by the total photon dose that can be applied before photodamage alters the sample, and photon shot noise sets the minimum achievable noise floor for most microscopes. Here we demonstrate a cavity-enhanced SRS microscope that is more sensitive than an equivalent conventional SRS microscope by up to 8.3(7) dB in spectroscopy and 8.6(1) dB in cell imaging. These results approach quantum limits on sensitivity and demonstrate that quantum states of light are sufficient but not necessary to enhance the sensitivity of microscopy techniques that are limited by photodamage.
Spectral imaging techniques extract spectral information using dispersive elements in combination with optical microscopes. For rapid acquisition, multiplexing spectral information along one dimension of imaged pixels has been demonstrated in hyperspectral imaging, as well as in Raman and Brillouin imaging. Full-field spectroscopy, i.e., multiplexing where imaged pixels are collected in 2D simultaneously while spectral analysis is performed sequentially, can increase spectral imaging speed, but so far has been attained at low spectral resolutions. Here, we extend 2D multiplexing to high spectral resolutions of ∼80 MHz (∼0.0001 nm) using high-throughput spectral discrimination based on atomic transitions.