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.
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.
In optical microscopy, as in any biological measurement performed with light, there is a compromise between increasing power to improve resolution and decreasing power to prevent photodamage to the biological structure. Quantum optics allows this compromise to be overcome, enabling improved precision without increased illumination intensity. This thesis demonstrates, for the first time, absolute quantum advantage in microscopy, with a specific focus on Stimulated Raman Scattering (SRS) microscopy.SRS microscopy is a powerful tool widely used in many fields, especially in biomedical sciences. It uses a two-photon process where the light is downshifted in frequency by being absorbed and subsequently inelastically scattered by the intrinsic molecular vibrations in the sample. Each element or molecule has a different response, allowing selective imaging of biological systems. SRS microscopy is particularly useful because it is label-free (no addition of dyes, beads or any implanted tracer), meaning that imaging contrast is achieved without modification of, or interference with, the specimen. Here I present our newly built state-of-the-art shot-noise limited SRS microscope. I also characterize the Raman microscope, describe the developped imaging system and show SRS spectroscopy and imaging of both polystyrene beads and living yeast cells.The quantisation of light introduces a fundamental uncertainty, or quantum noise, in both the amplitude and phase quadratures of an optical field. A squeezed state of light is a state of the optical field where the uncertainty of one of these quadratures is reduced below the quantum noise. Therefore, it provides an approach to reduce the noise level below this quantum noise limit and consequently increases measured signal-to-noise ratio. Here I describe the concepts involved in this technique and present our newly built light squeezer. I characterize the squeezing and show up to -1.3 dB of noise floor reduction for a 6 picosecond pulsed laser, which is the pulse duration that maximises the Raman interaction.In this thesis we apply squeezed states of light in SRS, developping a quantum-enhanced micro- scope. Our SRS Raman intensity noise is ∆I/I ≈ 10^(−7) and the measured quantum efficiency of the built photodiode is η = 72%. In this proof of principle we show a reduction of the noise floor of at least 13% below the quantum noise limit for Raman imaging of polystyrene beads and living yeast cells. This enhancement allowed us to resolve the cell membrane where with conventional microscope would be unresolvable. It also allowed sub-micron spatial resolution. The improved image contrast and reduced imaging time, while avoiding photodamage in the sample, surpasses the current state-of-the-art Raman microscopes and opens opportunities for new discoveries in biophysics.
Stimulated Raman scattering (SRS) microscopy is a powerful label-free imaging technique that probes the vibrational response of chemicals with high specificity and sensitivity. High-power, quantum-enhanced SRS microscopes have been recently demonstrated and applied to polymers and biological samples. Quantum correlations, in the form of squeezed light, enable the microscopes to operate below the shot noise limit, enhancing their performance without increasing the illumination intensity. This addresses the signal-to-noise ratio (SNR) and speed constraints introduced by photodamage in shot noise-limited microscopes. Previous microscopes have either used single-beam squeezing, but with insufficient brightness to reach the optimal ratio of pump-to-Stokes intensity for maximum SNR, or have used twin-beam squeezing and suffered a 3 dB noise penalty. Here we report a quantum-enhanced Raman microscope that uses a bright squeezed single-beam, enabling operation at the optimal efficiency of the SRS process. The increase in brightness leads to multimode effects that degrade the squeezing level, which we partially overcome using spatial filtering. We apply our quantum-enhanced SRS microscope to biological samples and demonstrate quantum-enhanced multispectral imaging of living cells. The imaging speed of 100×100 pixels in 18 seconds allows the dynamics of cell organelles to be resolved. The SNR achieved is compatible with video-rate imaging, with the quantum correlations yielding a 20% improvement in imaging speed compared to shot noise-limited operation.
Stimulated Raman scattering (SRS) microscopy has emerged as a powerful technique for probing the spatiotemporal dynamics of molecular bonds with exceptional sensitivity, resolution, and speed. However, classically, its performance remains fundamentally constrained by optical shot noise, which imposes a strict limit on detection sensitivity and speed. Here, we demonstrate a quantum-enhanced SRS microscopy platform that circumvents this barrier by harnessing amplitude-squeezed light. Specifically, we generate a Stokes beam with 5.2dB of amplitude squeezing using traveling-wave optical parametric amplification in second-order nonlinear waveguides, and combine it with a tunable coherent pump to access vibrational modes spanning from 1000 to 3100cm−1. Applied to quantum imaging of metabolites in biological tissue (pork muscle), our quantum-enhanced Raman microscope achieves an average noise suppression of 3.6dB and a 51% enhancement in signal-to-noise ratio (SNR) -- to the best of our knowledge, the largest improvement reported to date in quantum-enhanced SRS microscopy of biological samples.
Stimulated Raman scattering (SRS) microscopy is a powerful technique that allows for the visualization of molecular vibrational images for label-free imaging, metabolic imaging, and supermultiplex imaging. However, its sensitivity is mainly limited by the shot noise of laser pulses, and long acquisition times are sometimes required to detect weak SRS signals hidden by the shot noise. To overcome this limitation, quantum-enhanced (QE-) SRS microscopy has been demonstrated, while the low optical power of squeezed pulses limits the sensitivity. In this work, we present QE-SRS microscopy using quantum-enhanced balanced detection (QE-BD) scheme, where the sensitivity of balanced detection SRS microscope is enhanced by injecting squeezed vacuum, allowing for QE-SRS imaging with high power SRS pump pulses (typically several tens of milliwatt), while balanced detection causes 3 dB drawback in the signal-to-noise ratio. We experimentally demonstrate QE-SRS imaging with 2.6 dB noise reduction compared with shot-noise-limited balanced detection SRS. We also demonstrate hyperspectral QE-SRS imaging by fast wavelength tuning of Stokes pulses. These results show the potential feasibility of high-power QE-SRS whose sensitivity is beyond that of classical shot-noise-limited SRS microscopes.
We investigate the optimal pump intensity in quantum-enhanced stimulated Raman scattering (SRS) microscopy using an asymmetric beamsplitter. Specifically, we derive the intensity that maximizes the signal-to-noise ratio (SNR) under the condition where the total optical intensity of the pump and Stokes pulses is constrained to prevent photodamage, and under the condition without such constraints. In the absence of power constraints, the highest SNR is achieved by simply maximizing the pump intensity without using squeezed light. Under power constraints, the optimal pump intensity in classical SRS is known to be one-third of the photodamage threshold. We reveal that, in the quantum-enhanced regime, the optimal pump intensity is lower than the classical optimum, and as the squeezing level increases, this deviation from the classical optimum grows more pronounced. Our analysis provides valuable guidelines for achieving high-sensitivity SRS imaging with quantum enhancement.
Quantum-enhanced stimulated Raman scattering (QE-SRS) microscopy sits at the intersection of biomedical imaging, laser microscopy, and quantum optics toward realizing sensitive molecular-vibrational imaging. This Perspective explores the current status of this emerging field, integrating viewpoints from these diverse disciplines to highlight its potential for advancing biological and medical imaging. We discuss key challenges and future directions in leveraging quantum light to push the sensitivity limits of SRS microscopy, paving the way for next-generation imaging modalities.