An all-optical technique enables instantaneous single-shot demodulation of images at high frequency

Multiple-wavelength range-gated active imaging applied to the evaluation of simultaneous movement of millimeter-size objects moving in a given volume

The combined multiple-wavelength range-gated active imaging (WRAI) principle is able to determine the position of a moving object in a four-dimensional space and to deduce its trajectory and its speed independently of the video frequency. However, when the scene size is reduced and the objects have a millimeter size, the temporal values intervening on the depth of the visualized zone in the scene cannot be reduced further because of technological limitations. To improve the depth resolution, the illumination type of the juxtaposed style of this principle has been modified. Therefore, it was important to evaluate this new context in the case of millimeter-size objects moving simultaneously in a reduced volume. Based on the rainbow volume velocimetry method, the combined WRAI principle was studied in accelerometry and velocimetry with four-dimensional images of millimeter-size objects. This basic principle combining two wavelength categories determines the depth of moving objects in the scene with the warm color category and the precise moment of moving objects position with the cold color category. The difference in this new, to the best of our knowledge, method is at the level of the scene illumination, which is obtained transversally by a pulsed light source having a wide spectral band limited in warm colors in order to get a better depth resolution. For cold colors, the illumination with pulsed beams of distinct wavelengths remains unchanged. Thus, it is possible independently of the video frequency to know from a single recorded image the trajectory, the speed, and the acceleration of millimeter-size objects moving simultaneously in 3D space and also the chronology of their passages. The experimental tests validated this modified multiple-wavelength range-gated active imaging method and confirmed the possibility of avoiding confusion when the object trajectories intersect.

Time-modulated excitation for enhanced single-molecule localization microscopy

Structured illumination in single-molecule localization microscopy provides new information on the position of molecules and thus improves the localization precision compared to standard localization methods. Here, we used a time-shifted sinusoidal excitation pattern to modulate the fluorescence signal of the molecules whose position information is carried by the phase and recovered by synchronous demodulation. We designed two flexible fast demodulation systems located upstream of the camera, allowing us to overcome the limiting camera acquisition frequency and thus to maximize the collection of photons in the demodulation process. The temporally modulated fluorescence signal was then sampled synchronously on the same image, repeatedly during acquisition. This microscopy, called ModLoc, allows us to experimentally improve the localization precision by a factor of 2.4 in one direction, compared to classical Gaussian fitting methods. A temporal study and an experimental demonstration both show that the short lifetimes of the molecules in blinking regimes impose a modulation frequency in the kilohertz range, which is beyond the reach of current cameras. A demodulation system operating at these frequencies would thus be necessary to take full advantage of this new localization approach.

This article is part of the Theo Murphy meeting issue 'Super-resolution structured illumination microscopy (part 2)'.

Modal decomposition of complex optical fields using convolutional neural networks

Recent studies have shown convolutional neural networks (CNNs) can be trained to perform modal decomposition using intensity images of optical fields. A fundamental limitation of these techniques is that the modal phases cannot be uniquely calculated using a single intensity image. The knowledge of modal phases is crucial for wavefront sensing, alignment, and mode matching applications. Heterodyne imaging techniques can provide images of the transverse complex amplitude and phase profiles of laser beams at high resolutions and frame rates. In this work, we train a CNN to perform modal decomposition using simulated heterodyne images, allowing the complete modal phases to be predicted. This is, to our knowledge, the first machine learning decomposition scheme to utilize complex phase information to perform modal decomposition. We compare our network with a traditional overlap integral and center-of-mass centering algorithm and show that it is both less sensitive to beam centering and on average more accurate in our simulated images.

Resonant Electro-Optic Imaging for Microscopy at Nanosecond Resolution

We demonstrate an electro-optic wide-field method to enable fluorescence lifetime microscopy (FLIM) with high throughput and single-molecule sensitivity. Resonantly driven Pockels cells are used to efficiently gate images at 39 MHz, allowing fluorescence lifetime to be captured on standard camera sensors. Lifetime imaging of single molecules is enabled in wide field with exposure times of less than 100 ms. This capability allows combination of wide-field FLIM with single-molecule super-resolution localization microscopy. Fast single-molecule dynamics such as FRET and molecular binding events are captured from wide-field images without prior spatial knowledge. A lifetime sensitivity of 1.9 times the photon shot-noise limit is achieved, and high throughput is shown by acquiring wide-field FLIM images with millisecond exposure and >10⁸ photons per frame. Resonant electro-optic FLIM allows lifetime contrast in any wide-field microscopy method.

Differential wavefront sensing and control using radio-frequency optical demodulation

Differential wavefront sensing is an essential technique for optimising the performance of many precision interferometric experiments. Perhaps the most extensive application of this is for alignment sensing using radio-frequency beats measured with quadrant photodiodes. Here we present a new technique that uses optical demodulation to measure such optical beats at high resolutions using commercial laboratory equipment. We experimentally demonstrate that the images captured can be digitally processed to generate wavefront error signals and use these in a closed loop control system for correct wavefront errors for alignment and mode-matching a beam into an optical cavity to 99.9%. This experiment paves the way for the correction of even higher order errors when paired with higher order wavefront actuators. Such a sensing scheme could find use in optimizing complex interferometers consisting of coupled cavities, such as those found in gravitational wave detectors, or simply just for sensing higher order wavefront errors in heterodyne interferometric table-top experiments.

Photothermal Microscopy: Imaging the Optical Absorption of Single Nanoparticles and Single Molecules

The photothermal (PT) signal arises from slight changes of the index of refraction in a sample due to absorption of a heating light beam. Refractive index changes are measured with a second probing beam, usually of a different color. In the past two decades, this all-optical detection method has reached the sensitivity of single particles and single molecules, which gave birth to original applications in material science and biology. PT microscopy enables shot-noise-limited detection of individual nanoabsorbers among strong scatterers and circumvents many of the limitations of fluorescence-based detection. This review describes the theoretical basis of PT microscopy, the methodological developments that improved its sensitivity toward single-nanoparticle and single-molecule imaging, and a vast number of applications to single-nanoparticle imaging and tracking in material science and in cellular biology.