Publications

Transmission electron microscopy (TEM) of vitrified biological macromolecules (cryo-EM) is limited by the weak phase contrast signal that is available from such samples. Using a phase plate would thus substantially improve the signal-to-noise ratio. We have previously demonstrated the use of a high-power Fabry–Perot cavity as a phase plate for TEM. We now report improvements to our laser cavity that allow us to achieve record continuous wave intensities of over 450 GW/cm2, sufficient to produce the optimal 90° phase shift for 300 keV electrons. In addition, we have performed the first cryo-EM reconstruction using a laser phase plate, demonstrating that the stability of this laser phase plate is sufficient for use during standard cryo-EM data collection.

Stochastic distortion of light beams in scattering samples makes in-depth photoexcitation in brain tissue a major challenge. A common solution for overcoming scattering involves adaptive pre-compensation of the unknown distortion(1-3). However, this requires long iterative searches for sample-specific optimized corrections, which is a problem when applied to optical neurostimulation where typical timescales in the system are in the millisecond range. Thus, photoexcitation in scattering media that is independent of the properties of a specific sample would be an ideal solution. Here, we show that temporally focused two-photon excitation(4) with generalized phase contrast(5) enables photoexcitation of arbitrary spatial patterns within turbid tissues with remarkable robustness to scattering. We demonstrate three-dimensional confinement of tailored photoexcitation patterns >200 mu m in depth, both in numerical simulations and through brain slices combined with patch-clamp recording of photoactivated channelrhodopsin-2.

Utilizing quantum properties of light to break the diffraction limit has been the goal of intense research in the recent years. This paper is a progress report on a study aimed at experimentally demonstrating a superresolution microscopy technique enabled by photon antibunching, a non-classical emission statistics feature exhibited by most emitters used as fluorescent markers. We find that photon antibunching gives rise to correlations that encode high spatial frequency information on the distribution of fluorescent emitters. Detecting these correlations using photon counting instrumentation in a standard fluorescence microscope setting allows for three-dimensional superresolution imaging of fluorophore stained samples. The technique provides a quantum alternative to the established superresolution tools.

Ultrafast science is inherently, due to the lack of fast enough detectors and electronics, based on nonlinear interactions. Typically, however, nonlinear measurements require significant powers and often operate in a limited spectral range. Here we overcome the difficulties of ultraweak ultrafast measurements by precision time-domain localization of spectral components. We utilize this for linear self-referenced characterization of pulse trains having similar to 1 photon per pulse, a regime in which nonlinear techniques are impractical, at a temporal resolution of similar to 10 fs. This technique does not only set a new scale of sensitivity in ultrashort pulse characterization, but is also applicable in any spectral range from the near-infrared to the deep UV.

Biological photonic systems composed of anhydrous guanine crystals evolved separately in several taxonomic groups. Here, two such systems found in fish and spiders, both of which make use of anhydrous guanine crystal plates to produce structural colors, are examined. Measurements of the photonic-crystal structures using cryo-SEM show that the crystal plates in both fish skin and spider integument are similar to 20-nm thick. The reflective unit in the fish comprises stacks of single plates alternating with similar to 230-nm-thick cytoplasm layers. In the spiders the plates are formed as doublet crystals, cemented by 30-nm layers of amorphous guanine, and are stacked with similar to 200 nm of cytoplasm between crystal doublets. They achieve light reflective properties through the control of crystal morphology and stack dimensions, reaching similar efficiencies of light reflectivity in both fish skin and spider integument. The structure of guanine plates in spiders are compared with the more common situation in which guanine occurs in the form of relatively unorganized prismatic crystals, yielding a matt white coloration.

Solution of many problems of gas kinetics, transport phenomena, and spectroscopy can be significantly simplified using the linearized Boltzmann equation within so called one-dimensional approximation, in which the collision kernel is averaged over the transverse velocities. The error introduced by this approximation is usually considered to be small, however, no quantitative analysis of its value has been done so far. We study the accuracy of the one-dimensional approximation in a particular problem of calculating spectral line shapes in gases. The inaccuracy is shown to be small in the wings of line, in the cases of light buffer molecules or low collision frequency. For arbitrary parameters the error is calculated numerically within the rigid-sphere model. These results outline the area of applicability of computationally economic one-dimensional approach.

The Coulomb corrections (CC) to the processes of bremsstrahlung and pair production are investigated. The next-to-leading term in the high-energy asymptotics is found. This term becomes very essential in the region of intermediate energies. The influence of screening for CC is small for differential cross section, spectrum, and the total cross section of pair production. The same is true for the spectrum of bremsstrahlung, but not for the differential cross section, where the influence of screening can be very large. The corresponding screening corrections as well as the modification of the differential cross section of bremsstrahlung are found. A comparison of our results for the total cross section of pair production with the experimental data available is performed. This comparison has justified our analytical results and allowed to elaborate a simple ansatz for the next-to-leading correction. The influence of the electron beam shape on CC for bremsstrahlung is investigated. It turns out that the differential cross section is very sensitive to this shape.