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We propose to enhance the performance of localized plasmon structured illumination microscopy (LP-SIM) via intensity correlations. LP-SIM uses sub-wavelength illumination patterns to encode high spatial frequency information. It can enhance the resolution up to three-fold before gaps in the optical transfer function (OTF) support arise. For blinking fluorophores or for quantum antibunching, an intensity correlation analysis induces higher harmonics of the illumination pattern and enlarges the effective OTF. This enables ultrahigh resolutions without gaps in the OTF support, and thus a fully deterministic imaging scheme. We present simulations that include shot and external noise and demonstrate the resolution power under realistic photon budgets. The technique has potential in light microscopy where low-intensity illumination is paramount while aiming for high spatial but moderate temporal resolutions.

Ischemic cerebral stroke is one of the leading causes of death and disability in humans. However, molecular processes underlying the development of this pathology remain poorly understood. There are major gaps in our understanding of metabolic changes that occur in the brain tissue during the early stages of ischemia and reperfusion. In particular, it is generally accepted that both ischemia (I) and reperfusion (R) generate reactive oxygen species (ROS) that cause oxidative stress which is one of the main drivers of the pathology, although ROS generation during I/R was never demonstrated in vivo due to the lack of suitable methods. In the present study, we record for the first time the dynamics of intracellular pH and H2O2 during I/R in cultured neurons and during experimental stroke in rats using the latest generation of genetically encoded biosensors SypHer3s and HyPer7. We detect a buildup of powerful acidosis in the brain tissue that overlaps with the ischemic core from the first seconds of pathogenesis. At the same time, no significant H2O2 generation was found in the acute phase of ischemia/reperfusion. HyPer7 oxidation in the brain was detected only 24 h later. Comparison of in vivo experiments with studies on cultured neurons under I/R demonstrates that the dynamics of metabolic processes in these models significantly differ, suggesting that a cell culture is a poor predictor of metabolic events in vivo., Highlights • The dynamics of stroke-induced changes in intracellular pH and H2O2 was recorded. • Ischemia leads to immediate acidification of the affected part of the brain. • Only slight increase in H2O2 in the brain tissue occurs during ischemia/reperfusion. • The pronounced increase in H2O2 in the brain occurs next day after I/R. • Neurons show different I/R-induced pH and H2O2 dynamics in vitro and in vivo.

High-resolution thermal and magnetic vector gradient imaging is demonstrated using diamond quantum sensors in a microstructured fiber probe platform. These sensors are capable of in situ magnetic field and temperature measurements with 160 pT / Hz and 25 mK / Hz sensitivities.

By exploiting stimulated Raman scattering in long nitrogen-filled capillary fibers, we demonstrate continuous tunability of Yb laser systems over the 1.0-1.7 μm range, with conversion efficiency up to 82%, and an up to 10-fold pulse compression.

The spectrum of atomic hydrogen has long been viewed as a Rosetta stone that bears the key to decode the writings of quantum mechanics in a vast variety of physical, chemical, and biological systems. Here, we show that, in addition to its role as a basic model of quantum mechanics, the hydrogen atom provides a fundamental building block of quantum information. Through its electron and nuclear spin degrees of freedom, the hydrogen atom is shown to lend a physically meaningful frame and a suitable Hilbert space for bipartite entanglement, whose two-qubit concurrence and quantum coherence can be expressed in terms of the fundamental physical constants -- the Planck and Boltzmann constants, electron and proton masses, the fine-structure constant, as well as the Bohr radius and the Bohr magneton. The intrinsic, natural entanglement that the hyperfine-structure (HFS) states of the H atom store at low temperatures rapidly decreases with a growth in temperature, vanishing above a $\tau_c$ $\approx$ 5.35 $\mu$eV threshold. An external magnetic field, however, can overcome this thermal loss of HFS entanglement. As one of the central findings of this work, we show that an external magnetic field can induce and sustain an HFS entanglement, against all the odds of thermal effects, at temperatures well above the $\tau_c$ threshold, thus enabling magnetic-field-assisted entanglement engineering in low-temperature gases and solids., Comment: 6 pages, 4 figures

We present experiments on cell cultures and brain slices that demonstrate two-photon optogenetic pH sensing and pH-resolved brain imaging using a laser driver whose spectrum is carefully tailored to provide the maximum contrast of a ratiometric two-photon fluorescence readout from a high-brightness genetically encoded yellow-fluorescent-protein-based sensor, SypHer3s. Two spectrally isolated components of this laser field are set to induce two-photon-excited fluorescence (2PEF) by driving SypHer3s through one of two excitation pathways - via either the protonated or deprotonated states of its chromophore. With the spectrum of the laser field accurately adjusted for a maximum contrast of these two 2PEF signals, the ratio of their intensities is shown to provide a remarkably broad dynamic range for pH measurements, enabling high-contrast optogenetic deep-brain pH sensing and pH-resolved 2PEF imaging within a vast class of biological systems, ranging from cell cultures to the living brain. This article is protected by copyright. All rights reserved.

Lateral flow assay (LFA) has long been used as a biomarker detection technique. It has advantages such as low cost, rapid readout, portability, and ease of use. However, its qualitative readout process and lack of sensitivity are limiting factors. We report a photon-counting approach to accurately quantify LFAs while enhancing sensitivity. In particular, we demonstrate that the density of SARS-CoV-2 antibodies can be quantified and measured with an enhanced sensitivity using this simple laser optical analysis.

We demonstrate a reconnectable implantable ultraslim fiber-optic microendoscope that integrates a branching fiber bundle (BFB) with gradient-index fiber lenses, enabling a simultaneous fluorescence imaging of individual cells in distinctly separate brain regions, including brain structures as distant as the neocortex and hippocampus. We show that fluorescence images of individual calcium-indicator-expressing neurons in the brain of freely moving transgenic mice can be recorded, via the implanted BFB probe, in parallel with time- and cell-resolved traces of calcium signaling, thus enabling correlated circuit-dynamics studies at -multiple sites within the brain of freely moving animals.

Ultrashort high-energy pulses at wavelengths longer than 1 µm are now desirable for a vast variety of applications in ultrafast and strong-field physics. To date, the main answer to the wavelength tunability for energetic, broadband pulses still relies on optical parametric amplification (OPA), which often requires multiple and complex stages, may feature imperfect beam quality, and has limited conversion efficiency into one of the amplified waves. In this work, we present a completely different strategy to realize an energy-efficient and scalable laser frequency shifter. This relies on the continuous red shift provided by stimulated Raman scattering (SRS) over a long propagation distance in nitrogen-filled hollow-core fibers (HCF). We show a continuous tunability of the laser wavelength from 1030 nm up to 1730 nm with a conversion efficiency higher than 70% and high beam quality. The highly asymmetric spectral broadening, arising from the spatiotemporal nonlinear interplay between higher-order modes of the HCF, can be readily employed to generate pulses ( ∼ 20 f s ) significantly shorter than the pump ones ( ∼ 200 f s ) with high beam quality, and the pulse energy can further be scaled up to tens of millijoules. We envision that this technique, coupled with the emerging high-power Yb laser technology, has the potential to answer the increasing demand for energetic multi-TW few-cycle sources tunable in the near-IR.

We present an optical scientist vision for a photonic platform for real-time polymerase chain reaction (qPCR) and reverse-transcription qPCR (qRT-PCR) that integrates a rapid-cycle fiber-optic PCR chamber with feedback-controlled laser heating, online fluorometric detection of PCR products, and all-optical thermometry using an optical readout from color centers of diamond nanoparticles. We show that, with a careful optimization of laser-induced heating and heat-removal geometry, this approach can radically shorten the qPCR thermal cycle relative to standard PCR systems, paving the way toward credit-card-size testing kits for fast point-of-care infection diagnosis. Faster photonic qPCR/qRT-PCR will enable a rapid and highly specific early detection of infections, including new viruses, such as SARS-CoV-2 at very early stages-in most cases, even before the onset of disease, providing much-needed lead time for early treatment and disease spread prevention.

Remarkable advancements in the ability to create, manipulate, and measure quantum systems are paving the way to build next generations of devices based on quantum physics. Quantum technologies in Russia are on the list of strategically important cross-cutting directions in the framework of the National Technology Initiative programs and the Digital Economy National Program. The broad focus includes quantum computing and simulation, quantum communications, quantum metrology and sensing. This paper reviews existing research on quantum science and technologies in Russia and summarizes the main goals for the next few years that form the basis of an upcoming major national initiative.

Electron tunneling induced by a strong-field laser driver can lead to an ultrafast stepwise buildup of the photoelectron density \ensuremath{\rho}. When the laser field is strong enough, each such step in the temporal profile of \ensuremath{\rho} is confined well within the field cycle, providing an ultrabroadband, multidecade force that drives the photoelectron current $j$. However, whether or not the photocurrent can emit electromagnetic radiation with such an extraordinarily broad spectrum depends on the damping, which defines the low-frequency cutoff in the spectrum of this radiation. We show that, with a suitable choice of the gas pressure and parameters of the laser driver, the laser-induced tunneling photocurrent can serve as a source of a remarkably broadband electromagnetic radiation with a multidecade spectrum stretching from the vacuum ultraviolet all the way down to the microwave range. We demonstrate that the supercontinuum fields emitted by individual photocurrent steps, induced by different field half cycles, can coherently combine, giving rise to well-resolved high-order harmonics on the high-frequency end of the spectrum yoked to a bright midinfrared to microwave supercontinuum, dominating the long-wavelength part of the multidecade radiation output.

At the level of peak powers needed for a Kerr-lens mode-locked operation of solid-state soliton short-pulse lasers, a periodic perturbation induced by spatially localized pulse amplification in a laser cavity can induce soliton instability with respect to resonant dispersive-wave radiation, eventually leading to soliton blowup and pulse splitting of the laser output. Here, we present an experimental study of a high-peak-power self-mode-locking Cr:forsterite laser, showing that, despite its complex, explosion-like buildup dynamics, this soliton blowup can be captured and quantitatively characterized via an accurate cavity-dispersion- and gain-resolved analysis of the laser output. We demonstrate that, with a suitable cavity design and finely tailored balance of gain, dispersion, and nonlinearity, such a laser can be operated in a subcritical mode, right beneath the soliton blowup threshold, providing an efficient source of sub-100-fs 15–20 MHz repetition-rate pulses with energies as high as 33 nJ.

All-optical thermometry presents a promising avenue of investigation for sensitive biological and electronic systems, with methods free of radio-frequency interference of particular importance. In this work, an all-optical scanning fiber thermometer is tested and shown to possess $5\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ spatial and $20\phantom{\rule{0.2em}{0ex}}\mathrm{mK}/\sqrt{\mathrm{Hz}}$ temperature resolution. This probe is then used to generate noncontact scanning thermal images of active circuit elements with microscale thermal features.

We present an information-geometric perspective on a generic spectral-analysis task pertaining to a vast class of optical measurements in which a parameter θ needs to be evaluated from θ-dependent spectral features in a measurable optical readout. We show that the spectral shift and line broadening driven by small Δθ variations can be isolated as orthogonal components in a Pythagorean-theorem extension for a Euclidean distance in the space of probability distributions, representing the Δθ-induced information gain, expressible via the relative entropy and the pertinent Fisher information. This result offers important insights into the limits of optical signal analysis, as well as into the ultimate spectral resolution and the limiting sensitivity of a vast class of optical measurements. As one example, we derive a physically transparent closed-form analytical solution for the information-theory bound on the precision of all-optical temperature sensors based on color centers in diamond.

Thermogenetics is a promising innovative neurostimulation technique, which enables robust activation of single neurons using thermosensitive cation channels and IR stimulation. The main advantage of IR stimulation compared to conventional visible light optogenetics is the depth of penetration (up to millimeters). Due to physiological limitations, thermogenetic molecular tools for mammalian brain stimulation remain poorly developed. Here, we tested the possibility of employment of this new technique for stimulation of neocortical neurons. The method is based on activation gating of TRPV1-L channels selectively expressed in specific cells. Pyramidal neurons of layer 2/3 of neocortex were transfected at an embryonic stage using a pCAG expression vector and electroporation in utero. Depolarization and spiking responses of TRPV1L+ pyramidal neurons to IR radiation were recorded electrophysiologically in acute brain slices of adult animals with help of confocal visualization. As TRPV1L-expressing neurons are not sensitive to visible light, there were no limitations of the use of this technique with conventional fluorescence imaging. Our experiments demonstrated that the TRPV1-L+ pyramidal neurons preserve their electrical excitability in acute brain slices, while IR radiation can be successfully used to induce single neuronal depolarization and spiking at near physiological temperatures. Obtained results provide important information for adaptation of thermogenetic technology to mammalian brain studies in vivo.

The signal-to-noise contrast of the optical readout in broadband Raman quantum memories is analyzed as a function of the pulse widths and phase properties of tailored optical field waveforms used to write in and read out broadband photon wave packets. Based on this analysis, we quantify the tradeoff between the readout contrast and the speed of such memories. Off-resonance coherent four-wave mixing is shown to provide a source of noise photons, lowering the readout contrast in broadband Raman quantum memories. This noise cannot be suppressed by phase matching, but can be radically reduced with a suitable polarization arrangement and proper field-waveform tailoring.

We perform a detailed theoretical and experimental investigation of supercontinuum generation in large-mode-area photonic crystal fibers pumped by a high-energy, high-repetition rate picosecond Nd:YVO4 laser, with the goal of using it as the Stokes beam in coherent anti-Stokes Raman scattering setup. We analyze the influence of fiber structure and length on the supercontinuum power, spectral shape, and group delay dispersion. We identify the experimental conditions for stable supercontinuum generation, with microjoule-level pulse energy and the spectrum extending beyond 1600 nm, which allows excitation of Raman frequencies up to 3000 cm−1 and beyond. We demonstrate reliable and efficient operation of a coherent anti-Stokes Raman spectroscopy and microscopy setup using this supercontinuum source.

We demonstrate a multicompartment, power-scalable beam-line design in which sub-40-fs, terawatt field waveforms are tailored in space and time for downstream pulse compression to few-cycle pulse widths attained right at the site where the intense laser field interacts with a target. An accurate field-waveform characterization performed at multiple locations along the beam path shows that, despite all the complexity of their upstream nonlinear electrodynamics, field waveforms shorter than three field cycles can be delivered to a laser–matter interaction site in such a system, enabling a vast class of ultrafast strong-field laser–matter interaction studies.

We present high-resolution, all-optical thermometry based on ensembles of germanium-vacancy (GeV) color center in diamond and implement this method of thermometry in the fiber-optic format. Due to the unique properties of diamond, an all-optical approach using this method opens a way to produce back-action-free temperature measurements with resolution below 0.1 K in a wide range of temperatures.

A resonator–qubit system consisting of three resonators and two qubits is shown to enable a perfect swap and a faithful transfer of arbitrary unknown quantum states of photons, providing a much-needed building block for quantum information processing and quantum communication.

Optical field waveforms with an ultrabroad spectrum and a tailored phase are shown to enable selective excitation, coherent control, and attosecond spectrochronography of electron subshells in many-electron atomic systems. Analysis of the evolution of the density matrix of electron subshells in an atomic system driven by an ultrashort light pulse shows that the interference of different quantum pathways of electron dynamics plays a key role in the buildup of the nonlinear-optical response of such a system. Our analysis suggests a method whereby the attosecond dynamics of individual electron subshells in atoms can be coherently controlled with ultrashort laser pulses.

Thermogenetics is a promising innovative neurostimulation technique, which enables robust activation of neurons using thermosensitive transient receptor potential (TRP) cation channels. Broader application of this approach in neuroscience is, however, hindered by a limited variety of suitable ion channels, and by low spatial and temporal resolution of neuronal activation when TRP channels are activated by ambient temperature variations or chemical agonists. Here, we demonstrate rapid, robust and reproducible repeated activation of snake TRPA1 channels heterologously expressed in non-neuronal cells, mouse neurons and zebrafish neurons in vivo by infrared (IR) laser radiation. A fibre-optic probe that integrates a nitrogen−vacancy (NV) diamond quantum sensor with optical and microwave waveguide delivery enables thermometry with single-cell resolution, allowing neurons to be activated by exceptionally mild heating, thus preventing the damaging effects of excessive heat. The neuronal responses to the activation by IR laser radiation are fully characterized using Ca2+ imaging and electrophysiology, providing, for the first time, a complete framework for a thermogenetic manipulation of individual neurons using IR light., Current approaches to thermogenetic manipulation of neuronal activity lack sufficient spatiotemporal resolution. Here the authors show that neurons expressing snake TRPA1 channels are activated at high temporal resolution with IR light and this technique can be used to elicit behaviour in zebrafish larvae.

We present experimental studies of long-distance transmission of ultrashort mid-infrared laser pulses through atmospheric air, probing air dispersion in the 3.6–4.2-μm wavelength range. Atmospheric air is still highly transparent to electromagnetic radiation in this spectral region, making it interesting for long-distance signal transmission. However, unlike most of the high-transmission regions in gas media, the group-velocity dispersion, as we show in this work, is anomalous at these wavelengths due to the nearby asymmetric-stretch rovibrational band of atmospheric carbon dioxide. The spectrograms of ultrashort mid-infrared laser pulses transmitted over a distance of 60 m in our experiments provide a map of air dispersion in this wavelength range, revealing clear signatures of anomalous dispersion, with anomalous group delays as long as 1.8 ps detected across the bandwidth covered by 80-fs laser pulses.

We present brain imaging experiments on rat cortical areas, demonstrating that, when combined with a suitable high-brightness, cell-specific genetically encoded fluorescent marker, three-photon-excited fluorescence (3PEF), enables subcellular-resolution, cell-specific 3D brain imaging that is fully compatible and readily integrable with other nonlinear-optical imaging modalities, including two-photon-fluorescence and harmonic-generation microscopy. With laser excitation provided by sub-100-fs, 1.25-µm laser pulses, cell-specific 3PEF from astrocytes and their processes detected in parallel with a three-photon-resonance-enhanced third harmonic from blood vessels is shown to enable a high-contrast 3D imaging of gliovascular interfaces.

We demonstrate an accurate quantitative characterization of absolute two- and three-photon absorption (2PA and 3PA) action cross sections of a genetically encodable fluorescent marker Sypher3s. Both 2PA and 3PA action cross sections of this marker are found to be remarkably high, enabling high-brightness, cell-specific two- and three-photon fluorescence brain imaging. Brain imaging experiments on sliced samples of rat's cortical areas are presented to demonstrate these imaging modalities. The 2PA action cross section of Sypher3s is shown to be highly sensitive to the level of pH, enabling pH measurements via a ratiometric readout of the two-photon fluorescence with two laser excitation wavelengths, thus paving the way toward fast optical pH sensing in deep-tissue experiments.

We present an energy-efficient and scalable technique to realize energetic sub-100-fs pulses, continuously tunable in the 1030÷1730 nm range, by exploiting stimulated Raman scattering in long N2-filled hollow-core fibers.

We present a time-domain Schmidt-mode analysis of a broadband continuous-variable entanglement of photon pairs generated via a vectorial four-wave mixing (FWM) of ultrashort laser pulses in a highly nonlinear birefringent optical fiber. We demonstrate that the time-domain eigenmodes of high-purity two-photon states generated through vectorial FWM can be steered, by varying the pump wavelength and FWM polarization geometry, from a high-purity entangled ket to a high-entropy entangled state in a space of a very high dimensionality. Moreover, this pulse-mode analysis is shown to provide a clear physical perspective on how the entanglement structure of two-photon states builds up as a result of short-pulse FWM dynamics. This insight reveals a correspondence-type relation between the quantum and classical pictures of photon-pair generation. With an eye on practical applications, a clear understanding of the temporal profile of pulse modes representing high-purity two-photon states is central to a meaningful shaping of ultrashort photon-packet waveforms for super-resolving microscopy and multiphoton spectroscopy using quantum states of light.

Methods of nonlinear optics provide a vast arsenal of tools for label-free brain imaging, offering a unique combination of chemical specificity, the ability to detect fine morphological features, and an unprecedentedly high, subdiffraction spatial resolution. While these techniques provide a rapidly growing platform for the microscopy of neurons and fine intraneural structures, optical imaging of astroglia still largely relies on filament-protein-antibody staining, subject to limitations and difficulties especially severe in live-brain studies. Once viewed as an ancillary, inert brain scaffold, astroglia are being promoted, as a part of an ongoing paradigm shift in neurosciences, into the role of a key active agent of intercellular communication and information processing, playing a significant role in brain functioning under normal and pathological conditions. Here, we show that methods of nonlinear optics provide a unique resource to address long-standing challenges in label-free astroglia imaging. We demonstrate that, with a suitable beam-focusing geometry and careful driver-pulse compression, microscopy of second-harmonic generation (SHG) can enable a high-resolution label-free imaging of fibrillar structures of astrocytes, most notably astrocyte processes and their endfeet. SHG microscopy of astrocytes is integrated in our approach with nonlinear-optical imaging of red blood cells based on third-harmonic generation (THG) enhanced by a three-photon resonance with the Soret band of hemoglobin. With astroglia and red blood cells providing two physically distinct imaging contrasts in SHG and THG channels, a parallel detection of the second and third harmonics enables a high-contrast, high-resolution, stain-free stereoimaging of gliovascular interfaces in the central nervous system. Transverse scans of the second and third harmonics are shown to resolve an ultrafine texture of blood-vessel walls and astrocyte-process endfeet on gliovascular interfaces with a spatial resolution within 1 μm at focusing depths up to 20 μm inside a brain.

We present a physical model that enables a detailed quantitative analysis of propagation of broadband low-frequency electromagnetic field waveforms through the atmosphere. Our analysis reveals broadband atmospheric transparency regions within which the group-velocity of atmospheric air is unusually weak, enabling distortion-free long-distance transmission of broadband low-frequency electromagnetic pulses.

We demonstrate stain-free, high-contrast, subcellular-resolution imaging of astroglial cells using epi-detected third-harmonic generation (THG). The astrocyte-imaging capability of THG is verified by colocalizing THG images with fluorescence images of astrocytes expressing a genetically encodable fluorescent reporter. We show that THG imaging with an optimized point-spread function can reliably detect significant subcellular features of astrocytes, including cell nuclei, as well as the soma shape and boundaries.

A generation of optical harmonics by circularly polarized light, which is in general forbidden in an isotropic medium, was recently observed in molecular gases where molecules were aligned and oriented by another linearly polarized laser pulse [1] or by pulses with twisted polarization [2]. In order to generate nth-harmonic with circularly polarized light, a molecule has to "absorb" n photons, having the same handedness, and to emit a single nth harmonic photon while fulfilling the energy and the momentum conservation laws, meaning a necessity to excite the molecule to appropriate rotational state. One can do this by means of intrapulse rotational Raman scattering, which prepares the molecule in the excited state through a subsequent absorption of (n-1) same-handed photons [1]. All previously reported experimental studies on circular harmonic generation in molecular gases were performed with visible/near-IR drivers, in either gas jets or in standing cells and with pre-alignment of molecules by an additional laser pulses. Here we report on the studies of self-action of elliptically polarized mid-IR pulses on the generation of low-order harmonics in filaments ignited in air and in argon, representing a molecular and an atomic gas respectively. The harmonics were driven by 20-mJ, sub-100 fs mid-IR pulses, centred at 3.9 pm wavelength. In the experiments filamentation was assisted by focusing light with a spherical mirror of ROC=-2500 mm into an open-end tube, through which Ar gas was flown at slight overpressure. Polarization of the pulses was controlled by a broadband quarter wave plate (QWP). Spectra of 5th and 7th harmonics were recorded after the filaments with a grating spectrometer (HR4000, OceanOptics). Polarization state of the generated harmonics was analysed with a Glan-Taylor polarizer.

High-order harmonic generation (HHG) is one of the central effects in strong-field nonlinear-optical physics [1]. Within the past two decades, HHG has been at the heart of a paradigm-shifting breakthrough to attosecond physics [2]. Much less limelight has been given to the ability of HHG to serve as a highly sensitive and in many ways unique analytical tool. When used in this modality, HHG can help detect and understand ultrafast electron dynamics in complex gas-phase, solid-state, and laser-plasma systems [3]. Here, we extend this concept by using HHG as a probe for relativistic laser-matter interactions driven by ultrashort pulses in the mid-infrared.

Nonlinear microscopy techniques are widely used in the field of biophotonics thanks to their ability to image in highly scattering tissues such as brain [1]. Techniques that use intrinsic sources of contrast rather than exogenous labels are of great interest, because they are suitable not only for fundamental biological research but also for clinical applications [2]. Neurons are the most studied cells in the central nervous system of mammals. However, in certain parts of brain glial cells called astrocytes outnumber neurons. Historically, astrocytes considered to be simple support cells that structure the brain and control blood brain barrier, but recently were discovered to have great functional diversity and play many roles in healthy tissues as well as in CNS disorders [3].

We derive a closed-form analytical expression for the linear entropy of a multipartite qutrit state, providing a quantitative measure for quantum entanglement within the class of n-mode nonorthogonal qutrit states with any n. Conditions for enhanced and maximum quantum entanglement of multipartite qutrit states are identified. The usefulness of the introduced multipartite qutrit states as quantum communication channel resources is analyzed. The Hamiltonians allowing for the generation of multipartite qutrit states can be attained by combining optomechanical cavities with sequences of tunable beam splitters.

The canonical framework of optical physics connects the validity of perturbative nonlinear optics to the smallness of the optical driver field E compared to a characteristic field Eat that acts on electrons in an atom, a molecule, or a crystal lattice. However, in a vast area of strong-field optical science, the borderline between perturbative and nonperturbative nonlinear optics is defined as γ ~1, where γ is the Keldysh parameter. Not only is this criterion frequency-dependent, in a stark contrast with E/Eat ~1, but it also often dictates much weaker fields at which the perturbative treatment is still valid, leading to a dramatic shrinkage in the convergence radius of perturbation-theory expansions. Here, we identify the physics behind the gap between the E/Eat ~1 and γ ~1 conditions as the limits of perturbative nonlinear optics. We argue that, while the criterion E/Eat << 1 sets a universal upper-bound limit on the validity of perturbative nonlinear optics and its central concept of nonlinear-optical susceptibilities, optical nonlinearities related to photoionization pathways become nonperturbative in much weaker optical fields, with the limits of a perturbative treatment defined by the Keldysh parameter γ rather than the E/Eat ratio.

Formation of light bullets-tightly localized in space and time light packets, retaining their spatiotemporal shape during propagation-is, for the first time, experimentally observed and investigated in a new regime of mid-infrared filamentation in ambient air. It is suggested that the light bullets generated in ambient air by multi-mJ, positively chirped 3.9-μm pulses originate from a dynamic interplay between the anomalous dispersion in the vicinity of

The mid-infrared spectral range is unique in many ways. Within this region, electromagnetic radiation can resonate with the most intense signature molecular bands, thus drastically enhancing the coupling between the field and molecular motions. Electrons driven by intense ultrashort mid-IR field waveforms acquire unusually high pondermotive energies within a fraction of the field cycle, giving rise to new regimes of high-field nonlinear optics. The λ2 scaling of phasespace mode volume with radiation wavelength λ translates into the λ2 dependence of the self-focusing threshold, allowing much higher peak powers to be transmitted in a single laser filament in the mid-IR range without losing beam continuity and spatial coherence. Recent breakthroughs in ultrafast photonics in mid-IR help understand complex interactions of high-intensity ultrashort mid-IR pulses with matter, offer new approaches for x-ray generation, enable mid-IR laser filamentation in the atmosphere, facilitate lasing in filaments, give rise to unique regimes of laser−matter interactions, and reveal unexpected properties of materials in the mid-IR range. Motivated and driven by numerous applications and long-standing challenges in strong-field physics, molecular spectroscopy, semiconductor electronics, and standoff detection, ultrafast optical science is rapidly expanding toward longer wavelengths. Experiments reveal unique properties of filaments induced by ultrashort laser pulses in the mid-infrared, where the generation of powerful supercontinuum radiation is accompanied by unusual scenarios of optical harmonic generation, giving rise to remarkably broad radiation spectra, stretching from the visible to the mid-infrared. Generation of few- and even single-cycle midinfrared field waveforms has been demonstrated within a broad range of peak powers and central wavelengths. Belowthe- bandgap high-order harmonics generated by ultrashort mid-infrared laser pulses are shown to be ideally suited to probe the nonlinearities of electron bands, enabling an all-optical mapping of the electron band structure in bulk solids. As a part of a bigger picture, laser-induced filamentation in the mid-infrared and intraband high-harmonic generation using ultrashort mid-infrared pulses offer important physical insights into the general properties of the nonlinear-optical response of matter as a function of the wavelength. Unlike their near-infrared counterparts, which can be accurately described within the framework of perturbative nonlinear optics, mid-infrared filaments often entangle perturbative and nonperturbative nonlinear-optical effects, showing clear signatures of strong-field optical physics.

Secondary radiation emission of laser-induced filaments is revisited from a perspective of transient antenna radiation. Solutions for transient-antenna radiation fields are shown to provide an accurate description of the spectral and polarization properties, radiation patterns, and the angular dispersion of terahertz and microwave radiation emitted by laser filaments. Time-domain pulsed-antenna analysis offers a physically clear explanation for the bandwidth of this radiation, relating the low-frequency cutoff in its spectrum to the filament length, thus explaining efficient microwave generation in laser filamentation experiments.

We demonstrate a multimodal nonlinear-optical imaging that combines second- and third-harmonic generation (SHG and THG) with three-photon-excited fluorescence (3PEF) as a means to resolve fine details of the cell structure and trace its transformations throughout structurally complex episodes of cell-cycle dynamics, including the key stages and signatures in cell division. When zoomed in on cell mitosis, this technique enables a high-contrast multimodal imaging of intra- and extracellular signatures of cell division, detecting, via a multiplex, 3PEF/SHG/THG readout, a remarkable diversity of shapes, sizes, and symmetries in a truly single-beam setting, with no need for beam refocusing or field-waveform re-adjustment.

Multimodal nonlinear microscopy combining third-harmonic generation (THG) with two- and three-photon-excited fluorescence (2PEF and 3PEF) is shown to provide a powerful resource for high-fidelity imaging of nucleoli and nucleolar proteins. We demonstrate that, with a suitably tailored genetically encoded fluorescent stain, the 2PEF/3PEF readout from specific nucleolar proteins can be reliably detected against the extranucleolar 2PEF/3PEF signal, enabling high-contrast imaging of the key nucleolar ribosome biogenesis components, such as fibrillarin. THG is shown to provide a versatile readout for unstained nucleolus imaging in a vast class of biological systems as different as neurons in brain slices and cultured HeLa cells.

Spectral analysis of high-order harmonics generated by ultrashort mid-infrared pulses in molecular nitrogen reveals well-resolved signatures of inverse Raman scattering, showing up near the frequencies of prominent vibrational transitions of nitrogen molecules. When tuned on a resonance with the v ′ = 0 → v ′ ′ = 0 pathway within the B 3 Π g → C 3 Π u second positive system of molecular nitrogen, the eleventh harmonic of a 3.9 µm, 80 fs driver is shown to acquire a distinctive antisymmetric spectral profile with red-shifted bright and blue-shifted dark features as indicators of stimulated Raman gain and loss. This high-harmonic setting extends the inverse Raman effect to a vast class of strong-field light–matter interaction scenarios.

Cutting-edge methods of laser microscopy combined with fluorescent protein engineering and spectral analysis provide a unique resource for high-resolution neuroimaging, enabling a high-fidelity, high-contrast detection of fine structural details of neural cells and intracellular compartments. In addition to their extraordinary imaging abilities in real space, such methods can help resolve the neural states in a multidimensional space of neural responses whereby individual neurons and neural populations encode information on external stimuli. This study shows, however, that laser-induced biochemical processes in neural cells can give rise to an uncertainty of neural states, setting an upper bound on the information that optical measurements can provide on neural states, neural encodings, and neural dynamics. Comparison of absorbed laser power with the native biochemical energy budget of neuronal firing suggests that each readout photon in optical recording comes at a cost of precision of neural encoding and a loss of information encoded by the neural response. A quantitative measure for such a measurement-induced neural uncertainty can be defined, as this study shows, in terms of the Fisher information, relating the lower bound of this uncertainty to the loss of the Shannon information capacity of neural states.

We show that the full electron band structure of solids can be determined by means of high-order harmonic generation using ultrashort laser pulses in the mid-infrared range. Electron and hole wave packets whose direction of motion is controlled by the polarization of laser pulse relative to the symmetry axes of the Brillouin zone are shown to provide an effective probe for the electron

The propagation of high-power ultrashort light pulses involves intricate nonlinear spatio-temporal dynamics where various spectral–temporal field transformation effects are strongly coupled to the beam dynamics, which, in turn, varies from the leading to the trailing edge of the pulse. Analysis of this nonlinear dynamics, accompanied by spatial instabilities, beam breakup into multiple filaments, and unique phenomena leading to the generation of extremely short optical field waveforms, is equivalent in its computational complexity to a simulation of the time evolution of a few billion-dimensional physical system. Such an analysis requires exaflops of computational operations and is usually performed on high-performance supercomputers. Here, we present methods of physical modeling and numerical analysis that allow problems of this class to be solved on a laboratory computer boosted by a cluster of graphic accelerators. Exaflop computations performed with the application of these methods reveal new unique phenomena in the spatio-temporal dynamics of high-power ultrashort laser pulses. We demonstrate that unprecedentedly short light bullets can be generated as a part of that dynamics, providing optical field localization in both space and time through a delicate balance between dispersion and nonlinearity with simultaneous suppression of diffraction-induced beam divergence due to the joint effect of Kerr and ionization nonlinearities.

A bundle of individually addressable optical fibers is shown to enable three-dimensional optical readout from single neurons in live brain, as well as in intact brain extracted from transgenic mice. With individual fibers in the bundle being only a few microns in diameter, single neurons are readily resolved in brain images transmitted by the fiber bundle. The third dimension is added by scanning the fiber in the longitudinal direction, with the fluorescence return read out from only one of the fibers in the bundle.