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The effect of laser radiation with a wavelength near 1.55 µm on the photoluminescence properties of “nitrogen-vacancy” (NV) color centers in diamond has been studied experimentally. The effect of radiation of an infrared laser on the charge state of color centers redistributes the spectral intensity of photoluminescence in the ensemble of NV centers. In the case of neutral NV centers, the quenching of photoluminescence reaching about 60% has been observed. A feature of the laser excitation of NV centers by this method is the absence of competition with stimulated emission and two-photon excitation, which is observed with an increase in the power of infrared radiation at other wavelengths.

We present all-optical high-resolution temperature measurements in the brain of freely behaving mice using an implantable thermometer that integrates a reconnectable two-section fiber-optic probe a...

We identify experimental conditions for efficient generation of THz pulses by high power near IR laser radiation: 2 TW, 30 fs, 800-nm laser pulses with their second harmonic were slightly focused in a low-pressure gas cell in such a way to avoid multiple filamentation and excessive ionization. This two-color scheme yields a microjoule level of THz pulses which is two orders of magnitude higher than the signal generated in the atmosphere of ambient air.

Accurate, high spatial resolution thermal measurements have provided fundamental insights into many fields of study; however, existing thermometer technology often suffers from one or more limitati...

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.

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.

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.

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 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 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.

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.

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.

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.

We have fabricated the upconversion nanoparticles (NPs), which are nanocrystallites of 10 nm, doped with Yb3+ and Er3+ ions, prepared through hydrothermal method. Monitoring of their photophysical properties had been realized at different fabrication stages by using electron microscopy and luminescence spectroscopy under photoexcitation of the NPs with the 100 mW laser at 980 nm wavelength. The performed monitoring allows us to observe dramatic changes in the upconversion properties of the NPs during the sequence of the fabrication stages and revealed very strong correlation between the NPs’ upconversion efficiency and their structural specificities.

We generate fully controllable fs multimillijoule pulse bursts with the energy handling, throughput efficiency and frequency resolution substantially exceeding that achievable in spatial-light-modulator and interferometric techniques. The demonstrated proof-of-concept experiments include coherent control of nitrogen-ion emission via multiple-pulse excitation and generation of tunable narrowband THz pulses via optical rectification.

We present one- and two-photon-absorption fluorescence spectroscopic analysis of biliverdin (BV) chromophore-based single-domain near-infrared fluorescent proteins (iRFPs). The results of these studies are used to estimate the internal electric fields acting on BV inside iRFPs and quantify the electric dipole properties of this chromophore, defining the red shift of excitation and emission spectra of BV-based iRFPs. The iRFP studied in this work is shown to fit well the global diagram of the red-shift tunability of currently available BV-based iRFPs as dictated by the quadratic Stark effect, suggesting the existence of the lower bound for the strongest red shifts attainable within this family of fluorescent proteins. The absolute value of the two-photon absorption (TPA) cross section of a fluorescent calcium sensor based on the studied iRFP is found to be significantly larger than the TPA cross sections of other widely used genetically encodable fluorescent calcium sensors.

Analysis of optical properties of mid-infrared-transparent nonlinear crystals reveals octave phase matching for a highly efficient optical parametric amplification of single-cycle electromagnetic field waveforms within the 3- to 12-μm wavelength range, recently demonstrated in experiments.

We demonstrate a compact laser platform that integrates stimulated and coherent Raman microscopy with high-resolution nonlinear Raman spectroscopy. Ultrashort laser pulses with a carefully managed, broadly tunable chirp are shown to enable a comfortable switching between the high-contrast microscopy and high-resolution spectroscopy modalities in both stimulated and coherent versions of nonlinear Raman scattering. Sub-10-cm−1 spectral resolution in stimulated Raman spectroscopy is demonstrated through a careful compensation of nonlinear phase distortions of ultrashort laser pulses. This offers a powerful tool for a reliable identification of molecular vibrations with close frequencies, enhancing the chemical selectivity of spectroscopic analysis of complex multicomponent systems. Copyright © 2016 John Wiley & Sons, Ltd.

We demonstrate the physical principles of multimodal nonlinear optical microspectroscopy, integrating methods of coherent and stimulated Raman scattering of ultrashort chirped laser pulses in a single optical scheme. Nonlinear phase distortions of ultrashort laser pulses are accurately compensated within a broad spectral range in this scheme to enable a high-spectral-resolution laser microspectroscopy that can reliably resolve groups of fingerprint molecular vibrations with close frequencies, thus facilitating an analysis of complex multicomponent systems.

We study supercontinuum (SC) generation in large-mode-area (LMA) photonic crystal fibers with various core sizes and lengths, pumped by a picosecond Nd:YVO4 laser. Micro-joule level SC pulse energy is achieved, and the spectrum extends beyond 1600 nm, corresponding to an effective Raman detection range over 3000 wavenumbers. A multiplex CARS setup based on the SC source is constructed, and we demonstrate CARS acquisition in air, and compare the signal obtained with different LMA fiber parameters.

A monolithic, ultra-compact fiber probe integrating a nitrogen-vacancy diamond microcrystal, a dual-core photonic-crystal fiber (PCF), and a microwave transmission line is shown to enable a highly sensitive dual-channel vectorial magnetic field measurements by means of simultaneous optical detection of magnetic resonances through the two fiber cores. A reliable detection of microscale magnetic-field variations as low as ∼10 µT/(Hz)1/2 is demonstrated using a dual-core PCF with a core-to-core separation of 6 µm. Such a fiber probe is shown to provide a unique tool for a vectorial three-dimensional mapping of weak magnetic fields induced by microscale objects.

We report results of experimental investigation on polarization evolution and losses in linearly and circularly polarized mid-IR filaments in air under different focusing conditions.

We show that a broadly accepted criterion of laser-induced breakdown in solids, defining the laser-breakdown threshold in terms of the laser fluence or laser intensity needed to generate a certain fraction of the critical electron density rc within the laser pulse, fails in the case of high-intensity few-cycle laser pulses. Such laser pulses can give rise to subcycle oscillations of electron density ρ with peak ρ values well above ρc even when the total energy of the laser pulse is too low to induce a laser damage of material. The central idea of our approach is that, instead of the ρ = ρ c ratio, the laser-breakdown threshold connects to the total laser energy coupled to the electron subsystem and subsequently transferred to the crystal lattice. With this approach, as we show in this work, predictions of the physical model start to converge to the available experimental data.

Filamentation of Mid-IR pulses in ambient air is significantly more complicated and challenging as compared to near-IR filaments, due the changing sign of dispersion in the vicinity of 3.6 μm, substantially lower ionization rates and hence plasma density [1], as well as because of the presence of molecular resonances around the atmospheric transparency windows. Experimental observations reveal that despite its low concentration in ambient air, which is only 400–600 ppm (0.04–0.06%), CO2 strongly affects filamentation of 3.9-μm pulses by influencing spectral and temporal dynamics, as well as spatial beam transformations and propagation losses. As a main loss mechanism during filamentation of loosely focused 3.9-μm pulses in ambient air, dynamic absorption by CO2 is identified. Since in the case of loose focusing plasma density in the filament is negligible, pulses during filamentation in both main constituents, O2 and N2, experience pronounced red-sided broadening (Fig.la), which is governed by stimulated Raman scattering. In ambient air, newly generated spectral components roll over CO2 resonant absorption band situated in the vicinity of 4.2-μm and are effectively absorbed, which results in up to 50% filamentation losses. To our surprise, an increase of CO2 concentration to ∼5% level (by exhaling air [3] into 2-m long open-ended tube) leads to complete disappearance of losses.

Four-wave mixing (FWM) of femtosecond near-infrared laser pulses and its second harmonic in the filamentation regime is shown to give rise to ultrashort field waveforms in the mid-infrared with pulse widths as short as a half of the field cycle and produce ultrabroadband supercontinuum spectra stretching from the mid-IR to the terahertz region. Generation of 7-fs pulses centered at 4.35 μm is demonstrated by a two-color filamentation experiment, where the 25-fs, 800-nm fundamental-wavelength output of a Ti: Sapphire laser is mixed with its second harmonic. The spectral and temporal properties of the mid-IR waveforms, as well as their emission pattern, are consistent with the FWM scenario of frequency conversion generalized to include the Kerr effect and ionization-induced refractive-index modulation.

The methods of controlling the charge state of nitrogen–vacancy centers using pulsed infrared laser field are studied. The effect of radiation intensity at a wavelength of about 1 μm on the ionization of these color centers was shown. The obtained results can be useful for multiphoton microscopy of biological objects, high-resolution microscopy and sensory applications.

We demonstrate that a background-free readout of two-photon fluorescence from nitrogen-vacancy (NV) centers in a strongly fluorescing environment can be accomplished by all-optical means via a multiphoton charge-state modulation of NV centers in a mixture of negatively charged and neutral NV centers. A 100 fs, 1060 nm output of an ytterbium fiber laser is ideally suited for this modality of multiphoton microscopy, providing, as our experiments show, an efficient two-photon excitation of both NV− and NV0 charge states, but keeping the nonlinearity of n-photon ionization needed for NV−/NV0 charge-state modulation to a minimum, n=3.

Analysis of the complex refractive index of atmospheric air reveals remarkably broad and continuous regions of anomalous and near-zero group-velocity dispersion in the subterahertz (sub-THz) and millimeter-band atmospheric windows. One such broadband dispersion anomaly is shown to occur in the high-frequency wing of the 60-GHz band of molecular oxygen. Adjacent to this dispersion anomaly is a broadband atmospheric transparency region within which the group-velocity dispersion is unusually weak, enabling distortion-free long-distance transmission of broadband millimeter-wave field waveforms, as well as broadband remote sensing in the millimeter band. Although broad regions of anomalous and near-zero dispersion also exist in the sub-THz range, their utility for long-distance transmission and remote sensing is limited because of strong atmospheric absorption.

Ultrashort laser pulses can give rise to extremely brief' finely sculpted pulses of electric current in a solid [1]. Recently' full reconstruction of energy band structure of a crystal has been demonstrated using high-order harmonic generation (HHG) [2,3]. Here, we show that polarization maps of high-order harmonics can serve to fully characterize vectorial properties of petahertz electron currents generated in crystalline solids by an ultrashort optical driver. Our HHG experiments were performed with a 200-μm-thick zincblende ZnSe crystal. The [1,1,1] axis crystal was oriented along the wave vector of the driver, with the driver field E d making an angle ψ with the ΓΚΧ axis of the ZnSe Brillouin zone (Figs. 1a, 1b). The driver 160-fs-pulse with a central wavelength 3.6 μm and energy up to 100 μJ was generated as idler beam in 3 stages KTA-based optical parametric amplifier pumped by 1-kHz Yb-regenerative amplifier. The driver field intensity kept below 0.5 TW/cm2 in all experiments, to avoid a laser damage of the crystal.

Generation of ultrashort pulses of electromagnetic radiation in the range of wavelengths from 3 to 17 μm is experimentally demonstrated using difference-frequency mixing of the signal and idler waves delivered through an optical parametric amplification of a week broadband seed by high-power near-infrared ultrashort laser pulses. Cross-correlation measurements demonstrate that pulses as short as a few cycles of electromagnetic field are produced in the mid- and far-infrared ranges by means of difference-frequency generation.

We present a method by which the spectral intensity of an ultrafast laser pulse can be accumulated at selected frequencies by a controllable amount. Using a 4-f pulse shaper we modulate the phase of the frequency components of a femtosecond laser. By inducing femtosecond filamentation with the modulated pulse, we can concentrate the spectral amplitude of the pulse at various frequencies. The phase mask applied by the pulse shaper determines the frequencies for which accumulation occurs, as well as the intensity of the spectral concentration. This technique provides a way to obtain pulses with adjustable amplitude using only phase modulation and the nonlinear response of a medium. This provides a means whereby information which is encoded into spectral phase jumps may be decoded into measurable spectral intensity spikes.

Few-cycle high peak power mid-IR sources are required for strong field applications, such as driving HHG in the keV range [1] and high efficiency THz generation [2]. In order to expose target to a peak field without causing excessive ionization, ultrashort pulses consisting of a few-cycles are needed. Parametric sources based on KTA crystals have been successfully used for the generation of multi-mJ pulses in the 3–4 μm range [3], however, the duration is typically limited to sub-100 fs duration due to phase matching bandwidth limitations. A robust and popular scheme for the generation of few-cycle pulses is via post-compression in a hollow core fiber (HCF) filled with noble gas [4] that yields uniform pulse compression across the beam and the perfect beam profile at the output due to spatial filtering effects.

Different regimes of filamentation of multi-millijoule mid-infrared pulses in ambient air can be achieved by varying focusing conditions and chirp of the driving pulses. Dynamic absorption losses are identified as a possible mechanism of observed plasma-less filamentation.

We demonstrate a wavelength-tunable source of high-energy sub-100 fs light pulses where all the key components, including a master oscillator, an amplifier, and a tunable frequency shifter, are based on photonic-crystal fibers (PCFs). Soliton self-frequency shift in a specifically designed all-solid photonic band-gap fiber with an effective mode area of 110 mm 2 is used in this system to transform a 1040 nm, 50 MHz, 100 fs amplified output of a large-mode-area ytterbium-doped PCF oscillator–amplifier system into sub-100 fs, 6.4 nJ light pulses smoothly tunable within the range of wavelengths from 1160 to 1260 nm. & 2012 Published by Elsevier B.V.

Optically detected electron spin resonance in fiber-coupled nitrogen–vacancy (NV) centers of diamond is used to demonstrate a fiber-optic quantum thermometry of individual thermogenetically activated neurons. Laser-induced temperature variations read out from single neurons with the NV-diamond fiber sensor are shown to strongly correlate with the fluorescence of calcium-ion sensors, serving as online indicators of the inward Ca2+ current across the cell membrane of neurons expressing transient receptor potential (TRP) cation channels. Local laser heating above the TRP-channel activation threshold is shown to reproducibly evoke robust action potentials, visualized by calcium-ion-sensor-aided fluorescence imaging and detected as prominent characteristic waveforms in the time-resolved response of fluorescence Ca2+ sensors.

Extended coupled-wave analysis of optical parametric chirped-pulse amplification (OPCPA) reveals regimes whereby high-peak-power few-cycle pulses can be generated in the long-wavelength infrared (LWIR) spectral range. Broadband OPCPA in suitable nonlinear crystals pumped at around 2 μm and seeded either through the signal or the idler input is shown to enable the generation of high-power field waveforms with pulse widths shorter than two field cycles within the entire LWIR range.