The physical processes associated with propagation of a high-power (power > critical power for self-focusing) laser beam in water include nonlinear focusing, stimulated Raman scattering (SRS), optical breakdown and plasma formation. The interplay between nonlinear focusing and SRS is analyzed for cases where a significant portion of the pump power is channeled into the Stokes wave. Propagation simulations and an analytical model demonstrate that the Stokes wave can re-focus the pump wave after the power in the latter falls below the critical power. It is shown that this novel focusing mechanism is distinct from cross-phase focusing. While discussed here in the context of propagation in water, the gain-focusing phenomenon is general to any medium supporting nonlinear focusing and stimulated forward Raman scattering.
The random lasing (RL) emission of dyed vesicular polymeric films in diffusive regime of light propagation was investigated under helium temperature. It represents itself the coupling effect of stimulated emission and stimulated Raman scattering (SRS). Due to the effect a set of Raman lines reveal in RL spectrum. Distinguishing feature of the effect is the stimulated emission promotes the Raman lines revealing. Intensity of Raman lines have quadratic trend on pump intensity. Furthermore SRS in this condition has the more structured spectrum than spontaneous Raman one. It makes enable detection…
Raman sensing and Raman microscopy are amongst the most specific optical technologies to identify the chemical compounds of unknown samples, and to enable label-free biomedical imaging with molecular contrast. However, the high cost and complexity, low speed, and incomplete spectral information provided by current technology are major challenges preventing more widespread application of Raman systems. To overcome these limitations, we developed a new method for stimulated Raman spectroscopy and Raman imaging using continuous wave (CW), rapidly wavelength swept lasers. Our all-fiber, time-encod…
The inclusion of atomic inversion in Raman scattering can significantly alter field dynamics in plasmonic settings. Our calculations show that large local fields and femtosecond pulses combine to yield: (i) population inversion within hot spots; (ii) gain saturation; and (iii) conversion efficiencies characterized by a switch-like transition to the stimulated regime that spans twelve orders of magnitude. While in Raman scattering atomic inversion is usually neglected, we demonstrate that in some circumstances full accounting of the dynamics of the Bloch vector is required.
Stimulated Raman scattering (SRS) represents a powerful tool for accessing the vibrational properties of molecular compounds or solid state systems. From a spectroscopic perspective, SRS is able to capture Raman spectra free from incoherent background processes and typically ensures a signal enhancement of several orders of magnitude with respect to its spontaneous counterpart. Since its discovery in 1962, SRS has been applied to develop technological applications, such as Raman-based lasers, frequency shifters for pulsed sources and Raman amplifiers. For the full exploitation of their potenti…
We study forward stimulated Raman emission from weakly fluorescent dye 4'-diethylamino-N-methyl-4-stilbazolium tosylate (DEST) in 1,2,dichloroethane solution excited by a 28 ps, 532 nm Nd: YAG laser. Neat 1, 2, dichloroethane emits the first Stokes line at 631 nm with a spectral width of 1.6 nm corresponding to a Raman shift of 2956 per cm. We observe reduction of spectral width with the addition of DEST in 1, 2, dichloroethane solution. The single pass conversion efficiency for forward Raman emission is as high as 20 percent in a 1 cm path length sample. The pulse duration of forward stimulat…
We investigate for the first time the enhancement of the stimulated Raman scattering in slow-light Silicon-on-Insulator (SOI) photonic crystal line defect waveguides. By applying the Bloch-Floquet formalism to the guided modes in a planar photonic crystal, we develop a formalism that relates the intensity of the down-shifted Stokes signal to the pump intensity and the modal group velocities. The formalism is then applied to two prospective schemes for enhanced stimulated Raman generation in slow-light photonic crystal waveguides. The results demonstrate a maximum factor of 10^4 (66,000) enhanc…
Spectral compression of femtosecond pulses by second harmonic generation in the presence of substantial group velocity dispersion provides a convenient source of narrowband Raman pump pulses for femtosecond stimulated Raman spectroscopy (FSRS). We discuss here a simple and efficient modification that dramatically increases the versatility of the second harmonic spectral compression technique. Adding a spectral filter following second harmonic generation produces narrowband pulses with a superior temporal profile. This simple modification i) increases the Raman gain for a given pulse energy, ii…
We theoretically and experimentally investigated the transition between modulation instability and Raman gain in a small silica microcavity with a large free-spectral range (FSR), which reveals that we can selectively switch from a four-wave mixing dominant state to a stimulated Raman scattering dominant state. Both the theoretical analysis and the experiment show that a Raman-dominant region is present between transitions of Kerr combs with different free-spectral range spacings. We can obtain a stable Kerr comb and a stable Raman state selectively by changing the driving power, coupling betw…
A unified framework for sensitivity comparison across stimulated Raman, stimulated Raman photothermal, and mid-infrared photothermal microscopy
Institute for Photon Science and Technology, The University of Tokyo（东京大学光子科学研究所）；Department of Advanced Materials Science, The University of Tokyo（东京大学先进材料科学系）；RIKEN Center for Advanced Photonics, RIKEN（理化学研究所先进光子学中心）
We demonstrate sensitive high-resolution stimulated Raman measurements of hydrogen using a hollow-core photonic crystal fiber (HC-PCF). The Raman transition is pumped by a narrow linewidth (<50 kHz) 1064 nm continuous-wave (CW) fiber laser. The probe light is produced by a homebuilt CW optical parametric oscillator (OPO), tunable from around 800 nm to 1300 nm (linewidth ~ 5 MHz). These narrow linewidth lasers allow for an excellent spectral resolution of approximately 10^-4 cm^(-1). The setup employs a differential measurement technique for noise rejection in the probe beam, which also elimina…
Stimulated Raman scattering (SRS) microscopy allows for high-speed label-free chemical imaging of biomedical systems. The imaging sensitivity of SRS microscopy is limited to ~10 mM for endogenous biomolecules. Electronic pre-resonant SRS allows detection of sub-micromolar chromophores. However, label-free SRS detection of single biomolecules having extremely small Raman cross-sections (~10-30 cm2 sr-1) remains unreachable. Here, we demonstrate plasmon-enhanced stimulated Raman scattering (PESRS) microscopy with single-molecule detection sensitivity. Incorporating pico-Joule laser excitation, b…
We demonstrate the dynamics of stimulated Raman scattering in designed high-Q/Vm silicon photonic band gap nanocavities through the coupled-mode theory framework towards optically-pumped silicon lasing. The interplay of other chi(3) effects such as two-photon absorption and optical Kerr, related free-carrier dynamics, thermal effects, as well as linear losses such as cavity radiation and linear material absorption are included and investigated numerically. Our results clarify the relative contributions and evolution of the mechanisms, and demonstrate the lasing and shutdown thresholds. Our stu…
Hydrogen detection is of great importance in chemical and energy industries. Optical fiber hydrogen sensors show flexibility and compactness and have the potential for distributed analysis. However, traditional fiber sensors encounter a challenge for light to interact with hydrogen directly since hydrogen only display weak quadrupole absorption, and metallic palladium and platinum thin-film coatings are typically used as an optically detectable label. Here, based on stimulated Raman spectroscopy in hollow-core photonic crystal fibers, we investigate the label-free optical fiber distributed hyd…
In this work we extend low frequency impulsive stimulated Raman microspectroscopy to the pre-electronic resonance regime. We use a broadband two color collinear pump probe scheme which can be readily extended to imaging. We discuss the difficulties unique to this type of measurements in the form of competing resonant two-photon processes and the means to overcome them. We successfully reduce the noise which arises due to those competing processes by eliminating the detected spectral components which do not contribute to the vibrational signature of the sample though introduce most of the noise…
We demonstrate the stimulated Raman near-field microscopy of few molecules, measured only using near-field optical forces thereby eliminating the need for far-field optical detection. The molecules were excited in the near-field without resonant electronic enhancement. We imaged gold nanoparticles of 30 nm diameter functionalized with a self-assembled monolayer (SAM) of 4-nitrobenzenethiol (4-NBT) molecules. The maximum number of molecules detected by the gold-coated nano-probe at the position of maximum field enhancement could be fewer than 20 molecules. The molecules were imaged by vibrating…
High quality lithium niobate thin film microresonators provide an ideal platform for on-chip nonlinear optics applications. However, the phase matching condition for efficient parametric process sets a high requirement for fabrication control. Here we demonstrate a photonic molecular structure composed of two strongly coupled lithium niobate microdisks of different diameters fabricated using femtosecond laser micromachining and focused ion beam milling. With a continuous wave excitation, rich nonlinear optical processes including cascaded four-wave mixing and stimulated Raman scattering were o…
We demonstrate stimulated Raman gain using a broadband LED Stokes source to measure vibrational spectra of aqueous glucose solutions. This versatile and cost-effective method increases Raman signal for a variety of applications. We measured both stimulated Raman and spontaneous Raman spectra of glucose solutions with concentrations up to 10 mM with a photon counter and lock-in amplifier. We built partial least squares regression models based on both stimulated Raman and spontaneous Raman spectral data measured with each instrument for predicting concentrations of the glucose solutions. The sti…
Stimulated Raman spectroscopy has become a powerful tool to study the spatiodynamics of molecular bonds with high sensitivity, resolution and speed. However, sensitivity and speed of state-of-the-art stimulated Raman spectroscopy are currently limited by the shot-noise of the light beam probing the Raman process. Here, we demonstrate an enhancement of the sensitivity of continuous-wave stimulated Raman spectroscopy by reducing the quantum noise of the probing light below the shot-noise limit by means of amplitude squeezed states of light. Probing polymer samples with Raman shifts around 2950 $…
We investigate the evolution of coherence property of noise-seeded Stokes wave in short (< 1 ps) and long pulse (> 1 ps) regimes. Nonlinear equations expressing the evolution of pump and Stokes wave are solved numerically for both the regions. The simulations include quantum noise by incorporating noise seed in the pump field where one photon per mode with random phase. The spectral phase fluctuations of the Stokes wave for both the regions, are characterized by performing multiple simulations and finally, the degrees of first-order mutual coherence are calculated as a function of wavelength f…
Laser pulses interaction with tobacco mosaic virus (TMV) in Tris-HCl pH7.5 buffer and in water has been investigated. 20 ns ruby laser pulses have been used for excitation. Spectrum of the light passing through the sample was registered with the help of Fabri-Perot interferometer. In the case of TMV in water we observed in the spectrum only one line of the exciting laser light, for TMV in Tris-HCl pH7.5 buffer second line appeared, corresponding to the stimulated low-frequency Raman scattering (SLFRS) on the breathing radial mode of TMV. SLFRS frequency shift by 2 cm-1, (60 GHz), conversion ef…
We report on the generation of a Raman cascade spanning the wavelength range of 523 to 1750 nm wavelength range, in a standard telecommunication graded-index multimode optical fiber. Despite the highly multimode nature of the pump, the Raman peaks are generated in specific modes of the fiber, confirming substantial beam cleanup during the stimulated Raman scattering process.
Stimulated Raman scattering (SRS) in ultra-high-Q surface-tension-induced spherical and chip-based toroid microcavities is considered both theoretically and experimentally. These microcavities are fabricated from silica, exhibit small mode volume (typically 1000 $μm^{3}$) and possess whispering-gallery type modes with long photon storage times (in the range of 100 ns), significantly reducing the threshold for stimulated nonlinear optical phenomena. Oscillation threshold levels of less than 100 $μ$% -Watts of launched fiber pump power, in microcavities with quality factors of 100 million are ob…
We demonstrate the acquisition of stimulated Raman scattering spectra with the use of a single femtosecond pulse. High resolution vibrational spectra are obtained by shifting the phase of a narrow band of frequencies in the broadband input pulse spectrum, using spectral shaping. The vibrational spectrum is resolved by examining the amplitude features formed in the spectrum after interaction with the sample. Using this technique, low frequency Raman lines (<100cm^-1) are resolved in a straightforward manner.
The evolution versus pump power of the spectrum of a singly resonant optical parametric oscillator based on an MgO-doped periodically poled stoichiometric lithium tantalate crystal is observed. The onset of cascade Raman lasing due to stimulated Raman scattering in the nonlinear crystal is analyzed. Spurious frequency doubling and sum-frequency generation phenomena are observed and understood. A strong reduction of the intracavity Raman scattering is obtained by a careful adjustment of the cavity losses.
Backward stimulated Raman scattering in gases provides a promising route to compression and amplification of a Stokes seed-pulse by counter-propagating against a pump-pulse, as has been already demonstrated in various platforms, mainly in free-space. However, the dynamics governing this process when seeded by noise has not yet been investigated in a fully controllable collinear environment. Here we report the first unambiguous observation of efficient noise-seeded backward stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber. At high gas pressures, when the backw…
We report a shot noise limited high speed stimulated Raman microscopy platform allowing to acquire molecular vibrational spectra over \SI{200}{\per \cm} in \SI{12}{\micro \second} at a scan rate of 40kHz. Using spectral focusing together with optimized acousto-optics programmable dispersive filters the designed low noise imaging platform performs chemical imaging of dynamical processes such as Mannitol crystal hydratation and reaches a signal to noise ratio sufficient to perform label free histological imaging on frozen human colon tissue slides.
We investigate the influence of the stimulated Raman scattering on the formation of bright and dark localized states in all-fiber resonators subject to a coherent optical injection, when operating in the normal dispersion regime. In the absence of the Raman effect, and far from any modulational instability, localized structures form due to the locking of domain walls connecting two coexisting continuous wave states, and undergo a particular bifurcation structure known as collapsed snaking. The stimulated Raman scattering breaks the reflection symmetry of the system, and modifies the dynamics, …
Stimulated low frequency Raman scattering (SLFRS) in single-crystal diamond films (SCD) with a graphitized layer built-in is investigated. SLFRS process in all studied structures has frequency shift in gigahertz range (5.1-13.2 GHz). Coherent phonon mode excitation at a frequency of several GHz is a result of nonlinear interaction of high-power laser wave with submicron films. SLFRS conversion efficiency and threshold is estimated experimentally. Comparison of the obtained results with data on SLFRS properties in synthetic opal matrixes is made.
Stimulated Raman scattering (SRS) in its strongly nonlinear, kinetic regime is controlled by a technique of deterministic, strong temporal modulation and spatial scrambling of laser speckle patterns, called Spike Trains of Uneven Duration and Delay (STUD pulses) [B. Afeyan and S. Hüller, Phys. Rev. Lett. (submitted)]. Kinetic simulations show that use of STUD pulses may decrease SRS reflectivity by more than an order of magnitude over random-phase-plate (RPP) or induced-spatial-incoherence (ISI) beams of the same average intensity and comparable bandwidth.
We demonstrate a compact and versatile laser system for stimulated Raman spectroscopy (SRS). The system is based on a tunable continuous wave (CW) probe laser combined with a home-built semi-monolithic nanosecond pulsed pump Nd:YVO4 laser at 1064 nm. The CW operation of the probe laser offers narrow linewidth, low noise and the advantage that temporal synchronization with the pump is not required. The laser system enables polarization-sensitive stimulated Raman spectroscopy (PS-SRS) with fast high resolution measurement of the depolarization ratio by simultaneous detection of Raman scattered l…
We demonstrate an approach for background-free three-dimensional imaging of director fields in liquid crystals using stimulated Raman scattering microscopy. This imaging technique is implemented using a single femtosecond pulsed laser and a photonic crystal fiber, providing Stokes and pump frequencies needed to access Raman shifts of different chemical bonds of molecules and allowing for chemically selective and broadband imaging of both pristine liquid crystals and composite materials. Using examples of model three-dimensional structures of director fields, we show that the described techniqu…
Expressions for the two-dimensional Stimulated x-ray Raman Spectroscopy (2D-SXRS) signal obtained using attosecond x-ray pulses are derived. The 1D- and 2D-SXRS signals are calculated for trans-N-methyl acetamide (NMA) with broad bandwidth (FWHM ~14.2eV, 181 as) pulses tuned to the oxygen and nitrogen K-edges. Crosspeaks in 2D signals reveal electronic Franck-Condon overlaps between valence orbitals and relaxed orbitals in the presence of the core hole.
Temporal cavity solitons (CS) are optical pulses that can persist in passive resonators, and they play a key role in the generation of coherent microresonator frequency combs. In resonators made of amorphous materials, such as fused silica, they can exhibit a spectral red-shift due to stimulated Raman scattering. Here we show that this Raman-induced self-frequency-shift imposes a fundamental limit on the duration and bandwidth of temporal CSs. Specifically, we theoretically predict that stimulated Raman scattering introduces a previously unidentified Hopf bifurcation that leads to destabilizat…
We propose and theoretically analyze a new vibrational spectroscopy, termed electron- and light-induced stimulated Raman (ELISR) scattering, that combines the high spatial resolution of electron microscopy with the molecular sensitivity of surface-enhanced Raman spectroscopy. With ELISR, electron-beam excitation of plasmonic nanoparticles is utilized as a spectrally-broadband but spatially-confined Stokes beam in the presence of a diffraction-limited pump laser. To characterize this technique, we develop a numerical model and conduct full-field electromagnetic simulations to investigate two di…
Label-free Imaging of Single-Biomolecule Structure and Interaction by Stimulated Raman Photothermal Encoded Scattering
Boston University（波士顿大学）；Stowers Institute for Medical Research（斯图沃医学研究所）；University of Washington（华盛顿大学）；Northeastern University（东北大学）；Georgia Institute of Technology（佐治亚理工学院）；Emory University（埃默里大学）；University of Virginia School of Medicine（弗吉尼亚大学医学院）；Howard Hughes Medical Institute（霍华德·休斯医学研究所）
We have calculated the resonant and nonresonant contributions to attosecond impulsive stimulated electronic Raman scattering (SERS) in regions of autoionizing transitions. Comparison with Multiconfiguration Time-Dependent Hartree-Fock (MCTDHF) calculations find that attosecond SERS is dominated by continuum transitions and not autoionizing resonances. These results agree quantitatively with a rate equation that includes second-order Raman and first-and second-order photoionization rates. Such rate models can be extended to larger molecular systems. Our results indicate that attosecond SERS tra…
We propose a novel femtosecond stimulated Raman spectroscopy (FSRS) technique that combines entangled photons with interference detection to select matter pathways and enhance the resolution. Following photo excitation by an actinic pump, the measurement uses a pair of broadband entangled photons, one (signal) interacts with the molecule together with a third narrowband pulse induces the Raman process. The other (idler) photon provides a reference for the coincidence measurement. This interferometric photon-coincidence counting detection allows to separately measure Raman gain and loss signals…
We present multi-color imaging by stimulated Raman scattering (SRS) enabled by an ultrafast fiber-based light source with integrated amplitude modulation and frame-to-frame wavelength tuning. With a relative intensity noise level of -153.7 dBc/Hz at 20.25 MHz the light source is well suited for SRS imaging and outperforms other fiber-based light source concepts for SRS imaging. The light source is tunable in under 5 ms per arbitrary wavelength step between 700 cm$^{-1}$ and 3200 cm$^{-1}$, which allows for addressing Raman resonances from the fingerprint to the CH-stretch region. Moreover, the…
Stimulated Raman backscattering (RBS) of intense laser radiation confined by a single-mode plasma channel with a radial variation of plasma frequency greater than a homogeneous-plasma RBS bandwidth is characterized by a strong transverse localization of resonantly-driven electron plasma waves (EPW). The EPW localization reduces the peak growth rate of RBS and increases the amplification bandwidth. The continuum of non-bound modes of backscattered radiation shrinks the transverse field profile in a channel and increases the RBS growth rate. Solution of the initial-value problem shows that an el…
Nonstationary molecular states which contain electronic coherences can be impulsively created and manipulated by using recently-developed ultrashort optical and X-ray pulses via photoexcitation, photoionization and Auger processes. We propose several stimulated-Raman detection schemes that can monitor the phase-sensitive electronic and nuclear dynamics. Three detection protocols of an X-ray broadband probe are compared - frequency dispersed transmission, integrated photon number change, and total pulse energy change. In addition each can be either linear or quadratic in the X-ray probe intensi…
Time-resolved femtosecond stimulated Raman spectroscopy (FSRS) is a powerful tool for investigating ultrafast structural and vibrational dynamics in light absorbing systems. However, the technique generally requires exposing a sample to high laser pulse fluences and long acquisition times to achieve adequate signal-to-noise ratios. Here, we describe a timeresolved FSRS instrument built around a Yb ultrafast amplifier operating at 200 kHz, and address some of the unique challenges that arise at high repetition-rates. The setup includes detection with a 9 kHz CMOS camera and an improved dual-cho…
The hybrid Stimulated Raman Scattering - Two Plasmon Decay instability had shown to play the dominant role for plasma waves excitation and energy absorption at interaction of very intense femtosecond obliquely incident $p$-polarized laser pulse (intensity $10^{17}-5\cdot10^{19}$ W/cm$^2$) with a steep gradient ($L\sim λ$) plasma near the quarter critical surface, i.e. within the typical for modern experiments conditions. The plasmons are excited as two wave packets confined near this surface with very wide $\approx ω_0/c$ spatial spectra along its normal. Hence, phase matching conditions for 3…
Excited-state vibrations are crucial for determining photophysical and photochemical properties of molecular compounds. Stimulated Raman scattering can coherently stimulate and probe molecular vibrations with optical pulses, but it is generally restricted to ground state properties. Working in resonance conditions, indeed, enables cross-section enhancement and selective excitation to a targeted electronic level, but is hampered by an increased signal complexity due to the presence of overlapping spectral contributions. Here, we show how detailed information on ground and excited state vibratio…
We study localized wave on continuous wave background analytically in a nonlinear fiber with higher order effects such as higher order dispersion, Kerr dispersion, and stimulated inelastic scattering. We present an exact rational W-shaped soliton solutions, whose structural properties depend on the frequency of the background field. The hump value increase with the decrease of the background frequency in the certain regime. The highest value of the W-shaped soliton can be nine times the background's, and the distribution shape is identical with the one of well-known eyes-shaped rogue wave with…
Stimulated Raman adiabatic passage (STIRAP) describes adiabatic population transfer between two states coherently coupled via a mediating state that remains unoccupied. This renders STIRAP robust against loss in the mediating state, leading to profound applications in atomic- and molecular-beam research, trapped-ion physics, superconducting circuits, other solid-state systems, optics, in entanglement generation and qubit operations. STIRAP in optomechanics has been considered for optical frequency conversion where a mechanical mode provides the mediating state. Given the advances of optomechan…
Electron and vibrational dynamics of molecules are commonly studied by subjecting them to two interactions with a fast actinic pulse that prepares them in a nonstationary state and after a variable delay period $T$, probing them with a Raman process induced by a combination of a broadband and a narrowband pulse. This technique known as femtosecond stimulated Raman spectroscopy (FSRS) can effectively probe time resolved vibrational resonances. We show how FSRS signals can be modeled and interpreted using the stochastic Liouville equations (SLE) originally developed for NMR lineshapes. The SLE p…
Dynamics of solitons is considered in the framework of an extended nonlinear Schrodinger equation (NLSE), which is derived from a system of the Zakharov's type for the interaction between high- and low-frequency (HF and LF) waves. The resulting NLSE includes a pseudo-stimulated-Raman-scattering (pseudo-SRS) term, i.e., a spatial-domain counterpart of the SRS term, which is a known ingredient of the temporal-domain NLSE in optics. Also included is inhomogeneity of the spatial second-order dispersion (SOD) and linear losses of HF waves. It is shown that wavenumber downshift by the pseudo-SRS may…
Hyperspectral stimulated Raman scattering (SRS) microscopy is a label-free technique for biomedical and mineralogical imaging which can suffer from low signal to noise ratios. Here we demonstrate the use of an unsupervised deep learning neural network for rapid and automatic denoising of SRS images: UHRED (Unsupervised Hyperspectral Resolution Enhancement and Denoising). UHRED is capable of one-shot learning; only one hyperspectral image is needed, with no requirements for training on previously labelled datasets or images. Furthermore, by applying a k-means clustering algorithm to the process…
