Nonlinear Raman Process Research Articles

Surface Enhanced Nonlinear Raman Processes for Advanced Vibrational Probing.

Surface enhanced Raman scattering (SERS) is not restricted to the well-known one-photon excited spontaneous Raman process that gives information on molecular composition, structure, and interaction through vibrational probing with high sensitivity. The enhancement mainly originates in high local fields, specifically those provided by localized surface plasmon resonances of metal nanostructures. High local fields can particularly support nonlinear Raman scattering, as it depends on the fields to higher powers. By revealing plasmon-molecule interactions, nonlinear Raman processes provide a very sensitive access to the properties of metal nanomaterials and their interfaces with molecules and other materials. This Perspective discusses plasmon-enhanced spontaneous and coherent nonlinear Raman scattering with the aim of identifying advantages that lead to an advanced vibrational characterization of such systems. The discussion will highlight the aspects of vibrational information that can be gained based on specific advantages of different incoherent and coherent Raman scattering and their surface enhancement. While the incoherent process of surface enhanced hyper Raman scattering (SEHRS) gives highly selective and spectral information complementary to SERS, the incoherent process of surface enhanced pumped anti-Stokes Raman scattering (SEPARS) can help to infer effective nonresonant SERS cross sections and allows to see "hot" vibrational transitions. Surface enhanced coherent anti-Stokes Raman scattering (SECARS) and surface enhanced stimulated Raman scattering (SESRS) combine the advantages of high local fields and coherence, which gives rise to high detection sensitivity and offers possibilities to explore molecule-plasmon interactions for a comprehensive characterization of composite and hybrid structures in materials research, catalysis, and nanobiophotonics.

Efficient optical isolator via dual-Raman process with chiral nonlinearity

A scheme for optical nonreciprocity is proposed via dual-mode Raman process in thermal atoms. We investigate a far-detuned nonlinear Raman process, with the assistance of the Doppler effect to achieve chiral optical nonlinearity. The resulting Kerr nonlinear absorption is subject to a direction-dependent Doppler shifts seen by the thermal atoms. Moreover, the inter-channel Raman interference could significantly enhance Kerr nonlinear absorption for probe field, leading to the enhancement of the optical nonreciprocity. Simulation results show that the isolation ratio could reach 41 dB and the insertion loss is about 0.16 dB, with bandwidth up to 2π ×48 MHz. Furthermore, the sensitivity of the system is investigated under different strengths of the control and Raman fields.

Silent-enhancement of multiple Raman modes via tuning optical properties of graphene nanostructures

Raman scattering signal can be enhanced through localization of incident field into sub-wavelength hot-spots through plasmonic nano-structures (Surface-enhanced Raman scattering-SERS). Recently, further enhancement of SERS signal via quantum objects are proposed by [1] without increasing the hot-spot intensity (\textit{silent-enhancement}) where this suggestion prevents the modification of vibrational modes or the breakdown of molecules. The method utilizes path interference in the non-linear response of Stokes-shifted Raman modes. In this work, we extend this phenomenon to tune the spectral position of \textit{silent-enhancement} factor where the multiple vibrational modes can be detected with a better signal-to-noise ratio, simultaneously. This can be achieved in two different schemes by employing either (i) graphene structures with quantum emitters or (ii) replacing quantum emitters with graphene spherical nano-shell in \cite{Postaci2018}. In addition, the latter system is exactly solvable in the steady-state. These suggestions not only preserve conventional non-linear Raman processes but also provide flexibility to enhance (silently) multiple vibrational Raman modes due to the tunable optical properties of graphene.

Quantum non-demolition measurement based on an SU(1,1)-SU(2)-concatenated atom-light hybrid interferometer

Quantum non-demolition (QND) measurement is an important tool in the fields of quantum information processing and quantum optics. The atom-light hybrid interferometer is of great interest due to its combination of an atomic spin wave and an optical wave, which can be utilized for photon number QND measurement via the AC-Stark effect. In this paper, we present an SU(1,1)-SU(2)-concatenated atom-light hybrid interferometer, and theoretically study QND measurement of the photon number. Compared to the traditional SU(2) interferometer, the signal-to-noise ratio in a balanced case is improved by a gain factor of the nonlinear Raman process (NRP) in this proposed interferometer. Furthermore, the condition of high-quality QND measurement is analyzed. In the presence of losses, the measurement quality is reduced. We can adjust the gain parameter of the NRP in the readout stage to reduce the impact due to losses. Moreover, this scheme is a multiarm interferometer, which has the potential of multiparameter estimation with many important applications in the detection of vector fields, quantum imaging, and so on.

Cavity-Free Circulator with Low Insertion Loss Using Hot Atoms

Nonreciprocal optical devices, such as isolators and circulators, provide highly desirable functionalities in routing signals. This motivates the active research on high-efficiency nonreciprocal devices, especially for miniaturized and integrable magnetic-free schemes. All the present experimental demonstrations of circulators in optical domains are based on the high-$Q$ factor cavity. In this work, we demonstrate a cavity-free optical circulator with low insertion loss by making use of a far-detuned nonlinear Raman process with the assistance of the atomic Doppler effect. Due to the large detuning, the excited state can be adiabatically eliminated, resulting in nearly vanishing absorption. An optical circulator with average transmission of more than $85\mathrm{%}$ and fidelity above $0.98$ is well achieved in an experiment with hot atoms. Our scheme shows the possibility of cavity-free nonreciprocal optical devices with low insertion loss.

Theoretical principles of Raman spectroscopy

Abstract This contribution reports on the theoretical foundations of Raman spectroscopy. Since the discovery of the Raman effect in 1928, Raman spectroscopy with its linear and nonlinear variants has established itself as a powerful analytical tool in almost all scientific fields (chemistry, physics, material sciences, pharmacy, biology, (bio)medicine, geology, mineralogy, environmental sciences, etc.). First, a short introduction to linear Raman spectroscopy is given, followed by two approaches to increase the intrinsically weak Raman signal, namely resonance Raman and surface enhanced Raman spectroscopy. The last part of this contribution briefly introduces nonlinear Raman processes observed using pulsed lasers as excitation sources.

Investigation of X-ray Nonlinear Raman Processes

Investigation of X-ray Nonlinear Raman Processes

Q-switched mode-locking of second-Stokes pulses in a diode-pumped YVO4/Nd:YVO4/YVO4 self-Raman laser

A diode-end-pumped simultaneously Q-switched and mode-locked (QML) YVO4/Nd:YVO4/YVO4 self-Raman second-Stokes laser was demonstrated for the first time, to the best of the authors’ knowledge. The shortest mode-locked pulse width of the laser at 1313.4 nm was found to be ~5.7 ps. The maximum average output power, highest pulse energy and highest peak power were found to be 1.33 W, 14.63 μJ and 2.44 MW, respectively. The experimental results confirm that a nonlinear Raman process can greatly compress the QML Stokes pulse width.

Diode-pumped simultaneously Q-switched and mode-locked YVO_4/Nd:YVO_4/YVO_4 crystal self-Raman first-Stokes laser

A diode-end-pumped simultaneously Q-switched and modelocked self-Raman YVO(4)/Nd:YVO(4)/YVO(4) laser at first-Stokes wavelength of 1175.9 nm was demonstrated. The shortest mode-locked pulse width of the laser was obtained to be ~23.57 ps, with the corresponding timebandwidth product of ~0.51. The maximum average output power, the highest pulse energy and the highest peak power were obtained to be 1.83 W, 6.1 μJ and 220 kW, respectively. The nonlinear Raman process improved the Q-switched mode-locking performance of the Stokes pulses.

Opto-mechanical estimation of micro-trap with cold atoms via nonlinear stimulated Raman scattering spectrum

High-gain resonant nonlinear Raman scattering on trapped cold atoms within a high-fineness ring optical cavity is simply explained under a nonlinear opto-mechanical mechanism, and a proposal using it to detect frequency of micro-trap on atom chip is presented. The enhancement of scattering spectrum is due to a coherent Raman conversion between two different cavity modes mediated by collective vibrations of atoms through nonlinear opto-mechanical couplings. The physical conditions of this technique are roughly estimated on Rubidium atoms, and a simple quantum analysis as well as a multi-body semiclassical simulation on this nonlinear Raman process is conducted.

Control of magnetic dipole terahertz radiation by cavity-based phase modulation

Although it is well accepted that the ultrafast manipulation of spins or magnetization in solid promises potential applications in coherent terahertz (THz) radiation source, spintronics and quantum information processing, their performance is significantly limited by the weak coupling between radiation field and magnetic dipole oscillation. For such 'weak' magnetic system, we propose an effective and simple route based on the cavity-based phase modulation technique towards the efficient energy extraction, demonstrated via controlling the magnetic dipole THz radiation generated in the nonlinear Raman process from antiferromagnetic (AFM) NiO. An asymmetric coupled Fabry-Pérot (FP) cavity is constituted by simply placing a metallic planar mirror in the vicinity of a NiO slab. The energy-extraction (THz radiation) can be effectively manipulated by changing the NiO-mirror distance to modulate the phase relation between the magnetic wave and the induced magnetization in NiO. The distinct radiation control can be observed and the experiments are well explained by numerically analyzing the radiation dynamics that highlights the role of phase modulation during the radiation process.

Cooperative optical effects in volumes embedded in layered media

A cluster of point sources can generate optical radiation in a manner substantially different from what characterizes the emission of a single point source. Such differences are mainly caused by the cooperation of the sources and are even more remarkable under particular electromagnetic boundary conditions. Furthermore, the geometry of the problem cannot be ignored as it makes an important contribution to the contrast between single source and collective behaviour. This paper tries to explore the subject in view of its applications to coherent scattering that is typical of non-linear Raman processes. To this end, the classical theories of electromagnetic radiation from a point source (treated as a randomly oriented Hertzian dipole) and of partial coherence are joined into a unified formalism to evaluate light emission from a volume seen as a collection of point sources embedded in a layered medium. In certain reasonable circumstances and beyond the undeniable complexity of the problem, the formalism leads to the relevant advantage of analytical results for the power radiated outside the medium. In other more general cases, fast Fourier transforms can be used in principle. The possibility and convenience of using the theory to model micro-CARS imaging are illustrated and discussed. Copyright © 2010 John Wiley & Sons, Ltd.

Parametric interactions and nonlinear Raman processes of vibronic excitons in polydiacetylene studied with a two‐optical‐cycle laser

AbstractSum and difference frequency mixing, which are the analogues of the second‐order nonlinear optical processes were for the first time found in vibronic exciton systems under high‐density excitations. Cascaded nonlinear Raman scattering was for the first time observed in the exciton system of PDA. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fifth-order nonlinear Raman processes in molecular liquids using quasi-cw noisy light. II. Experiment

Fifth-order analogs of coherent Raman scattering generated in a number of molecular liquids using broadband quasi-cw noisy light are presented. It is seen how the signal for the direct fifth-order process, which probes the dynamics of both a fundamental vibration and its overtone, is often contaminated by a sequential process, that is only capable of probing the vibrational dynamics of the fundamental. Although these two processes are virtually indistinguishable when a single Raman resonance is excited, we find that when a second Raman resonance is available within the experimental window governed by the bandwidth of the noisy light, new frequency components in the signal arise and the two competing fifth-order processes become distinguishable. These new frequency components, as well as their decay, are explained in terms of spectral filtering of the noisy light by the Raman resonances. This spectral filter analogy predicts which of the two competing processes dominates in an equimolar mixture of benzene-h6 and benzene-d6, and also in neat pyridine (each providing two available fundamentals). It is found by this new method that the sequential event is present and apparently fully dominates the direct process for all molecular liquids studied here.

Fifth-order nonlinear Raman processes in molecular liquids using quasi-cw noisy light. I. Theory

Fifth-order nonlinear Raman processes using broadband, incoherent light are treated for a multiply resonant, multicomponent mixture. In particular, the theoretical development of the direct and the sequential fifth-order analogs of coherent Raman scattering is presented. Of the complete formalism, only the dominant doubly Raman resonant hyperpolarizability contributions to the signal intensity are discussed in this article. Furthermore, application is made to simulate fifth-order signals from a variety of hypothetical molecular liquids. It is seen how the direct and the sequential processes can distinguish themselves in a mixture, in a neat liquid with more than one Raman coherence, and also whenever the Raman active modes are taken to be anharmonic. This theoretical treatment anticipates experimental results presented in the following paper.

Femtosecond fifth-order nonlinear response of nuclear motion in liquids investigated by incoherent laser light. Part II. Experiment

A setup for detection of a fifth-order nonlinear Raman process based on incoherent laser light is developed, enabling the introduction of two independent delay times. Depending on phase matching conditions, interference of nonlinear processes of third- and fifth-order or a dominance of one of these processes can be obtained. Possibilities to distinguish between them are discussed and signals of fifth-order Raman-type processes are identified. Regarding the chosen time variable the delay curves are symmetric. Because the correlation time of the incoherent radiation is in the order of magnitude of the fast dephasing time in the liquids, a necessary deconvolution procedure is developed and applied to extract molecular data. A fast dephasing time obtained for nitrobenzene agrees with published data. In mixtures of nitrobenzene with trifluoroethanol the dephasing time decreases, additional excitation of an intramolecular vibration causes an increased effective dephasing times. Both effects could be simulated by independent molecular responses for both kinds of molecules or independent relaxation mechanisms, respectively. From the dependence of the measured FWHM times on the delay time between the two excitations, relations of two relaxation times can be deduced. This information is going beyond that obtainable from third-order coherent techniques.

High-contrast ratio subpicosecond Nd:glass laser with Raman master oscillator

High-energy, high contrast ultrashot laser pulses have been generated by amplification of a Raman-shifted ruby laser radiation in a Nd:glass amplifier. This optical scheme exploits a fortuitous coincidence between the wavelengths of the separate lasers, and the output pulse is dramatically sharpened by two nonlinear Raman processes. A 0.8 ps, 0.15 mJ pulse at the wavelength of 1.056 μm was generated by stimulated Raman scattering of a 2 mJ, 25 ps mode-locked ruby laser pulse in liquid SF 6 and pressured hydrogen. Amplification of the pulse in Nd:phosphate glass rod amplifiers increased the energy to 20 mJ. The brightness of the amplified pulse is 5 × 10 17W cm −2 sr −1. The measured ratio of the amplified pulse brightness to that of ASE exceeded 10 12.

Theoretical analysis of photoacoustic Raman spectroscopy

The theoretical basis for photoacoustic Raman spectroscopy (PARS) is developed. An expression for the Raman gain coefficient is derived. This gain coefficient is used to determine the change in the internal translational energy of a gaseous sample that is produced by illumination with two laser beams whose frequency difference corresponds to a Raman frequency shift. The magnitude of the pressure change associated with this nonlinear Raman process was deduced for a simple quasi-equilibrium model appropriate for modulated cw laser excitation and for a kinetic model applicable to pulsed laser excitation. The kinetic model explicitly accounts for the various pumping and relaxation rates associated with the PARS process.

Resonant four-photon parametric oscillation in the presence of a strong field due to a neighboring transition

An analysis is made of generation of the sum frequency ω3 = 2ω1 + ω2 in the case when the pump frequency coincides with that of a two-photon transition 2ω1 = ω21, and the frequency ω3 is tuned near the resonance line of an atom. Under conditions of resonance with the frequency of the generated field, the parametric process has some characteristic features associated with a strong competition between effects involving increases in the nonlinear susceptibility, absorption, and changes in the phase matching. Moreover, in the presence of a fairly high-intensity field due to a neighboring transition at the frequency ω2, a nonlinear Raman process (which changes the characteristics of the medium at the frequency ω3) begins to play an important part. It is found that the highest parametric oscillation powers over a wide spectral range may be achieved by appropriately varying the intensity of the field due to the neighboring transition, the atom concentration, or the interaction length.