Nonlinear Optical Experiments Research Articles

Nonlinear optics in hollow-core photonic bandgap fibers

Hollow-core photonic-bandgap fibers provide a new geometry for the realization and enhancement of many nonlinear optical effects. Such fibers offer novel guidance and dispersion properties that provide an advantage over conventional fibers for various applications. In this review we summarize the nonlinear optics experiments that have been performed using these hollow-core fibers.

Semiclassical treatments for small-molecule dynamics in low-temperature crystals using fixed and adiabatic vibrational bases

Time-resolved coherent nonlinear optical experiments on small molecules in low-temperature host crystals are exposing valuable information on quantum mechanical dynamics in condensed media. We make use of generic features of these systems to frame two simple, comprehensive theories that will enable the efficient calculations of their ultrafast spectroscopic signals and support their interpretation in terms of the underlying chemical dynamics. Without resorting to a simple harmonic analysis, both treatments rely on the identification of normal coordinates to unambiguously partition the well-structured guest-host complex into a system and a bath. Both approaches expand the overall wave function as a sum of product states between fully anharmonic vibrational basis states for the system and approximate Gaussian wave packets for the bath degrees of freedom. The theories exploit the fact that ultrafast experiments typically drive large-amplitude motion in a few intermolecular degrees of freedom of higher frequency than the crystal phonons, while these intramolecular vibrations indirectly induce smaller-amplitude--but still perhaps coherent--motion among the lattice modes. The equations of motion for the time-dependent parameters of the bath wave packets are fairly compact in a fixed vibrational basis/Gaussian bath (FVB/GB) approach. An alternative adiabatic vibrational basis/Gaussian bath (AVB/GB) treatment leads to more complicated equations of motion involving adiabatic and nonadiabatic vector potentials. Computational demands for propagation of the parameter equations of motion appear quite manageable for tens or hundreds of atoms and scale similarly with system size in the two cases. Because of the time-scale separation between intermolecular and lattice vibrations, the AVB/GB theory may in some instances require fewer vibrational basis states than the FVB/GB approach. Either framework should enable practical first-principles calculations of nonlinear optical signals from molecules in cryogenic matrices and their semiclassical interpretation in terms of electronic and vibrational decoherence and vibrational population relaxation, all within a pure-state description of the macroscopic many-body complex.

Focal-plane imaging of crossed beams in nonlinear optics experiments

An application of focal-plane imaging that can be used as a real time diagnostic of beam crossing in various optical techniques is reported. We discuss two specific versions and demonstrate the capability of maximizing system performance with an example in a combined dual-pump coherent anti-Stokes Raman scattering-interferometric Rayleigh scattering experiment (CARS-IRS). We find that this imaging diagnostic significantly reduces beam alignment time and loss of CARS-IRS signals due to inadvertent misalignments.

Nonlinear optics and spectroscopy at the nanoscale with a hollow-pyramid aperture SNOM

We report on a novel near-field microscope in which ultrashort laser pulses are coupled into hollow-pyramid cantilever probes. The high throughput, absence of polarization pinning and absence of chirping, which are premium features of such probes, enable obtaining sufficient peak power in the near-field to perform nonlinear optical experiments. We show experimental results on second-harmonic generation from metal nanostructures and two-photon excitation of fluorescent conjugated polymers on the subwavelength scale.

Dark and bright vortex solitons in electromagnetically induced transparent media

We show that dark and bright vortex solitons can exist in three-state electromagnetically induced transparent media under some appropriate conditions. We also analyse the stability of the dark and bright vortex solitons. This work may provide other research opportunities in nonlinear optical experiments and may result in a substantial impact on technology.

Bright, dark, bistable bright, and vortex spatial-optical solitons in a cold three-state medium

We show the formation of bright, dark, bistable bright, and vortex spatial-optical solitons in a lifetime-broadened three-state atomic medium. We demonstrate, both analytically and numerically, that these solitons can exist under appropriate conditions. This work may provide other research opportunities in nonlinear optical experiments and may result in a substantial impact on technology.

Optical Structures and their Control in a Liquid-Crystal-Light-Valve Experiment

We review the conditions for the control of optical structures in a nonlinear optical experiment, which is a Liquid-Crystal-Light-Valve with optical feedback. The optical structures we deal with may be either localized states or spatially extended patterns. Indeed, the localized structures here described represent the localized solutions of a pattern-forming system. We will show that there exists different regions of existence of such localized states, and that different types of localized structures can be selected depending on the choice of the control parameters. Localized states, stable for some range of parameters, are in general characterized by complex spatio-temporal dynamics. In the same way, spatially extended patterns show space-time chaotic behaviors. We show that pattern targeting and control may be achieved by adding an extra control loop to the experiment. Different target patterns can be stabilized in real time and synchronization between space-time chaotic signals is demonstrated.

Localized structures in an optical feedback interferometer: properties and interactions

The occurrence of localized structures in a nonlinear optical experiment is a robust phenomenon, persisting in a wide region of the parameter space. We demonstrate how different control parameters determine several properties of localized structures; among these of particular relevance are the contrast and the amplitude and frequency of the oscillations appearing on the tails of the structures. Tuning these oscillations modifies the interactions between pairs of localized structures, thus resulting in a substantial modification of their bound-state spectrum.

Spontaneous nucleation of localized peaks in a multistable nonlinear system

In a nonlinear optical experiment we report a unique class of localized structures, which appears as localized peaks of a pattern nucleating over another pattern. We show that this occurs when the system is driven through three pattern branches of solutions, accompanied by the appearance of localized peaks with two different amplitudes. Spontaneous creation and motion of localized peaks are triggered by amplitude and phase fluctuations of the underlying pattern. The scenario is universal and applies whenever a subcritical bifurcation exists between two different pattern solutions.

Electromagnetically induced transparency: Optics in coherent media

Coherent preparation by laser light of quantum states of atoms and molecules can lead to quantum interference in the amplitudes of optical transitions. In this way the optical properties of a medium can be dramatically modified, leading to electromagnetically induced transparency and related effects, which have placed gas-phase systems at the center of recent advances in the development of media with radically new optical properties. This article reviews these advances and the new possibilities they offer for nonlinear optics and quantum information science. As a basis for the theory of electromagnetically induced transparency the authors consider the atomic dynamics and the optical response of the medium to a continuous-wave laser. They then discuss pulse propagation and the adiabatic evolution of field-coupled states and show how coherently prepared media can be used to improve frequency conversion in nonlinear optical mixing experiments. The extension of these concepts to very weak optical fields in the few-photon limit is then examined. The review concludes with a discussion of future prospects and potential new applications.

Enhancement of Raman scattering for an atom or molecule near a metal nanocylinder: Quantum theory of spontaneous emission and coupling to surface plasmon modes

An analytic expression for the electromagnetic enhancement of the spontaneous emission rate and Raman scattering cross-section for an excited atom or molecule in close proximity to a metal nanocylinder has been derived by quantum theory. Coupling of the atomic or molecular optical radiation into the TM0 surface plasmon mode of the nanocylinder results in reradiation by the cylinder, a process that is most efficient when the incident radiation is linearly polarized, with the electric field oriented parallel to the axis of the nanocylinder. For a silver cylinder having a radius and length of 5 and 20 nm, respectively, the enhancement in the spontaneous emission rate is >10(7) for variant Planck's over 2pi omega0 approximately 2.4 eV (lambda=514 nm), which corresponds to an increase of approximately 10(14) in the Raman scattering cross section. This result, as well as the prediction that the atomic dipole generates broadband, femtosecond pulses, are in qualitative agreement with previously reported experiments involving metal nanoparticle aggregates. The theoretical results described here are expected to be of value in guiding future nonlinear optical experiments in which carbon nanotubes or metal nanowires with controllable physical and electrical characteristics are patterned onto a substrate and coupled with emitting atoms or molecules.

Characterization of femtosecond light pulses coupled to hollow-pyramid near-field probes: Localization in space and time

We perform the in situ characterization, by second-order optical autocorrelation, of femtosecond pulses transmitted by near-field optical probes. We demonstrate that transmission through hollow pyramid probes with diameter down to 65 nm has negligible effects on the duration of pulses as short as 30 fs. We also show that such probes allow obtaining, at their output, sufficient peak power to perform nonlinear optical experiments in the near field on such a space and time scale.

Nonlinear optical spectroscopy of chiral molecules

We review nonlinear optical processes that are specific to chiral molecules in solution and on surfaces. In contrast to conventional natural optical activity phenomena, which depend linearly on the electric field strength of the optical field, we discuss how optical processes that are nonlinear (quadratic, cubic, and quartic) functions of the electromagnetic field strength may probe optically active centers and chiral vibrations. We show that nonlinear techniques open entirely new ways of exploring chirality in chemical and biological systems: The cubic processes give rise to nonlinear circular dichroism and nonlinear optical rotation and make it possible to observe dynamic chiral processes at ultrafast time scales. The quadratic second-harmonic and sum-frequency-generation phenomena and the quartic processes may arise entirely in the electric-dipole approximation and do not require the use of circularly polarized light to detect chirality. They provide surface selectivity and their observables can be relatively much larger than in linear optical activity. These processes also give rise to the generation of light at a new color, and in liquids this frequency conversion only occurs if the solution is optically active. We survey recent chiral nonlinear optical experiments and give examples of their application to problems of biophysical interest.

Selection rules for probing biexcitons and electron spin transitions in isotropic quantum dot ensembles

Three-dimensional rotational averages are evaluated for third-order nonlinear spectroscopic measurements of quantum dots. Photon echo, transient grating, and transient absorption are explicitly considered. It is shown that (a) biexciton formation can be suppressed relative to other contributions to nonlinear spectroscopies for isotropic nanocrystal ensembles by choice of polarizations for the excitation pulses; (b) circularly polarized excitation light can differentiate between exciton spin states in nonlinear optical experiments; and (c) electron spin state flip kinetics can be probed directly in an isotropic quantum dot system by using certain sequences of linear cross-polarized pulses.

Dynamical Models in the Two‐Dimensional Infrared Spectroscopy of Peptides

AbstractA brief description of the signals in multidimensional infrared spectroscopy is given with emphasis on the various types of experiments that can be done, what they measure, and their relationship to other kinds of resonant nonlinear optical experiments. The predictions from Bloch dynamics and responses including spectral diffusion are discussed. Experiments with the amide‐I transitions of N‐methylacetamide and the alanine dipetide show that Bloch dynamics are inadequate, but for small values of waiting times can give a reasonable description of the 2D spectra. Both single frequency and dual frequency 2D experiments that excite both the amide‐I and amide‐II modes are reported.

Theory of Ultrafast Nonlinear Refraction in Semiconductor Superlattices

A theoretical study is undertaken into the Kerr nonlinear refraction observable in the transparency window in semiconductor heterostructures. Symmetry considerations indicate that the number of nonzero, independent tensor components increases from three to eight for single wavelength third-order nonlinear processes with the usual growth orientation. These are related to the observable phenomena of self-phase-modulation, cross-phase-modulation, and four-wave-mixing of the orthogonal polarization components in the usual waveguide geometry. For a 14:14 GaAs-AlAs superlattice, all eight independent elements are calculated. It is shown that the nonlinear response is highly anisotropic. The modification of the nonlinear coefficients with quantum well intermixing is investigated, with a several-fold reduction in value predicted. This modification could be exploited in some long-standing and novel nonlinear optical experiments.

Water dynamics: dependence on local structure probed with vibrational echo correlation spectroscopy

Ultrafast infrared vibrational echo correlation spectroscopy with full phase information of the entire O–D stretching band of HOD in H 2O and molecular dynamics simulations are employed to investigate water dynamics. The wavelength dependence of the measured dynamics demonstrates that different hydrogen bonded water species are subject to distinct ultrafast (∼100 fs) local fluctuations and essentially identical slower (0.4 ps to ∼2 ps) structural rearrangements. Simulations provide insights into the nature of the very fast and slower dynamics. The results also show that the theoretical methods that are widely used in the description of nonlinear optical experiments need to be advanced to adequately describe water dynamics.

A Nonlinear Optical Experiment: Stimulated Raman Scattering in Benzene and Deuterated Benzene

A nonlinear optical experiment is presented that utilizes a doubled Nd:YAG laser to produce stimulated Raman scattering (SRS) in benzene and deuterated benzene. A sampling arrangement is described that results in strong stimulated Raman emission involving symmetric CC and CH(CD) stretching modes in both C6H6 and C6D6. The wavelengths of the coherent beams are easily measured with a simple transmission grating and meter stick. Observations of both collinear and conical stimulated Raman beams are made and the reasons for these discussed. The measured frequencies permit calculation of CC and CH force constants in benzene.

Spatial synchronization of regular optical patterns

We investigate an extended, nonlinear optical experiment, exhibiting the spontaneous formation of hexagonal patterns out of a stationary bifurcation. The system is exposed to a two-dimensional spatially periodic forcing, namely, static hexagonal patterns, under variation of their spatial periodicity. Parameters are the strength of the forcing and the distance to pattern forming threshold. The system response is quantitatively characterized with different methods. We observe several locking regimes, where the system is entrained by the forcing. Most of the locking regimes can be related to resonances between the different critical wave numbers and the forcing wave number or its spatial harmonics. One particular locking appears to result from two of these simple resonances in a kind of generalized order m:n synchronization. The width of the locking regimes increases with forcing strength, apparently representing a spatial analog of Arnold tongues.

Generation and characterization of spatially and temporally localized few-cycle optical wave packets

Direct experimental evidence of localized few-cycle wave packets with a characteristic spatiotemporal X shape is reported. These wave packets are also referred to as X pulses. For their generation, a Ti:sapphire laser beam was transformed into single and array-shaped Bessel-like beams by using broadband refractive and reflective thin-film axicons. Extremely small conical angles allow a reduction of the spatially induced group velocity dispersion and therefore enhance the contrast. Complete first and second order spatiotemporal autocorrelation maps of sub-10-fs localized wave packets are measured and are compared with numerical simulations. High-power X pulses open further prospects for advanced nonlinear optical experiments like the formation of spatiotemporal solitons.