Nonlinear Optical Experiments Research Articles

Spectral Dynamics of Spatially Incoherent Modulation Instability

To date, all experiments in nonlinear statistical optics have relied on beams whose transverse spatial statistics were Gaussian. Here, we present a new technique to generalize these studies by using a spatial light modulator to create spatially incoherent beams with arbitrary spectral distributions. As a specific example of the new dynamics possible, we consider the spatial modulation instability of a partially coherent beam. We show that, for statistical beams of uniform intensity and equal correlation length, the underlying spectral shape determines the threshold and visibility of intensity modulations as well as the spectral profile of the growing sidebands. We demonstrate the behavior using statistical light, but the results will hold for any wave-kinetic system, such as plasma, ultracold gases, and turbulent acoustic waves.

Two-dimensional micro-Raman mapping of stress and strain distributions in strained silicon waveguides

Micro-Raman scattering experiments were performed on strained silicon waveguides designed for nonlinear optical experiments. Thin stressing silicon nitride (Si3N4 or SiNx) cladding layers, deposited on a light-guiding silicon core layer, strain silicon to enable χ(2), the second-order nonlinear susceptibility. Different deposition treatments allow varying the applied stress. The resulting strained waveguides are investigated by micro-Raman confocal spectroscopy performed on the waveguide facet. By modelling the measured Raman shifts, the local stress and strain are extracted. Thus, two-dimensional maps of the stress distribution as a function of the SiNx deposition parameters are drawn. A comparison of different samples allows underlying the non-uniformity of resulting strain.

Self-referenceable frequency comb from an ultrafast thin disk laser

We present the first measurement of the carrier envelope offset (CEO) frequency of an ultrafast thin disk laser (TDL). The TDL used for this proof-of-principle experiment was based on the gain material Yb:Lu(2)O(3) and delivered 7 W of average power in 142-fs pulses, which is more than two times shorter than previously realized with this material. Using only 65 mW of the output of the laser, we generated a coherent octave-spanning supercontinuum (SC) in a highly nonlinear photonic crystal fiber (PCF). We detected the CEO beat signal using a standard f-to-2f interferometer, achieving a signal-to-noise ratio of >25 dB (3 kHz resolution bandwidth). The CEO frequency was tunable with the pump current with a slope of 33 kHz/mA. This result opens the door towards high-power frequency combs from unamplified oscillators. Furthermore, it confirms the suitability of these sources for future intralaser extreme nonlinear optics experiments such as high harmonic generation and VUV frequency comb generation from compact sources.

Phase Transition Diagnostic in Iron Telluride by Nonlinear Optical Experiments

I.V. Kityk, R. Viennois and K. Plucinski Electrical Engineering Dept., Czestochowa University of Technology, Armii Krajowej 17, Czestochowa, Poland Institut Charles Gerhardt, University Montpellier 2 and CNRS, Pl. E. Bataillon, Montpellier, France Condensed Matter Physics Department, University of Geneva, 21 Quai E. Ansermet, Geneva, Switzerland Electronic Department, Military University of Technology, Kaliskiego 2, 00-908, Warsaw, Poland

Study of photonic crystal fiber with an air core

Photonic crystal fibers with an air hole in the core are analyzed by finite-element method. The relations of mode field distribution, confinement loss and dispersion characteristics with fiber structure parameter and wavelength are achieved. The principle of guiding light in air hole is explained by diffraction and the characteristics of photonic crystal fiber. For the fiber with low loss, single-mode, tightly confined light in the air core, the ranges of structure parameters and wavelengths are obtained. An optimal fiber structure is designed, of which the mode is tightly confined in the air core, and the confinement loss α=5.9× 10-5 dB/km. These provide theoretical instruction for the design and fabrication of photonic crystal fiber with an air hole in the core. The high intensity in an air hole, coupled with long interaction length, promises a new class of experiments in light-matter interaction and nonlinear fiber optics.

Giant nonlinear optical activity in a plasmonic metamaterial

In 1950, a quarter of a century after his first-ever nonlinear optical experiment when intensity-dependent absorption was observed in uranium-doped glass, Sergey Vavilov predicted that birefringence, dichroism and polarization rotatory power should be dependent on light intensity. It required the invention of the laser to observe the barely detectable effect of light intensity on the polarization rotatory power of the optically active lithium iodate crystal, the phenomenon now known as the nonlinear optical activity, a high-intensity counterpart of the fundamental optical effect of polarization rotation in chiral media. Here we report that a plasmonic metamaterial exhibits nonlinear optical activity 30 million times stronger than lithium iodate crystals, thus transforming this fundamental phenomenon of polarization nonlinear optics from an esoteric phenomenon into a major effect of nonlinear plasmonics with potential for practical applications.

Light confinement by a cylindrical metallic waveguide in a dense buffer-gas environment

We report on the implementation of metallic microtubes in a system of rubidium vapour at 230\,bar of argon buffer gas. The high buffer gas pressure leads to a widely pressure broadened linewidth of several nanometers, interpolating between the sharp atomic physics spectra and the band structure of solid state systems. Tube-like metallic waveguide structures have been inserted in the high pressure buffer gas system, allowing for an enhancement of the atom-light interaction over an optical guiding length in the tube of up to 1\,mm. The system holds promise for nonlinear optics experiments and the study of atom-light polariton condensation.

Condensation and thermalization of classsical optical waves in a waveguide

We consider the long-term evolution of a random nonlinear wave that propagates in a multimode optical waveguide. The optical wave exhibits a thermalization process characterized by an irreversible evolution toward an equilibrium state. The tails of the equilibrium distribution satisfy the property of energy equipartition among the modes of the waveguide. As a consequence of this thermalization, the optical field undergoes a process of classical wave condensation, which is characterized by a macroscopic occupation of the fundamental mode of the waveguide. Considering the nonlinear Schr¨ odinger equation with a confining potential, we formulate a wave turbulence description of the random wave into the basis of the eigenmodes of the waveguide. The condensate amplitudeiscalculatedanalyticallyasafunctionofthewaveenergy,anditisfoundinquantitativeagreementwith the numerical simulations. The analysis reveals that the waveguide configuration introduces an effective physical frequency cutoff, which regularizes the ultraviolet catastrophe inherent to the ensemble of classical nonlinear waves. The numerical simulations have been performed in the framework of a readily accessible nonlinear fiber optics experiment.

Manifestation of Hamiltonian Monodromy in Nonlinear Wave Systems

We show that the concept of dynamical monodromy plays a natural fundamental role in the spatiotemporal dynamics of counterpropagating nonlinear wave systems. By means of an adiabatic change of the boundary conditions imposed to the wave system, we show that Hamiltonian monodromy manifests itself through the spontaneous formation of a topological phase singularity (2π- or π-phase defect) in the nonlinear waves. This manifestation of dynamical Hamiltonian monodromy is illustrated by generic nonlinear wave models. In particular, we predict that its measurement can be realized in a direct way in the framework of a nonlinear optics experiment.

Analysis of the effect of phase-mismatch and material absorption on the terahertz-wave generation from GaSe

We analyzed the effect of phase-mismatch and material absorption on the terahertz-wave difference-frequency generation from GaSe theoretically. We calculated the best length of crystal and the corresponding terahertz power under four different conditions and the effect of angle-mismatch on phase-mismatch. The result provided a theoretical basis and reference to nonlinear optical difference-frequency experiments.

Relaxation processes in systems strongly coupled to a harmonic bath

The simulation of nonlinear optical experiments in the condensed phase often requires including relaxation between different states of matter due to interaction with an external bath. This is commonly done using second-order Redfield or Lindblad equations. Here, we derive closed expressions for relaxation equations to any order in the coupling to a harmonic bath. This is done using a compact superoperator notation combined with reduced equations of motions based on Time Convolutionless theory (TCL), partial ordering prescription.

Partial-coherence method to model experimental free-electron laser pulse statistics

A general numerical approach is described that allows obtaining model sets of temporal pulse shapes of free-electron lasers (FELs) operating in the self-amplified spontaneous emission mode. Based on a random partial-coherence approach, sets of pulse shapes can be calculated that satisfy statistical criteria of FEL light predicted by established FEL theory. Importantly, the numerically retrieved sets of pulses reproduce the experimentally accessible FEL light characteristics as measured at the Free-electron LASer at Hamburg (FLASH), such as the average spectrum, single-shot spectral shape, and pulse duration. The high-precision agreement with the experimental average spectral shape, without further knowledge of FEL machine parameters, makes this approach a convenient tool for the analysis and theoretical modeling of nonlinear optical or pump-probe experiments with FEL light.

Correlated fluctuations in the exciton dynamics and spectroscopy of DNA

The absorption of ultraviolet light creates excitations in DNA, which subsequently start moving in the helix. Their fate is important for an understanding of photodamage, and is determined by the interplay of electronic couplings between bases and the structure of the DNA environment. We model the effect of dynamical fluctuations in the environment and study correlation, which is present when multiple base pairs interact with the same mode in the environment. We find that the correlations strongly affect the exciton dynamics, and show how they are observed in the decay of the anisotropy as a function of coherence and population time in a nonlinear optical experiment.

Boundary-induced localized structures in a nonlinear optical feedback experiment

Experimental and numerical evidence of symmetry-breaking bifurcations of a circular dissipative soliton with additional boundary conditions in the feedback of a liquid crystal light valve are reported. By tuning the strength of the nonlinearity or the size of the additional boundaries, the circular structure breaks up into polygonal symmetries and the system exhibits multistability. The experimental results are confirmed by numerical simulations with different configurations of the polarizers thus demonstrating the universality of the phenomenon.

Defects in nanothin crystalline layers and multilayer structures formed of them

Optical spectra of multilayer structures of nanothin films with nanocrystals of defined size and undoped crystals with given content of the impurities were compared. Relationships among the content of impurities, colloids and properties of the initial crystals, and multilayer structures were examined. The influence of surface effects on size-dependent properties of nanothin layers with LiF, CaF2, CdS and CdSe nanocrystals were investigated. Microstructure testing, optical absorption, photoluminescence and nonlinear optical experiments indicate that the possible origins of nanothin layers peculiarities are their purification and the metal excess accumulation in their boundaries.

Absolute measurement of the nonlinear refractive indices of reference materials

We report absolute measurements of the nonlinear refractive index on carbon disulfide (CS2) and fused silica. These materials are commonly used as standard references in nonlinear optical experiments. To obtain more accurate values than those usually used, we have combined the z-scan method inside a 4-f imaging system (in order to analyze the spatial distortion of the diffracted pump beam) with the “Kerr shutter” experiment (to evaluate the temporal pulse width durations for three different wavelengths such as 1064, 532, and 355 nm). We obtained surprisingly n2 values one order of magnitude less than the one usually taken into account in the picosecond regime and a more significant dispersion of the nonlinear refraction index. Experimental and simulated Z-scan transmittance profiles as well as acquired autocorrelation functions in the Kerr-gating experiments are presented here in order to validate our measurements.

Push–pull structures with a pyrazine core and hexatriene chain: synthesis and light-emitting properties

In this paper, we describe the synthesis of various push–pull molecules with a central pyrazine unit connected to a hexatriene chain terminated by various p-substituted phenyl groups. The key steps involve metallation and subsequent transmetallation of 2-chloro and 2,6-dichloropyrazine followed by a Negishi cross-coupling reaction of the intermediate organozinc derivative with (2 E,4 E)-5-bromopentadienal. The aldehydes are then submitted to a Wittig reaction with the appropriate phosphonium salts readily obtained from various substituted benzyl alcohols. The light-emitting properties of the so obtained molecules are then investigated in terms of absorption and emission spectra and non-linear optics experiments have been carried out in two cases.

Detuning management of optical solitons in coupled quantum wells

We show the possibility to generate bright and dark optical solitons based on bound-to-bound intersubband transitions in an asymmetric three-coupled quantum well structure. By presenting the concept of detuning management, we show that the bright optical soliton can evolve into a dark one by adiabatically changing the corresponding one- and two-photon detunings. With adjustable carrier frequencies within the terahertz regime, we also demonstrate numerically shape-recovered collisions of two solitons in our proposed quantum wells. The present investigation may provide research opportunities in nonlinear optical experiments and may have impact on the technology for the design of semiconductor devices.

Isochronic carrier-envelope phase-shift compensator

A concept for orthogonal control of phase and group delay inside a laser cavity by a specially designed compensator assembly is discussed. Similar to the construction of variable polarization retarder, this assembly consists of two thin wedge prisms made from appropriately chosen optical materials. Being shifted as a whole, the assembly allows changing the phase delay with no influence on the cavity round-trip time, whereas relative shifting of the prisms enables adjustment of the latter. This scheme is discussed theoretically and verified experimentally, indicating a factor 30 reduction of the influence on the repetition rate compared to the commonly used silica wedge pair. For a 2pi adjustment of the carrier-envelope phase shift, single-pass timing differences are reduced to the single-femtosecond regime. With negligible distortions of timing and dispersion, the described compensator device greatly simplifies carrier-envelope phase control and experiments in extreme nonlinear optics.

Theory and phase-cycling scheme selection principles of collinear phase coherent multi-dimensional optical spectroscopy

We present the theory and the selection procedure of phase-cycling schemes for phase coherent multidimension optical spectroscopy. We apply our selection procedure to determine the phase-cycling schemes with the least number of steps needed to measure the two-dimensional spectra of various time resolved four wave mixing optical processes. The phase-cycling scheme selection procedure presented in this paper can be applied to higher order nonlinear optical experiments involving more optical pulses that measure higher dimensional optical spectra.