Coherent Optical Fields Research Articles

Search for Methods of Restoring Spatial Phase Distribution of Complex Optical Fields

The paper presents principal approaches to diagnosing the structure forming skeleton of the complex optical field. An optimal approach is chosen, which allow to bring much closer the solution of the phase problem of localization speckle-field special points. This paper deals with the development of these concepts and presents different methods for tackling the problem of diagnosing spatial (coordinate) phase distribution with the help of registered intensity distribution in complex optical fields. The results of the computer simulation of the experi- ment fulfilled within the framework of scalar approximation form the basis of the investigation. The algorithm of solving the phase problem in the approximation of scalar coherent optical fields can be divided into several stages. The first one is to obtain complex amplitude-phase transformations in

Tunable terahertz coherent perfect absorption in a monolayer graphene.

Coherent perfect absorber (CPA) was proposed as the time-reversed counterpart to laser: a resonator containing lossy medium instead of gain medium can absorb the coherent optical fields completely. Here, we exploit a monolayer graphene to realize the CPA in a nonresonant manner. It is found that quasi-CPA point exists in the terahertz regime for suspending monolayer graphene, and the CPA can be implemented with the assistance of proper phase modulation among two incident beams at the quasi-CPA frequencies. The graphene-based CPA is found of broadband angular selectivity: CPA point splits into two frequency bands for the orthogonal s and p polarizations at oblique incidence, and the two bands cover a wide frequency range starting from zero frequency. Furthermore, the coherent absorption can be tuned substantially by varying the gate-controlled Fermi energy. The findings of CPA with nonresonant graphene sheet can be generalized for potential applications in terahertz/infrared detections and signal processing with two-dimensional optoelectronic materials.

Optical wavelength conversion via optomechanical coupling in a silica resonator

In an optomechanical resonator, an optically active mechanical mode can couple to any of the optical resonances via radiation pressure. This unique property can enable a remarkable phenomenon: conversion of optical fields via optomechanical coupling between vastly different wavelengths. Here we expand an earlier experimental study [Science 338, 1609 (2012)] on classical wavelength conversion of coherent optical fields by coupling two optical modes to a mechanical breathing mode in a silica resonator. Heterodyne detection of the converted optical fields shows that the wavelength conversion process is coherent and bidirectional. The conversion efficiency obtained features a distinct saturation behavior that arises from optomechanical impedance matching. A measurement of the coherent mechanical excitation involved in the wavelength conversion process also provides additional insight on the underlying optomechanical interactions.

Interconnection of polarization properties and coherence of optical fields.

Theoretical and experimental approaches to diagnosing internal spin and orbital optical flows and the corresponding optical forces caused by these flows are offered. These approaches are based on the investigation of the motion of the particles tested in the formed optical field. The dependence of the above-mentioned forces upon the size and optical properties of the particles is demonstrated. The possibility of using kinematic values defining the motion dynamics of particles of the Rayleigh light scattering mechanism to make a quantitative assessment of the degree of coherence of mutually orthogonal waves that are linearly polarized in the incidence plane is demonstrated. The feasibility of using the above mentioned approach, its shortcomings, and its advantages over the interfering method for estimating the degree of coherence are analyzed.

Wavelength-Tunable Spasing in the Visible

A SPASER, short for surface plasmon amplification by stimulated emission of radiation, is key to accessing coherent optical fields at the nanoscale. Nevertheless, the realization of a SPASER in the visible range still remains a great challenge because of strong dissipative losses. Here, we demonstrate that room-temperature SPASER emission can be achieved by amplifying longitudinal surface plasmon modes supported in gold nanorods as plasmon nanocavities and utilizing laser dyes to supply optical gain for compensation of plasmon losses. By choosing a particular organic dye and adjusting the doping level, the resonant wavelength of the SPASER emission can be tuned from 562 to 627 nm with a spectral line width narrowed down to 5-11 nm. This work provides a versatile route toward SPASERs at extended wavelength regimes.

Spectral coherence anomalies

We describe the anomalous behavior of the spectral degree of coherence as a function of light frequency in the vicinity of phase singularities of partially spatially coherent, polychromatic wave fields. We distinguish the discovered spectral coherence anomalies from conventional spectral anomalies realized with fully spatially coherent optical fields. We also demonstrate how the previously reported spectral anomalies can be engineered in partially spatially coherent fields.

Wiener–Khintchin theorem for spatial coherence of optical wave field

The application of the Wiener–Khintchin theorem for determining the functions of the transversal and longitudinal spatial coherence of the optical field, depending on the parameters of the angular and temporal frequency spectra of the field, is considered. Based on the Wiener–Khintchin theorem, the possibility to derive the van Cittert–Zernike theorem for transversal and longitudinal spatial coherence of the optical field of a spatially incoherent quasi-monochromatic light source is shown. An expression for the dependence of the longitudinal coherence length of the optical field on the width of the frequency and width of the angular spectra of the field is obtained. The results of the interference experiment confirming the dependence of the longitudinal coherence length on the parameters of angular and frequency spectra of the field are presented.

Coherent destruction of tunneling of two interacting bosons in a tight-binding lattice

Recent works [Y.D. Chong {\it et al.}, Phys. Rev. Lett. {\bf 105}, 053901 (2010); W. Wan {\it et al.}, Science {\bf 331}, 889 (2011)] have shown that the time-reversed process of lasing at threshold realizes a coherent perfect absorber (CPA). In a CPA, a lossy medium in an optical cavity with a specific degree of dissipation, equal in modulus to the gain of the lasing medium, can perfectly absorb coherent optical waves at discrete frequencies that are the time-reversed counterpart of the lasing modes. Here the concepts of time-reversal of lasing and CPA are extended for optical radiation emitted by a laser operated in an arbitrary (and generally highly-nonlinear) regime, i.e. for transient, chaotic or periodic coherent optical fields. We prove that any electromagnetic signal $E(t)$ generated by a laser system \textbf{S} operated in an arbitrary regime can be perfectly absorbed by a CPA device $\bf{S'}$ which is simply realized by placing inside \textbf{S} a broadband linear absorber (attenuator) of appropriate transmittance. As examples, we discuss CPA devices that perfectly absorb a chaotic laser signal and a frequency-modulated optical wave.

Composite stokes singularities in coherently and incoherently superimposed vector optical fields

The composite Stokes singularities formed in the coherent and incoherent superposition of two vector monochromatic coherent optical fields are studied and compared analytically and numerically. It is shown that the dynamic behavior of composite Stokes singularities depends on the superposition scheme (coherent or incoherent superposition), initial spatial distributions of the two superimposed fields, and propagation distance, and for the coherent superposition additionally depends on the relative phase of the two fields. There exist S12 (C-points), S31, S23 composite singularities, and L-lines, in particular, V-points and C-lines at the transverse plane. Generally, both V-points and C-lines are unstable. For the coherent superposition the degree of polarization P=1, whereas for the incoherent superposition P may be less than 1 and is variable upon propagation. The results are illustrated analytically and numerically.

Generalized optical interferometry for modal analysis in arbitrary degrees of freedom

We generalize the traditional concept of temporal optical interferometry to any degree of freedom of a coherent optical field. By identifying the structure of a unitary optical transformation that we designate the generalized phase operator, we enable optical interferometry to be carried out in any modal basis describing a degree of freedom. The structure of the generalized phase operator is that of a fractional optical transform, thus establishing the connection between fractional transforms, optical interferometry, and modal analysis.

Coherent perfect absorbers for transient, periodic, or chaotic optical fields: Time-reversed lasers beyond threshold

Recent works [Chong et al., Phys. Rev. Lett. 105, 053901 (2010); Wan et al., Science 331, 889 (2011)] have shown that the time-reversed process of lasing at threshold realizes a coherent perfect absorber (CPA). In a CPA, a lossy medium in an optical cavity with a specific degree of dissipation, equal in modulus to the gain of the lasing medium, can perfectly absorb coherent optical waves at discrete frequencies that are the time-reversed counterpart of the lasing modes. Here the concepts of time reversal of lasing and CPA are extended for optical radiation emitted by a laser operated in an arbitrary regime, i.e., for transient, chaotic, or periodic coherent optical fields. We prove that any electromagnetic signal $E(t)$ generated by a laser system $S$ operated in an arbitrary regime can be perfectly absorbed by a CPA device ${S}^{\ensuremath{'}}$ which is simply realized by placing inside $S$ a broadband linear absorber (attenuator) of appropriate transmittance. As examples, we discuss CPA devices that perfectly absorb a chaotic laser signal and a frequency-modulated optical wave.

Joint segmentation of collectively moving objects using a bag-of-words model and level set evolution

In scenes with collectively moving objects, to disregard the individual objects and take the entire group into consideration for motion characterization is a promising approach with wide application prospects. In contrast to studies on the segmentation of independently moving objects, our purpose is to construct a segmentation of these objects to characterize their motions at a macroscopic level. In general, the collectively moving objects in a group have very similar motion behavior with their neighbors and appear as a kind of global collective motion. This paper presents a joint segmentation approach for these collectively moving objects. In our model, we extract these macroscopic movement patterns based on optical flow field sequences. Specifically, a group of collectively moving objects correspond to a region where the optical flow field has high magnitude and high local direction coherence. As a result, our problem can be addressed by identifying these coherent optical flow field regions. The segmentation is performed through the minimization of a variational energy functional derived from the Bayes classification rule. Specifically, we use a bag-of-words model to generate a codebook as a collection of prototypical optical flow patterns, and the class-conditional probability density functions for different regions are determined based on these patterns. Finally, the minimization of our proposed energy functional results in the gradient descent evolution of segmentation boundaries which are implicitly represented through level sets. The application of our proposed approach is to segment and track multiple groups of collectively moving objects in a large variety of real-world scenes.

Use of motion peculiarities of test particles for estimating degree of coherence of optical fields

We discuss interconnections between the depth of spatial inhomogeneity of energy distribution of the optical fields and the velocity of motion of test particles for the cases of different light scattering mechanisms during interaction with light. We suggest an additional tool for determining the degree of coherence of superposing waves that propagate along mutually orthogonal directions and have orthogonal polarisations, being linearly polarised in the incidence plane. The use of velocity of the test particles while estimating the degree of coherence of optical fields is suggested for the first time.

Probing degenerate two-level atomic media by coherent optical heterodyning

We analyse how light-induced coherent population oscillations and ground-state Zeeman coherence in an atomic medium with degenerate two-level transitions can modify spectra of applied cw resonant radiation at the sub-mW power level. The use of mutually coherent optical fields and heterodyne detection schemes allows spectral resolution at a kHz level, well below the laser linewidth. We find that ground-state Zeeman coherence may facilitate nonlinear wave mixing, while coherent population oscillations are responsible for phase and amplitude modulation of the applied fields. Conditions for the generation of new optical fields by nonlinear wave mixing in degenerate two-level atomic media are formulated.

Effects of phase modulation used for SBS suppression on phase noise in an optical fibre

Phase modulation is used for stimulated Brillouin scattering (SBS) suppression in fibre systems. A novel means is proposed to choose the modulation index and frequency of phase modulation. This means is based on the measurement of phase noise in fibre systems. The phase noise decreases due to SBS suppression first and then increases due to laser linewidth broadening with the modulation index. The phase noise decreases first and then exhibits fluctuations with the modulation frequency. The optimum modulation index and frequency increase with the input power. The visibility of the coherent optical field can be treated as a sign of the laser linewidth. The results provide a guide to the practical design of long-haul interferometric fibre sensing systems.

Infrared antenna measurement of the spatial coherence function

The degree of coherence of a partially coherent monochromatic optical field is measured with a dual-dipole phased-array antenna coupled to a metal-oxide-metal tunnel diode detector. For a two-element phased-array, the degree of coherence is a measure of the correlation of electric fields received by the antennas as a function of the element separation. To extract the coherence function from the measured antenna response, a calibration method is developed to remove propagation loss and device nonuniformity. Measurements at 10.6 µm are substantiated by electromagnetic simulations and compared to the result derived from the Van Cittert-Zernike theorem.

Concentration of Non-maximally Entangled States via Entanglement Swapping in Cavity QED

Using the highly detuned interaction between Λ-type three-level atoms and coherent optical fields in cavity QED, we can create a maximally entangled state between an atom and a cavity mode from a non-maximally entangled atomic state and a non-maximally entangled coherent state via entanglement swapping. The averaged output entanglement of the scheme is the product of the initial two non-maximal entanglements rather than the sum of them. The impact made by the spontaneous emission from the atomic excited levels has been canceled here, which makes the current protocol more feasible.

Room-temperature sub-diffraction-limited plasmon laser by total internal reflection

Plasmon lasers are a new class of coherent optical amplifiers that generate and sustain light well below its diffraction limit. Their intense, coherent and confined optical fields can enhance significantly light-matter interactions and bring fundamentally new capabilities to bio-sensing, data storage, photolithography and optical communications. However, metallic plasmon laser cavities generally exhibit both high metal and radiation losses, limiting the operation of plasmon lasers to cryogenic temperatures, where sufficient gain can be attained. Here, we present a room-temperature semiconductor sub-diffraction-limited laser by adopting total internal reflection of surface plasmons to mitigate the radiation loss, while using hybrid semiconductor-insulator-metal nanosquares for strong confinement with low metal loss. High cavity quality factors, approaching 100, along with strong λ/20 mode confinement, lead to enhancements of spontaneous emission rate by up to 18-fold. By controlling the structural geometry we reduce the number of cavity modes to achieve single-mode lasing.

Controlling metamaterial resonances with light

We investigate the use of coherent optical fields as a means of dynamically controlling the resonant behaviour of a variety of composite metamaterials, wherein the metamaterial structures are embedded in a dispersive dielectric medium. Control and switching is implemented by coherently driving the resonant permittivity of the embedding medium by applied optical radiation. The effect of embedding Split ring resonators (SRR) in a frequency- dispersive medium with Lorentz-like dispersion or with dispersion engineered by electromagnetic induced transparency (EIT), is manifested in the splitting of the negative permeability band, the modified (frequency-dependent) filling fractions and dissipation factors. The modified material parameters are strongly linked to the resonant frequencies of the medium, while for an embedding medium exhibiting EIT, also to the strength and detuning of the control field. The robustness of control against the deleterious influence of dissipation associated with the metallic structures as well as the inhomogeneous broadening due to structural imperfections is demonstrated. Studies on plasmonic metamaterials that consist of metallic nanorods arranged in loops and exhibit a collective magnetic response at optical frequencies are presented. Control and switching in this class of plasmonic nanorod metamaterials is shown to be possible, for example, by embedding these arrays in a Raman active liquid like CS$_2$ and utilizing the Inverse Raman Effect.

Relationship between Complete Coherence in the Space-Time and in the Space-Frequency Domains

We present some new results relating to properties of completely coherent optical fields. Our analysis elucidates the relationship between the theories of such fields in the space-time and in the space-frequency domains. We also show that the concept of cross-spectral purity, introduced by L. Mandel many years ago, plays an important role in clarifying this relationship.