Linear Optical Regime Research Articles

High-g-factor phase-matched second harmonic generation near photonic bandgap of polar cholesteric liquid crystals

ABSTRACT The discovery of polar liquid crystals, such as ferroelectric and helielectric liquid crystals, provides fascinating platforms for designing and modulating polarisation structures that generate strong second harmonic generation. Here, we report that the helielectric nematics, i.e. the polar cholesteric liquid crystals, realise an unprecedented phase-matching condition for the second harmonic generation in the photonic bandgap. Like the circular dichroism in the linear optical regime, the phase-matched second harmonic generation generated in a helielectric nematic medium exhibits a circular polarisation selectivity in the nonlinear optical regime. The maximum g-factor for the second harmonic generation is over ~ 1.95.

Nonlinear control of topological corner and edge states in a loop resonator

The modulation and transformation of topological states play a crucial role in controlling light transport in optical communication system. Herein, we construct a Su-Schrieffer-Heeger (SSH) lattice beyond the linear optical regime. The SSH model has resulted in the topological edge states (TESs) and the topological corner states (TCSs). It is demonstrated that the second-harmonic generation (SHG) and third-harmonic generation (THG) can occur in a topological edge loop based on the frequency matching in all-dielectric photonic crystals. Different from previous studies in which the excitation of TESs and TCSs is achieved through the selection of frequencies, in this paper the excitation of TESs and TCSs has been achieved through the harmonic generations with different orders. More importantly, there is a transformation from the trapping of optical signal on the TCS to the releasing of optical signal on the TES. The mode transformation between TESs and TCSs has important applications in optical flow control, optical signal storing and releasing.

Nonlinear Optical Wavefront Engineering with Geometric Phase Controlled All‐Dielectric Metadevice

AbstractOptical wavefront engineering with artificial nanostructures has important applications in laser manufacturing, bioimaging, optical communications, etc. In the linear optical regime, metasurfaces are widely utilized to realize wavefront engineering functionalities, such as imaging with a flat lens and generation of vortex beams and holographic images. On the other hand, it is highly desirable to directly control the wavefront of light during nonlinear optical processes. Here, the nonlinear optical wavefront engineering with geometric phase controlled all‐dielectric metadevices is experimentally demonstrated. This is realized by integrating the multifunctional dielectric metasurface with the conventional nonlinear optical crystals. It is demonstrated that the nonlinear metadevice can be used to generate optical vortices and nonreciprocal holographic images at second harmonic frequencies. The proposed all‐dielectric metadevice represents an important strategy to develop compact and multifunctional nonlinear optical sources.

Thickness-dependent terahertz emission from Bi2S3 films under excitation below and above the band gap

Two-dimensional (2D) materials demonstrate fascinating thickness-dependent optical properties due to their van der Waals interaction and quantum confinement in the linear optical regime. However, the thickness-dependent nonlinear optical response is more complicated as virtual carriers (dipoles) and real carriers can be generated by below- and above-band-gap excitation, which calls for a more comprehensive understanding of these carriers in 2D materials. Herein, a direct band gap ${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ is utilized to investigate the thickness-dependent nonlinear optical response by a terahertz (THz) emission spectroscopy. The results suggest that the THz emission intensity decreases with the increase of band gap energy under an 800-nm femtosecond laser excitation, which can be described by an empirical exponential equation. Under below-band-gap excitation, the optical rectification effect induced by instantaneous polarization (virtual carriers) dominates the THz emission mechanism in thin ${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ films. In contrast, under above-band-gap excitation, real carriers are controlled by both resonant optical rectification and surface depletion electric field effects, resulting in shift and drift currents in thick ${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ films. The contribution ratio of the shift and drift currents is $\ensuremath{\sim}1:1$ at ${45}^{\ensuremath{\circ}}$ oblique incidence. This competition between shift and drift currents results in the elliptically polarized THz wave generation, and the major axis orientation and ellipticity of the ellipse could be manipulated by changing the polarization of the pump light. In this paper, we imply the potential for designing tunable on-chip THz sources and nonlinear optoelectronic devices based on thickness-dependent 2D materials.

Optical interpretation of linear-optics superradiance and subradiance

Super- and subradiance are usually described in the framework of Dicke collective states, which is an ``atomic picture'' in which the electromagnetic field only provides an effective interaction between the atoms. Here, we discuss a complementary picture, in which we describe the propagation and scattering of light in the atomic medium, which provides a complex susceptibility and scatterers. This ``optical picture'' is valid in the linear-optics regime for disordered samples and is mainly relevant at low density, when the susceptibility and scattering cross-section can be computed from simple textbook formulas. In this picture, superradiance is a dispersion effect due to a single scattering event dressed by an effective refractive index, whereas subradiance is due to multiple scattering. We present numerical and experimental data supporting our interpretation.

Nonlinear Chiroptical Holography with Pancharatnam–Berry Phase Controlled Plasmonic Metasurface

AbstractMetasurface optical holography represents an important technique for developing ultrathin diffractive optical elements. Among various designs, the metasurface devices with strong chiroptical responses offer a new degree of freedom to manipulate light fields. However, in a linear optical regime, it is usually complicated to realize strong chiroptical properties with metasurfaces. Here, nonlinear chiroptical holography using the Pancharatnam–Berry phase controlled achiral metasurface is reported. The metasurface consists of mirror symmetry breaking plasmonic meta‐atoms. Under the pumping of a fundamental wave, the achiral meta‐atoms radiate second harmonic waves with strong circular dichroism and spin dependent Pancharatnam–Berry phase. Both the amplitude and phase of the second harmonic waves can be independently and simultaneously controlled at the subwavelength scale. As a proof of concept, the spin selective nonlinear chiroptical holography is experimentally demonstrated through the second harmonic process on gold plasmonic metasurfaces. The proposed nonlinear metasurface with Pancharatnam–Berry phase and chiroptical properties open new avenues for multifunctional wavefront engineering and optical information processing.

Nonlinear vectorial holography with quad-atom metasurfaces

Vectorial optical holography represents a solution to control the polarization and amplitude distribution of light in the Fourier space. While vectorial optical holography has been experimentally demonstrated in the linear optical regime, its nonlinear counterpart, which can provide extra degrees of freedom of light-field manipulation through the frequency conversion processes, remains unexplored. Here, we experimentally demonstrate the nonlinear vectorial holography through the second harmonic generation process on a quad-atom plasmonic metasurface. The quad-atom metasurface consists of gold meta-atoms with threefold rotational symmetry. Based on the concept of nonlinear geometric phase, we can simultaneously manipulate the phase and amplitude of the left and right circularly polarized second harmonic waves generated from the quad-atom metasurface. By superposing the two orthogonal polarization components, the quad-atom metasurface can produce nonlinear holographic images with vectorial polarization distributions. The proposed metasurface platform may have important applications in vectorial polarization nonlinear optical source, high-capacity optical information storage, and optical encryption.

Second Harmonic Optical Circular Dichroism of Plasmonic Chiral Helicoid-III Nanoparticles.

While plasmonic particles can provide optical resonances in a wide spectral range from the lower visible up to the near-infrared, often, symmetry effects are utilized to obtain particular optical responses. By breaking certain spatial symmetries, chiral structures arise and provide robust chiroptical responses to these plasmonic resonances. Here, we observe strong chiroptical responses in the linear and nonlinear optical regime for chiral L-handed helicoid-III nanoparticles and quantify them by means of an asymmetric factor, the so-called g-factor. We calculate the linear optical g-factors for two distinct chiroptical resonances to -0.12 and -0.43 and the nonlinear optical g-factors to -1.45 and -1.63. The results demonstrate that the chirality of the helicoid-III nanoparticles is strongly enhanced in the nonlinear regime.

Subradiance in dilute atomic ensembles: Role of pairs and multiple scattering

We study numerically the slow (subradiant) decay of the fluorescence of motionless atoms after a weak pulsed excitation. We show that, in the linear-optics regime and for an excitation detuned by several natural linewidths, the slow decay rate can be dominated by close pairs of atoms (dimers) forming superradiant and subradiant states. However, for a large-enough resonant optical depth and at later time, the dynamics is dominated by collective many-body effects. In this regime, we study the polarization and the spectrum of the emitted light, as well as the spatial distribution of excitation inside the sample, as a function of time during the decay dynamics. The behavior of these observables is consistent with what would be expected for radiation trapping of nearly resonant light. This finding sheds light on subradiance in dilute samples by providing an interpretation based on the light behavior of the system (multiple scattering) which is complementary to the more commonly used picture of the collective atomic Dicke state.

Mechano‐opticalbehavior in poly (ethylene terephthalate)/poly (ether imide) blends

AbstractMechano‐optical behavior and related structural evolution during uniaxial stretching of melt miscible poly (ethylene terephthalate) (PET)/poly (ether imide) (PEI) blends were studied near their glass transition temperature using an instrumented machine that measures true stress, true strain and spectral birefringence simultaneously. Stretching from amorphous state, two distinct stress‐optical regimes were observed at temperatures between Tgand Tcc(cold crystallization). Near Tg, a typical photoelastic behavior persists until a critical temperature above which temperature independent initial stress optical behavior is observed. At those temperatures above Tg, where glassy behavior is observed, decreasing stretching rate was also found to eliminate this glassy photo elastic regime leading to the observation of a linear initial stress optical behavior that becomes temperature independent as expected from linear stress optical rule. Increasing PEI concentration in the blends suppresses crystallizability and increases temperature at which initial elastic region disappears giving way to pure liquid behavior where linear stress optical behavior is observed. This is attributed to the increase and broadening of the glass transition temperature with the addition of noncrystallizable PEI. In PET/PEI blends, the stress‐optical coefficient (SOC), determined in a linear stress optical regime, was found to increase linearly with the increase in PEI concentration.

Effect of the thermal–optical nonlinearity on optical trapping Rayleigh particles

In femtosecond-pulsed optical tweezers, the high peak intensity of each laser pulse results in the instantaneous trapping of a particle, while the high-repetition-rate ensures the repetitive trapping of a particle by pulse trains. Therefore, the high peak intensity and high-repetition-rate inevitably lead to the third-order optical nonlinearity and the thermal–optical nonlinearity, respectively. Herein, we investigate the optical forces on the Rayleigh particle using a high-repetition-rate femtosecond-pulsed Gaussian beam when the Kerr nonlinearity and thermal–optical nonlinearity are considered simultaneously. With the help of thermally induced nonlinearity of the surrounding medium to change the relative refractive index of particles, the three-dimensional stable trapping of slightly low-refractive-index particles is realized by the use of high-repetition-rate femtosecond-pulsed Gaussian beams. Besides, we study the influence of thermal–optical nonlinearity on optical trapping of high-refractive-index particles. It is shown that the trapping stiffness increases nonlinearly with the increase of power due to the thermal–optical​ nonlinearity. This work opens up the possibility of trapping the low-refractive-index beyond the linear optics regime, successfully explains the reported experimental observations, and provides the theoretical support for tapping nonlinear optical particles in femtosecond optical tweezers.

Внутридопплеровские резонансы затухания свободной поляризации в ультратонких газовых ячейках

The linear-optical effect of the free polarization decay (FPD) in an ultrathin gas cell, the internal thickness of which is less than or of the order of the wavelength of the exciting monochromatic laser radiation, transmitted orthogonally to the plane-parallel walls of this cell, is theoretically investigated. The considered coherent FPD signal is emitted immediately after an abrupt interruption of such a relatively weak stationary radiation. Sub-Doppler resonances arising at the central frequencies of the studied quantum transitions in the spectra of the intensity and energy of the FPD as a result of the transit-time relaxation of atoms in the gas cell are established and analyzed. A significant dependence of these resonances and the dynamics of the FPD on the ratio of the inner thickness of the ultrathin cell to the wavelength of the exciting monochromatic radiation is shown. The narrow, high-contrast sub-Doppler resonances of the FPD detected and investigated in the linear-optical regime can be used in ultrahigh-resolution atomic (molecular) spectroscopy, as well as references for compact frequency standards.

Nonlinear accelerated orbiting motions of optical trapped particles through two-photon absorption.

Vortex beams carrying optical angular momentum (AM) could drive the orbital motion of a small particle around the optical axis. In general, the orbital rotation speed of trapped particles increases linearly with the increasing laser power. Beyond the linear optics regime, in this work, we investigate both the optical force and torque on a two-photon absorbing Rayleigh particle produced by the tightly focused femtosecond-pulsed circularly polarized vortex beam. Different from the trapping dynamics of particles without two-photon absorption (TPA), it is shown that the orbital motion of trapped particles with TPA accelerates nonlinearly as the laser power increases. Moreover, the orbital motion acceleration of trapped particles is proportional to the TPA coefficient. The corresponding underlying mechanism is discussed in detail. Our results may find interesting applications in the characterization of the optical nonlinearity of a single nanoparticle, and AM manipulation and particle transportation in the nonlinear optics regime.

Nonlinear optical properties of halide perovskites and their applications

Nonlinear optics has undergone dramatic developments in the past 60 years, which has revolutionized the photonic and optoelectronic fields with many essential applications such as electro-optic switching, frequency mixing, optical parametric oscillation, optical phase conjugation, and so forth. As one of the new and promising candidates for both next-generation photovoltaic and optoelectronic devices, halide perovskite semiconductors have attracted extensive research attention because of their excellent electrical and optical properties demonstrated in the linear optical regime. In the past five years, halide perovskites have become a new research frontier of nonlinear optical materials because their highly tunable chemical components and multiple structures provide a variety of outstanding nonlinear optical properties, which support a broad scope of nonlinear optical applications. In this review, we have summarized the nonlinear optical properties of halide perovskites categorized according to the second-, third-, and high-order processes. Aside from the more conventional nonlinear effects, such as sum and difference frequency generation, this review also pays attention to the lesser known but important nonlinear phenomena, such as linear and circular photogalvanic effects, the high-order shift current effect, and the multi-photon pumped photoluminescence. We have also reviewed and summarized the nonlinear applications of halide perovskites, including multi-photon pumped photoluminescence imaging, multi-photon pumped amplified spontaneous emission and lasing, sub-bandgap and self-powered photodetection, all-optical and electro-optic modulation, saturable absorption, optical limiting, and so on. It is our belief that halide perovskites have proven to be excellent candidates for promoting the upgrading and updating of nonlinear optical devices with greatly improved performance and novel functionalities.

Nano‐Kirigami Metasurface with Giant Nonlinear Optical Circular Dichroism

AbstractIn linear optical regime, it is shown that the polarization, amplitude, and phase of light can be easily controlled by using optical metasurfaces, thus enabling the powerful designs of metalenses, high‐efficiency holograms, and so on. In comparison, the manipulation of nonlinear optical processes on the metasurfaces is much more challenging. This is mainly because the metasurface nonlinear source is also modulating the waves emitted from it. Thus, designing nonlinear meta‐atoms with multiple optical functionalities is highly desirable. Herein, the broadband and near‐unity nonlinear optical circular dichroism (CD) of second harmonic generation from a nano‐kirigami plasmonic metasurface is experimentally demonstrated, which is free‐standing with total thickness of about one‐third of the excitation wavelength. The broadband and near unity nonlinear CD is attributed to both the strong CD of fundamental wavelengths and spin‐controlled second harmonic generations from the nano‐kirigami meta‐atoms with threefold rotational symmetries. The proposed nonlinear nano‐kirigami metasurface provides a novel platform for realizing various nonlinear optical functionalities at the nanoscale.

Observation of Floquet solitons in a topological bandgap.

Topological protection is a universal phenomenon that applies to electronic, photonic, ultracold atomic, mechanical, and other systems. The vast majority of research in these systems has explored the linear domain, where interparticle interactions are negligible. We experimentally observed solitons-waves that propagate without changing shape as a result of nonlinearity-in a photonic Floquet topological insulator. These solitons exhibited distinct behavior in that they executed cyclotron-like orbits associated with the underlying topology. Specifically, we used a waveguide array with periodic variations along the waveguide axis, giving rise to nonzero winding number, and the nonlinearity arose from the optical Kerr effect. This result applies to a range of bosonic systems because it is described by the focusing nonlinear Schrödinger equation (equivalently, the attractive Gross-Pitaevskii equation).

Topological insulators go nonlinear

Topological Optics Whereas solid-state insulators tend to be fixed by material properties, photonic topological insulators can be designed at will to mimic a variety of scenarios and complex interactions. Mukherjee and Rechtsman go beyond the linear optical regime that has been studied to date and show that photonic topological insulators can also exhibit nonlinear optical features (see the Perspective by Ablowitz and Cole). Their array of laser-written waveguides can support solitons, which are also found to exhibit topological features, performing cyclotron-like orbits associated with the topology of the lattice. The nonlinear properties provide a rich playground for further exploration, with the possibility of mimicking other interacting bosonic systems. Science , this issue p. [856][1]; see also p. [821][2] [1]: /lookup/doi/10.1126/science.aba8725 [2]: /lookup/doi/10.1126/science.abb5162

Multiplexed Nondiffracting Nonlinear Metasurfaces

AbstractMetasurfaces and planar photonic nanostructures have drawn great interest from the optical scientific community due to their diverse abilities of manipulating electromagnetic waves and high integration. Most metasurfaces launch diffracting waves, and thus suffer from divergence, short working distance, and instability. Although much effort has been devoted to researching nondiffracting metasurfaces which can launch electromagnetic waves with constant transverse intensity profiles in free‐space propagation, the number of working channels is inherently limited as these meta‐devices are implemented in the linear optical regime. Here, the multiplexed nondiffracting nonlinear metasurfaces are theoretically proposed and experimentally realized, which can generate the representative nondiffracting Bessel beam and Airy beam. Three Bessel beams with different numerical apertures and topological charges and three Airy beams with different propagation curves and focal lengths can be generated by a combination of different spins and wavelengths. The complex properties of the nondiffracting beams can be designed and detected in a more comprehensive and concise way with Fourier analysis. This proof‐of‐concept represents a new strategy for realizing multiplexed nondiffracting metasurfaces with advantages of ultracompactness, high‐pixelation, and easy integration and paves the way for multi‐channel optical communication and manipulation.

Polarization-dependent orbital angular momentum flipping in fibers with acousto-optic interaction

In this paper we consider the evolution of linearly polarized optical vortices in circular optical fibers with a propagating fundamental flexural acoustic wave. A new polarization-dependent mode conversion is found out, in which the sign of the topological charge (and orbital angular momentum) of the outgoing vortex beam is governed by the direction of the incident linear polarization. This effect can be used for implementing polarization-controlled orbital angular momentum flipping. This paves the way to implementation of the all-fibre stable controlled-NOT gate, in which the linear polarization carries the control qubit and the topological charge carries the target. Such a gate is able to produce optical beams with entanglement between polarization and orbital degrees of freedom in regime of linear optics. Yet, such orbital angular momentum controlling should be useful in micromechanics, classical and quantum information encoding, and classical simulation of quantum algorithms.

Deep subwavelength lithography via tunable terahertz plasmons

A scheme to overcome diffraction limit in optical lithography via tunable plasmons is proposed. The plasmons are generated by a current-driven instability and are resonance amplified between the drain and source barriers of the transistor. A series of discrete deep subwavelength can be obtained by controlling the gate voltage. Thus, it is possible to realize lithography with a resolution over 1/100 vacuum wavelength and achieve arbitrary one-dimensional and even simple two-dimensional patterns. Our scheme works in the linear optics regime and is easy to be experimentally realized.