Laser harmonic generation with independent control of frequency and orbital angular momentum

The non-linear optical process of laser harmonic generation (HG) enables the creation of high quality pulses of UV or even X-ray radiation, which have many potential uses at the frontiers of experimental science, ranging from lensless microscopy to ultrafast metrology and chiral science. Although many of the promising applications are enabled by generating harmonic modes with orbital angular momentum (OAM), independent control of the harmonic frequency and OAM level remains elusive. Here we show, through a theoretical approach, validated with 3D simulations, how unique 2-D harmonic progressions can be obtained, with both frequency and OAM level tuned independently, from tailored structured targets in both reflective and transmissive configurations. Through preferential selection of a subset of harmonic modes with a specific OAM value, a controlled frequency comb of circularly polarised harmonics can be produced. Our approach to describe HG, which simplifies both the theoretical predictions and the analysis of the harmonic spectrum, is directly applicable across the full range of HG mechanisms and can be readily applied to investigations of OAM harmonics in other processes, such as OAM cascades in Raman amplification, or the analysis of harmonic progressions in nonlinear optics.

Electrically interfaced Brillouin-active waveguide for microwave photonic measurements

New strategies for converting signals between optical and microwave domains could play a pivotal role in advancing both classical and quantum technologies. Traditional approaches to optical-to-microwave transduction typically perturb or destroy the information encoded on intensity of the light field, eliminating the possibility for further processing or distribution of these signals. In this paper, we introduce an optical-to-microwave conversion method that allows for both detection and spectral analysis of microwave photonic signals without degradation of their information content. This functionality is demonstrated using an optomechanical waveguide integrated with a piezoelectric transducer. Efficient electromechanical and optomechanical coupling within this system permits bidirectional optical-to-microwave conversion with a quantum efficiency of up to -54.16 dB. Leveraging the preservation of the optical field envelope in intramodal Brillouin scattering, we demonstrate a multi-channel microwave photonic filter by transmitting an optical signal through a series of electro-optomechanical waveguide segments, each with distinct resonance frequencies. Such electro-optomechanical systems could offer flexible strategies for remote sensing, channelization, and spectrum analysis in microwave photonics.

SRS-Net: a universal framework for solving stimulated Raman scattering in nonlinear fiber-optic systems by physics-informed deep learning

As a crucial nonlinear phenomenon, stimulated Raman scattering (SRS) plays multifaceted roles involved in forward and inverse problems. In fibre-optic systems, these roles range from detrimental interference that impairs optical performance to beneficial effects that enables various devices such as Raman amplifier. To obtain solutions of SRS, various numerical methods customized for different scenarios have been proposed. However, these methods are time-consuming, low-efficiency, and experience-orientated, particularly in combined scenarios consisting of both forward and inverse problems. Inspired by physics-informed neural networks, we propose SRS-Net, which combines the efficient automatic differentiation and powerful representation ability of neural networks with the regularization of SRS physical laws, to obtain universal solutions for SRS of forward, inverse, and combined problems. We showcase the intuitive solving procedure and high-speed performance of SRS-Net through extensive simulations covering different scenarios. Additionally, we validate its capabilities in experiments involving the high-fidelity modelling of a wavelength division multiplexing system spanning the C + L-band with approximately 10 THz. The versatility of the SRS-Net framework extends beyond SRS, indicating its potential as a promising universal solution in other engineering problems with nonlinear dynamics governed by partial differential equations.

Voltage-controlled nonlinear optical properties in gold nanofilms via electrothermal effect

Dynamic control of the optical properties of gold nanostructures is crucial for advancing photonics technologies spanning optical signal processing, on-chip light sources and optical computing. Despite recent advances in tunable plasmons in gold nanostructures, most studies are limited to the linear or static regime, leaving the dynamic manipulation of nonlinear optical properties unexplored. This study demonstrates the voltage-controlled Kerr nonlinear optical response of gold nanofilms via the electrothermal effect. By applying relatively low voltages (~10 V), the nonlinear absorption coefficient and refractive index are reduced by 40.4% and 33.1%, respectively, due to the increased damping coefficient of gold nanofilm. Furthermore, a voltage-controlled all-fiber gold nanofilm saturable absorber is fabricated and used in mode-locked fiber lasers, enabling reversible wavelength-tuning and operation regimes switching (e.g., mode-locking-Q-switched mode-locking). These findings advance the understanding of electrically controlled nonlinear optical responses in gold nanofilms and offer a flexible approach for controlling fiber laser operations.

Programmable generation of counterrotating bicircular light pulses in the multi-terahertz frequency range

The manipulation of solid states using intense infrared or terahertz light fields is a pivotal area in contemporary ultrafast photonics research. While conventional circular polarization has been well explored, the potential of counterrotating bicircular light remains widely underexplored, despite growing interest in theory. In the mid-infrared or multi-terahertz region, experimental challenges lie in difficulties in stabilizing the relative phase between two-color lights and the lack of available polarization elements. Here, we successfully generated phase-stable counterrotating bicircular light pulses in the 14-39 THz frequency range circumventing the above problems. Employing spectral broadening, polarization pulse shaping with a spatial light modulator, and intra-pulse difference frequency generation leveraging a distinctive angular-momentum selection rule within the nonlinear crystal, we achieved direct conversion from near-infrared pulses into the designed counterrotating bicircular multi-terahertz pulses. Use of the spatial light modulator enables programmable control over the shape, orientation, rotational symmetry, and helicity of the bicircular light field trajectory. This advancement provides a novel pathway for the programmable manipulation of light fields, and marks a significant step toward understanding and harnessing the impact of tailored light fields on matter, particularly in the context of topological semimetals.

Many-body enhancement of high-harmonic generation in monolayer MoS2

Many-body effects play an important role in enhancing and modifying optical absorption and other excited-state properties of solids in the perturbative regime, but their role in high harmonic generation (HHG) and other nonlinear response beyond the perturbative regime is not well-understood. We develop here an ab initio many-body method to study nonperturbative HHG based on the real-time propagation of the non-equilibrium Green's function with the GW self energy. We calculate the HHG of monolayer MoS2 and obtain good agreement with experiment, including the reproduction of characteristic patterns of monotonic and nonmonotonic harmonic yield in the parallel and perpendicular responses, respectively. Here, we show that many-body effects are especially important to accurately reproduce the spectral features in the perpendicular response, which reflect a complex interplay of electron-hole interactions (or exciton effects) in tandem with the many-body renormalization and Berry curvature of the independent quasiparticle bandstructure.

Unraveling the structure-property relationship of novel thiophene and furan-fused cyclopentadienyl chromophores for nonlinear optical applications

Development of organic nonlinear optical materials has become progressively more important due to their emerging applications in new-generation photonic devices. A novel series of chromophores based on innovative thiophene and furan-fused cyclopentadienyl bridge with various powerful donor and acceptor moieties were designed and theoretically investigated for applications in nonlinear optics. To unravel the structure-property relationship between this new push-pull conjugated systems and their nonlinear optical property, multiple methods, including density of states analysis, coupled perturbed Kohn-Sham (CPKS) method, sum-over-states (SOS) model, the two-level model (TSM), hyperpolarizability density analysis, and the (hyper)polarizability contribution decomposition, were performed to comprehensively investigated the nonlinear optical and electronic properties of this new π-system. Due to excellent charge transfer ability of new bridge and distinctive structure of donor and acceptor, the designed chromophores exhibit deep HOMO levels, low excitation energy, high dipole moment difference and large hyperpolarizability, indicating the appealing air-stable property and remarkable electrooptic performance of them. Importantly, THQ-CS-A3 and PA-CS-A3 shows outstanding NLO response properties with βtot value of 6953.9 × 10-30 and 5066.0 × 10-30 esu in AN, respectively. The influence of the push-pull strength, the heterocycle and the π-conjugation of new bridge on the nonlinear optical properties of this novel powerful systems are clarified. This new series of chromophores exhibit remarkable electro-optical Pockels and optical rectification effect. More interestingly, PA-CS-A3 and THQ-CS-A2 also show appealing SHG effect. This study will help people understand the nature of nonlinear optical properties of innovative heteroarene-fused based cyclopentadienyl chromophores and offer guidance for the rational design of chromophores with outstanding electrooptic (EO) performance in the future.

Optical vortex-antivortex crystallization in free space

Stable vortex lattices are basic dynamical patterns which have been demonstrated in physical systems including superconductor physics, Bose-Einstein condensates, hydrodynamics and optics. Vortex-antivortex (VAV) ensembles can be produced, self-organizing into the respective polar lattices. However, these structures are in general highly unstable due to the strong VAV attraction. Here, we demonstrate that multiple optical VAV clusters nested in the propagating coherent field can crystallize into patterns which preserve their lattice structures over distance up to several Rayleigh lengths. To explain this phenomenon, we present a model for effective interactions between the vortices and antivortices at different lattice sites. The observed VAV crystallization is a consequence of the globally balanced VAV couplings. As the crystallization does not require the presence of nonlinearities and appears in free space, it may find applications to high-capacity optical communications and multiparticle manipulations. Our findings suggest possibilities for constructing VAV complexes through the orbit-orbit couplings, which differs from the extensively studied spin-orbit couplings.

Coherence-controlled chaotic soliton bunch

Controlling the coherence of chaotic soliton bunch holds the promise to explore novel light-matter interactions and manipulate dynamic events such as rogue waves. However, the coherence control of chaotic soliton bunch remains challenging, as there is a lack of dynamic equilibrium mechanism for stochastic soliton interactions. Here, we develop a strategy to effectively control the coherence of chaotic soliton bunch in a laser. We show that by introducing a lumped fourth-order-dispersion (FOD), the soliton oscillating tails can be formed and generate the potential barriers among the chaotic solitons. The repulsive force between neighboring solitons enabled by the potential barriers gives rise to an alleviation of the soliton fusion/annihilation from stochastic interactions, endowing the capability to control the coherence in chaotic soliton bunch. We envision that this result provides a promising test-bed for a variety of dynamical complexity science and brings new insights into the nonlinear behavior of chaotic laser sources.

Electrically tunable third-harmonic generation using intersubband polaritonic metasurfaces

Nonlinear intersubband polaritonic metasurfaces, which integrate giant nonlinear responses derived from intersubband transitions of multiple quantum wells (MQWs) with plasmonic nanoresonators, not only facilitate efficient frequency conversion at pump intensities on the order of few tens of kW cm-2 but also enable electrical modulation of nonlinear responses at the individual meta-atom level and dynamic beam manipulation. The electrical modulation characteristics of the magnitude and phase of the nonlinear optical response are realized through Stark tuning of the resonant intersubband nonlinearity. In this study, we report, for the first time, experimental implementations of electrical modulation characteristics of mid-infrared third-harmonic generation (THG) using an intersubband polaritonic metasurface based on MQW with electrically tunable third-order nonlinear response. Experimentally, we achieved a 450% modulation depth of the THG signal, 86% suppression of zero-order THG diffraction tuning based on local phase tuning exceeding 180 degrees, and THG beam steering using phase gradients. Our work proposes a new route for electrically tunable flat nonlinear optical elements with versatile functionalities.

Pulse-doubling perovskite nanowire lasers enabled by phonon-assisted multistep energy funneling

Laser pulse multiplication from an optical gain medium has shown great potential in miniaturizing integrated optoelectronic devices. Perovskite multiple quantum wells (MQWs) structures have recently been recognized as an effective gain media capable of doubling laser pulses that do not rely on external optical equipment. Although the light amplifications enabled with pulse doubling are reported based on the perovskite MQWs thin films, the micro-nanolasers possessed a specific cavity for laser pulse multiplication and their corresponding intrinsic laser dynamics are still inadequate. Herein, a single-mode double-pulsed nanolaser from self-assembled perovskite MQWs nanowires is realized, exhibiting a pulse duration of 28 ps and pulse interval of 22 ps based on single femtosecond laser pulse excitation. It is established that the continuous energy building up within a certain timescale is essential for the multiple population inversion in the gain medium, which arises from the slowing carrier localization process owning to the stronger exciton-phonon coupling in the smaller-n QWs. Therefore, the double-pulsed lasing is achieved from one fast energy funnel process from the adjacent small-n QWs to gain active region and another slow process from the spatially separated ones. This report may shed new light on the intrinsic energy relaxation mechanism and boost the further development of perovskite multiple-pulse lasers.

Second-harmonic generation tensors from high-throughput density-functional perturbation theory

Optical materials play a key role in enabling modern optoelectronic technologies in a wide variety of domains such as the medical or the energy sector. Among them, nonlinear optical crystals are of primary importance to achieve a broader range of electromagnetic waves in the devices. However, numerous and contradicting requirements significantly limit the discovery of new potential candidates, which, in turn, hinders the technological development. In the present work, the static nonlinear susceptibility and dielectric tensor are computed via density-functional perturbation theory for a set of 579 inorganic semiconductors. The computational methodology is discussed and the provided database is described with respect to both its data distribution and its format. Several comparisons with both experimental and ab initio results from literature allow to confirm the reliability of our data. The aim of this work is to provide a relevant dataset to foster the identification of promising nonlinear optical crystals in order to motivate their subsequent experimental investigation.

Inversion Symmetry-Broken Tetralayer Graphene Probed by Second-Harmonic Generation

Stacking orders provide a unique way to tune the properties of two-dimensional materials. Recently, ABCB-stacked tetralayer graphene has been predicted to possess atypical elemental ferroelectricity arising from its symmetry breaking but has been experimentally explored very little. Here, we observe pronounced nonlinear optical second-harmonic generation (SHG) in ABCB-stacked tetralayer graphene while absent in both ABAB- and ABCA-stacked allotropes. Our results provide direct evidence of symmetry breaking in ABCB-stacked tetralayer graphene. The remarkable contrast in the SHG spectra of tetralayer graphene allows straightforward identification of ABCB domains from the other two kinds of stacking order and facilitates the characterization of their crystalline orientation. The employed SHG technique serves as a convenient tool for exploring the intriguing physics and novel nonlinear optics in ABCB-stacked graphene, where spontaneous polarization and intrinsically gapped flat bands coexist. Our results establish ABCB-stacked graphene as a unique platform for studying the rare ferroelectricity in noncentrosymmetric elemental structures.

Highly Efficient Ultraviolet Third-Harmonic Generation in an Isolated Thin Si Meta-Structure

Nonlinear nanophotonic devices have shown great potential for on-chip information processing, quantum source, 3D microfabrication, greatly promoting the developments of integrated optics, quantum science, nanoscience and technologies, etc. To promote the applications of nonlinear nanodevices, improving the nonlinear efficiency, expanding the spectra region of nonlinear response and reducing device thickness are three key issues. Herein, this study focuses on the nonlinear effect of third-harmonic generation (THG), and present a thin Si meta-sructure to improve the THG efficiency in the ultraviolet (UV) region. The measured THG efficiency is up to 10-5 at an emission wavelength of 309 nm. Also, the THG nanosystem is only 100 nm in thickness, which is two-five times thinner than previous all-dielectric nanosystems applied in THG studies. These findings not only present a powerful thin meta-structure with highly efficient THG emission in UV region, but also provide a constructive avenue for further understanding the light-matter interactions at subwavelength scales, guiding the design and fabricating of advanced photonic devices in future.

Surface photogalvanic effect in Ag2Te

The bulk photovoltaic effect (BPVE) in non-centrosymmetric materials has attracted significant attention in recent years due to its potential to surpass the Shockley-Queisser limit. Although these materials are strictly constrained by symmetry, progress has been made in artificially reducing symmetry to stimulate BPVE in wider systems. However, the complexity of these techniques has hindered their practical implementation. In this study, we demonstrate a large intrinsic photocurrent response in centrosymmetric topological insulator Ag2Te, attributed to the surface photogalvanic effect (SPGE), which is induced by symmetry reduction of the surface. Through diverse spatially-resolved measurements on specially designed devices, we directly observe that SPGE in Ag2Te arises from the difference between two opposite photocurrent flows generated from the top and bottom surfaces. Acting as an efficient SPGE material, Ag2Te demonstrates robust performance across a wide spectral range from visible to mid-infrared, making it promising for applications in solar cells and mid-infrared detectors. More importantly, SPGE generated on low-symmetric surfaces can potentially be found in various systems, thereby inspiring a broader range of choices for photovoltaic materials.

Superluminal light propagation in a three-level ladder system

Superluminal light propagation is typically accompanied by significant absorption that might prevent its observation in realistic samples. We propose an all-optical implementation exploiting the two-photon resonance in three-level media to overcome this problem. With several computational methods, we analyze three possible configurations of optically-dressed systems and identify an optimal configuration for superluminal propagation. Due to the far-detuned operating regime with low absorption, this scenario avoids the usual need for population inversion, gain assistance or nonlinear optical response. Our analysis covers a broad parameter space and aims for the identification of conditions where significant pulse advancement can be achieved at high transmission levels. In this context, a figure of merit is introduced accounting for a trade-off between the desired group-index values and transmission level. This quantity helps to identify the optimal characteristics of the dressing beam.

Replica symmetry breaking in 1D Rayleigh scattering system: theory and validations

Spin glass theory, as a paradigm for describing disordered magnetic systems, constitutes a prominent subject of study within statistical physics. Replica symmetry breaking (RSB), as one of the pivotal concepts for the understanding of spin glass theory, means that under identical conditions, disordered systems can yield distinct states with nontrivial correlations. Random fiber laser (RFL) based on Rayleigh scattering (RS) is a complex disordered system, owing to the disorder and stochasticity of RS. In this work, for the first time, a precise theoretical model is elaborated for studying the photonic phase transition via the platform of RS-based RFL, in which we clearly reveal that, apart from the pump power, the photon phase variation in RFL is also an analogy to the temperature term in spin-glass phase transition, leading to a novel insight into the intrinsic mechanisms of photonic phase transition. In addition, based on this model and real-time high-fidelity detection spectral evolution, we theoretically predict and experimentally observe the mode-asymmetric characteristics of photonic phase transition in RS-based RFL. This finding contributes to a deeper understanding of the photonic RSB regime and the dynamics of RS-based RFL.

Tunable entangled photon-pair generation in a liquid crystal

Liquid crystals, with their ability to self-assemble, strong response to an electric field and integrability into complex systems, are key materials in light-beam manipulation1. The recently discovered ferroelectric nematic liquid crystals2,3 also have considerable second-order optical nonlinearity, making them a potential material for nonlinear optics4,5. Their use as sources of quantum light could considerably extend the boundaries of photonic quantum technologies6. However, spontaneous parametric down-conversion, the basic source of entangled photons7, heralded single photons8 and squeezed light9, has so far not been observed in liquid crystals-or in any liquids or organic materials. Here we implement spontaneous parametric down-conversion in a ferroelectric nematic liquid crystal and demonstrate electric-field tunable broadband generation of entangled photons, with an efficiency comparable to that of the best nonlinear crystals. The emission rate and polarization state of photon pairs is markedly varied by applying a few volts or twisting the molecular orientation along the sample. A liquid-crystal source enables a special type of quasi-phase matching10, which is based on the molecular twist structure and is therefore reconfigurable for the desired spectral and polarization properties of photon pairs. Such sources promise to outperform standard nonlinear optical materials in terms of functionality, brightness and the tunability of the generated quantum state. The concepts developed here can be extended to complex topological structures, macroscopic devices and multi-pixel tunable quantum light sources.

Power-dependent study of the photothermal response of several alcoholic media with femtosecond laser-induced thermal lens

We explore the photothermal response of methanol, ethylene glycol, and glycerol using the femtosecond laser-induced thermal lens spectroscopy (FTLS) technique. A mode mismatched pump-probe spectroscopic technique was utilized to analyze the influence of localized thermal heating on the photothermal response of solvents. The findings revealed a strong dependence on both the input pump power and the molecular characteristics of the solvents. At significantly high pump power, the excess heat load deposited to the solvent is found to be responsible for the induction of the convection currents in the heat transfer mechanisms. Our results highlight that the influence of pump power on photothermal and thermal lens characteristics is intricately linked to the natural drifting and heat transfer mechanisms of solvent molecules. The molecular motion and existing connective processes were correlated with the molecular characteristics of the samples. The present finding reveals that FTLS is a sensitive probe for comprehending the impact of input laser power, molecular structure, and intermolecular H bonding on the photothermal characteristics and thermo-optical properties of the alcoholic medium.

Observation of discrete-light temporal refraction by moving potentials with broken Galilean invariance

Refraction is a basic beam bending effect at two media's interface. While traditional studies focus on stationary boundaries, moving boundaries or potentials could enable new laws of refractions. Meanwhile, media's discretization plays a pivotal role in refraction owing to Galilean invariance breaking principle in discrete-wave mechanics, making refraction highly moving-speed dependent. Here, by harnessing a synthetic temporal lattice in a fiber-loop circuit, we observe discrete time refraction by a moving gauge-potential barrier. We unveil the selection rules for the potential moving speed, which can only take an integer v = 1 or fractional v = 1/q (odd q) value to guarantee a well-defined refraction. We observe reflectionless/reflective refractions for v = 1 and v = 1/3 speeds, transparent potentials with vanishing refraction/reflection, refraction of dynamic moving potential and refraction for relativistic Zitterbewegung effect. Our findings may feature applications in versatile time control and measurement for optical communications and signal processing.

Amplification of electromagnetic fields by a rotating body

In 1971, Zel'dovich predicted the amplification of electromagnetic (EM) waves scattered by a rotating metallic cylinder, gaining mechanical rotational energy from the body. This phenomenon was believed to be unobservable with electromagnetic fields due to technological difficulties in meeting the condition of amplification that is, the cylinder must rotate faster than the frequency of the incoming radiation. Here, we measure the amplification of an electromagnetic field, generated by a toroid LC-circuit, scattered by an aluminium cylinder spinning in the toroid gap. We show that when the Zel'dovich condition is met, the resistance induced by the cylinder becomes negative implying amplification of the incoming EM fields. These results reveal the connection between the concept of induction generators and the physics of this fundamental physics effect and open new prospects towards testing the Zel'dovich mechanism in the quantum regime, as well as related quantum friction effects.

Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb3SeI7

Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus Nb3SeI7, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered Nb3SeI7 with C6v symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χzxx. Meanwhile, although odd-layered Nb3SeI7 with C3v symmetry has both OP and IP dipole components (χzxx and χyyy), the value of χzxx is 1 order of magnitude greater than that of χyyy, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χzxx and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs Nb3SeI7 can facilitate the rapid development of chip-integrated NLO devices.

Room temperature, cascadable, all-optical polariton universal gates

Today, almost all information processing is performed using electronic logic circuits operating at several gigahertz frequency. All-optical logic holds the promise to allow for up to three orders of magnitude higher speed. Whereas essential all-optical transistor functionalities were demonstrated across a range of platforms, utilising them to implement a complete Boolean logic gate set and in particular negation, i.e. switching off an optical signal with another, weaker, optical signal, poses a major challenge. Here, we realize a cascadable NOT gate by introducing the concept of non-ground-state polariton amplification in organic semiconductor microcavities under non-resonant optical excitation. We unravel the importance of vibron-mediated stimulated scattering in room temperature operation of the inverter. Moreover, we extend the concept to a multi-input universal NOR logic gate, where in the presence of any of the input signals non-ground-state amplification supersedes spontaneous ground-state condensation, resulting in a NOR gate with ~1 ps switching time. The realisation of an ultrafast universal logic gate constitutes an essential step for more complex optical circuitry that could boost information processing applications.

Ultrafast Opto-Electronic and Thermal Tuning of Third-Harmonic Generation in a Graphene Field Effect Transistor

Graphene is a unique platform for tunable opto-electronic applications thanks to its linear band dispersion, which allows electrical control of resonant light-matter interactions. Tuning the nonlinear optical response of graphene is possible both electrically and in an all-optical fashion, but each approach involves a trade-off between speed and modulation depth. Here, lattice temperature, electron doping, and all-optical tuning of third-harmonic generation are combined in a hexagonal boron nitride-encapsulated graphene opto-electronic device and demonstrate up to 85% modulation depth along with gate-tunable ultrafast dynamics. These results arise from the dynamic changes in the transient electronic temperature combined with Pauli blocking induced by the out-of-equilibrium chemical potential. The work provides a detailed description of the transient nonlinear optical and electronic response of graphene, which is crucial for the design of nanoscale and ultrafast optical modulators, detectors, and frequency converters.

ITO-Induced Nonlinear Optical Response Enhancement of Titanium Nitride Thin Films

A series of TiN/ITO composite films with various thickness of ITO buffer layer were fabricated in this study. The enhancement of optical properties was realized in the composite thin films. The absorption spectra showed that absorption intensity in the near-infrared region was obviously enhanced with the increase of ITO thickness due to the coupling of surface plasma between TiN and ITO. The epsilon-near-zero wavelength of this composite can be tuned from 935 nm to 1895 nm by varying the thickness of ITO thin films. The nonlinear optical property investigated by Z-scan technique showed that the nonlinear absorption coefficient (β = 3.03 × 10-4 cm/W) for the composite was about 14.02 times greater than that of single-layer TiN films. The theoretical calculations performed by finite difference time domain were in good agreement with those of the experiments.