Optical Harmonics Research Articles

Even-order optical harmonics generated from centrosymmetric-material metasurfaces

Generation of even-order optical harmonics requires noncentrosymmetric structures being conventionally observed in crystals lacking the center of inversion. In centrosymmetric systems, even-order harmonics may arise, e.g., at surfaces but such effects are usually very weak. Here we observe optical harmonics up to fourth order generated under the normal incidence from centrosymmetric dielectric metasurfaces empowered by resonances. We design silicon metasurfaces supporting optical quasibound states in the continuum and guided-mode resonances, and demonstrate the enhancement of second-harmonic signals by over three orders of magnitude compared to nonresonant thin films. Under the optimal conditions, the brightness of the second harmonic approaches that of the third harmonic, and the fourth-order harmonic becomes detectable. Published by the American Physical Society 2024

Grayscale to Multicolor Laser Writing Inside a Label‐Free Metal‐Organic Frameworks

AbstractDirect laser writing (DLW), being a universal tool for fast creating colorless/color images on different substrates, still suffers from simultaneous writing grayscale and color images inside the transparent media. Here, it is discovered that a unique set of porosity, coordination bonding between organic and inorganic building blocks, and the lack of inversion symmetry of the label‐free metal‐organic frameworks (MOFs), on the one hand, provides the possibility of laser writing the grayscale images through the amorphization/carbonization. On the other hand, the reduction of the laser writing power leads to controllable creation of color images via defect formation with sub‐diffraction resolution inside the MOF crystals. The latter is due to the processes of self‐absorption of generated optical harmonics by nonlinear MOFs within the visible spectral range. As a result, simultaneous grayscale and multicolor writing of QR codes and images are demonstrated with up to 400 nm resolution inside optically transparent MOF crystals, thereby discovering a new family of functional materials for DLW.

Manifestation of Local Asymmetric Regions in Centrosymmetric Phase of Ferroelectric Crystals in Brillouin Scattering and Second Optical Harmonic Generation

Manifestation of Local Asymmetric Regions in Centrosymmetric Phase of Ferroelectric Crystals in Brillouin Scattering and Second Optical Harmonic Generation

Demonstration of Improving the Performance of a Fibre Optic Displacement Sensor Using the Optical Harmonic Vernier Effect by Cascading Tapered Optical Single-Mode Fibres

Demonstration of Improving the Performance of a Fibre Optic Displacement Sensor Using the Optical Harmonic Vernier Effect by Cascading Tapered Optical Single-Mode Fibres

High-harmonic generation from a subwavelength dielectric resonator.

Higher-order optical harmonics entered the realm of nanostructured solids being observed recently in optical gratings and metasurfaces with a subwavelength thickness. Structuring materials at the subwavelength scale allows us toresonantly enhance the efficiency of nonlinear processes and reduce the size of high-harmonic sources. We report the observation of up to a seventh harmonic generated from a single subwavelength resonator made of AlGaAs material. This process is enabled by careful engineering of the resonator geometry for supporting an optical mode associated with a quasi-bound state in the continuum in the mid-infrared spectral range at around λ=3.7 μm pump wavelength. The resonator volume measures ~0.1 λ3. The resonant modes are excited with an azimuthally polarized tightly focused beam. We evaluate the contributions of perturbative and nonperturbative nonlinearities to the harmonic generation process. Our work proves the possibility to miniaturize solid-state sources of high harmonics to the subwavelength volumes.

High Optical Harmonics Generation on Solid Surfaces Irradiated by Mid-IR Femtosecond Laser Pulses

We obtained the spectra of high optical harmonics produced by subrelativistic femtosecond pulses on the surface of polystyrene, CaF2, BK7, and Al solid targets. High harmonics of up to the 51st order of radiation with central 3.85 µm wavelength were observed. The highest order harmonics were generated from the polystyrene target surface. The harmonics energy versus their numbers is shown to fit well a decaying power law with the exponent ranging from 4 to 8/3.

High Optical Harmonics Generation on Solid Surfaces Irradiated by Mid-IR Femtosecond Laser Pulses

High Optical Harmonics Generation on Solid Surfaces Irradiated by Mid-IR Femtosecond Laser Pulses

Large-tunable microwave photonics frequency multiplier overcoming the fixed filter bandwidth limitation on improved feed-forward modulation technology

Improved feed-forward modulation (IFFM) technology is a two-stage modulation photonics design that achieves coherent frequency up-conversion based on incoherent optical carriers. For the scheme, at least two optical fixed filters are required; the first is used to extract desired optical sidebands for the second-stage modulation’s driving signal generation, and the second filter is to suppress unwanted optical harmonics for high-quality heterodyne detection. This is for a fixed-frequency signal generation. To accommodate tunable frequency signal generation, at least two tunable optical filers are needed. In this paper, a wide frequency tunable method for an IFFM-based frequency multiplier with only a fixed bandwidth optical filter is proposed. A proof-of-concept experiment successfully demonstrates the tuning mechanism of the scheme. The scheme exploits the two pairs of coherent optical sidebands generated from two incoherent light sources to realize coherent addition of the two beat signals from each pair to attain a high-power frequency-multiplied signal for enhanced transmission power budget. From the simulation, a frequency multiplication factor of 16 and a 160-GHz signal with a stable frequency and a power level enhanced by 19 dB are achieved by employing two incoherent lasers. By adjusting the output frequency of the local oscillator, we show that multi-band signals generation, including microwave, millimeter- and terahertz-wave, can be realized, and the performance of the generated signals is also investigated.

Effect of the Length and Pressure of a Gas Jet on Optical Harmonics Generation by 4.5-μm Femtosecond Laser Radiation of a Fe:ZnSe Laser System

The effect of the length and pressure of an argon gas jet on low-order harmonics (5th, 7th, 9th, and 11th) generation by 4.5-μm femtosecond laser radiation of a Fe:ZnSe laser system has been studied experimentally. It has been shown that an increase in the length of the generation medium up to the waist length allows one to increase the generation efficiency by a factor of 12. It has also been demonstrated that a change in the length of the gas medium changes the pressure dependence of the energy of the generated radiation because of change in the phase matching conditions, whose correct simulation requires the inclusion of nonlinear propagation effects for pump pulse and generated harmonics.

Proposal for High-Energy Cutoff Extension of Optical Harmonics of Solid Materials Using the Example of a One-Dimensional ZnO Crystal.

We propose a novel approach based on the subcycle injection of carriers to extend the high-energy cutoff in solid-state high harmonics. The mechanism is first examined by employing the standard single-cell semiconductor Bloch equation (SC SBE) method for one-dimensional (1D) Mathieu potential model for ZnO subjected to the intense linearly polarized midinfrared laser field and extreme-ultraviolet pulse. Then, we use coupled solution of Maxwell propagation equation and SC SBE to propagate the fundamental laser field through the sample, and find that the high-harmonics pulse train from the entrance section of the sample can inject carriers to the conduction bands with attosecond timing, subsequently leading to a dramatic extension of high-energy cutoff in harmonics from the backside. We predict that for a peak intensity at 2×10^{11} W/cm^{2}, as a result of the self-seeding, the high-energy cutoff shifts from 20th (7.75eV) order to around 50th (19.38eV) order harmonics.

Optical Harmonics Generation under the Interaction of Intense (up to 1014 W/cm2) Mid-Infrared Femtosecond Laser Radiation of a Fe:ZnSe Laser System with a Dense Laminar Gas Jet

Optical Harmonics Generation under the Interaction of Intense (up to 1014 W/cm2) Mid-Infrared Femtosecond Laser Radiation of a Fe:ZnSe Laser System with a Dense Laminar Gas Jet

Nonlinear light scattering spectra in water: Dependence on the content of dissolved gas and effects of non‐stationarity

AbstractHere, we study stimulated nonlinear scattering in the spectral range near the second optical harmonic of a laser pump in water samples containing gas nanobubbles. It was found that the spectral and spatial characteristics of nonlinear scattering radiation undergo significant changes upon settling of initially degassed water samples in a flask, open to the atmospheric air. It turned out that the contact with the atmospheric air results in the growth of the volume number density of gas nanobubbles. It appeared that the physical mechanism of nonlinear scattering changes upon increasing the nanobubble content. In fact, four‐photon parametric scattering in degassed water, which occurs due to the cubic nonlinear susceptibility χ(3) of medium, transforms upon increasing the content of nanobubbles into stimulated hyper‐Raman scattering controlled by the constant (∂χ(2)/∂q), where χ(2) is the quadratic nonlinear susceptibility and q is coordinate of a nucleus. The physical nature of the observed phenomenon is the generation of a surface plasmon arising due to the optical breakdown inside a gas nanobubble, which leads to an effective buildup of vibrations of molecules/ions localized in the surface layer of nanobubble. The plasmon bandwidth Δω was theoretically estimated. It turned out that Δω ≈ ω0, where ω0 is the resonant plasmon wavenumber, that is, in our case, the quality factor of the plasmon oscillator is very low.

High-Harmonic Generation from Resonant Dielectric Metasurfaces Empowered by Bound States in the Continuum

The concept of optical bound states in the continuum (BICs) underpins the existence of strongly localized waves embedded into the radiation spectrum that can enhance the electromagnetic fields in subwavelength photonic structures. Early studies of optical BICs in waveguides and photonic crystals uncovered their topological properties, and the concept of quasi-BIC metasurfaces facilitated applications of strong light-matter interactions to biosensing, lasing, and low-order nonlinear processes. Here we employ BIC-empowered dielectric metasurfaces to generate efficiently high optical harmonics up to the 11th order. We optimize a BIC mode for the first few harmonics and observe a transition between perturbative and nonperturbative nonlinear regimes. We also suggest a general strategy for designing subwavelength structures with strong resonances and nonperturbative nonlinearities. Our work bridges the fields of perturbative and nonperturbative nonlinear optics on the subwavelength scale.

Maximum RF Input Signal on the Electro-Optic Modulator Transmission Boundary Area for Harmonic Distortion Compensation in Analog Radio Over Fiber

In this study, we perform an analytical investigation of the electro-optic (EO), Mach-Zehnder modulator (MZM) transmission boundary area with a nonlinear model. We propose a nonlinear model that represents the allowable RF input amplitude within the MZM transmission boundary area. We investigate the maximum RF input amplitude and confine its harmonics using nonlinear model theory, RoF simulation, and MZM characteristic experiments. The maximum RF input amplitude in the MZM transmission boundary area is a convenient technique that can confine harmonics without an optical amplifier, optical filter, or other optical devices in RoF links. We select several RF input amplitudes within the MZM boundary area and account for harmonic distortion in the investigation. The RF input amplitude is 0.8 times of the EO modulator switching voltages. The boundary areas show that near the switching voltage value, the harmonics-to-main RF ratio (HMR) is less than 10%. Furthermore, the maximum RF inputs are still appropriate for QPSK modulation. We evaluate the nonlinear model of these modulation signals in terms of the error vector magnitude and constellation diagram. After considering the optical fiber impairment, thermal noise, and harmonics distortion in the nonlinear model with the modulation signal of QPSK, the ACLR and EVMs satisfy the 3GPP specification. The nonlinear models are appropriate for a small or medium sized of radio-over-fiber links.

A New and Simple Frequency Quadrupling Millimeter-Wave Signal Generation Enabled by a Single Mach-Zehnder Modulator Without Optical Filter

In this work, we have designed and proposed a new optical method to realize the generating of frequency quadrupling millimeter-wave (mm-wave) only employing one Mach-Zehnder modulator (MZM), and no any optical filter is needed. Compared with the previously reported quadruple frequency scheme, this work greatly simplifies the structure of the system, and makes the performance better and the cost lower. In the scheme, the MZM is operated at peak bias point, which is applied to retain the even order optical sidebands and wipe off the odd order optical harmonics. By tuning-up the optical shifter and optical attenuator (OATT) to control the zeroth order optical carrier returns zero, only ±2nd order optical tones are generated. We demonstrated and discussed the presented scheme feasibility based on the detailed mathematical formula derivation and numerical simulation, the simulation results of which show that employing a sinusoidal radio frequency (RF) source with frequency at 10 GHz to drive a push-pull MZM, a 40 GHz millimeter-wave (mm-wave) signal can be obtained. In the work, the performance of the generated quadrupling signal is that the optical sideband suppression ratio (OSSR) is 34.8 dB and the radio frequency sideband suppression ratio (RFSSR) can reach 27.2 dB.

Generation of the Second Optical Harmonic by a Layered Plasmon Nanoparticle

Generation of the Second Optical Harmonic by a Layered Plasmon Nanoparticle

A compact and stable incidence-plane-rotating second harmonics detector.

We describe a compact and stable setup for detecting the optical second harmonics, in which the incident plane rotates with respect to the sample. The setup is composed of rotating Fresnel rhomb optics and a femtosecond ytterbium-doped fiber laser source operating at the repetition frequency of 10MHz. The setup including the laser source occupies an area of 1 m2 and is stable so that the intensity fluctuation of the laser harmonics can be less than 0.2% for 4 h. We present the isotropic harmonic signal of a gold mirror of 0.5 pW and demonstrate the integrity and sensitivity of the setup. We also show the polarization-dependent six-fold pattern of the harmonics of a few-layer WSe2, from which we infer the degree of local-field effects. Finally, we describe the extensibility of the setup to investigate the samples in various conditions such as cryogenic, strained, ultrafast non-equilibrium, and high magnetic fields.

Application of P(VDF-TrFE) Glass Coating for Robust Harmonic Nanoparticles Characterization.

Polyvinylidene fluoride and its copolymers are a well-known family of low-cost ferroelectric materials widely used for the fabrication of devices for a wide range of applications. A biocompatibility, high optical quality, chemical and mechanical durability of poly(vinylidene fluoride–trifluoroethylene), (P(VDF–TrFE)), makes it particularly attractive for designing of effective coating layers for different diagnostic techniques. In the present work, the nonlinear optical characterization of P(VDF-TrFE)-coating films deposited onto a glass substrate was done. Advantages of the coating application for cells/substrates in the field of multiphoton imaging the efficiency of such coating layer for long-duration characterization of so-called harmonic nanoparticles (HNPs) were shown. The influence of glass surface protection by P(VDF-TrFE) film from an effect of HNPs sticking to the walls of the flow-cell was analyzed for effective studying of the optical harmonics generation efficiency of HNPs making the analysis more robust.

Magnetic Field Effects in Optical Harmonics Generation by Excitons

Mechanisms that are responsible for the optical harmonics generation at exciton transitions are analyzed in several classes of materials. In the cubic GaAs semiconductor, the magneto-induced optical second-harmonic generation second optical harmonic is observed in the region of orbital quantization of the valence and conduction bands. An unusually strong amplitude of the optical third-harmonic generation in an external magnetic field in the region of 1s exciton due to exciton–polariton resonance is found. The optical second-harmonic generation due to 1s, 2s, and 2p exciton resonances in a magnetic field is revealed in a hexagonal wide band-gap ZnO semiconductor. Depending on the symmetry of the exciton states, the mechanisms of the optical second-harmonic generation involve the spin and orbital Zeeman effects, and the magneto-Stark effect. The magneto-induced contribution to second harmonic generation (SHG) in the region of exciton transitions in the Cr2O3 antiferromagnet placed in an external magnetic field is studied.

Role of gas nanobubbles in nonlinear hyper-Raman scattering of light in water

In this work, we studied stimulated hyper-Raman scattering in the spectral range near the second optical harmonic of a laser pump in water samples containing gas nanobubbles (bubstons). The spectrum of the hyper-Raman scattering from nanobubble-free water samples was found to differ from that of nanobubbles containing water samples. To interpret these results, we put forward a hypothesis that the detected effect is related to the huge enhancement caused by the generation of a resonance surface plasmon inside a gas nanobubble during optical (laser-induced) breakdown. The plasmon bandwidth Δ ω was theoretically estimated. It turned out that Δ ω ≈ ω 0 , where ω 0 is the resonant plasmon frequency, i.e., in our case, the quality factor of the plasmon oscillator is very low. The estimate of the enhancement factor, which was made based on experimental data, gives ≈ 1.5 × 10 15 .