Nonlinear Frequency Conversion Research Articles

Hyperparametric frequency noise eater

Nonlinear optical frequency conversion can result in redistribution of the noise of the coherent pump light among generated optical harmonics leading to improved quality of laser emission. The property is useful for making narrow line lasers suitable for various metrology applications on a chip. We show theoretically that the resonant hyperparametric scattering allows reducing optical frequency noise in one of the generated harmonics with respect to the coherent pump light. The reduction calls for significantly dissimilar quality factors of the resonator modes and appears at the cost of enhanced frequency sensitivity to power fluctuations of the pump light. The artificial suppression of the quality factor of one of the modes participating in the scattering does not lead to significant increase of the oscillation threshold and broadens the dynamic range of the phase matching of the process. The technique is promising for creation of chip-scale parametric frequency ``noise eaters.''

Zinc oxide based dielectric nanoantennas for efficient nonlinear frequency conversion

We present the enhancement of frequency doubled and tripled light in zinc oxide (ZnO) by utilizing arrays of Mie-resonant dielectric nanoantennas. The cylindrical antennas exhibit resonances at the fundamental excitation wavelength, enabling strong localization of light and redistribution of the electric field inside the structures. Linear transmission spectroscopy reveals the resonant behavior of the fabricated nanoantennas and confirms the performed theoretical simulations. Nonlinear spectroscopic measurements show direct dependency between the resonant behavior and the amplification of nonlinear signals. Comparing the unpatterned ZnO thin films with the dielectric nanostructures shows a maximum enhancement by factors of about 22 for SHG and 7 for THG from resonant excitations of the nanoantennas.

Frequency doubling of twisted light independent of the integer topological charge

Light with orbital angular momentum (OAM), or twisted light, is widely investigated in the fields of optical communications, quantum information science, and nonlinear optics by harnessing its unbounded dimension. For nonlinear frequency conversion of twisted light, the efficiency usually decreases exponentially with increasing topological charge, which severely degrades the quality of the output modes. We here conceive and develop a method of eliminating the dependence of the conversion efficiency on the topological charge in second-harmonic generation using a single lens imaging technique. We first give the theoretical model and simulate the evolution of light beams near the image point of the lens, then the independence of the SHG conversion efficiency from topological charge is verified for different integer values of the topological charge; this independence is confirmed for varying pump power. The method can be generalized to other light–matter interactions and is a promising method of achieving higher efficiency and fidelity in the interaction of twisted light with matter.

Phase-matched nonlinear second-harmonic generation in plasmonic metasurfaces

Abstract The phase matching between the propagating fundamental and nonlinearly generated waves plays an important role in the efficiency of the nonlinear frequency conversion in macroscopic crystals. However, in nanoscale samples, such as nanoplasmonic structures, the phase-matching condition is often ignored due to the sub-wavelength nature of the materials. Here, we first show that the phase matching of the lattice plasmon modes at the fundamental and second-harmonic frequencies in a plasmonic nanoantenna array can effectively enhance the surface-enhanced second-harmonic generation. Additionally, a significant enhancement of the second-harmonic generation is demonstrated using stationary band-edge lattice plasmon modes with zero phase.

High-power vortex beam generation enabled by a phased beam array fed at the nonfocal-plane.

High-power vortex beams have extensive applications in optical communication, nonlinear frequency conversion, and laser processing. To overcome a single beam's power limitation, generating vortex beams, based on a phased beam array, is an intuitive idea that requires locking each beamlet's phase to a specific different value. Conventionally, the intensity profiles of the focal plane (far field) are used for extracting the cost functions in active phase control systems. However, as for generating vortex beams, the cost function extraction method at the focal plane suffers because the same intensity profile of the beam array could correspond to different phase distributions in near field. Thus, the accurate phase control signals are difficult to obtain. In this paper, a new concept of extracting cost functions at the non-focal-plane is firstly presented and analyzed in detail by numerical simulation. This cost function extraction method is an efficient way of generating vortex beams with different topological charges, including second-order Bessel-Gaussian beams. The new concept could provide a valuable reference and contribute to the practical implementation of generating vortex beams by coherent beam combining technology.

Ultra-Broadband Second-Harmonic Generation in ZnO Nano-Tetrapod With Over-One-Octave Bandwidth

Broadband nonlinear optical frequency conversion has attracted considerable interest in many applications, such as optical communication, frequency comb generation, and so on. In this letter, we report on the ultra-broadband second-harmonic generation (SHG) from the zinc oxide nano-tetrapod with less stringent phase-matching requirement. Using a super-continuum source together with an optical parametric oscillator as the excitation light, a broadband SHG with over-one-octave bandwidth spanning from the ultraviolet to near-infrared (375–1100 nm) is readily obtained. The SHG dependence of pump polarization and pump power is also characterized systematically. This nanoscale ultra-broadband optical converter may open a new way for various applications ranging from imaging to on-chip all-optical signal processing operations.

Third-order nonlinear optical properties of calcite crystal in UV region

Different from cascaded second-order nonlinear frequency conversion which is a common way to obtain ultraviolet (UV) laser only through noncentrosymmetric crystal, the optical properties of the phase-matched direct third-harmonic generation (THG) in the centrosymmetric calcite crystal was studied. A femtosecond laser was taken as the pumping source in the type-I (ooo→e) and type-II (ooe→e) phase-matched experiment, and a nanosecond laser was also used for comparison. Polarization angle ρ, phase-matching angle θ and azimuth angle φ in direct THG were determined and verified. Strong third-harmonic at 266 nm was observed in calcite crystal pumped by femtosecond laser. Compared with type-I phase-matched, Type-II was more efficient for energy conversion. The shortest harmonic wavelength obtained was about 213 nm, which was close to deep-UV region. The maximal output energy was 19.3 μJ (average power, ～19.3 mW) and the maximal conversion efficiency was 1.25% at 266 nm, which was the highest conversion efficiency at present. These results may indicate a wider application field for centrosymmetric crystals and another method to produce laser in UV and even deep-UV region.

An All‐Optical, Actively Q‐Switched Fiber Laser by an Antimonene‐Based Optical Modulator

AbstractQ‐switched fiber lasers are of great interest in industrial material processing, nonlinear frequency conversion, spectroscopy etc. However, passive Q‐switching possesses drawbacks of degradation and failure of the saturable absorber and the difficulty in accurate modification of the repetition rate. To overcome these issues, active Q‐switching that can normally modulate the cavity quality‐factor by an externally‐driven Q‐switcher is in high demand. Herein, an actively Q‐switched laser with antimonene‐based all‐optical modulator is devised based on the high photo‐thermal efficiency (48%) and broadband response in antimonene. It is demonstrated that this actively modulated laser represents all‐optically tunable output parameters (e.g., output repetition rate), environmental stability, and easy synchronization. It is anticipated that this actively antimonene‐based all‐optical modulator with advantages of large modulation depth, low energy consumption, and high conversion efficiency has great potential in all‐optical information processing and pulsed laser engineering.

Temporal solitons in free-space femtosecond enhancement cavities

Temporal dissipative solitons in nonlinear optical resonators are self-compressed, self-stabilizing and indefinitely circulating wave packets. Owing to these properties, they have been harnessed for the generation of ultrashort pulses and frequency combs in active and passive laser architectures, including mode-locked lasers1–4, passive fibre resonators5 and microresonators6–11. Here, we demonstrate the formation of temporal dissipative solitons in a free-space enhancement cavity with a Kerr nonlinearity and a spectrally tailored finesse. By locking a 100-MHz-repetition-rate train of 350-fs, 1,035-nm pulses to this cavity-soliton state, we generate a 37-fs sech²-shaped pulse with a peak-power enhancement of 3,200, which exhibits low-frequency intensity-noise suppression. The power scalability unique to free-space cavities, the unprecedented combination of peak-power enhancement and temporal compression, and the cavity-soliton-specific noise filtering attest to the vast potential of this platform of optical solitons for applications including spatiotemporal filtering and compression of ultrashort pulses and cavity-enhanced nonlinear frequency conversion. Temporal dissipative soliton formation in a free-space femtosecond enhancement cavity with a thin Kerr medium is reported. Locking a 350-fs, 1,035-nm pulse train with a repetition rate of 100 MHz to this cavity-soliton state generates a 37-fs sech2-shaped pulse with a peak-power enhancement of 3,200.

Nonlinear frequency conversion and manipulation of vector beams in a Sagnac loop.

Vector beams (VBs) are widely investigated for their special intensities and polarization distributions, which are useful in optical micromanipulation, optical microfabrication, optical communication, and single molecule imaging. To date, nonlinear frequency conversion (NFC) and manipulation of VBs remain challenging because of the polarization sensitivity of most nonlinear processes. Here we report an experimental realization of NFC and manipulation of VBs that can be used to expand the available frequency band. The main idea of our scheme is the introduction of a Sagnac loop to solve the polarization dependence problem of NFC in nonlinear crystals. Additionally, we find that a linearly polarized VB should be transformed into a hybrid-polarized VB in exponential form before performing NFC. The experimental results agree well with those of our theoretical model. The proposed method is also applicable to other wavebands and second-order nonlinear processes, and may be generalized to the quantum regime for single photons.

Effect of Absorption on the Efficiency of Terahertz Radiation Generation in the Metal Waveguide Partially Filled with Nonlinear Crystal LiNbO3, Dast or ZnTe

The influence of terahertz (THz) radiation absorption on the efficiency of generation of coherent THz radiation in the system ‘nonlinear-optical crystal partially filling the cross section of a rectangular metal waveguide’ has been investigated. The efficiency of the nonlinear frequency conversion of optical laser radiation to the THz range depends on the loss in the system and the fulfillment of the phase-matching (FM) condition in a nonlinear crystal. The method of partially filling of a metal waveguide with a nonlinear optical crystal is used to ensure phase matching. The phase matching is achieved by numerical determination of the thickness of the nonlinear crystal, that is the degree of partial filling of the waveguide. The attenuation of THz radiation caused by losses both in the metal walls of the waveguide and in the crystal was studied, taking into account the dimension of the cross section of the waveguide, the degree of partial filling, and the dielectric constant of the crystal. It is shown that the partial filling of the waveguide with a nonlinear crystal results in an increase in the efficiency of generation of THz radiation by an order of magnitude, owing to the decrease in absorption.

Frequency-stepped pulse train generation in an amplified frequency-shifted loop for oxygen A-band spectroscopy.

We have demonstrated a frequency-stepped pulse train (FSPT) generation system enabling direct oxygen A-band spectroscopy in the time domain. The FSPT is formed by circulating the initial single-frequency pulses at ~1529.5 nm in an amplified frequency-shifted loop (AFSL). An FSPT with up to 80 equidistant optical frequencies can be generated with a frequency spacing of 200 MHz in this configuration. The fundamental wavelength of the FSPT is then frequency doubled to ~764.75 nm, covering one absorption peak in oxygen A-band, for the proof-of-principle experiment in spectroscopy. The oxygen transmission curves retrieved from FSPT-based differential absorption measurement are in good agreement with the calculated results from HITRAN database. The root-mean-squared errors between the measured and calculated results are 0.68% and 0.55% for clean air at 1 atm. and 1.5 atm., respectively. The wavelength of such an FSPT can be further extended to match the absorption lines of various gases by selecting proper gain media or through nonlinear frequency conversion, thereby making the FSPT-based spectroscopy a promising candidate for trace-gas remote sensing.

Dewetting mechanisms and their exploitation for the large-scale fabrication of advanced nanophotonic systems

ABSTRACTRecent progress in submicron- and nano-fabrication technologies has led to the emergence of novel photonic structures such as optical nanoantennas and metasurfaces. Real-life applications of these advanced photonic structures still require substantial improvement of the fabrication processes, in terms of their throughput and cost-effectiveness. Because of its simplicity and effectiveness, dewetting of a thin film has attained increasing attention as a feasible process for improving the scalability and productivity. Here, we provide an overview of the mechanisms and phenomenologies of dewetting to foster an improved fundamental understanding necessary for the optimisation of the dewetting process condition and template design. We then review the strategies demonstrating the use of templated-dewetting for producing well-aligned arrays of submicron- and nanostructures with great control over their size, shape and arrangement. Recent applications of dewetted structures in advanced nanophotonics are reviewed with an emphasis on the exploitation of dewetting mechanisms. Special attention is given to the fabrication of resonant optical nanoantennas and nanophotonic applications in which high repeatability and throughput are important parameters: sensing, colourisation, photovoltaics and nonlinear light frequency conversion. We expect this review to provide a basis for the use of thin-film dewetting to realise the industrial-level fabrication of various practical advanced photonic systems.

Experimental Observation of Toroidal Dipole Modes in All‐Dielectric Metasurfaces

AbstractThe study of toroidal dipole modes has attracted a growing attention due to the specific properties of the toroidal electromagnetic response which differs from more familiar electric and magnetic dipole modes. Herein, toroidal dipole modes generated by metasurfaces composed of trimer clusters of high‐index dielectric particles are observed. Both far‐field transmission measurements and direct near‐field mapping of the electromagnetic fields are performed in microwave experiments, and two distinct types of the toroidal dipole modes are observed in a single geometry of the metasurface design, where the toroidal modes are generated either inside of the three‐particle clusters (the so‐called intra‐cluster toroidal modes) or between the neighboring particles in the clusters (inter‐cluster toroidal modes). A transient response of the toroidal dipole modes excited by a pulse is studied in detail. Since the metasurface is composed of simple dielectric disks without the use of any metallic components, the proposed design can be feasibly scalable to both micro‐ and nanometer‐size dielectric structures, and it can be employed in the flat‐optics platform for realizing the beams shaping and efficient light–matter interaction for multiple hotspot energy localization, nonlinear frequency conversion, and highly efficient trapping of light.

Nonlinear frequency conversion and manipulation of vector beams.

Vector beams have been extensively investigated in recent years because of their fascinating vector character across the beam transverse section, which is demonstrated to be useful for optical micro-manipulation, optical micro-fabrication, optical communication, single molecule imaging, and so on. To date, it is still a challenge to realize nonlinear frequency conversion and manipulation of such vector beams because of the polarization sensitivity in most nonlinear processes. Here, for the first time, to the best of our knowledge, we generate second-harmonic vector beams by using three-wave mixing processes in our experiment, which occur in two orthogonally placed nonlinear crystals, and the vector property is recognized by using a Glan-Taylor polarizer. This nonlinear frequency conversion process enables vector beams to be obtained at new wavelengths, and opens up new possibilities for all-optical switching and manipulation of vector beams.

The manipulation of second-order nonlinear harmonic wave by structured material and structured light

Nonlinear frequency conversion is the important method to get coherent radiation in all-optical waveband. The intentional manipulation of nonlinear harmonic wave, which has some special phase, amplitude, polarization, and so on, is also important to realize all kinds of optical micro-manipulation, optical micro-fabrication, and optical communication in all-optical waveband. There are three methods to manipulate such nonlinear harmonic wave, which are tailoring the functional facet, the structure of crystals, and the structure of the incident light. In this review, we will systematically introduce the fundamental principle and latest advances in this field and summarize the respective characteristics of these three methods.

Cascaded crystalline Raman lasers for extended wavelength coverage: continuous-wave, third-Stokes operation

The development of high-power laser sources with narrow emission, tunable within the water transmission window around 1.7 μm, is of interest for applications as diverse as medical imaging and atmospheric sensing. Where suitable laser gain media are not available, operation in this spectral region is often achieved via nonlinear frequency conversion, and optical parametric oscillators (OPOs) are a common solution. A practical alternative to OPOs, to avoid birefringent- or quasi-phase-matching requirements, is the use of stimulated Raman scattering within a suitable material to convert a pump source to longer wavelengths via one or more Stokes shifts; however, as this is a χ(3) nonlinear process, such frequency conversion is usually the preserve of high-energy pulsed lasers. Semiconductor disk lasers (SDLs), on the other hand, have very high-finesse external resonators, suitable for efficient intracavity nonlinear conversion even in continuous-wave (CW) operation. Here we report, to the best of our knowledge, the first continuous-wave third-Stokes crystalline Raman laser and the longest emission wavelength from an SDL-pumped Raman laser, achieving high power, CW output, and broad wavelength tuning around 1.73 μm. The KGd(WO4)2 (KGW) Raman laser, which was intracavity-pumped by a 1.18 μm InGaAs-based SDL, demonstrated cascaded CW Stokes oscillation at 1.32 μm, 1.50 μm, and 1.73 μm with watt-level output achievable at each wavelength. The 1.73 μm Stokes emission was diffraction limited (M2<1.01) and narrow linewidth (<46 pm FWHM; measurement limited). By rotation of a birefringent filter placed within the fundamental resonator, we attained three tunable emission wavelength bands, one centred at each Stokes component, and achieved up to 65 nm tuning for the third-Stokes Raman laser from 1696 nm to 1761 nm. We have thus demonstrated a platform laser technology that takes well-developed InGaAs-based SDLs and provides spectral coverage and high performance in the near-infrared water transmission windows using commercially available components.

Heterogeneous Thin-Film Lithium Niobate Integrated Photonics for Electrooptics and Nonlinear Optics

Ion-sliced thin-film lithium niobate (LN) compact waveguide technology has facilitated the resurgence of integrated photonics based on the material. The thin-film waveguides offer over an order of magnitude improvement in optical confinement and bending radius compared to conventional LN waveguides. The thin-film technology can also be implemented on versatile silicon substrates. Harnessing the improved confinement, a variety of miniaturized and efficient photonic devices has been realized. The state of the art of thin-film LN integrated photonics is reviewed, focusing on heterogeneous integration, electrooptic modulation, and nonlinear frequency conversion. The potential applications and associated challenges of next-generation LN-based integrated photonics are discussed.

Widely tunable, continuous-wave, intra-cavity optical parametric oscillator based on an Yb-doped fiber laser.

We report an optical parametric oscillator (OPO) intra-cavity pumped by an Yb-doped fiber laser. In comparison to an intra-cavity OPO based on a solid-state laser gain medium, the benefits of using a fiber-based scheme include a superior beam quality of the generated mid-infrared idler light at high power levels, a more efficient process of nonlinear frequency conversion, and the prospect of scaling to higher power. In a preliminary experiment, we obtained a slope efficiency of the down-converted power of as high as 66% with respect to the absorbed laser diode power. To the best of our knowledge, this is the first demonstration of an intra-cavity OPO based on a fiber laser.

Enhanced Second Harmonic Generation in Double‐Resonance Colloidal Metasurfaces

AbstractA key challenge for optical circuits is the ability to integrate nonlinear optical signal processing components such as optical modulators and frequency mixers at the chip scale. Optical antennas that focus light into nanoscale volumes can be utilized to shrink the footprint and increase the efficiency of these components. Multiresonant antennas that enhance both optical absorption and emission process are recently demonstrated to enable efficient nonlinear frequency conversion at the nanoscale and are promising as structures for second harmonic generation (SHG) and upconversion. Here, the ability of colloidal metasurfaces fabricated by self‐assembly as on‐chip platforms for enhanced SHG is demonstrated. These metasurfaces exhibit high spatial overlap of multiple surface plasmon modes whose frequencies can be independently tuned through appropriate design of colloidal and metasurface geometries. It is demonstrated that these bottom‐up structures rival lithographic nonlinear optical antennas in SHG efficiency, suggesting the potential for these colloidal metasurfaces in integrated on‐chip architectures.