Efficient Nonlinear Frequency Conversion Research Articles

Chirality-controlled second-order nonlinear frequency conversion in lithium niobate film metasurfaces.

The high-quality factor resonant metasurfaces have extensive applications in enhancing nonlinear frequency conversion efficiency at the subwavelength scale. However, methods for actively modulating the frequency conversion process are limited. We design a chiral lithium niobate film metasurface and investigate the photonic spin as a new degree of freedom to dynamically control the second-order nonlinear frequency conversion, without reconfiguring the structure by using external stimuli. The chiral resonance with circular dichroism (CD) of 0.62 gives rise to a high nonlinear CD of 0.84 in second-harmonic generation efficiency. Interestingly, combining the chiral resonance and an achiral quasi-bound state in the continuum enables us to investigate the photonic-spin-controlled sum-frequency generation and the photon pair generation from the spontaneous parametric downconversion process. Owing to the ultrahigh quality factor exceeding 103 both for two resonances, the second-order nonlinear frequency conversion occurs at a wavelength region of 0.2 nm, suggesting good monochromaticity. Our work opens new, to our knowledge, avenues for practical implementation of dynamically controlled nonlinear optical devices and will find utility in holography, switchable light sources, and information processing.

Second Harmonic Generation in Apodized Chirped Periodically Poled Lithium Niobate Loaded Waveguides Based on Bound States in Continuum

With the rapid development of optical communication and quantum information, the demand for efficient and broadband nonlinear frequency conversion has increased. At present, most single-frequency conversion processes in lithium niobate on insulator (LNOI) waveguides suffer from lateral leakage without proper design, leading to an additional increase in propagation loss. Achieving broadband frequency conversion also encounters this problem in that there are no relevant works that have solved this yet. In this paper, we theoretically propose an efficient and flat broadband second harmonic generation (SHG) in silicon nitride loaded apodized chirped periodically poled LNOI waveguides. By using a bound states in the continuum (BICs) mechanism to reduce the propagation loss and utilizing the characteristic that the BICs are insensitive to wavelength, an ultra-low-loss wave band of 80 nm is realized. Then, by employing an apodized chirped design, a flat broadband SHG is achieved. The normalized conversion efficiency (NCE) is approximately 222%W−1cm−2, and the bandwidth is about 100 nm. Moreover, the presented waveguides are simple and can be fabricated without direct etching of lithium niobate, exhibiting excellent fabrication tolerance. Our work may open a new avenue for exploring low-loss and flat broadband nonlinear frequency conversion on various on-chip integrated photonic platforms.

High-efficiency anti-stokes Raman blue laser in CO2 enables high-luminance RGB laser-driven white light

Stimulated Raman scattering (SRS) is an efficient nonlinear frequency conversion method, enabling simultaneous generation of red, green, and blue (RGB) lasers. In order to synthesize a white light source for laser display by SRS, a 532 nm laser was used as pump source, and high purity gaseous carbon dioxide (CO2) was used as the Raman active medium. First, by optimizing experimental parameters, with an f = 1.5 m focal lens, in 0.8 atm CO2 pumped at 304 mJ, a first-order anti-Stokes (AS1) 495 nm blue laser was achieved, with an energy of 31.9 mJ, peak power of 10.9 MW and conversion efficiency (CE) of 10.5 %. Then, Then, the second-order Stokes light (S2) at 624 nm, residual pump laser (S0) at 532 nm, and AS1 laser were utilized as RGB primary colors. By the variation of pressure below 1 atmosphere (atm), the laser-driven white light (LDWL) with adjustable correlated color temperature (CCT) below 4700 K were simulated. Finally, LDWL up to 2.6 × 1018cd/m2 of a CCT of 3300 K could be synthesized at an RGB power ratio of PR: PG: PB = 0.447:0.094:0.459, resulting in a white light power CE of 44.4 % and luminous efficacy of 113.7 lm/W. In addition, use of the 574 nm yellow Raman light is expected to realize a four-primary (RGBY) laser display scheme with higher Luminance and broader color gamut. Moreover, the feasibility of using a 515 nm Yb: YAG laser as pump source to widen the range of CCT and improve the brightness of LDWL was discussed.

Low-repetition-rate optical frequency comb

Reducing the pulse repetition rate of an optical frequency comb increases the pulse energy for a given average power. This enhances the efficiency of nonlinear frequency conversion and it facilitates extending the accessible wavelength range, for example, into the extreme ultraviolet (XUV). The resulting spectrally dense frequency comb can still be used for precision spectroscopy of narrow atomic or molecular transitions. In this paper, we demonstrate a low-noise infrared frequency comb with a repetition rate as low as 40 kHz using a Yb:KYW mode-locked laser, pulse picking, and subsequent amplification. The frequency comb structure is confirmed by generating a beat note with a continuous wave reference laser. A comb mode is actively stabilized to the reference laser, and the integrated rms phase noise from 20 Hz to 20 kHz is measured to be 195 mrad.

Periodically-Poled Lithium Tantalate Ridge Waveguides for Efficient Nonlinear Frequency Conversion in the Near UV

Optical damage resistant ridge waveguides for blue and near UV wavelengths have been fabricated using high-temperature Zr and Zn diffusion doping and vapor transport equilibration (VTE) of congruent LiTaO3 crystals. For both dopants high optical damage thresholds >10 MW/cm2 for 532 nm light were demonstrated at room temperature, which can be increased by a factor ~3 when heating the samples to ~150°C. Ridge waveguides with low optical losses of ~0.4 dB/cm were fabricated using diamond-blade dicing. First-order periodic poling with grating periods of ~3 μm can be used for efficient nonlinear frequency conversion for both SHG (800 nm pump) or SPDC (400 nm pump) processes.

Achieving the full-wavelength phase-matching for efficient nonlinear optical frequency conversion in C(NH2)3BF4

Achieving the full-wavelength phase-matching for efficient nonlinear optical frequency conversion in C(NH2)3BF4

Extremely high-Q quasi-BICs induced by simultaneously broken out-of- and in-plane symmetries in dielectric metasurfaces of low-index materials

Generating high-Q resonances is a difficult task to realize with low-index materials as the building blocks of metasurfaces, despite of its significance. Up to date, various schemes for obtaining strong resonances with low-index materials have been reported, while the highest record of Q-factor still remains in the order of several thousands. This work shows that quasi-bound states in the continuum (quasi-BICs) with extremely high Q-factors in the orders of 105∼107 can be obtained in the low-index metasurfaces, taking lithium niobate (LN) metasurfaces supported by silica substrate as examples. In such metasurfaces, BICs with inversion symmetric distributions evolve to quasi-BICs with extremely high Q-factors by simultaneously breaking out-of- and in-plane symmetries due to multiple symmetry protectedness. Dramatically high enhancement of mode intensity in the order of 104∼106 due to the high Q-factor enables highly efficient nonlinear frequency conversion: the results based on the self-consistent approach show that the total efficiency of second-harmonic generation reaches around 9.4% for pump intensity of 0.5 W/cm2. As the first report on extremely high-Q quasi-BICs in low-index metasurfaces, the presented results pave the way towards broad applications for ultraviolet nanophotonics.

Frequency Converters for the Terahertz and Infrared Ranges

A method for solving the problem of nonlinear diffraction on two-dimensional periodic gratings of graphene ribbons has been developed. The third-order nonlinear conductivity of graphene under the action of two waves is taken into account, which is determined by the field of the pump wave, for which we use the field on graphene ribbons obtained by solving the linear diffraction problem. Numerical analysis shows the efficiency of nonlinear frequency conversion in the terahertz and infrared ranges when the frequencies of the incident pump and signal waves coincide with the resonant frequencies of the fundamental and higher order modes of surface plasmon polaritons in graphene ribbons.

Intracavity third-harmonic generation in a continuous-wave/self-mode-locked semiconductor disk laser

The high peak power of picosecond pulses produced by a self-mode-locked semiconductor disk laser can effectively improve the efficiency of nonlinear frequency conversion. This paper presents the intracavity frequency tripling in a self-mode-locked semiconductor disk laser, and a picosecond pulse train at 327 nm wavelength is achieved. The pulse repetition rate is 0.49 GHz, and the pulse width is 5.0 ps. The obtained maximum ultraviolet output power under mode locking is 30.5 mW, and the corresponding conversion efficiency is obviously larger than that of continuous-wave operation. These ultraviolet picosecond pulses have high spatial and temporal resolution and can be applied in some emerging fields.

Thin-film lithium niobate photonic integrated devices: Advances and oppotunities

<sec>Lithium niobate, known as one of the most widely used nonlinear optical crystals, has recently received significant attention from both academia and industrial circles. The surge in interest can be attributed to the commercial availability of thin-film lithium niobate (TFLN) wafers and the rapid advancements in nanofabrication techniques. A milestone was achieved in 2020 with the successful fabrication of wafer-scale TFLN photonic integrated circuits, which paved the way for mass-producible and cost-effective manufacturing of TFLN-based products.</sec><sec>At present, the majority of research on TFLN photonic integrated devices focuses on light manipulation, i.e. field modulation and frequency conversion. The electro-optic, acousto-optic, photo-elastic and piezo-electric effects of lithium niobate are harnessed to modulate the amplitude, phase and frequency of light. The second-order and third-order nonlinearities of lithium niobate enable frequency conversion processes, which leads to the development of frequency converters, optical frequency combs, and supercontinuum generation devices. These exceptional optical properties of lithium niobate enable the electromagnetic wave to manipulate covering from radio-frequency to terahertz, infrared, and visible bands. Using the outstanding performance of TFLN photonic integrated devices, including remarkable modulation rate, wide operation bandwidth, efficient nonlinear frequency conversion, and low power consumption, diverse applications, such as spanning optical information processing, laser ranging, optical frequency combs, microwave optics, precision measurement, quantum optics, and quantum computing, are demonstrated.</sec><sec>Additionally, it is reported that TFLN-based lasers and amplifiers have made remarkable progress, and both optical and electrical pumps are available. These achievements include combining gain materials, such as rare-earth ions or heterostructures, with III-V semiconductors. The integration of low-dimensional materials or absorptive metals with TFLN can also realize TFLN-based detectors. These significant developments expand the potential applications of TFLN photonic integrated devices, thus paving the way for monolithic TFLN chips.</sec><sec>The versatility and high performances of TFLN photonic integrated devices have made revolutionary progress in these fields, opening up new possibilities for cutting-edge technologies and their practical implementations. In this point of view, we briefly introduce the development of TFLN nanofabricationn technology. Subsequently, we review the latest progress of TFLN photonic integrated devices, including lasers, functional nonlinear optical devices, and detectors. Finally, we discuss the future development directions and potential ways of TFLN photonics.</sec>

Second-harmonic generation of embedded plasmonic nanoparticle arrays via interparticle coupling

Efficient nonlinear frequency conversion, such as second-harmonic generation in ultracompact structures, is essential for the development of modern nanophotonic devices. Here, we demonstrate intense second-harmonic emission in scalable embedded Ag nanoparticle arrays fabricated by ion implantation into BK7 glass. The interparticle coupling effect significantly enhances the local field at the nanogap (gap size ∼ 1 nm) of two neighboring Ag nanoparticles and finally amplifies second-harmonic emission generated at the surface of plasmonic nanoparticles. Notably, the intensity of second-harmonic emission in embedded Ag nanoparticle arrays is comparable to that of two-dimensional transition metal dichalcogenides under the excitation of a fundamental wave at 1064 nm and independent of the incident polarization angles. Our work offers a promising strategy on the rapid fabrication of low-cost nonlinear optical nanostructures with great environmental stability.

Mid-Infrared Nonlinear Frequency Conversion Using Monolithic Barium Titanate on Silicon-on-Insulator

Efficient nonlinear frequency conversion in the mid-infrared (mid-IR) region was demonstrated by a barium titanate (BTO) on a silicon-on-insulator (SOI) substrate. The BTO thin film was epitaxially grown by pulsed-laser deposition (PLD). The tensorial second order optical nonlinearity and the ferroelectric domain property of the BTO were characterized by the polarimetric mid-IR second harmonic generation (SHG) measurements. Azimuthal-dependent polarized SHG was modeled at different combinations of the nonlinear coefficient <formula><tex>$d_{ij}$</tex></formula> and the domain fraction factor <formula><tex>$\Delta D^{Y}/\Delta D^{X}$</tex></formula>. Strong SHG intensity was experimentally observed over a broad spectrum at &#x03BB; = 3.1 &#x03BC;m - 3.7 &#x03BC;m. A characteristic linear dependence between the SHG signal and the square of the pumping laser intensity was observed. The monolithic integration between the epitaxial BTO and the Si provides a compact platform for efficient on-chip light generation and quantum photonic technologies.

150 kHz, 300 ps green laser frequency doubled from a linearly polarized passively Q-switched Nd:YAG/Cr4+:YAG microchip oscillator and a Nd:YVO4 amplifier

We report the design and performance of a compact solid-state green laser capable of producing a 300 ps pulse width (PW) at a pulse repetition rate (PRR) of 150 kHz. The green laser was comprised of a thermally bonded [111]-cut Nd:YAG and [110]-cut Cr4+:YAG microchip, a Nd:YVO4 crystal, and a hydrothermal grey-track resisted KTP crystal, serving as the oscillator, amplifier and nonlinear frequency converter, respectively. The optimal working conditions for efficient laser amplification and nonlinear frequency conversion were studied by numerical solving the rate equations and coupled wave equations, respectively. Experimental setups were constructed thereafter according to the simulated results. Stable single-longitudinal-mode-operated, linearly polarized (LP) pulses with PW of ∼ 350 ps, PRR of ∼ 150 kHz, and polarization extinction ratio > 13 dB were obtained from the oscillator. The average power was boosted from 285 mW to 1.8 W by the amplifier and then frequency doubled to 532 nm with an average power of 970 mW, resulting in a shorter PW of ∼ 300 ps. The long term power stability of 0.63% (rms) was measured in 200 min indicating its great potential for practical Lidar applications. To the best of our knowledge, these results are among the highest PRR from a LP Nd:YAG/Cr4+:YAG based green laser with PW ≤ 300 ps. The design procedure has also provided an effective way towards the construction of compact solid-state laser systems with oscillation, amplification, and nonlinear frequency conversion units.

Second-harmonic and sum-frequency generation based on birefringence phase matching of BaMgF4 crystal.

BaMgF4 is a ferroelectric nonlinear crystal with a very wide transparency window ranging from 125 nm to 13µm of the wavelength. Therefore, it is a candidate material to generate ultraviolet or deep ultraviolet laser, which is very important in lithography, semiconductor manufacturing, and advanced instrument development. Here, the second-order birefringence phase-matching processes of the BaMgF4 crystal were studied, including second-harmonic generation (SHG) and sum-frequency generation (SFG). In the experiments, we measured the phase-matching angle, nonlinear frequency conversion efficiency, and angle bandwidth of SHG and SFG processes of BaMgF4 crystal, which are in well agreement with the theoretical calculations. This study may promote the research of nonlinear optical process of BaMgF4 crystal and also the further development of all-solid-state vacuum ultraviolet lasers.

Broadband Mid-IR Light Sources From Difference Frequency Generators Based on a 2-mm-Long Aperiodically-Poled Lithium-Niobate Crystal

We present a difference frequency generator (DFG) based on a 2-mm-long MgO-doped aperiodically-poled lithium-niobate (APPLN) crystal with a linearly-varying poling-period. The DFG was pumped by a femtosecond fiber-laser system with a repetition-rate of 53.4 MHz. It produced broadband mid-infrared light source having an instantaneous-bandwidth covering 1.9–4.6 μm, with an average power of 30 mW. Moreover, by numerical simulations, we revealed that due to the relatively high intensity of the incident signal wave, an APPLN crystal with a length of a few milli-meters was sufficient to produce efficient and broadband nonlinear frequency conversion in a high-repetition-rate, ultra-short-pulsed DFG system.

Efficient third-harmonic generation of broadband picosecond radiation by employing spectrally noncritical phase-matching KDP crystal and cascaded LBO crystals

To improve the nonlinear frequency conversion efficiency of a broadband picosecond fiber laser, the spectrally noncritical phase matching for frequency doubling, dual-crystal cascaded design for frequency tripling is employed. By this method, we obtain 2.39 W of output power at the third-harmonic wavelength, when the spectral full width at half maximum of the fundamental light is 5.5 nm and the input fundamental light power is 8.6 W. The frequency conversion efficiency of IR-UV is 27.8%. To the best of our knowledge, this is the first time of the broadband third harmonic picosecond ultraviolet laser generation by the combination of the spectrally noncritical phase matching for frequency doubling and dual-crystal cascaded design for frequency tripling.

Nanowire-assisted microcavity in a photonic crystal waveguide and the enabled high-efficiency optical frequency conversions

We report an indium phosphide nanowire (NW)-induced cavity in a silicon planar photonic crystal (PPC) waveguide to improve the light–NW coupling. The integration of NW shifts the transmission band of the PPC waveguide into the mode gap of the bare waveguide, which gives rise to a microcavity located on the NW section. Resonant modes with Q factors exceeding 10 3 are obtained. Leveraging on the high density of the electric field in the microcavity, the light–NW interaction is enhanced strongly for efficient nonlinear frequency conversion. Second-harmonic generation and sum-frequency generation in the NW are realized with a continuous-wave pump laser in a power level of tens of microwatts, showing a cavity-enhancement factor of 112. The hybrid integration structure of NW-PPC waveguide and the self-formed microcavity not only opens a simple strategy to effectively enhance light–NW interactions, but also provides a compact platform to construct NW-based on-chip active devices.

Nonlinear frequency conversions via weak surface polaritonic wave breaking in a hybrid plasmonic waveguide.

Material design and input field properties limit high-harmonic excitation efficiency of surface-plasmon polaritons (SPPs) in a nanoscopic device. We remedy these limitations by developing a concept for a plasmonic waveguide that exploits spatiotemporal control of a weak surface polaritonic field to create efficient four-wave mixing (FWM) and periodic phase singularities. Our configuration comprises four-level double Λ-type atomic medium (4Λ As) doped in a lossless dielectric situated above a negative-index metamaterial (NIMM) layer. We report the coherent excitation and propagation of the multiple surface polaritonic shock waves (SWs) and establish the highly efficient frequency combs by surface polaritonic wave (SPW) breaking. Consequently, multiple FWM and periodic plasmonic phase singularity patterns generate through nonlinear self-defocusing control commensurate with the plasmonic noise within the atomic electromagnetically induced transparency (EIT) window. Our work introduces SW formation within the subwavelength scale and generates efficient nonlinear frequency conversion, thereby opening prospects for designing fast optical modulators and nonlinear plasmonic gates.

Ultrabroadband nonlinear optics in nanophotonic periodically poled lithium niobate waveguides

Quasi-phase-matched interactions in waveguides with quadratic nonlinearities enable highly efficient nonlinear frequency conversion. In this paper, we demonstrate the first generation of devices that combine the dispersion engineering available in nanophotonic waveguides with quasi-phase-matched nonlinear interactions available in periodically poled lithium niobate (PPLN). This combination enables quasi-static interactions of femtosecond pulses, reducing the pulse energy requirements by several orders of magnitude compared to conventional devices, from picojoules to femtojoules. We experimentally demonstrate two effects associated with second harmonic generation (SHG). First, we observe efficient quasi-phase-matched SHG with &lt; 100 f J of pulse energy. Second, in the limit of strong phase-mismatch, we observe spectral broadening of both harmonics with as little as 2 pJ of pulse energy. These results lay a foundation for a new class of nonlinear devices, in which coengineering of dispersion with quasi-phase-matching enables efficient nonlinear optics at the femtojoule level.

Phase-matched second-harmonic generation in coupled nonlinear optical waveguides

Phase matching is crucial for effective nonlinear frequency conversion. Recent advances in photonic miniaturization greatly promote implementations of efficient nonlinear frequency conversion in integrated photonic devices. Here, we investigate second-harmonic generation in coupled nonlinear waveguides. We develop a nonlinear optical coupled-mode theory and implement the concept of supermode to analyze the phase-matching condition, which greatly facilitates deep understanding of physics therein and optimization of device design. We have successfully identified six phase-matching conditions for the first time. The dynamic nonlinear optical processes are analytically modeled, which explicitly demonstrates that the phase-matching condition can be realized by extra momentum compensating due to the interwaveguide coupling. The influences of input pump amplitude and relative phase difference are successively investigated. The theoretical model presented here should be readily generalized to other nonlinear optical processes (e.g., difference frequency generations) and will remain valid in asymmetric coupled waveguide structures or even beyond a weakly coupling regime. Our results will be useful for constructing highly efficient integrated nonlinear optical conversion devices with extended functionality.