High-efficiency Second-harmonic Generation Research Articles

Self-injection-locked near-visible laser at 781 nm by frequency-doubling in a quasi-singly resonant high-Q micro-cavity.

Based on a quasi-singly resonant plane-concave Fabry-Perot (FP) micro-cavity with a high Q-factor of 9.1 M and a volume of <0.12 mL, we have demonstrated self-injection-locked (SIL) second-harmonic generation (SHG) at near-visible frequencies. With an incident SIL pump power of 80 mW at 1561.8 nm, the maximum output SHG power at 780.9 nm reached 16.9 mW, achieving a high conversion efficiency of 21.1%. The intrinsic linewidth of the SIL pump laser was reduced from 1.1 MHz to about 130 Hz, and the corresponding linewidth of the SIL-SHG light was about 520 Hz as expected. Most importantly, high SHG efficiency was observed at any pump frequency of the corresponding FP resonant modes. Compared to the recently reported results of SIL-SHG using microring cavities, our SIL-SHG design with a quasi-singly resonant FP cavity offers higher conversion efficiency and output SIL-SHG power, along with a potential of continuous tunability of the SHG frequency. Thanks to its significantly larger mode-field volume, it can handle higher input power without incurring unwanted nonlinear effects.

Merging bound states in the continuum for high-efficiency second-harmonic generation

Merging bound states in the continuum for high-efficiency second-harmonic generation

High-efficiency second-harmonic generation in reverse-polarization dual-layer lithium niobate waveguides.

Second-harmonic (SH) generation is an important way to generate short-wavelength light sources. Thin-film lithium niobate (TFLN) resonators and waveguides featuring tight light confinement are flexible to realize mode- or quasi-phase-matching (MPM or QPM) and thus efficient SH generation. Here, we report an efficient SH with an absolute conversion efficiency of 45% in a reverse-polarized double-layer TFLN waveguide. Such a high conversion efficiency without periodic poling was realized with the pump of 67 mW in the 1550 nm band, benefiting from the employment of the largest nonlinear coefficient d33 and the significant nonlinear interaction of the fundamental transverse electric (TE) mode and the first-order TE mode in the vertical direction. This work paves the way toward on-chip integrated nonlinear optics in terms of harmonic generation and quantum light sources across the telecom and visible bands.

High-efficiency second harmonic generation in a micro-resonator on dual-layered lithium niobate.

High-quality micro-resonators on thin-film lithium niobate (TFLN) have emerged as an ideal platform for on-chip nonlinear optical applications due to their strong light confinement and excellent natural nonlinear optical properties. Here, we present high-efficiency second-harmonic generation (SHG) in micro-resonators on a TFLN based on the modal phase matching and natural quasi-phase matching. By optimizing the phase-matching conditions through thermal tuning, we demonstrate an on-chip SHG efficiency of 149,000%/W in the low power regime. Furthermore, we achieve an absolute conversion efficiency of 10.3% with a 0.3 mW pump power. Our work paves the way toward future efficient on-chip frequency conversion of classical and quantum light without the need for poling of the LN films.

Analytical solution to incident angle quasi-phase-matching engineering for second harmonic generation in a periodic-poled lithium niobate crystal

Phase matching or quasi-phase matching (QPM) is of significant importance to the conversion efficiency of second harmonic generation (SHG) in artificial nonlinear crystals like lithium niobate (LN) crystal or microstructured nonlinear crystals like periodic-poled lithium niobate (PPLN) crystals. In this paper, we propose and show that the incident angle of pump laser light can be harnessed as an alternative versatile tool to engineer QPM for high-efficiency SHG in a PPLN crystal, in addition to conventional means of period adjusting or temperature tuning. A rigorous model is established and analytical solution of the nonlinear conversion efficiency under the small and large signal approximation theory is obtained at different incident angles. The variation of phase mismatching and walk-off length with incident angle or incident wavelength are also explored. Numerical simulations for a PPLN crystal with first order QPM structure are used to confirm our theoretical predictions based on the exact analytical solution of the general large-signal theory. The results show that the narrow-band tunable SHG output covers a range of 532 nm–552.8 nm at the ideal incident angle from 0° to 90°. This theoretical scheme, fully considering the reflection and transmission at the air-crystal interface, would offer an efficient theoretical system to evaluate the nonlinear frequency conversion and help to obtain the maximum SHG conversion efficiency by selecting an optimum incident wavelength and incident angle in a specially designed PPLN crystal, which would be very helpful for the design of tunable narrow-band pulse nanosecond, picosecond, and femtosecond laser devices via PPLN and other microstructured LN crystals.

Topological slow light enhanced second harmonic generation in double-resonant topological photonic crystal

Generally, the second-harmonic generation and slow light of multi-band topological edge states (TESs) have been studied separately. Therefore, the influence of simultaneous slow light and topology protection on second-harmonic generation (SHG) is deficient. Here, we propose a high-efficiency SHG using dual-frequency TESs in topological photonic crystals (TPCs) with slow-light conditions. The wave vector matching condition and energy conservation condition (frequency doubling) can be achieved by adequately adjusting the overall structural parameters of TPCs. The double-resonant nonlinear interaction between two TESs is enabled using a square lattice TPC. Due to the topological localization of the TES and the long interaction time of slow-light effect, the energy densities of the fundamental wave and SHG are significantly increased. Consequently, the high intrinsic efficiency of SHG can be obtained in the order of 7.40 × 10−4. Our work opens new avenues for using topological protected and slow light enhanced nonlinear frequency conversion in a TPC system.

Bright Second Harmonic Emission from Photonic Crystal Vertical Cavity

AbstractThis study presents photonic vertical cavities consisting of nonlinear materials embedded in photonic crystals (PhCs) for resonantly enhancing second harmonic generation (SHG). Previous attempts at SHG in such structures have been limited to efficiencies of 10−7 to 10−5, but a high SHG efficiency of 0.28% by constructing a vertical cavity with a lithium niobate membrane placed between two PhCs is demonstrated, which exhibits high‐quality resonances. The results open up new possibilities for compact laser frequency converters that can have a revolutionary impact on the fields of nonlinear optics and photonics.

High-Efficiency Second-Harmonic Generation Using Quasi-Bound State in LiNbO3 Metasurface

We numerically demonstrated a high-efficiency second-harmonic generation (SHG) using quasi-bound state in the continuum (quasi–BIC) in thin film LiNbO3 (TFLN) metasurface. The TFLN possessed exceptionally high second-order nonlinear coefficients, contributing to the enhanced SHG performance. An eccentric cylinder unit cell was presented to achieve high Q–factor resonances associated with the asymmetric parameter introduced. Simulations showed that the high efficiency of the second-harmonic conversion was obtained by using the high Q–factor of the asymmetric dielectric cylinder metasurface, and it achieved a high SHG efficiency of 6.5% at pump intensities as low as 1 MW/cm2 at a normal incident. Furthermore, the simulation results indicated that breaking the symmetry through oblique incidence was more effective in achieving a higher Q–factor compared to altering the structural parameters. Specifically, under 1° oblique incidences, the conversion efficiency could reach 1.2% at an incident power of 1 kW/cm². We have proposed a method to achieve a high conversion efficiency of second-harmonic generation in low-refractive-index materials. Our work not only offers theoretical support but also provides valuable insights for the advancement of efficient nonlinear frequency doubling technology, optical communication, and sensing applications.

Hybrid Germanium Bromide Perovskites with Tunable Second Harmonic Generation

AbstractGe‐based hybrid perovskite materials have demonstrated great potential for second harmonic generation (SHG) due to the geometry and lone‐pair induced non‐centrosymmetric structures. Here, we report a new family of hybrid 3D Ge‐based bromide perovskitesAGeBr3,A=CH3NH3(MA), CH(NH2)2(FA), Cs and FAGe0.5Sn0.5Br3, crystallizing in polar space groups. These compounds exhibit tunable SHG responses, where MAGeBr3shows the strongest SHG intensity (5×potassium dihydrogen phosphate, KDP). Structural and theoretical analysis indicate the high SHG efficiency is attributed to the displacement of Ge2+along [111] direction and the relatively strong interactions between lone pair electrons of Ge2+and polar MA cations along thec‐axis. This work provides new structural insights for designing and fine‐tuning the SHG properties in hybrid metal halide materials.

High-efficiency second-harmonic generation in coupled nano Fabry-Perot thin resonators.

In this paper we experimentally demonstrate second-harmonic generation (SHG) enhancement in thin 1D periodic plasmonic nanostructures on GaAs in the infrared spectral range. Due to the properly designed coupling of horizontal Fabry-Perot nanoresonators that occurs inside these structures, the obtained conversion efficiencies go up to the 10-7 W-1 range. Moreover, we demonstrate that the engineering of the plasmonic nanoantenna dimensions on the same GaAs layer can lead to SHG enhancement for pump wavelengths ranging from 2.8 µm to 3.3 µm.

Dimethylaniline-Based Hybrid Compounds of Cadmium Diiodide: Synthesis, Crystal Structure, and Physical Properties

In this work, synthesis, crystal structure, and selected properties of two new organic–inorganic compounds based on cadmium iodide with 2,5- and 2,6-dimethylaniline: CdI2(2,5-DMA)2 1 and CdI2(2,6-DMA)2 2, respectively, were investigated. Both compounds were obtained using the same methodology, and their crystal structures were determined from powder (1) and single-crystal (2) X-ray diffraction experiments. The examined materials crystallize in the noncentrosymmetric space group Fdd2 and contain large polarizable iodine ions and electron-rich aromatic ligands. Second harmonic generation (SHG) tests performed with urea as a reference revealed high SHG efficiency of powdered samples 1 and 2. SHG signals measured for samples 1 and 2 are, respectively, around 10 and 8 times larger than the signal for urea. The experimental part was supplemented with theoretical calculations of the second-order electric susceptibility tensor components, χijk(2). Calculated values of the χ113(2) tensor component around 1.2–1.4 pm·V–1 suggest that both materials are relatively efficient second harmonic generators. Fibrous morphology of the materials was revealed by scanning electron microscopy (SEM), and the observed bending of the fibers suggested their significant flexibility. Samples 1 and 2 were also characterized by spectroscopic techniques (IR, RS, UV–Vis-DRS) and their thermal stability and decomposition rates were researched using XRPD and TG/DSC, revealing that both investigated materials are thermally stable up to around 100 °C.

High-Efficiency Second-Harmonic and Sum-Frequency Generation in a Silicon Nitride Microring Integrated with Few-Layer GaSe

Silicon nitride (SiN) photonics platform has attributes of ultra-low linear and nonlinear propagation losses and CMOS-compatible fabrication process, promising large-scale multifunctional photonic circuits. However, the centrosymmetric nature of SiN inhibits second-order nonlinear optical responses in its photonics platform, which is desirable for developing efficient nonlinear active devices. Here, we demonstrate high-efficiency second-order nonlinear processes in SiN photonics platform by integrating a few-layer GaSe flake on a SiN microring resonator. With the pump of microwatts continuous-wave lasers, second-harmonic generation and sum-frequency generation with the conversion efficiencies of 849%/W and 123%/W, respectively, are achieved, which benefit from the ultrahigh second-order nonlinear susceptibility of GaSe, resonance enhanced GaSe-light interaction, and phase-matching condition satisfied by the mode engineering. Combining with the easy integration, the GaSe-assisted high-efficiency second-order nonlinear processes offer a new route to enriching already strong functionality of SiN photonics platform in nonlinear optics.

Influence of thermal lens effect on second harmonic process in semi-monolithic cavity scheme

Second harmonic generation (SHG) is an effective way to generate short wavelength laser with high power. The SHG is accompanied with the absorptions of fundamental waves and harmonic waves, which converts a fraction of the two waves deposit energy into heat, causing a temperature gradient along the radial direction of the periodically poled potassium titanyl phosphate (PPKTP) crystal. The inhomogeneous temperature distribution causes thermal lensing in the crystal. The thermal lensing effect will deform the spatial mode of the SHG cavity and result in the mode-mismatching of the fundamental wave to the SHG cavity, and therefore the conversion efficiency of SHG process is reduced. Moreover, with the increase of injected fundamental wave power, the influence caused by thermal lens becomes more and more serious. In order to obtain a high-efficiency frequency conversion, it is necessary to take the measure to minimize the effect caused by thermal lensing. In this paper, we report on a high efficiency generation of green laser at 532 nm by external cavity SHG process with a semi-monolithic standing cavity. The influences of thermal lens effect on the optimal conversion efficiency in different semi-monolithic cavities are theoretically analyzed. The variations of conversion efficiency with the pump power in “plane-concave” semi-monolithic cavity based on parallel crystal and also in “concave-concave” semi-monolithic cavity based on concave crystal are quantitatively analyzed. In experiments, two types of cavity structures are built to measure the variation of frequency doubling conversion efficiency with pump power. For the “plane-concave” semi-monolithic cavity, the maximum green laser power of 747 mW is obtained and the corresponding conversion efficiency reaches 93.4%±3%, with 800 mW infrared laser injected. For the “concave-concave” semi-monolithic cavity, the maximum green laser power of 529 mW is obtained and the corresponding conversion efficiency is 88.2% ± 3%, with 600 mW infrared laser injected. The results show that the thermal lens affects the optimal conversion efficiency more seriously in “concave-concave” semi-monolithic cavity than in “plane-concave” semi-monolithic cavity. Furthermore, the influence of thermal lens effect turns higher and higher with the increase of the loss in the cavity. It is obvious that the “plane-concave” semi-monolithic cavity is more suitable for the SHG process and has many potential applications in quantum optics and cold atom physics and provides a guidance for future research on high-efficiency SHG process.

Second Harmonic Generation in Germanium Quantum Wells for Nonlinear Silicon Photonics

Second-harmonic generation (SHG) is a direct measure of the strength of second-order nonlinear optical effects, which also include frequency mixing and parametric oscillations. Natural and artificial materials with broken center-of-inversion symmetry in their unit cell display high SHG efficiency, however, the silicon-foundry compatible group IV semiconductors (Si, Ge) are centrosymmetric, thereby preventing full integration of second-order nonlinearity in silicon photonics platforms. Here we demonstrate strong SHG in Ge-rich quantum wells grown on Si wafers. Unlike Si-rich epilayers, Ge-rich epilayers allow for waveguiding on a Si substrate. The symmetry breaking is artificially realized with a pair of asymmetric coupled quantum wells (ACQW), in which three of the quantum-confined states are equidistant in energy, resulting in a double resonance for SHG. Laser spectroscopy experiments demonstrate a giant second-order nonlinearity at mid-infrared pump wavelengths between 9 and 12 μm. Leveraging on the strong intersubband dipoles, the nonlinear susceptibility χ(2) almost reaches 105 pm/V, 4 orders of magnitude larger than bulk nonlinear materials for which, by the Miller’s rule, the range of 10 pm/V is the norm.

Second‐Harmonic Generation Based on the Dual‐Band Second‐Order Topological Corner States

Coherent frequency conversions using multiband topological edge states in photonic structures made of nonlinear optical materials have been extensively investigated. In practice, however, coherent sources localized at the subwavelength scale are of great importance. Herein, efficient second‐harmonic generation (SHG) using dual‐band second‐order topological corner states is reported. By properly adjusting the structural parameters, dual‐band corner states can be obtained in the first and second bandgaps, when the eigenfrequency of the corner state in the higher bandgap is twice that of the corner one in the lower gap. In a square lattice made of tellurium as an example, high SHG efficiency in the order of 10−1% can be obtained for a peak pump intensity of around 20 GW cm−2. The results presented provide the realizability of subwavelength‐scale coherent sources, which are robust against the structural disorder or defects.

High-Efficiency Spatial-Wave Frequency Multiplication Using Strongly Nonlinear Metasurface.

In the past decades, metasurfaces have opened up a promising venue for manipulating lights and electromagnetic (EM) waves. In the field of nonlinearity, second‐harmonic generation (SHG) is a research focus due to its diverse applications. There have been many researches for realizing SHG in optical regime using nonlinear characteristics of optical materials, but its efficiency is low. In microwave frequencies, SHGs are basically studied in the guided‐wave systems. Here, high‐efficiency SHGs of spatial waves are presented in the microwave frequency using nonlinear metasurface loaded with active chips at the subwavelength scale. The nonlinear meta‐atom is composed of receiving antenna, transmitting antenna, and active circuit of frequency multiplier, which can realize strongly nonlinear response and link the EM signals from the receiving to transmitting antennas. Correspondingly, to achieve the function of spatial‐wave frequency multiplication, the working frequency of the transmitting antenna in the meta‐atom should be twice as that of the receiving antenna, and hence the active chip is well matched to obtain the signal transforming with high efficiency. Good performance of the spatial‐wave frequency multiplication is demonstrated in the proof‐of‐concept experiments with the best transform efficiency of 85.11% under normal incidence, validating the proposed method.

Robust modal phase matching in subwavelength x-cut and z-cut lithium niobate thin-film waveguides

We use the nonlinear coupled-mode theory to theoretically investigate second-harmonic generation (SHG) in subwavelength x-cut and z-cut lithium niobate (LN) thin-film waveguides and derive the analytical formula to calculate SHG efficiency in x-cut and z-cut LN thin-film waveguides explicitly. Under the scheme of optimal modal phase matching (MPM), two types of LN thin films can achieve highly efficient frequency doubling of a 1064 nm laser with a comparable conversion efficiency due to very consistent modal field distribution of the fundamental wave and second-harmonic wave with efficient overlap between them. Such a robust MPM for high-efficiency SHG in both the subwavelength x-cut and z-cut LN thin-film waveguides is further confirmed in a broad wavelength range, which might facilitate design and application of micro–nano nonlinear optical devices based on the subwavelength LN thin film.

Realizing high efficiency 532 nm laser by optimizing the mode- and impedance-matching

Increasing the conversion efficiency of second harmonic generation (SHG) is an area of interest in research. We report a high-efficiency 532 nm laser generation, with a conversion efficiency of 94.04 ± 0.115% from the pump depletion of 98.1% ± 0.1%, by accurately quantifying the round-trip loss and the transmissivity of the input mirror using our proposed scheme. The optimal conversion efficiency of the cavity-enhanced frequency doubling process is independent of the waist and is determined by the pump depletion, round-trip loss, and transmissivity of the input mirror. These results show that the cavity-enhanced frequency doubling process is not necessary to set the focusing parameter at the optimal single-pass conversion. These results provide a guide for future research on high-efficiency SHG.

Experimental realization of high-efficiency blue light at 426 nm by external frequency doubling resonator

Second harmonic generation (SHG) is used to get continuous wave laser with a lot of applications, it is a major way to provide pump power for generating nonclassical states, especially for squeezed states and entanglement states. High-efficiency SHG resonant on atoms lines also provides laser sources for atomic entanglement generation, light-atom interaction and high-speed quantum memory. For the frequency-doubling process at 426 nm, the major challenge of increasing the conversion efficiency is the thermal effect caused by the absorption in crystal. The degradation of mode-match efficiency induced by the severely thermal effect limits the conversion efficiency of the second harmonic generator. Furthermore, the blue light induced infrared absorption (BLIIRA) in the nonlinear crystal intensifies the thermal effect, it makes the conversion efficiency of the frequency-doubling cavity and the stability of the output blue laser worse, and it is more serious at high input power. Based on the theoretical analysis of thermal lens, we find that the thermal lens should not be placed at the center of the crystal, the location of the equivalently thermals lens has a deviation from the center of the crystal. Follow the theoretical analysis of thermal lens, we design a ring cavity with a 10 mm-long periodically poled potassium titanyle phosphate (PPKTP) crystal to reduce the thermal lens effect induced mode-mismatch. The location of nonlinear crystal is adjusted precisely to reduce the mode-mismatch caused by the thermal lens under our theoretical analysis. Finally, we realized a high conversion efficiency blue laser at 426 nm with the conversion efficiency up to 83.1% with an output power of 428 mW after the adjustment of the crystal location, corresponding to our theoretical analysis well. The measured beam quality factors (&lt;i&gt;M&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; value) of the generated blue laser are &lt;inline-formula&gt;&lt;tex-math id="Z-20200217024354-1"&gt;\begin{document}$ M^2(x) = 1.05 $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; and &lt;inline-formula&gt;&lt;tex-math id="Z-20200217024354-2"&gt;\begin{document}$ M^2(y) = 1.02 $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, respectively. The measured power stability of Generated Blue laser in 15 mins is 1.25%. The output power of the SHG is strong enough to provide pump power for the generation of the continuous variable squeezed vacuum state at 852 nm and the long-term stability of the output blue laser is also measured to be fine. To the best of our knowledge, the conversion efficiency is the highest-reported one at this wavelength. We believe that such high-performance frequency doubling system is a fundamental building block for quantum information science based non-classical states.

Periodically poled thin-film lithium niobate microring resonators with a second-harmonic generation efficiency of 250,000%/W

Lithium niobate (LN), dubbed by many as the silicon of photonics, has recently risen to the forefront of chip-scale nonlinear optics research since its demonstration as an ultralow-loss integrated photonics platform. Due to its significant quadratic nonlinearity ($\chi^{(2)}$), LN inspires many important applications such as second-harmonic generation (SHG), spontaneous parametric down-conversion, and optical parametric oscillation. Here, we demonstrate high-efficiency SHG in dual-resonant, periodically poled z-cut LN microrings, where quasi-phase matching is realized by field-assisted domain engineering. Meanwhile, dual-band operation is accessed by optimizing the coupling conditions in fundamental and second-harmonic bands via a single pulley waveguide. As a result, when pumping a periodically poled LN microring in the low power regime at around 1617nm, an on-chip SHG efficiency of 250,000%/W is achieved, a state-of-the-art value reported among current integrated photonics platforms. An absolute conversion efficiency of 15% is recorded with a low pump power of 115$\mu$W in the waveguide. Such periodically poled LN microrings also present a versatile platform for other cavity-enhanced quasi-phase matched $\chi^{(2)}$ nonlinear optical processes.