Second-harmonic Conversion Efficiency Research Articles

Second harmonic of higher-order Poincaré sphere beam with two orthogonal 5%MgO:PPLN crystals

In this work, the second harmonic (SH) of higher-order Poincaré sphere (HOPS) beam was introduced and demonstrated with two orthogonal 5%MgO:PPLN crystals. Based on the quasi-phase-matching technique, the vectorial coupled wave equations were derived to simulate the SH of HOPS beams through the two crystals, including the cylindrical vector beams (CVBs), elliptically polarized CVBs (EPCVBs), and circularly polarized vortex beams. Then, the experimental setup was established to reveal that the SH of CVBs and EPCVBs present the four-lobed structure and still exhibit vector characteristics. Meanwhile, the circularly polarized vortex beams become the linearly polarized vortex beams with double phase topology, confirming the conservation of orbital angular momentum. Moreover, the maximum SH conversion efficiency of CVBs, EPCVBs, and circularly polarized vortex beams can reach 25.3%, 23.4%, and 29.4%, respectively, which may be instructive for promoting the SH generation of vector vortex beams with high efficiency.

A kinetic theory approach to second harmonic generation by high power laser in magnetized plasma with nonextensive distribution

The present study presents a kinetic theory approach to second-harmonic generation (SHG) through high-power laser in collisionless plasma with nonextensively-distributed electrons. The study applied a relativistic Vlasov equation to obtain nonlinear current density and second-harmonic (SH) conversion efficiency. It was observed that the power conversion efficiency (PCE) of second-harmonic generation depends on plasma and laser parameters such as dimensionless cyclotron frequency, plasma density, normalized thermal velocity, the nonextensive q-parameter, and laser intensity. Moreover, cyclotron frequency associated with plasma electrons has been found to play an important role in increasing the power conversion efficiency of SH generation.

Control of second-harmonic generation in all-dielectric intersubband metasurfaces by controlling the polarity of χ(2).

All-dielectric metasurfaces have recently led to a paradigm shift in nonlinear optics as they allow for circumventing the phase matching constraints of bulk crystals and offer high nonlinear conversion efficiencies when normalized by the light-matter interaction volume. Unlike bulk crystals, in all-dielectric metasurfaces nonlinear conversion efficiencies primarily rely on the material nonlinearity, field enhancements, and the modal overlaps, therefore most efforts to date have only focused on utilizing these degrees of freedom. In this work, we demonstrate that for second-harmonic generation in all-dielectric metasurfaces, an additional degree of freedom is the control of the polarity of the nonlinear susceptibility. We demonstrate that semiconductor heterostructures that support resonant nonlinearities based on quantum-engineered intersubband transitions provide this new degree of freedom. We can flip and control the polarity of the nonlinear susceptibility of the dielectric medium along the growth direction and couple it to the Mie-type photonic modes. Here we demonstrate that engineering the χ(2) polarity in the meta-atom enables the control of the second-harmonic radiation pattern and conversion efficiency. Our results therefore open a new direction for engineering and optimizing second-harmonic generation using all-dielectric intersubband nonlinear metasurfaces.

In-fibre second-harmonic generation with embedded two-dimensional materials

Silica-based optical fibres are a workhorse of nonlinear optics, providing ready access to a range of nonlinear phenomena including solitons and self-phase modulation. However, they have one fundamental limitation: due to the amorphous nature of silica, they do not exhibit second-order nonlinearity, except for negligible contributions from surfaces. Here we demonstrate second-harmonic generation in functionalized optical fibres by using a monolayer of highly nonlinear MoS2 directly grown on the fibre’s core. The MoS2-functionalized fibre exhibits a second-order susceptibility (χ(2)) value of 44 pm V–1 and a second-harmonic generation conversion efficiency of 0.2 × 10–3 m−2 W−1. This approach is scalable and can be generalized to other transition metal dichalcogenides and a wide range of waveguide systems. Our results demonstrate a new approach towards efficient in-fibre second-harmonic generation sources and may establish a platform for χ(2)-based nonlinear fibre optics, optoelectronics, photonics platforms, integrated optical architectures and active fibre networks.

Broadband second-harmonic generation in step-chirped periodically poled lithium niobate waveguides.

Periodically poled lithium niobate (PPLN) structures on a chip enable efficient second-order nonlinear optical effects, benefiting from the tight light confinement and the utilization of d33. Here, we report a broadband second-harmonic (SH) generation in a step-chirped PPLN waveguide on X-cut lithium niobate on insulator (LNOI). The high fidelity of the poling period is demonstrated over the whole length of 7 mm using a non-destructive technique of piezoresponse force microscopy. The SH signal was continuously observed in the step-chirped PPLN waveguides while scanning the wavelength of the pump laser from 1550 nm to 1660 nm. The SH conversion efficiency was measured to be 9.6 % W-1 cm-2 at 1642 nm. This work will benefit wavelength conversions of light sources with wideband spectra.

Highly efficient second harmonic generation of thin film lithium niobate nanograting near bound states in the continuum

Bound states in the continuum (BICs) are ubiquitous physical phenomena where such states occur due to strong coupling between leaky modes in side lossy systems. BICs in meta-optics and nanophotonics enable optical mode confinement to strengthen local field enhancement in nonlinear optics. In this study, we numerically investigate second-harmonic generation (SHG) in the vicinity of BICs with a photonic structure comprising one-dimensional nanogratings and a slab waveguide made of lithium niobate (LiNbO3, LN). By breaking the symmetry of LN nanogratings, BICs transition to quasi-BICs, which enable strong local field confinement inside LN slab waveguide to be supported, thereby resulting in improving SHG conversion with lower pump power of fundamental frequency (FW). With a peak intensity of 1.33 GW cm−2 at the FW, our structure features a second-harmonic conversion efficiency up to 8.13 × 10−5 at quasi-BICs. We believe that our results will facilitate the application of LN in integrated nonlinear nanophotonic.

Toward 1% single-photon anharmonicity with periodically poled lithium niobate microring resonators

The absence of the single-photon nonlinearity has been a major roadblock in developing quantum photonic circuits at optical frequencies. In this paper, we demonstrate a periodically poled thin film lithium niobate microring resonator (PPLNMR) that reaches 5,000,000%/W second-harmonic conversion efficiency—almost 20-fold enhancement over the state-of-the-art—by accessing its largest χ ( 2 ) tensor component d 33 via quasi-phase matching. The corresponding single-photon coupling rate g / 2 π is estimated to be 1.2 MHz, which is an important milestone as it approaches the dissipation rate κ / 2 π of best-available lithium niobate microresonators developed in the community. Using a figure of merit defined as g / κ , our device reaches a single-photon nonlinear anharmonicity approaching 1%. We show that, by further scaling of the device, it is possible to improve the single-photon anharmonicity to a regime where photon blockade effect can be manifested.

Analytical approximation of the second-harmonic conversion efficiency.

The second-harmonic generation process of a focused laser beam inside a nonlinear crystal is described by the Boyd-Kleinman theory. Calculating the actual conversion efficiency and upconverted power requires the solution to a double integral that is analytically intractable. We provide an expression that predicts the exact gain coefficient within an error margin of less than 2% over several orders of magnitude of the confocal parameter and as a function of the walk-off parameter. Our result allows for readily tuning the beam parameters to optimize the performance of the upconversion process and improve optical system designs.

Second-harmonic generation of temporally low-coherence light

In this Letter, we study the second-harmonic (SH) generation of temporally low-coherence light using the statistical optics method. By introducing the statistical characteristics of low-coherence light into the nonlinear coupled wave equation, we predict the self-convolution relationship between the power spectral density of the second-harmonic and that of the fundamental wave and demonstrate it in experiments. The effects of phase matching and group velocity matching on the second-harmonic bandwidth and conversion efficiency are analyzed, which is beneficial to improve the efficiency of low-coherent SH light. Combining the statistics optics and nonlinear optics, our research provides a suitable mathematical model for the study of nonlinear frequency conversion processes of temporally low-coherence light. Due to its statistical characteristics and temporal incoherence, low-coherence SH light would have further applications in uniform illumination, quantum optics, metrology, and so on.

Shallow-etched thin-film lithium niobate waveguides for highly-efficient second-harmonic generation.

High-fidelity periodic poling over long lengths is required for robust, quasi-phase-matched second-harmonic generation using the fundamental, quasi-TE polarized waveguide modes in a thin-film lithium niobate (TFLN) waveguide. Here, a shallow-etched ridge waveguide is fabricated in x-cut magnesium oxide doped TFLN and is poled accurately over 5 mm. The high fidelity of the poling is demonstrated over long lengths using a non-destructive technique of confocal scanning second-harmonic microscopy. We report a second-harmonic conversion efficiency of up to 939 %.W-1 (length-normalized conversion efficiency 3757 %.W-1.cm-2), measured at telecommunications wavelengths. The device demonstrates a narrow spectral linewidth (1 nm) and can be tuned precisely with a tuning characteristic of 0.1 nm/°C, over at least 40 °C without measurable loss of efficiency.

Second-harmonic generation from 2D photonic crystal waveguide with simultaneous near-flat dispersions at fundamental frequency and second harmonic

Photonic crystal waveguides (PCW) have shown great potential to generate second harmonic (SH), but the available schemes with cavity or defect suffer from a lack of multiple wavelengths operation and excessive phase-matching requirement. Here we propose a 2D PCW structure comprised of a modulated single line defect, which is capable of independently controlling the fundamental frequency (FF) and SH modes. Therefore, two near-flat group velocity dispersions can be achieved for these modes, and they simultaneously satisfy phase matching conditions. Based on multiply scattering method, we theoretically demonstrate that the SH resonant modes originated from the nonradiative near-flat dispersion have high quality factors (nearly 6.5×104 for the maximum value). And the maximum SH conversion efficiency for these resonant modes are 2 orders of magnitude higher than that of off-resonant modes. In addition, benefitting from the matching of the group velocities of the second harmonic and the fundamental, the central wavelength of SH can be modulated within wavelength range ∼3nm. These results can provide potential applications in on-chip integration of photonics and quantum optics.

Optimization of a double-pass scheme for thermal de-phasing compensation in high-power second-harmonic generation

The performance of a double-pass scheme is theoretically investigated for the efficiency enhancement of a second-harmonic (SH) beam generated by using a high-power fundamental laser beam. Based on a modified version of coupled equations that include the possible effects leading to the thermal de-phasing, the Collins integral is used for optimizing the focusing optics to obtain maximum efficiency of SH conversion. We found through simulation that at a fundamental power of 40 W, when the focusing regime is set for the loose condition, a conversion efficiency of 74% can be reached. It is possible if the focusing points of the first and second passes are designed to be located at 19 mm and 6 mm away from the input face of the crystal, respectively.

Class-J23 Power Amplifiers

This paper presents the theoretical introduction and experimental validation of the “Class-J2 Mode Power Amplifier,” which provides higher efficiency and output power compared with conventional class-J mode counterpart. This mode of operation is realized by injection of the second-harmonic current to drain node of a class-J power amplifier (PA) to reduce the 45° phase shift between drain current and voltage signals. Similar to class-J PAs, the second-harmonic impedance of class-J2 PAs is purely reactive to simplify the design of the output matching network. The auxiliary second-harmonic injection circuit comprises a transistor biased in class-B mode followed by a class-C biased amplifier to achieve high second-harmonic current conversion efficiency. Theoretical formulations suggest that a 5% improvement in drain efficiency ( $\eta _{D}$ ) as well as a 1.5-dB increase in output power can be achieved for the class-J2 mode in comparison with the typical class-J operation. To check the accuracy of theoretical predictions, a proof-of-concept 1-GHz class-J2 PA with 12.2-dBm output power and 43% PAE is implemented in a 0.18- $\mu \text{m}$ CMOS technology. For better comparison, a 1-GHz class-J PA with the same transistor size and bias condition as the class-J2 PA is also fabricated. The output power and PAE of the reference class-J PA are 11.4 dBm and 40.6%, respectively, which are in agreement with theoretical predictions.

Ultrahigh-Q photonic crystal nanocavities based on 4H silicon carbide

Photonic nanocavities with high quality (Q) factors are essential components for integrated optical circuits. The use of crystalline silicon carbide (SiC) for such nanocavities enables the realization of devices with superior properties. We fabricate ultrahigh-Q SiC photonic crystal nanocavities by etching air holes into a 4H-SiC slab that is prepared without using hydrogen ion implantation, which usually causes higher absorption losses. In addition, compared to usual designs, a relatively thin slab is utilized to avoid losses through cross-polarized mode coupling induced by the tapered air holes. We obtain a heterostructure nanocavity with a high experimental Q factor of 6.3×105, which is 16 times larger than the highest Q among the previously reported values for nanocavities based on crystalline SiC. We also show that our nanocavity exhibits a high normalized second-harmonic conversion efficiency of 1900%/W.

17 000%/W second-harmonic conversion efficiency in single-crystalline aluminum nitride microresonators

High quality factor optical microcavities have been employed in a variety of material systems to enhance nonlinear optical interactions. While single-crystalline aluminum nitride microresonators have recently emerged as a low loss platform for integrated nonlinear optics such as four wave mixing and Raman lasing, few studies have investigated this material for second-harmonic generation. In this letter, we demonstrate an optimized fabrication of dually resonant phase-matched ring resonators from epitaxial aluminum nitride thin films. An unprecendented second-harmonic generation efficiency of 17 000%/W is obtained in the low power regime, and pump depletion is observed at a relatively low input power of 3.5 mW. This poses epitaxial aluminum nitride as the highest efficiency second-harmonic generator among current integrated platforms.

Inverse-designed photonic fibers and metasurfaces for nonlinear frequency conversion [Invited

Typically, photonic waveguides designed for nonlinear frequency conversion rely on intuitive and established principles, including index guiding and bandgap engineering, and are based on simple shapes with high degrees of symmetry. We show that recently developed inverse-design techniques can be applied to discover new kinds of microstructured fibers and metasurfaces designed to achieve large nonlinear frequency-conversion efficiencies. As a proof of principle, we demonstrate complex, wavelength-scale chalcogenide glass fibers and gallium phosphide three-dimensional metasurfaces exhibiting some of the largest nonlinear conversion efficiencies predicted thus far, e.g., lowering the power requirement for third-harmonic generation by 104 and enhancing second-harmonic generation conversion efficiency by 107. Such enhancements arise because, in addition to enabling a great degree of tunability in the choice of design wavelengths, these optimization tools ensure both frequency- and phase-matching in addition to large nonlinear overlap factors.

Second-harmonic generation of a linearly polarized laser pulse propagating through magnetized plasma in the presence of a planar magnetostatic wiggler

This paper presents the second-harmonic generation of a linearly polarized laser pulse propagating through magnetized plasma embedded in a planar magnetostatic wiggler. Using the perturbation theory, the dispersion relation and non-linear current density of the laser-plasma interaction have been derived in the presence of the wiggler field. Moreover, the conversion efficiency and the amplitude of the phase mismatch for the second harmonic have been obtained. Numerical results indicate that the second-harmonic conversion efficiency significantly increases with increasing the wiggler strength. Furthermore, it is seen that the peak of the second-harmonic conversion efficiency has an increasing trend with respect to the increase in the wiggler strength. In addition, it is found that increasing the plasma frequency (or plasma density), the peak of the second-harmonic conversion efficiency gradually increases.

Compact burst-mode Nd:YAG laser for kHz-MHz bandwidth velocity and species measurements.

A compact-footprint (0.18 m2) flash-lamp-pumped, burst-mode Nd:YAG-based master-oscillator pulsed-amplifier laser is reported with a fundamental 1064nm output of over 14J per burst. A directly modulated diode laser seed source is used to generate 10ms duration arbitrary sequences of 500kHz doublet or MHz singlet pulses for flow-field velocity or species measurements, respectively. Flexible pulse widths are used to balance the energy distribution of pulse doublets and achieve second-harmonic conversion efficiencies up to 42%. Burst-mode laser performance characteristics, measurement accuracies in turbulent flows, and prospects for kHz-MHz flow-field diagnostics are discussed.

Theoretical investigation of phase-mismatched second-harmonic conversion efficiency in BBO crystal

We report a modified general relation of nanosecond second harmonic conversion efficiency relation to determine efficiency at any value of power density, laser wavelength, crystal length and beam divergence when transmission value are relative high >90%, which make it a reference for practical study comparison. We apply this study on BBO crystal and study the efficiency as function of both crystal length (0–0.7 cm) and laser wavelength (460–1400 nm), (527–1400 nm) in 3-D plot for phase matching types I and II respectively. Finally, we study the effect of crystal length and power density at generated deep-ultraviolet cut-off type I wavelength 205 nm.

A numerical investigation of enhanced backward second-harmonic generation in one-dimensional PIM/NIM structure

In this paper, we have demonstrated a numerical investigation of an enhanced backward second-harmonic generation (BSHG) effect in a one-dimensional positive-index material/negative-index material (1D-PIM/NIM) structure with nonlinear deep grating. The 1D-PIM/NIM structure composed of common linear PIM layers and NIM layers, whose electric permittivity and magnetic permeability are described by Drude model for allowing negative refractive index behavior, embedded in nonlinear χ(2) material in periodically arrangement. To model BSHG phenomenon, we have developed a completed set of nonlinear coupled-mode equations (NCMEs) by perturbing nonlinear wave equation by a small factor with appropriate scale following a way of multiple-scale approach (MSA). Then, we have numerically solved the NCMEs to achieve the second-harmonic frequency output and conversion efficiency of backward field. We have also discussed a backward phase-matching (BWPM) condition, which has been satisfied by tuning a fundamental frequency (FF) at ω in a negative refractive index region and second-harmonic (SH) frequency at 2ω in a positive refractive index region, and band-edge local field enhancement condition, which has been created by rearranging PIM and NIM layers in optimal periodic fashion. By using both conditions, a conversion efficiency of BSHG can be dramatically enhanced.