Efficient and highly tunable second-harmonic generation in Z-cut periodically poled lithium niobate nanowaveguides

Invertible all-optical logic gate on chip

We demonstrate an invertible all-optical gate on chip, with the roles of control and signal switchable by slightly adjusting their relative arrival time at the gate. It is based on the quantum Zeno blockade (QZB) driven by sum-frequency generation (SFG) in a periodically poled lithium niobate microring resonator. For two nearly identical nanosecond pulses, the later arriving pulse is modulated by the earlier arriving one, resulting in 2.4 and 3.9 power extinction between the two, respectively, when their peak powers are 1 mW and 2 mW, respectively. Our results, while to be improved and enriched, herald a new, to the best of our knowledge, paradigm of logical gates and circuits for exotic applications.

Broadband frequency comb generation through cascaded quadratic nonlinearity in thin-film lithium niobate microresonators

Broadband frequency comb generation through cascaded quadratic nonlinearity remains experimentally untapped in free-space cavities with bulk χ(2) materials mainly due to the high threshold power and restricted ability of dispersion engineering. Thin-film lithium niobate (LN) is a good platform for nonlinear optics due to the tight mode confinement in a nano-dimensional waveguide, the ease of dispersion engineering, large quadratic nonlinearities, and flexible phase matching via periodic poling. Here we demonstrate broadband frequency comb generation through dispersion engineering in a thin-film LN microresonator. Bandwidths of 150 nm (80 nm) and 25 nm (12 nm) for center wavelengths at 1560 and 780 nm are achieved, respectively, in a cavity-enhanced second-harmonic generation (doubly resonant optical parametric oscillator). Our demonstration paves the way for pure quadratic soliton generation, which is a great complement to dissipative Kerr soliton frequency combs for extended interesting nonlinear applications.

Ultra-efficient second harmonic generation via mode phase matching in integrated lithium niobate racetrack resonators

High-efficiency second harmonic generation (SHG) relying solely on intermodal dispersion engineering remains a challenge. Here, we realize highly efficient SHG using a double-waveguide coupled racetrack microring resonator on X-cut lithium niobate on insulator (LNOI), where both pump and second harmonic (SH) approach critical coupling. Through precise temperature tuning, simultaneous pump and SH resonance is attained in the resonator, dramatically enhancing SHG efficiency. With low pump power, a normalized conversion efficiency of 9972%/W is achieved. Moreover, the resonator provides a 25.73 dB enhancement in SHG efficiency compared to a 4 mm straight waveguide with identical phase matching in our experiment. This work enables efficient wavelength conversion and quantum state generation on integrated X-cut LNOI platforms.

Broadband second-harmonic generation in an angle-cut lithium niobate-on-insulator waveguide by a temperature gradient

Frequency conversion via nonlinear wave mixing is an important technology to broaden the spectral range of lasers, propelling their applications in optical communication, spectroscopy, signal processing, and quantum information. Many applications require not only a high conversion efficiency but also a broad phase matching bandwidth. Here, we demonstrate broadband birefringence phase matching (BPM) second-harmonic generation (SHG) in angle-cut lithium niobate-on-insulator (LNOI) ridge waveguides based on a temperature gradient scheme. The bandwidth and shift of the phase matching spectrum can be effectively tuned by controlling the temperature gradient of the waveguide. Broadband SHG of a telecom C-band femtosecond laser is also demonstrated. The approach may open a new avenue for tunable broadband nonlinear frequency conversion in various integrated photonics platforms.

Efficient second harmonic generation by harnessing bound states in the continuum in semi-nonlinear etchless lithium niobate waveguides

Integrated photonics provides unprecedented opportunities to pursue advanced nonlinear light sources with low-power consumptions and small footprints in a scalable manner, such as microcombs, chip-scale optical parametric oscillators and integrated quantum light sources. Among a variety of nonlinear optical processes, high-efficiency second harmonic generation (SHG) on-chip is particularly appealing and yet challenging. In this work, we present efficient SHG in highly engineerable semi-nonlinear waveguides consisting of electron-beam resist waveguides and thin-film silicon nitride (SiN)/lithium niobate (LN). By carefully designing octave-separating bound states in the continuum (BICs) for the nonlinear interacting waves in such a hybrid structure, we have simultaneously optimized the losses for both fundamental frequency (FF) and second harmonic (SH) waves and achieved modal phasing matching and maximized the nonlinear modal overlap between the FF and SH waves, which results in an experimental conversion efficiency up to 4.05% W^-1cm^-2. Our work provides a versatile and fabrication-friendly platform to explore on-chip nonlinear optical processes with high efficiency in the context of nanophotonics and quantum optics.

Thermally tunable and efficient second-harmonic generation on thin-film lithium niobate with integrated micro-heater

In this Letter, we report thermo-optic tunable and efficient second-harmonic generation (SHG) based on an X-cut periodically poled lithium niobate (PPLN) waveguide. By applying an on-chip heater with thermo-isolation trenches and combining a type-0 quasi-phase matching mechanism, we experimentally achieve a high on-chip SHG conversion efficiency of 2500-3000% W^-1 cm^-2 and a large tuning power efficiency of 94 pm/mW inside a single 5-mm-long straight PPLN waveguide. Our design is for energy-efficient, high-performance nonlinear applications, such as wavelength conversion, highly tunable coherent light sources, and photon-pair generation.

Ultra-broadband mid-infrared generation in dispersion-engineered thin-film lithium niobate

Thin-film lithium niobate (TFLN) is an emerging platform for compact, low-power nonlinear-optical devices, and has been used extensively for near-infrared frequency conversion. Recent work has extended these devices to mid-infrared wavelengths, where broadly tunable sources may be used for chemical sensing. To this end, we demonstrate efficient and broadband difference frequency generation between a fixed 1-µm pump and a tunable telecom source in uniformly-poled TFLN-on-sapphire by harnessing the dispersion-engineering available in tightly-confining waveguides. We show a simultaneous 1-2 order-of-magnitude improvement in conversion efficiency and ∼5-fold enhancement of operating bandwidth for mid-infrared generation when compared to equal-length conventional lithium niobate waveguides. We also examine the effects of mid-infrared loss from surface-adsorbed water on the performance of these devices.

All-optical wavelength conversion of a 92-Gb/s 16-QAM signal within the C-band in a single thin-film PPLN waveguide

Tunable all-optical wavelength conversion (AOWC) within 151 nm bandwidth is demonstrated in a thin-film periodically poled lithium niobate (PPLN) waveguide, which utilizes the cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) process. Also, in the same waveguide, AOWC of a 92-Gb/s 16-ary quadrature amplitude modulated (16-QAM) signal within the C-band is successfully achieved. For Bit-error ratio (BER) measurements, we obtain a negligible optical signal-to-noise ratio (OSNR) penalty (<0.2 dB) for the converted idler wave at a BER of 1e-3.

Ultra-low-power second-order nonlinear optics on a chip

Second-order nonlinear optical processes convert light from one wavelength to another and generate quantum entanglement. Creating chip-scale devices to efficiently control these interactions greatly increases the reach of photonics. Existing silicon-based photonic circuits utilize the third-order optical nonlinearity, but an analogous integrated platform for second-order nonlinear optics remains an outstanding challenge. Here we demonstrate efficient frequency doubling and parametric oscillation with a threshold of tens of micro-watts in an integrated thin-film lithium niobate photonic circuit. We achieve degenerate and non-degenerate operation of the parametric oscillator at room temperature and tune its emission over one terahertz by varying the pump frequency by hundreds of megahertz. Finally, we observe cascaded second-order processes that result in parametric oscillation. These resonant second-order nonlinear circuits will form a crucial part of the emerging nonlinear and quantum photonics platforms.

Here, the authors demonstrate a chip-scale device that realizes a comprehensive set of resonant second order nonlinear processes including optical parametric oscillation with a threshold power of 70 microwatts.

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 tunable birefringent phase-matched second-harmonic generation in an angle-cut lithium niobate-on-insulator ridge waveguide

Phase-matched nonlinear wave mixing, e.g., second-harmonic generation (SHG), is crucial for frequency conversion for integrated photonics and applications, where phase matching wavelength tunability in a wide manner is important. Here, we propose and demonstrate a novel design of angle-cut ridge waveguides for SHG on the lithium niobate-on-insulator (LNOI) platform via type-I birefringent phase matching (BPM). The unique strong birefringence of LN is used to achieve flexible temperature tuning. We experimentally demonstrate a normalized BPM conversion efficiency of 2.7%W^-1cm^-2 in an angle-cut LN ridge waveguide with a thermo tuning slope of 1.06 nm/K at the telecommunication C band. The approach effectively overcomes the spatial walk-off effect and avoids the need for periodic domain engineering. Furthermore, the angle-cut ridge waveguide scheme can be universally extended to other on-chip birefringent platforms where domain engineering is difficult or immature. The approach may open up an avenue for tunable nonlinear frequency conversion on integrated photonics for broad applications.

Second and cascaded harmonic generation of pulsed laser in a lithium niobate on insulator ridge waveguide

Nonlinear crystalline ridge waveguides, e.g., lithium niobate-on-insulator ridge waveguides, feature high index contrast and strong optical confinement, thus dramatically enhancing nonlinear interaction and facilitating various nonlinear effects. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and cascaded fourth-harmonic generation (FHG) in a periodically poled lithium niobate (PPLN) ridge waveguide pumped with pulsed laser at the quasi-phase matching (QPM) wavelength, as well as simultaneous SHG and cascaded third-harmonic generation (THG) waves when pumped at the non-QPM wavelength. Furthermore, the ridge waveguide achieves an efficient single-pass SHG conversion efficiency of picosecond pulsed laser at ∼62%. These results may be beneficial for on-chip nonlinear frequency conversion.

Noncritical phasematching behavior in thin-film lithium niobate frequency converters

We present a study of noncritical phasematching behavior in thin-film, periodically poled lithium niobate (PPLN) waveguides. Noncritical phasematching refers to designing waveguides so that the phasematching is insensitive to variations in waveguide thickness, width, or other parameters. For waveguide thickness (the dimension with greatest nonuniformity due to fabrication), we found that phasematching sensitivity can be minimized but not eliminated. We estimate limits on the acceptable thickness variation and discuss scaling with device length for second-harmonic generation and sum-frequency generation in thin-film PPLN frequency converters.

Second harmonic generation in gallium phosphide nano-waveguides

We designed, fabricated and tested gallium phosphide (GaP) nano-waveguides for second harmonic generation (SHG). We demonstrate SHG in the visible range around 655 nm using modal phase matching. We observe phase matched SHG for different combinations of interacting modes by varying the widths of the waveguides and tuning the wavelength of the pump. We achieved a normalized internal SHG conversion efficiency of 0.4% W^-1cm^-2 for a continuous-wave pump at wavelength of 1283.5 nm, the highest reported in the literature for a GaP waveguide. We also demonstrated temperature tuning of the SHG wavelength with a slope of 0.17 nm/^°C. The presented results contribute to the development of integrated photonic platforms with efficient nonlinear wave-mixing processes for classical and quantum applications.

Analysis of perovskite oxide etching using argon inductively coupled plasmas for photonics applications

We analyzed the dry etching of perovskite oxides using argon-based inductively coupled plasmas (ICP) for photonics applications. Various chamber conditions and their effects on etching rates have been demonstrated based on Z-cut lithium niobate (LN). The measured results are predictable and repeatable and can be applied to other perovskite oxides, such as X-cut LN and barium titanium oxide (BTO). The surface roughness is better for both etched LN and BTO compared with their as-deposited counterparts as confirmed by atomic force microscopy (AFM). Both the energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) methods have been used for surface chemical component comparisons, qualitative and quantitative, and no obvious surface state changes are observed according to the measured results. An optical waveguide fabricated with the optimized argon-based ICP etching was measured to have -3.7 dB/cm loss near 1550 nm wavelength for Z-cut LN, which validates this kind of method for perovskite oxides etching in photonics applications.

Ultrabright Quantum Photon Sources on Chip

Quantum photon sources of high rate, brightness, and purity are increasingly desirable as quantum information systems are quickly scaled up and applied to many fields. Using a periodically poled lithium niobate microresonator on chip, we demonstrate photon-pair generation at high rates of 8.5 and 36.3 MHz using only 3.4 and 13.4  μW pump power, respectively, marking orders of magnitude improvement over the state of the art, across all material platforms. These results constitute the first direct measurement of the device's giant single photon nonlinearity. The measured coincidence to accidental ratio is well above 100 at those high rates and reaches 14682±4427 at a lower pump power. The same chip enables heralded single-photon generation at tens of megahertz rates, each with low autocorrelation g_{H}^{(2)}(0)=0.008 and 0.097 for the microwatt pumps, which marks a new milestone. Such distinct performance, facilitated by the chip device's noiseless and giant optical nonlinearity, will contribute to the forthcoming pervasive adoption of quantum optical information technologies.