arXiv subjects

Xianwen Liu

Publications and source records attributed to Xianwen Liu.

13 recordsLinked to original sources

arXiv2026

Chip-based f-2f interferometry in periodically tapered lithium niobate nanophotonic waveguides

Nanophotonic supercontinuum generation offers a practical route to chip-based f-2f interferometry by leveraging coexisting chi(2) and chi(3) nonlinearities. In conventional uniform waveguides, the phase-matching bandwidth for second-harmonic generation (SHG) is intrinsically narrow, restricting the spectral overlap factor for heterodyne beating. To address this limitation, we introduce a periodically-tapered nanophotonic waveguide made from MgO-doped, z-cut thin-film lithium niobate for energy-efficient and fabrication-robust f-2f operation. By adiabatically varying the waveguide width within a dual phase-matching window that supports concurrent dispersive wave (DW) emission and SHG, we routinely achieved a broad spectral overlap between the SHG and DW components. This capability enables robust detection of the carrier-envelope offset frequency (fceo) at substantially lower pulse energies than that in uniform-waveguide approaches. We further developed a compact waveguide module that operates reliably under temperature fluctuations and is capable of interfacing with high-repetition-rate (500 MHz) mode-locked lasers, enabling detection and phase locking of fceo with a signal-to-noise ratio of 48 dB. These results highlight the potential of nanophotonic chips for developing compact, field-deployable frequency comb systems.

physics.optics↗

arXiv2021

Efficient and tunable blue light generation using lithium niobate nonlinear photonics

Thin film lithium niobate (LN) has recently emerged as a playground for chip-scale nonlinear optics and leads to highly efficient frequency conversions from near infrared to near-visible bands. For many nonlinear and quantum photonics applications, it is desirable to operate deep into the visible band within LN's transparency window. However, the strong material dispersion at short wavelengths makes phase-matching difficult, necessitating sub-micron scale control of domain structures for efficient quasi-phase-matching (QPM). Here we report the operation of thin film LN in the blue wavelength and high fidelity poling of thin-film LN waveguide to this regime. As a result, quasi-phase matching is realized between IR (871nm) and blue (435.5nm) wavelengths in a straight waveguide and prompts strong blue light generation with a conversion efficiency $2900\pm400\%W^{-1}cm^{-2}$

physics.optics↗

arXiv2020

III-Nitride nanophotonics for beyond-octave soliton generation and self-referencing

Frequency microcombs, successors to mode-locked laser and fiber combs, enable miniature rulers of light for applications including precision metrology, molecular fingerprinting, and exoplanet discoveries. To enable the frequency ruling function, microcombs must be stabilized by locking their carrier-envelop offset frequency. So far, the microcomb stabilization remains compounded by the elaborate optics external to the chip, thus evading its scaling benefit. To address this challenge, here we demonstrate a nanophotonic chip solution based on aluminum nitride thin films, which simultaneously offer optical Kerr nonlinearity for generating octave soliton combs and Pockels nonlinearity for enabling heterodyne detection of the offset frequency. The agile dispersion control of crystalline III-Nitride photonics permits high-fidelity generation of solitons with features including 1.5-octave comb span, dual dispersive waves, and sub-terahertz repetition rates down to 220 gigahertz. These attractive characteristics, aided by on-chip phase-matched aluminum nitride waveguides, allow the full determination of the offset frequency. Our proof-of-principle demonstration represents an important milestone towards fully-integrated self-locked microcombs for portable optical atomic clocks and frequency synthesizers.

physics.app-ph↗

arXiv2020

Ground-state Pulsed Cavity Electro-optics for Microwave-to-optical Conversion

In the development of quantum microwave-to-optical (MO) converters, excessive noise induced by the parametric optical drive remains a major challenge at milli-Kelvin temperatures. Here we study the extraneous noise added to an electro-optic transducer in its quantum ground state under an intense pulsed optical excitation. The integrated electro-optical transducer leverages the inherent Pockels effect of aluminum nitride microrings, flip-chip bonded to a superconducting resonator. Applying a pulsed optical drive with peak power exceeding the cooling power of the dilution refrigerator at its base temperature, we observe efficient bi-directional MO conversion, with near-ground state microwave thermal excitation ($\bar{n}_\mathrm{e}=0.09\pm0.06$). Time evolution study reveals that the residual thermal excitation is dominated by the superconductor absorption of stray light scattered off the chip-fiber interface. Our results shed light on suppressing microwave noise in a cavity electro-optic system under intense optical drive, which is an essential step towards quantum state transduction between microwave and optical frequencies.

quant-ph↗

arXiv2020

Photonic dissipation control for Kerr soliton generation in strongly Raman-active media

Microcavity solitons enable miniaturized coherent frequency comb sources. However, the formation of microcavity solitons can be disrupted by stimulated Raman scattering (SRS), particularly in the emerging crystalline microcomb materials with high Raman gain. Here, we propose and implement dissipation control---tailoring the energy dissipation of selected cavity modes---to purposely raise/lower the threshold of Raman lasing in a strongly Raman-active lithium niobate microring resonator, and realize on-demand soliton mode-locking or Raman lasing. Numerical simulations are carried out to confirm our analyses and agree well with experiment results. Our work demonstrates an effective approach to address strong SRS for microcavity soliton generation.

physics.optics↗

arXiv2020

Pockels Soliton Microcomb

Kerr soliton microcombs have recently emerged as a prominent topic in integrated photonics and enabled new horizons for optical frequency metrology. Kerr soliton microcombs, as its name suggests, are based on the high-order cubic optical nonlinearity. It is desirable to exploit quadratic photonic materials, namely Pockels materials, for soliton generation and on-chip implementation of 1f-2f comb self-referencing. Such quadratically-driven solitons have been theoretically proposed, but have not yet been observed in a nanophotonic platform despite of recent progresses in quadratic comb generation in free-space and crystalline resonators. Here we report photonic chip-based Pockels microcomb solitons driven by three-wave mixing in an aluminum nitride microring resonator. In contrast to typical Kerr solitons, our Pockels soliton features unity soliton generation fidelity, two-by-two annihilation of multi-soliton states, favorable tuning dynamics, and high pump-to-soliton conversion efficiency.

physics.optics↗

arXiv2019

Beyond 100 THz-spanning ultraviolet frequency combs in a non-centrosymmetric crystalline waveguide

Ultraviolet frequency combs enable applications ranging from precision spectroscopy to atomic clocks by addressing the electronic transitions of atoms and molecules. Access to ultraviolet light via integrated nonlinear optics is usually hampered by the strong material dispersion and large waveguide attention in the ultraviolet region. Here we demonstrate a simple route to chip-scale ultraviolet comb generators, simultaneously showing a gap-free frequency span of ~128 terahertz and supercontinuum sourced by an ultrafast fiber laser. The simultaneous cubic and quadratic nonlinear processes are implemented in single-crystalline aluminum nitride thin films, where a chirp-modulated taper waveguide is patterned to ensure a broad phase matching. The heterodyne beatnote characterization suggests that both the near-visible and ultraviolet supercontinuum combs maintain a high degree of coherence. Our approach is also adaptable to other non-centrosymmetric photonic platforms for ultrafast nonlinear optics with scalable bandwidth.

physics.optics↗

arXiv2019

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 ($χ^{(2)}$), 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$μ$W in the waveguide. Such periodically poled LN microrings also present a versatile platform for other cavity-enhanced quasi-phase matched $χ^{(2)}$ nonlinear optical processes.

physics.optics↗

arXiv2019

On-chip $χ^{(2)}$ microring optical parametric oscillator

Optical parametric oscillators (OPOs) have been widely used for decades as tunable, narrow linewidth, and coherent light sources for reaching long wavelengths and are attractive for applications such as quantum random number generation and Ising machines. To date, waveguide-based OPOs have suffered from relatively high thresholds on the order of hundreds of milliwatts. With the advance in integrated photonic techniques demonstrated by high-efficiency second harmonic generation in aluminum nitride (AlN) photonic microring resonators, highly compact and nanophotonic implementation of parametric oscillation is feasible. Here, we employ phase-matched AlN microring resonators to demonstrate low-threshold parametric oscillation in the telecom infrared band with an on-chip efficiency up to 17% and milliwatt-level output power. A broad phase-matching window is observed, enabling tunable generation of signal and idler pairs over a 180 nm bandwidth across the C band. This result establishes an important milestone in integrated nonlinear optics and paves the way towards chip-based quantum light sources and tunable, coherent radiation for spectroscopy and chemical sensing.

physics.optics↗

arXiv2018

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.

physics.optics↗

arXiv2018

Ultra-high-Q UV microring resonators based on single-crystalline AlN platform

Development of low-loss photonic components in the ultraviolet (UV) band will open new prospects for classical and quantum optics. Compared with other integrated platforms, aluminum nitride (AlN) is particularly attractive as it features an enormous bandgap of ~6.2 eV and intrinsic chi(2) and chi(3) susceptibilities. In this work, we demonstrate a record quality factor of 2.1 x 105 (optical loss ~ 8 dB/cm) at 390 nm based on single-crystalline AlN microrings. The low-loss AlN UV waveguide represents a significant milestone toward UV photonic integrated circuits as it features full compatibility for future incorporation of AlGaN-based UV emitters and receivers. On-chip UV spectroscopy, nonlinear optics and quantum information processing can also be envisioned.

physics.app-ph↗

arXiv2016

Broadband frequency comb generation in aluminum nitride-on-sapphire microresonators

Development of chip-scale optical frequency comb with the coverage from ultra-violet (UV) to mid-infrared (MIR) wavelength is of great significance. To expand the comb spectrum into the challenging UV region, a material platform with high UV transparency is crucial. In this paper, crystalline aluminum nitride (AlN)-onsapphire film is demonstrated for efficient Kerr frequency comb generation. Near-infrared (NIR) comb with nearly octave-spanning coverage and low parametric threshold is achieved in continuous-wave pumped high-quality-factor AlN microring resonators. The competition between stimulated Raman scattering (SRS) and hyperparametric oscillation is investigated, along with broadband comb generation via Raman-assisted four-wave mixing (FWM). Thanks to its wide bandgap, excellent crystalline quality as well as intrinsic quadratic and cubic susceptibilities, AlN-on-sapphire platform should be appealing for integrated nonlinear optics from MIR to UV region.

physics.optics↗

arXiv2016

Continuous-wave Raman Lasing in Aluminum Nitride Microresonators

We report the first investigation on continuous-wave Raman lasing in high-quality-factor aluminum nitride (AlN) microring resonators. Although wurtzite AlN is known to exhibit six Raman-active phonons, single-mode Raman lasing with low threshold and high slope efficiency is demonstrated. Selective excitation of A$_1^\mathrm{TO}$ and E$_2^\mathrm{high}$ phonons with Raman shifts of $\sim$612 and 660 cm$^{-1}$ is observed by adjusting the polarization of the pump light. A theoretical analysis of Raman scattering efficiency within ${c}$-plane (0001) of AlN is carried out to help account for the observed lasing behavior. Bidirectional lasing is experimentally confirmed as a result of symmetric Raman gain in micro-scale waveguides. Furthermore, second-order Raman lasing with unparalleled output power of $\sim$11.3 mW is obtained, which offers the capability to yield higher order Raman lasers for mid-infrared applications.

physics.optics↗