Hojoong Jung

Optical frequency comb generation from aluminum nitride microring resonator

Aluminum nitride (AlN) is an appealing nonlinear optical material for on-chip wavelength conversion. Here we report optical frequency comb generation from high-quality-factor AlN microring resonators integrated on silicon substrates. By engineering the waveguide structure to achieve near-zero dispersion at telecommunication wavelengths and optimizing the phase matching for four-wave mixing, frequency combs are generated with a single-wavelength continuous-wave pump laser. Further, the Kerr coefficient (n₂) of AlN is extracted from our experimental results.

Green, red, and IR frequency comb line generation from single IR pump in AlN microring resonator

On-chip frequency comb generations enable compact broadband sources for spectroscopic sensing and precision spectroscopy. Recent microcomb studies focus on the infrared spectral regime and have difficulty accessing the visible regime. We demonstrate comb-like visible frequency line generation through second-harmonic, third-harmonic, and sum-frequency conversion of a Kerr comb within a high-Q aluminum nitride (AlN) microring resonator pumped by a single telecom laser. The strong power enhancement, in conjunction with the unique combination of Pockels (χ2) and Kerr (χ3) optical nonlinearity of AlN, leads to cascaded frequency conversions in the visible spectrum. High-resolution spectroscopic study of the visible frequency lines indicates matched free spectrum range over all the bands. This frequency doubling and tripling effect in a single microcomb structure offers great potential for comb spectroscopy and self-referencing comb.

Non-reciprocal nonlinear optic induced transparency and frequency conversion on a chip

Developments in photonic chips have spurred photon based classical and quantum information processing, attributing to the high stability and scalability of integrated photonic devices [1, 2]. Optical nonlinearity [3] is indispensable in these complex photonic circuits, because it allows for classical and quantum light sources, all-optical switch, modulation, and non-reciprocity in ambient environments. It is commonly known that nonlinear interactions are often greatly enhanced in the microcavities [4]. However, the manifestations of coherent photon-photon interaction in a cavity, analogous to the electromagnetically induced transparency [5], have never been reported on an integrated platform. Here, we present an experimental demonstration of the coherent photon-photon interaction induced by second order optical nonlinearity (χ) on an aluminum nitride photonic chip. The non-reciprocal nonlinear optic induced transparency is demonstrated as a result of the coherent interference between photons with different colors: ones in the visible wavelength band and ones in the telecom wavelength band. Furthermore, a wide-band frequency conversion with an almost unit internal (0.14 external) efficiency and a bandwidth up to 0.76GHz is demonstrated.

Parametric down-conversion photon-pair source on a nanophotonic chip

Quantum-photonic chips, which integrate quantum light sources alongside active and passive optical elements, as well as single-photon detectors, show great potential for photonic quantum information processing and quantum technology. Mature semiconductor nanofabrication processes allow for scaling such photonic integrated circuits to on-chip networks of increasing complexity. Second-order nonlinear materials are the method of choice for generating photonic quantum states in the overwhelming majority of linear optic experiments using bulk components, but integration with waveguide circuitry on a nanophotonic chip proved to be challenging. Here, we demonstrate such an on-chip parametric down-conversion source of photon pairs based on second-order nonlinearity in an aluminum-nitride microring resonator. We show the potential of our source for quantum information processing by measuring the high visibility anti-bunching of heralded single photons with nearly ideal state purity. Our down-conversion source yields measured coincidence rates of 80 Hz, which implies MHz generation rates of correlated photon pairs. Low noise performance is demonstrated by measuring high coincidence-to-accidental ratios. The generated photon pairs are spectrally far separated from the pump field, providing great potential for realizing sufficient on-chip filtering and monolithic integration of quantum light sources, waveguide circuits and single-photon detectors.

Electrical tuning and switching of an optical frequency comb generated in aluminum nitride microring resonators

Aluminum nitride (AlN) has been shown to possess both strong Kerr nonlinearity and electro-optic Pockels effect. By combining these two effects, here we demonstrate on-chip reversible on/off switching of the optical frequency comb generated by an AlN microring resonator. We optimize the design of gating electrodes and the underneath resonator structure to effectively apply an electric field without increasing the optical loss. The switching of the comb is monitored by measuring one of the frequency comb peaks while varying the electric field. The controlled comb electro-optic response is investigated for direct comparison with the transient thermal effect.

Electrochemically sliced low loss AlGaN optical microresonators

High quality single crystal III-Nitride films are often formed over a thick buffer to reduce growth induced defects on a lattice mismatched substrate. However, it is challenging to fabricate nanophotonic waveguiding structures directly from this very top layer. Here, we demonstrate electrochemical slicing of high quality AlGaN thin films and its subsequent transfer to a lower index oxided silicon substrate for lithographic patterning of photonic waveguide and microresonators. TEM analysis of the nanomembrane waveguide demonstrates an AlGaN layer free of misfit dislocations commonly found in conventional epitaxial AlGaN grown on sapphire or Si. We probe the low material optical loss (1.22 dB/cm) of the nanomembrane by measuring the optical quality (Q) factor at 780 nm. High intrinsic quality factors of 680 000 are achieved after optimizing fabrication process. This versatile, low loss AlGaN device opens applications for nonlinear photonics at visible wavelengths.

Phase-dependent interference between frequency doubled comb lines in a χ (2) phase-matched aluminum nitride microring

Nonlinear optical conversion with frequency combs is important for self-referencing and for generating shorter wavelength combs. Here we demonstrate efficient frequency comb doubling through the combination of second-harmonic generation (SHG) and sum-frequency generation (SFG) of an input comb with a high Q, phase-matched χ(2) microring resonator. Phase coherence of the SHG and SFG nonlinear conversion processes is confirmed by sinusoidal phase-dependent interference between frequency doubled comb lines.

In-resonator variation of waveguide cross-sections for dispersion control of aluminum nitride micro-rings

We propose and demonstrate a dispersion control technique by combination of different waveguide cross sections in an aluminum nitride micro-ring resonator. Narrow and wide waveguides with normal and anomalous dispersion, respectively, are linked with tapering waveguides and enclosed in a ring resonator to produce a total dispersion near zero. The mode-coupling in multimoded waveguides is also effectively suppressed. This technique provides new degrees of freedom and enhanced flexibility in engineering the dispersion of microcomb resonators.

Switchable optical frequency comb in aluminum nitride microring resonator

Aluminum nitride is a promising nonlinear optical material with its strong Kerr and Pockels effects. Here we report optical frequency comb generation from aluminum nitride microring resonators and electrical switching of the comb.

Efficient Generation of a Near-visible Frequency Comb via Cherenkov-like Radiation from a Kerr Microcomb

Optical frequency combs enable state-of-the-art applications including frequency metrology, optical clocks, astronomical measurements, and sensing. Recent demonstrations of microresonator-based Kerr frequency combs or microcombs pave the way to scalable and stable comb sources on a photonic chip. Generating microcombs in the short-wavelength range, however, has been limited by large material dispersion and optical loss. Here we demonstrate a scheme for efficiently generating microcombs near the edge of the visible spectrum in a high-Q aluminum nitride microring resonator. The enhanced Pockels effect strongly couples infrared and near-visible modes into hybrid mode pairs, which participate in the Kerr microcomb generation process and lead to strong Cherenkov-like radiation in the near-visible band an octave apart. A surprisingly high on-chip conversion efficiency of 22% is achieved from a pulsed pump laser to the near-visible comb. As a result of pulse pumping, the generated microcombs are in the chaotic state. We further demonstrate a robust frequency tuning of the near-visible comb by more than one free spectral range and apply it to the absorption spectroscopy of a water-based dye molecule solution. Our work is a step towards high-efficiency visible microcomb generation and its utilization, and it also provides insights into the significance of the Pockels effect and its strong coupling with Kerr nonlinearity in a single microcavity device.