Xufeng Zhang

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.

Strongly coupled magnons and cavity microwave photons.

We realize a cavity magnon-microwave photon system in which a magnetic dipole interaction mediates strong coupling between the collective motion of a large number of spins in a ferrimagnet and the microwave field in a three-dimensional cavity. By scaling down the cavity size and increasing the number of spins, an ultrastrong coupling regime is achieved with a cooperativity reaching 12,600. Interesting dynamic features including classical Rabi-like oscillation, magnetically induced transparency, and the Purcell effect are demonstrated in this highly versatile platform, highlighting its great potential for coherent information processing.

Magnon dark modes and gradient memory.

Extensive efforts have been expended in developing hybrid quantum systems to overcome the short coherence time of superconducting circuits by introducing the naturally long-lived spin degree of freedom. Among all the possible materials, single-crystal yttrium iron garnet has shown up recently as a promising candidate for hybrid systems, and various highly coherent interactions, including strong and even ultrastrong coupling, have been demonstrated. One distinct advantage in these systems is that spins form well-defined magnon modes, which allows flexible and precise tuning. Here we demonstrate that by dissipation engineering, a non-Markovian interaction dynamics between the magnon and the microwave cavity photon can be achieved. Such a process enables us to build a magnon gradient memory to store information in the magnon dark modes, which decouple from the microwave cavity and thus preserve a long lifetime. Our findings provide a promising approach for developing long-lifetime, multimode quantum memories.

High-Q silicon optomechanical microdisk resonators at gigahertz frequencies

We report disk-shaped silicon optomechanical resonators with frequency up to 1.75 GHz in the ultrahigh frequency band. Optical transduction of the thermal motion of the disks’ in-plane vibrational modes yields a displacement sensitivity of 4.1 × 10−17 m/Hz1/2. Due to the reduced clamping loss, these disk resonators possess high mechanical quality factors (Q), with the highest value of 4370 for the 1.47 GHz mode measured in ambient air. Numerical simulation on the modal frequency and mechanical Q for disks of varying undercut shows modal coupling and suggests a realistic pedestal size to achieve the highest possible Q.

Electric-Field Coupling to Spin Waves in a Centrosymmetric Ferrite

We experimentally demonstrate that the spin-orbit interaction can be utilized for direct electric-field tuning of the propagation of spin waves in a single-crystal yttrium iron garnet magnonic waveguide. Magnetoelectric coupling not due to the spin-orbit interaction and, hence, an order of magnitude weaker leads to electric-field modification of the spin-wave velocity for waveguide geometries where the spin-orbit interaction will not contribute. A theory of the phase shift, validated by the experiment data, shows that, in the exchange spin wave regime, this electric tuning can have high efficiency. Our findings point to an important avenue for manipulating spin waves and developing electrically tunable magnonic devices.

Nonlinear optical effects of ultrahigh- Q silicon photonic nanocavities immersed in superfluid helium

Photonic nanocavities are a key component in many applications because of their capability of trapping and storing photons and enhancing interactions of light with various functional materials and structures. The maximal number of photons that can be stored in silicon photonic cavities is limited by the free-carrier and thermo-optic effects at room temperature. To reduce such effects, we performed the first experimental study of optical nonlinearities in ultrahigh-Q silicon disk nanocavities at cryogenic temperatures in a superfluid helium environment. At elevated input power, the cavity transmission spectra exhibit distinct blue-shifted bistability behavior when temperature crosses the liquid helium lambda point. At even lower temperatures, the spectra restore to symmetric Lorentzian shapes. Under this condition, we obtain a large intracavity photon number of about 40,000, which is limited ultimately by the local helium phase transition. These new discoveries are explained by theoretical calculations and numerical simulations.

A superhigh-frequency optoelectromechanical system based on a slotted photonic crystal cavity

We develop an all-integrated optoelectromechanical system that operates up to 4.20 GHz. The in-plane bulk acoustic modes of a photonic crystal membrane are electrocapacitively actuated and optically detected by a high-Q slotted photonic crystal cavity.

Superstrong coupling of thin film magnetostatic waves with microwave cavity

We experimentally demonstrated the strong coupling between a microwave cavity and standing magnetostatic magnon modes in a yttrium iron garnet film. Such strong coupling can be observed for various spin wave modes under different magnetic field bias configurations, with a coupling strength inversely proportional to the transverse mode number. A comb-like spectrum can be obtained from these high order modes. The collectively enhanced magnon-microwave photon coupling strength is comparable with the magnon free spectral range and therefore leads to the superstrong coupling regime. Our findings pave the road towards designing a new type of strongly hybridized magnon-photon system.

A 1.16-μm-radius disk cavity in a sunflower-type circular photonic crystal with ultrahigh quality factor

We present a 1.16-μm-radius disk cavity with ultrahigh quality (Q) factor by embedding the disk into a sunflower-type circular photonic crystal (CPC). The bandgap of the CPC reduces the bending loss of the whispering-gallery mode of the disk, leading to a simulated Q of 10(7), at least an order of magnitude higher than a bare disk of the same size. The design is experimentally verified with a record high loaded Q of 7.4×10(5) measured from an optimized device fabricated on a silicon-on-insulator substrate.

Triply resonant cavity electro-optomechanics at X-band

Optomechanical microcavities with high-frequency mechanical resonances facilitate experimental access to mechanical states with low phonon occupation and also hold promise for practical device applications including compact microwave sources. However, the weak radiation pressure force poses practical limits on achievable amplitudes at super high frequencies. Here, we demonstrate a piezoelectric force enhanced microcavity system that simultaneously supports microwave, optical and mechanical resonant modes. The combination of the highly sensitive optical readout and resonantly enhanced strong piezoelectric actuation enables us to build a microwave oscillator with excellent phase noise performance, which pushes the micromechanical signal source into microwave X-band.