Shu-Wei Huang

Pressure Dependence of Superconductivity in Single-Phase K3C60

The superconducting compound K3C60 (with transition temperature Tc = 19.3 kelvin at ambient pressure), formed as a single phase by reaction of alkali vapor with solids of the icosahedral C60 molecule (buckminsterfullerene), shows a very large decrease of Tc with increasing pressure. Susceptibility measurements on sintered pellets showing bulk superconductivity are reported up to 21 kilobars of pressure, where Tc is already less than 8 kelvin. The results are consistent with a piling up of the density of states at the Fermi level.

Normal-State Magnetic Properties of K3C60

We report measurements of the bulk magnetic susceptibility, the EPR susceptibility, and the 13C nuclear spin-lattice relaxation and paramagnetic shift in the normal state of K3C60. Many aspects of the results indicate that the conduction electron properties can be modeled by Fermi-liquid behavior in a narrow band. Some weak-temperature dependences are observed that deviate from simple metallic behavior and may reflect interactions in the system. The parameters we obtain from a nearly-free-electron analysis are in reasonable agreement with band structure calculations, photoemission experiments, and the superconducting-state parameters.

Non-Korringa 13C Nuclear Relaxation in the Normal State of the K3C60 Superconductor

We present 13C NMR results on K3C60 showing that nuclear relaxation is exponential only above 250 K where molecular motion narrows the NMR line. The non-exponential nuclear relaxation at low temperatures is an intrinsic property of the frozen state and is associated with intramolecular variations of the electron spin density distribution. Dipolar coupling to the electron spins at the Fermi level of the carbon pπ-t1u band is identified as the dominant source of 13C relaxation. The measured mean value of (T1T)-1 is determined not only by the total density of states at the Fermi level, but also by the intramolecular electron spin density distribution.

A broadband chip-scale optical frequency synthesizer at 2.7 × 10−16 relative uncertainty

Frequency stabilization to 0.3 parts per quadrillion is demonstrated on chip, opening new frontiers in precision metrology. Optical frequency combs—coherent light sources that connect optical frequencies with microwave oscillations—have become the enabling tool for precision spectroscopy, optical clockwork, and attosecond physics over the past decades. Current benchmark systems are self-referenced femtosecond mode-locked lasers, but Kerr nonlinear dynamics in high-Q solid-state microresonators has recently demonstrated promising features as alternative platforms. The advance not only fosters studies of chip-scale frequency metrology but also extends the realm of optical frequency combs. We report the full stabilization of chip-scale optical frequency combs. The microcomb’s two degrees of freedom, one of the comb lines and the native 18-GHz comb spacing, are simultaneously phase-locked to known optical and microwave references. Active comb spacing stabilization improves long-term stability by six orders of magnitude, reaching a record instrument-limited residual instability of 3.6mHz/τ. Comparing 46 nitride frequency comb lines with a fiber laser frequency comb, we demonstrate the unprecedented microcomb tooth-to-tooth relative frequency uncertainty down to 50 mHz and 2.7 × 10−16, heralding novel solid-state applications in precision spectroscopy, coherent communications, and astronomical spectrography.Optical frequency combs, coherent light sources that connect optical frequencies with microwave oscillations, have become the enabling tool for precision spectroscopy, optical clockwork and attosecond physics over the past decades. Current benchmark systems are self-referenced femtosecond mode-locked lasers, but four-wave-mixing in high-Q resonators have emerged as alternative platforms. Here we report the generation and full stabilization of CMOS-compatible optical frequency combs. The spiral microcombs two degrees-of-freedom, one of the comb line and the native 18 GHz comb spacing, are first simultaneously phase-locked to known optical and microwave references. Second, with pump power control, active comb spacing stabilization improves the long-term stability by six orders-of-magnitude, reaching an instrument-limited 3.6 mHz/sqrt(t) residual instability. Third, referencing thirty-three of the nitride frequency comb lines against a fiber comb, we demonstrate the comb tooth-to-tooth frequency relative inaccuracy down to 53 mHz and 2.8x10-16, heralding unprecedented chip-scale applications in precision spectroscopy, coherent communications, and astronomical spectrography.

An integrated low phase noise radiation-pressure-driven optomechanical oscillator chipset.

High-quality frequency references are the cornerstones in position, navigation and timing applications of both scientific and commercial domains. Optomechanical oscillators, with direct coupling to continuous-wave light and non-material-limited f × Q product, are long regarded as a potential platform for frequency reference in radio-frequency-photonic architectures. However, one major challenge is the compatibility with standard CMOS fabrication processes while maintaining optomechanical high quality performance. Here we demonstrate the monolithic integration of photonic crystal optomechanical oscillators and on-chip high speed Ge detectors based on the silicon CMOS platform. With the generation of both high harmonics (up to 59th order) and subharmonics (down to 1/4), our chipset provides multiple frequency tones for applications in both frequency multipliers and dividers. The phase noise is measured down to −125 dBc/Hz at 10 kHz offset at ~400 μW dropped-in powers, one of the lowest noise optomechanical oscillators to date and in room-temperature and atmospheric non-vacuum operating conditions. These characteristics enable optomechanical oscillators as a frequency reference platform for radio-frequency-photonic information processing.

Stability and Intrinsic Fluctuations of Dissipative Cavity Solitons in Kerr Frequency Microcombs

The generation of dissipative cavity solitons is one of the most intriguing features of microresonator-based Kerr frequency combs, enabling effective mode locking of comb modes and synthesis of ultrafast pulses. With the Lugiato-Lefever model, here, we conduct detailed theoretical investigations on the transient dynamics of dissipative cavity solitons and describe how several intrinsic effects of the Kerr comb disturb the stability of cavity solitons, including soliton breathing, higher order dispersion, dispersive wave emission, and cavity mode coupling. Our results and analysis agree well with recent measurements and provide insight into some as yet unexplained observations.

Stabilized chip-scale Kerr frequency comb via a high-Q reference photonic microresonator.

We stabilize a chip-scale Si3N4 phase-locked Kerr frequency comb via locking the pump laser to an independent stable high-Q reference microresonator and locking the comb spacing to an external microwave oscillator. In this comb, the pump laser shift induces negligible impact on the comb spacing change. This scheme is a step toward miniaturization of the stabilized Kerr comb system as the microresonator reference can potentially be integrated on-chip. Fractional instability of the optical harmonics of the stabilized comb is limited by the microwave oscillator used for a comb spacing lock below 1 s averaging time and coincides with the pump laser drift in the long term.

Clustered frequency comb.

We show theoretically that it is feasible to generate a spectrally broad Kerr frequency comb consisting of several spectral clusters phase matched due to interplay among second- and higher-order group velocity dispersion contributions. We validate the theoretical analysis experimentally by driving a magnesium fluoride resonator, characterized with 110 GHz free spectral range, with a continuous wave light at 1.55 μm and observing two comb clusters separated by nearly two-thirds of an octave.

Globally Stable Microresonator Turing Pattern Formation for Coherent High-Power THz Radiation On-Chip

In nonlinear microresonators driven by continuous-wave (cw) lasers, Turing patterns have been studied in the formalism of Lugiato-Lefever equation with emphasis on its high coherence and exceptional robustness against perturbations. Destabilization of Turing pattern and transition to spatio-temporal chaos, however, limits the available energy carried in the Turing rolls and prevents further harvest of their high coherence and robustness to noise. Here we report a novel scheme to circumvent such destabilization, by incorporating the effect of local mode hybridizations, and attain globally stable Turing pattern formation in chip-scale nonlinear oscillators, achieving a record high power conversion efficiency of 45% and an elevated peak-to-valley contrast of 100. The stationary Turing pattern is discretely tunable across 430 GHz on a THz carrier, with a fractional frequency sideband non-uniformity measured at 7.3x10^-14. We demonstrate the simultaneous microwave and optical coherence of the Turing rolls at different evolution stages through ultrafast optical correlation techniques. The free running Turing roll coherence, 9 kHz in 200 ms and 160 kHz in 20 minutes, is transferred onto a plasmonic photomixer for one of the highest power THz coherent generation at room-temperature, with 1.1% optical-to-THz power conversion. Its long-term stability can be further improved by more than two orders of magnitude, reaching an Allan deviation of 6x10^-10 at 100 s, with a simple computer-aided slow feedback control. The demonstrated on-chip coherent high-power Turing-THz system is promising to find applications in astrophysics, medical imaging, and wireless communications.

Panoramic-reconstruction temporal imaging for seamless measurements of slowly-evolved femtosecond pulse dynamics

Single-shot real-time characterization of optical waveforms with sub-picosecond resolution is essential for investigating various ultrafast optical dynamics. However, the finite temporal recording length of current techniques hinders comprehensive understanding of many intriguing ultrafast optical phenomena that evolve over a timescale much longer than their fine temporal details. Inspired by the space-time duality and by stitching of multiple microscopic images to achieve a larger field of view in the spatial domain, here a panoramic-reconstruction temporal imaging (PARTI) system is devised to scale up the temporal recording length without sacrificing the resolution. As a proof-of-concept demonstration, the PARTI system is applied to study the dynamic waveforms of slowly evolved dissipative Kerr solitons in an ultrahigh-Q microresonator. Two 1.5-ns-long comprehensive evolution portraits are reconstructed with 740 fs resolution and dissipative Kerr soliton transition dynamics, in which a multiplet soliton state evolves into a stable singlet soliton state, are depicted.Real-time characterization of ultrafast dynamics comes with a tradeoff between temporal resolution and recording length. Here, Li et al. use a temporal reconstruction technique inspired by panoramic microscopy to image the dynamics of slowly evolved dissipative Kerr solitons in a microresonator.