Integrated and DC-powered superconducting microcomb.

Frequency combs, specialized laser sources emitting multiple equidistant frequency lines, have revolutionized science and technology with unprecedented precision and versatility. Recently, integrated frequency combs are emerging as scalable solutions for on-chip photonics. Here, we demonstrate a fully integrated superconducting microcomb that is easy to manufacture, simple to operate, and consumes ultra-low power. Our turnkey apparatus comprises a basic nonlinear superconducting device, a Josephson junction, directly coupled to a superconducting microstrip resonator. We showcase coherent comb generation through self-started mode-locking. Therefore, comb emission is initiated solely by activating a DC bias source, with power consumption as low as tens of picowatts. The resulting comb spectrum resides in the microwave domain and spans multiple octaves. The linewidths of all comb lines can be narrowed down to 1 Hz through a unique coherent injection-locking technique. Our work represents a critical step towards fully integrated microwave photonics and offers the potential for integrated quantum processors.

Toroidic phase transitions in a direct-kagome artificial spin ice.

Ferrotoroidicity-the fourth form of primary ferroic order-breaks both space and time-inversion symmetry. So far, direct observation of ferrotoroidicity in natural materials remains elusive, which impedes the exploration of ferrotoroidic phase transitions. Here we overcome the limitations of natural materials using an artificial nanomagnet system that can be characterized at the constituent level and at different effective temperatures. We design a nanomagnet array as to realize a direct-kagome spin ice. This artificial spin ice exhibits robust toroidal moments and a quasi-degenerate ground state with two distinct low-temperature toroidal phases: ferrotoroidicity and paratoroidicity. Using magnetic force microscopy and Monte Carlo simulation, we demonstrate a phase transition between ferrotoroidicity and paratoroidicity, along with a cross-over to a non-toroidal paramagnetic phase. Our quasi-degenerate artificial spin ice in a direct-kagome structure provides a model system for the investigation of magnetic states and phase transitions that are inaccessible in natural materials.

High-uniformity and high-performance waveguide Ge photodetectors for the O and C bands.

This paper presents the test results for high-performance and high-uniformity waveguide silicon-based germanium (Ge) photodetectors (PDs) for the O band and C band. Both wafer-scale and chip-scale test results are provided. The fabricated lateral p-i-n (LPIN) PDs exhibit a responsivity of 0.97 A/W at a bias of -2V, a bandwidth of 60 GHz, and a no-return-to-zero (NRZ) eye diagram rate of 53.125 Gb/s. Additionally, an average dark current of 22.4 nA was obtained in the vertical p-i-n (VPIN) PDs at -2V by optimizing the doping process. The device can reach an average responsivity of 0.9 A/W in the O band. The standard deviation in a wafer with a dark current and responsivity is as low as 7.77 nA and 0.03 A/W at -2V, respectively.

Unconventional Superconducting Diode Effects via Antisymmetry and Antisymmetry Breaking.

Symmetry breaking plays a pivotal role in unlocking intriguing properties and functionalities in material systems. For example, the breaking of spatial and temporal symmetries leads to a fascinating phenomenon: the superconducting diode effect. However, generating and precisely controlling the superconducting diode effect pose significant challenges. Here, we take a novel route with the deliberate manipulation of magnetic charge potentials to realize unconventional superconducting flux-quantum diode effects. We achieve this through suitably tailored nanoengineered arrays of nanobar magnets on top of a superconducting thin film. We demonstrate the vital roles of inversion antisymmetry and its breaking in evoking unconventional superconducting effects, namely a magnetically symmetric diode effect and an odd-parity magnetotransport effect. These effects are nonvolatilely controllable through in situ magnetization switching of the nanobar magnets. Our findings promote the use of antisymmetry (breaking) for initiating unconventional superconducting properties, paving the way for exciting prospects and innovative functionalities in superconducting electronics.

Beam Steering Technology of Optical Phased Array Based on Silicon Photonic Integrated Chip.

Light detection and ranging (LiDAR) is widely used in scenarios such as autonomous driving, imaging, remote sensing surveying, and space communication due to its advantages of high ranging accuracy and large scanning angle. Optical phased array (OPA) has been studied as an important solution for achieving all-solid-state scanning. In this work, the recent research progress in improving the beam steering performance of the OPA based on silicon photonic integrated chips was reviewed. An optimization scheme for aperiodic OPA is proposed.

PIC-integrable high-responsivity germanium waveguide photodetector in the C + L band.

We report the demonstration of a germanium waveguide p-i-n photodetector (PD) for the C + L band light detection. Tensile strain is transferred into the germanium layer using a SiN stressor on top surface of the germanium. The simulation and experimental results show that the trenches must be formed around the device, so that the strain can be transferred effectively. The device exhibits an almost flat responsivity with respect to the wavelength range from 1510 nm to 1630 nm, and high responsivity of over 1.1 A/W is achieved at 1625 nm. The frequency response measurement reveals that a high 3 dB bandwidth (f3dB) of over 50 GHz can be obtained. The realization of the photonic-integrated circuits (PIC)-integrable waveguide Ge PDs paves the way for future telecom applications in the C + L band.

High-speed compact folded Michelson interferometer modulator.

We propose and experimentally demonstrate a novel compact folded Michelson interferometer (FMI) modulator with high modulation efficiency. By folding the 0.5 mm-long phase shift arms, the length of the modulation area of the FMI modulator is only 0.25 mm. Meanwhile, the traveling wave electrode (TWE) is also shorter, which decreases the propagation loss of the RF signal and contributes to a small footprint. The Vπ-L of the present device is as low as 0.87 V·cm at -8 V bias voltage. The minimum optical insertion loss is 3.7 dB, and the static extinction ratio (ER) is over 25 dB. The measured 3-dB electro-optical (EO) bandwidth is 17.3 GHz at a -6 V bias. The OOK eye diagram up to 40 Gb/s is demonstrated under 2 V driver voltage.

80Gb/s NRZ Ge waveguide electro-absorption modulator.

We demonstrate a Ge electro-absorption modulator (EAM) in L band with a 3 dB electro-optical bandwidth beyond 67 GHz at -3 V bias voltage. The Eye diagram measurement shows a data rate of over 80 Gbps for non-return-to-zero on-off keying (NRZ-OOK) modulation at a voltage swing of 2.3 Vpp and the wavelength of 1605 nm. Through the comparison of multi-device results, it is proved that the introduction of the annealing process after CMP can increase the mean static extinction ratio of the EAM from 7.27 dB to 11.83 dB, which confirms the manufacturability of the device. The dynamic power consumption of the device is 6.348 fJ/bit. The performance of our device is comprehensive. The Ge EAM device also has excellent performance as a photodetector (PD) in the C and L communication bands. The responsivity of the device is 1.04 A/W at the wavelength of 1610 nm, resulting in ∼0.87 mW of static power consumption at -3 V bias voltage under 0.28 mW of optical input and the 3 dB opto-electric bandwidth of the devices are beyond 43 GHz at -3 V bias.

Crystallizing Kagome Artificial Spin Ice.

Artificial spin ices are engineered arrays of dipolarly coupled nanobar magnets. They enable direct investigations of fascinating collective phenomena from their diverse microstates. However, experimental access to ground states in the geometrically frustrated systems has proven difficult, limiting studies and applications of novel properties and functionalities from the low energy states. Here, we introduce a convenient approach to control the competing diploar interactions between the neighboring nanomagnets, allowing us to tailor the vertex degeneracy of the ground states. We achieve this by tuning the length of selected nanobar magnets in the spin ice lattice. We demonstrate the effectiveness of our method by realizing multiple low energy microstates in a kagome artificial spin ice, particularly the hardly accessible long range ordered ground state-the spin crystal state. Our strategy can be directly applied to other artificial spin systems to achieve exotic phases and explore new emergent collective behaviors.

Increasing wavelength-controlled steering range of an optical phased array by using two subarrays.

We demonstrate an optical phased array that consists of two subarrays based on the silicon on insulator (SOI) platform, each subarray including 16 independent channels. The demonstrated field of view of the optical phased array is 36.6∘×32.6∘ with a spot size of 1.68∘×0.0673∘. A steering range of 32.6° is achieved by combining two subarrays with different periods and tuning the wavelength from 1500 nm to 1600 nm. In another dimension, the steering is realized by introducing phase differences between channels.

Silicon nitride assisted 1×64 optical phased array based on a SOI platform.

We demonstrate a 1×64 optical phased array (OPA) based on a silicon on insulator (SOI) platform with integrated silicon nitride. The input port of the OPA is fabricated using a silicon nitride waveguide due to its advantage of allowing more optical power. The phase shifter is a silicon waveguide with heater because of the higher thermo-optic coefficient of silicon. And a double layer silicon nitride assisted grating is used in the emitter to reduce the emission strength and then increase the length of emitter to reduce the spot size. The length of the grating emitter is 1.5 mm and the measured field of view of this optical phased array is 35.5°×22.7° with spot size of 0.69°×0.075°.

Broadband and compact polarization beam splitter based on an asymmetrical directional coupler with extra optimizing designs.

In this paper, a novel, to the best of our knowledge, polarization beam splitter (PBS) based on an asymmetrical directional coupler (DC) was proposed, which consists of a strip waveguide (WG) and a ${{\rm Si}_3}{{\rm N}_4}$Si3N4 loaded horizontal slot WG. By carefully adjusting the geometric parameters of the DC, the phase match condition between these two WGs can be satisfied for the transverse magnetic (TM) polarization, while the coupling efficiency of the transverse electric (TE) polarization is frustrated due to the large phase mismatch. The extra optimizing designs include adding filters to the output ports as well as introducing the tapered structure into the DC, which is settled by the particle swarm optimizing (PSO) algorithm so that the performance of the proposed PBS is improved over a broadband range. Numerical simulations show that the bandwidths for the extinction ratio (ER) $ \gt {20}\;{\rm dB}$>20dB, 30 dB, and 40 dB are 160 nm, 95 nm, and 50 nm, respectively, with insertion loss (IL) $ \lt {1}\;{\rm dB}$<1dB for the wavelength of 1.49-1.58 µm. The analysis of the deviations demonstrates that the proposed PBS allows high fabrication tolerances.

Dual-layer waveguide grating antenna with high directionality for optical phased arrays.

Development of the waveguide grating antenna with high directionality is significantly important for the optical phased array. A Si3N4/Si dual-layer structure with the grating pattern on the Si3N4 layer is proposed to improve the directionality of the waveguide grating antenna. High directionality of more than 89% can be achieved, and the length of the waveguide grating antenna is longer than 4 mm.

Approach to calculate normal modes by decomposing the dyadic Green's function.

Normal mode is a very fundamental notion in quantum and classical optics. In this paper, we present a method to calculate normal modes by decomposing dyadic Green's function, where the modes are excited by dipoles. The modes obtained by our method can be directly normalized and their degeneracies can be easily removed. This method can be applied to many theoretical descriptions of cavity electrodynamics and is of interest to nanophotonics.