research Highlights

A Silicon Photonic-CMOS Transceiver for 5G Radio-over-Fiber Communications

The proposed millimeter-wave-over-fiber system is illustrated below. Based on the analog RoF architecture, the system acts as a remote miniature antenna module. Frequency converters and DSP are not required that considerably simplify the system complexity, cost, and power consumption. By spatially arrangement of the RAUs, the network coverage can be significantly improved. The core of the system is a silicon photonic (SiPh)-CMOS transceiver and an antenna module as shown in the figure. In the downlink path, a 28-GHz optical signal from the CO is carried through an optical fiber array to the SiPh chip where a photodetector (PD) converts the optical signal to a photocurrent. This photocurrent is further processed and amplified by a CMOS transmitter and radiated by an integrated antenna for wireless transmission to the end user. The CMOS transmitter consists of a transimpedance amplifier (TIA) and a power amplifier (PA). In the uplink path, the antenna module receives an electronic millimeter-wave signal from the end-user equipment. This signal then is amplified by a CMOS receiver and fed into an optical modulator through a voltage driver. The CMOS receiver consists of a low-noise amplifier (LNA) and a modulator driver. The upstream signal is modulated by an optical modulator and is transmitted to the CO through the fiber array. The antenna is fabricated on a printed circuit board (PCB) and the fiber array is edge coupled to the SiPh chips.

Ultra-wide broadband inductive-peaking Si microring modulators

An operation scheme using electrical peaking to engineer the modulation band of a Si microring modulator is presented. By incorporating an inductor design at the metal traces of a Si microring modulator, the driving signal can be magnified near the peaking frequency.  We accomplish a Si microring modulator design with a wide and flat transmission band over 95 GHz, which is potentially applied for a non-return-to-zero (NRZ) data transmission over 120 Gbit/s without extra signal post-compensation.

3D integrated photonics

A new technology using hydrogen annealing and thermal oxidation is proposed to fabricate a very compact two-dimensional multimode interference coupler monolithically integrated with two-layer silicon photonic wires. Two devices, a 1×1 cross power coupler and a 1×22 power splitter, are presented and characterized. The experimental result shows that a 3-dB transmission bandwidth of 1.1 nm and the on-chip coupler losses of 8.6 dB and 2.84 dB were accomplished for the 1×1 cross power coupler and the 1×22 power splitter. This work demonstrates the possibility of realizing three-dimensional photonic integration for silicon-on-insulator lightwave circuits. 

A self-assembled microbonded germanium/silicon heterojunction photodiode 

A novel technique is presented for fabricating high performance Ge/Si photodiodes using surface tension to locally bond germanium (Ge) on silicon (Si). Surface tension is a cohesive force among liquid molecules that tends to bring contiguous objects contact to maintain a minimum surface energy. One could take advantage of this mechanism to fabricate a heterojunction optoelectronic device where the lattice constants of joined semiconductors are different. A high-speed Ge/Si heterojunction waveguide photodiode is presented by microbonding a beam-shaped Ge, first grown by rapid-melt-growth (RMG) method, on top of a Si waveguide via surface tension. Excellent device performances with an operating bandwidth of 32 GHz. 

Ultra-smooth sidewall of Si photonics devices by hydrogen annealing 

A fast, effective process using hydrogen annealing is introduced to perform profile transformation on silicon-on- insulator (SOI) and to reduce sidewall roughness of silicon photonics devices. Utilizing this technique, we also observe the root-mean-square (rms) sidewall roughness is dramatically reduced from 20 to 0.26 nm. A theoretical model is presented to analyze the profile transformation, and experimental results show this process can be engineered by several parameters including temperature, pressure, and time.

Application of strong transverse magneto-optical Kerr effect on high sensitive surface plasmon grating sensors

A high sensitive sensor is demonstrated by exploiting strong transverse magneto-optical Kerr effect on a ferromagnetic surface plasmon grating. The surface plasmon grating, made of a hybridized Au/Fe/Au layer, exhibits a very dispersive Kerr parameter variation near the surface plasmon polariton (SPP) wavelength via coherent scattering of the SPP on the grating structure. Interrogating this Kerr parameter can be utilized for detecting chemical or biological objects in a fluid medium. The experiment results show the minimal detectable mass concentration of sodium chloride in a saline solution is 4.27 × 10E-3 %, corresponding to a refractive index change of 7.60 × 10E-6 RIU. For an avidin-biotin interaction experiment, the sensitivity of avidin detection in PBS solution is 1.97 nM, which is limited by the index fluctuation of flowing media during measurement. 

High-speed Si Mach-Zehnder modulators

Silicon traveling-wave Mach-Zehnder modulators (MZM) have been demonstrated for high-speed data transmission over 50 Gb/s in optical communication and optical interconnects. In addition to 50 Gb/s NRZ reported for state-of-the-art Si modulators, four-level pulse amplitude modulation (PAM-4) actually is desired to boost the transmission data rate up to 100 Gb/s per channel. In this study, we show a dual-drive Si MZM with capability of PAM-4 modulation and integrated with CMOS driver circuits. 

Directed Self-Assembly of Star-Block Copolymers by Topographic Nanopatterns through Nucleation and Growth Mechanism

Exploring the ordering mechanism and dynamics of self-assembled block copolymer (BCP) thin films under confined conditions are highly essential in the application of BCP lithography. In this study, it is aimed to examine the self-assembling mechanism and kinetics of silicon-containing 3-arm star-block copolymer composed of polystyrene (PS) and poly(dimethylsiloxane) blocks as nanostructured thin films with perpendicular cylinders and controlled lateral ordering by directed self-assembly using topographically patterned substrates. The ordering process of the star-block copolymer within fabricated topographic patterns with PS-functionalized sidewall can be carried out through the type of secondary (i.e., heterogeneous) nucleation for microphase separation initiated from the edge and/or corner of the topographic patterns, and directed to grow as well-ordered hexagonally packed perpendicular cylinders. The growth rate for the confined microphase separation is highly dependent upon the dimension and also the geometric texture of the preformed pattern. Fast self-assembly for ordering of BCP thin film can be achieved by lowering the confinement dimension and also increasing the concern number of the preformed pattern, providing a new strategy for the design of BCP lithography from the integration of top-down and bottom-up approaches.

Optomechanical Self-Regulated Coupling of a  Suspended Microsphere Cavity and a Waveguide in the Aqueous Medium

An on-chip integrated operating platform comprising waveguides and microelectromechanical systems is employed to study the cavity-enhanced optical gradient force on colloidal microspheres, owing to the ability to precisely control the distance between a suspended microsphere and a waveguide through dielectrophoretic force. We introduce two kinds of optomechanical coupling mechanisms at resonance, depending on the initial coupling gap without inclusion of the optical gradient force. One is self-adjusted coupling, where the coupling gap of a suspended microsphere continuously varies with the optical input power, and the other is bistable coupling, where the coupling gap hops from one state to the other as the input power exceeds over a threshold value, which is caused by the nature of nonlinear gap-dependent optical gradient force. 

Manipulation of micro-particles through optical interference patterns generated by integrated photonic devices

Micro-particle transport and switch governed by guided-wave optical interference are presented. The optical interference, occurring in a directional coupler and a multi-mode interferometer made by inverted rib waveguides, results in a specific evanescent field dependent on wavelength. Through a detailed theoretical analysis, the field of induced optical force shows a correlative pattern associated with the evanescent field. Experimental results demonstrate that 10 mm polystyrene beads are propelled with a trajectory subject to the interference pattern accordingly. By launching different wavelengths, the polystyrene beads can be delivered to different output waveguide ports. Massive micro-particle manipulation is applicable.

Enhanced light outcoupling in a thin film by texturing meshed surfaces

We have demonstrated the light-extraction enhancement of meshed surfaces fabricated on a PDMS sub- strate. The outcoupling coefficient of OLEDs with this meshed structure enhances up to 46%. In addition, the ex- perimental result shows that the outcoupling eficiency is in- sensitive to optical wavelength. This property is beneficial to preserve the color spectrum of light-emitting devices. A sa- lient feature of this technology is that PDMS meshed sur- faces can be fabricated separately and laminated on the large-area glass substrate of organic electroluminescent de- vices.