Minghao Qi

A three-dimensional optical photonic crystal with designed point defects

Photonic crystals offer unprecedented opportunities for miniaturization and integration of optical devices. They also exhibit a variety of new physical phenomena, including suppression or enhancement of spontaneous emission, low-threshold lasing, and quantum information processing. Various techniques for the fabrication of three-dimensional (3D) photonic crystals—such as silicon micromachining, wafer fusion bonding, holographic lithography, self-assembly, angled-etching, micromanipulation, glancing-angle deposition and auto-cloning—have been proposed and demonstrated with different levels of success. However, a critical step towards the fabrication of functional 3D devices, that is, the incorporation of microcavities or waveguides in a controllable way, has not been achieved at optical wavelengths. Here we present the fabrication of 3D photonic crystals that are particularly suited for optical device integration using a lithographic layer-by-layer approach. Point-defect microcavities are introduced during the fabrication process and optical measurements show they have resonant signatures around telecommunications wavelengths (1.3–1.5 µm). Measurements of reflectance and transmittance at near-infrared are in good agreement with numerical simulations.

An All-Silicon Passive Optical Diode

A Passive Optical Diode Electrical diodes are at the core of microelectronics. The optical equivalent, however, has been difficult to realize owing to the time-reversal symmetry of Maxwells equations that describe electromagnetic propagation. Usually, a control input in the form of a magnetic field is required that breaks that symmetry. Such inputs are not practical for optical integrated circuits. Fan et al. (p. 447, published online 22 December) developed a silicon-based microresonator device that could control the asymmetric transmission of light through it. The passive optical diode was compatible with current complementary metal-oxide semiconductor processing technology and thus should be readily integrated into optoelectronic circuitry. A silicon-based device is developed that allows the asymmetric propagation of light. A passive optical diode effect would be useful for on-chip optical information processing but has been difficult to achieve. Using a method based on optical nonlinearity, we demonstrate a forward-backward transmission ratio of up to 28 decibels within telecommunication wavelengths. Our device, which uses two silicon rings 5 micrometers in radius, is passive yet maintains optical nonreciprocity for a broad range of input power levels, and it performs equally well even if the backward input power is higher than the forward input. The silicon optical diode is ultracompact and is compatible with current complementary metal-oxide semiconductor processing.

Top-gated graphene field-effect-transistors formed by decomposition of SiC

Top-gated, few-layer graphene field-effect transistors (FETs) fabricated on thermally decomposed semi-insulating 4H-SiC substrates are demonstrated. Physical vapor deposited SiO2 is used as the gate dielectric. A two-dimensional hexagonal arrangement of carbon atoms with the correct lattice vectors, observed by high-resolution scanning tunneling microscopy, confirms the formation of multiple graphene layers on top of the SiC substrates. The observation of n-type and p-type transition further verifies Dirac Fermions’ unique transport properties in graphene layers. The measured electron and hole mobilities on these fabricated graphene FETs are as high as 5400 and 4400cm2∕Vs, respectively, which are much larger than the corresponding values from conventional SiC or silicon.

Atomic-layer-deposited nanostructures for graphene-based nanoelectronics

Graphene is a hexagonally bonded sheet of carbon atoms that exhibits superior transport properties with a velocity of 108cm∕s and a room-temperature mobility of >15000cm2∕Vs. How to grow gate dielectrics on graphene with low defect states is a challenge for graphene-based nanoelectronics. Here, we present the growth behavior of Al2O3 and HfO2 films on highly ordered pyrolytic graphite (HOPG) by atomic layer deposition (ALD). To our surprise, large numbers of Al2O3 and HfO2 nanoribbons, with dimensions of 5–200nm in width and >50μm in length, are observed on HOPG surfaces at growth temperature between 200 and 250°C. This is due to the large numbers of step edges of graphene on HOPG surfaces, which serve as nucleation sites for the ALD process. These Al2O3 and HfO2 nanoribbons can be used as hard masks to generate graphene nanoribbons or as top-gate dielectrics for graphene devices. This methodology could be extended to synthesize insulating, semiconducting, and metallic nanostructures and their combinations.

Multiple-channel silicon micro-resonator based filters for WDM applications

We demonstrate predictable resonance wavelength shifts in silicon micro-resonators by varying their perimeters using high-resolution lithography. The linear coefficient between the resonance wavelength shifts and the perimeter changes is determined with detailed experiments, and found to be nearly constant across the C and L bands in telecommunications. This empirical coefficient is also compared to that obtained from simulations on straight waveguides. Based on the linear model, without post-fabrication trimming or tuning, an eight-channel wavelength de-multiplexer with reasonably predicted average channel spacing ~ 1.8+/-0.1 nm (3dB bandwidth ~ 0.7+/-0.1 nm) is demonstrated at telecommunication bands in a silicon chip for the first time. This filter has out-of-band rejection ratio ~ 40 dB, low channel crosstalk </= 30 dB and low channel dropping loss </= 4+/-1 dB except for degraded performance in one channel due to fabrication imperfections.

Mode-locked dark pulse Kerr combs in normal-dispersion microresonators

Kerr frequency combs from microresonators are now extensively investigated as a potentially portable technology for a variety of applications. Most studies employ anomalous dispersion microresonators that support modulational instability for comb initiation, and mode-locking transitions resulting in coherent bright soliton-like pulse generation have been reported. However, some experiments show comb generation in normal dispersion microresonators; simulations suggest the formation of dark pulse temporal profiles. Excitation of dark pulse solutions is difficult due to the lack of modulational instability in the effective blue-detuned pumping region; an excitation pathway has been demonstrated neither in experiment nor in simulation. Here we report experiments in which dark pulse combs are formed by mode-interaction-aided excitation; for the first time, a mode-locking transition is observed in the normal dispersion regime. The excitation pathway proposed is also supported by simulations.

A highly compact third-order silicon microring add-drop filter with a very large free spectral range, a flat passband and a low delay dispersion

We demonstrate highly compact third-order silicon microring add-drop filters. The microring resonator has a small radius of 2.5 mum and a very large free spectral range of 32 nm at 1.55 mum. Experimental results show a low add-drop crosstalk of around -20 dB. Box-like channel dropping response is demonstrated, and it has a passband of ~ 1 nm (125 GHz), fast rolling-off (slope ~ 0.2 dB/GHz), high out-of-band signal rejection of around 40 dB and a low drop loss. Simulation agrees well with experiments in power transmission, and the group delay is also simulated and the variation is less than 1 ps within the passband. The propagation loss in microring resonators is optimized.

Modeling and measurement of losses in silicon-on-insulator resonators and bends

We present an analytical model to quantify losses in resonators and bends without uncertain contributions from fiber coupling in/out or waveguide cleavage facets. With resonators in add-drop configuration, intrinsic losses are calculated from the free spectral range, through-port extinction and drop-port bandwidth. We fabricated and characterized silicon-on-insulator resonator for loss analysis. At 1.55 mum, racetrack resonators with a bending radius of 4.5 mum show intrinsic losses as small as 0.14+/-0.014 dB/round-trip. Meanwhile, intrinsic losses increase up to 1.23 dB/round-trip in the racetrack resonator that has a bending radius of 2.25 mum. Losses in a 180 degrees bend are estimated as a half of the intrinsic losses in these racetrack resonators, i.e., 0.07+/-0.007 dB/turn for a bending radius of 4.5 mum and 0.62 dB/turn for a bending radius of 2.25 mum. Loss in a 90 degrees bend with a radius of 4.5 mum is determined to be 0.06+/-0.006 dB/turn at 1.55 mum. The losses in 180 degrees or 90 degrees bends are found to be mainly due to the transition loss between waveguide bends and straight waveguides.

Compact silicon microring resonators with ultra-low propagation loss in the C band.

The propagation loss in compact silicon microring resonators is optimized with varied ring widths as well as bending radii. At the telecom band of 1.53-1.57 mum, we demonstrate as low as 3-4 dB/cm propagation losses in compact silicon microring resonators with a small bending radius of 5 mum, corresponding to a high intrinsic quality factor of 200,000-300,000. The loss is reduced to 2-3 dB/cm for a larger bending radius of 10 mum, and the intrinsic quality factor increases up to an ultrahigh value of 420,000. Slot-waveguide microring resonators with around 80% optical power confinement in the slot are also demonstrated with propagation losses as low as 1.3+/-0.2 dB/mm at 1.55 mum band. These loss numbers are believed to be among the lowest ones ever achieved in silicon microring resonators with similar sizes.

Silicon-on-Insulator Microring Add-Drop Filters With Free Spectral Ranges Over 30 nm

We demonstrate highly compact optical add-drop filters based on silicon-on-insulator microring resonators. The microring resonators have a small radius of 2.5 mum and a very large free spectral range ~ 32 nm at the 1.55 mum communication band. The propagation loss in such small micoring resonators was experimentally determined and shown to be extremely important in designing microring add-drop filters with low add-drop crosstalk, low drop loss, and maximally flat drop passband. For box-like channel dropping responses, second-order optical add-drop filters with two coupled microring resonators are designed and demonstrated, and the simulation matches well with the experiment. Devices were patterned with electron-beam lithography. Two fabrication procedures utilizing different polarity of resists were introduced and compared, and the process with negative resist resulted in much smaller sidewall roughness of waveguides, thus reducing the propagation loss in microring resonators.