Chuantong Cheng

A highly efficient thermo-optic microring modulator assisted by graphene

Graphenes remarkable electrical and optical properties afford great potential for constructing various optoelectronic devices, including modulators, photodetectors and pulse lasers. In particular, graphene-based optical modulators were demonstrated to be featured with a broadband response, small footprint, ultrafast speed and CMOS-compatibility, which may provide an alternative architecture for light-modulation in integrated photonic circuits. While on-chip graphene modulators have been studied in various structures, most of them are based on a capacitance-like configuration subjected to complicated fabrication processes and providing a low yield of working devices. Here, we experimentally demonstrate a new type of graphene modulator by employing graphenes electrical and thermal properties, which can be achieved with a simple fabrication flow. On a graphene-coated microring resonator with a small active area of 10 μm(2), we have obtained an effective optical modulation via thermal energy electrically generated in a graphene layer. The resonant wavelength of the ring resonator shifts by 2.9 nm under an electrical power of 28 mW, which enables a large modulation depth of 7 dB and a broad operating wavelength range of 6.2 nm with 3 dB modulation. Due to the extremely high electrical and thermal conductivity in graphene, the graphene thermo-optical modulator operates at a very fast switching rate compared with the conventional silicon thermo-optic modulator, i.e. 10%-90% rise (90%-10% fall) time of 750 ns (800 ns). The results promise a novel architecture for massive on-chip modulation of optical interconnects compatible with CMOS technology.

Optical breakdown for silica and silicon with double femtosecond laser pulses.

The optical breakdown thresholds (OBTs) of typical dielectric and semiconductor materials are measured using double 40-fs laser pulses. By measuring the OBTs with different laser energy and different time delays between the two pulses, we found that the total energy of breakdown decrease for silica and increase for silicon with the increase of the first pulse energy.

CMOS-Compatible Vertical Grating Coupler With Quasi Mach–Zehnder Characteristics

A vertical grating coupler on silicon-on-insulator substrates has been designed and demonstrated. The light from a vertical fiber can be coupled in and split equally into two arms with the fiber placed in the grating center. An optical combiner is used to collect the transmission from the two arms. The measured peak coupling efficiency is 37%. Our device can also function like a Mach-Zehnder interferometer. In a device with an arm difference of 30 μm, the normalized transmission spectra of 20-nm free spectral range and more than 12-dB extinction ratio at 1567 nm are obtained.

Monolithic Integration of Si 3 N 4 Microring Filters With Bulk CMOS IC Through Post-Backend Process

An experimental demonstration of backend monolithic integration of Si3N4 microring filter with bulk complementary metal-oxide-semiconductor (CMOS) integrated circuit (IC) is accomplished using CMOS postbackend process. Si3N4 photonic layer is integrated on the top surface of CMOS IC die which is manufactured in commercial CMOS foundry. The Si3N4 microring filters in photonic layer are fabricated using CMOS postbackend process with only two additional lithography steps. The filters can be thermally tuned by microheaters integrated in CMOS circuits, which are controlled by transmission gates. A measured optical transmission spectrum and a dynamic characteristic of the integrated filter are provided.

Bidirectional grating coupler based optical modulator for low-loss Integration and low-cost fiber packaging

We proposed and demonstrated a novel optical modulator based on a bidirectional grating coupler designed for perfectly vertical fiber coupling. The grating functions as the fiber coupler and 3-dB splitter. To observe the interference, an arm difference of 30μm is introduced. As a result of the high coupling efficiency and near perfect split ratio of the grating coupler, this device exhibits a low on-chip insertion loss of 5.4dB (coupling loss included) and high on-off extinction ratio more than 20dB. The modulation efficiency is estimated to be within 3-3.84V•cm. In order to investigate the fiber misalignment tolerance of this modulator, misalignment influence of the static characteristics is analyzed. 10Gb/s Data transmission experiments of this device are performed with different fiber launch positions. The energy efficiency is estimated to be 8.1pJ/bit.

A Pure Frequency Tripler Based on CVD Graphene

In this letter, the chemical vapor deposition grown single-layer graphene with micrometer scale graphene flakes interspersed on the surface has been first used to implement field-effect transistors (FETs) for the purpose of frequency tripling. Two Dirac points can be observed in the transfer curves of the designed FETs, which are essential to generate pure third harmonic with ambipolar graphene. With an input of 1 kHz, more than 94% of the output signal RF power is concentrated at 3 kHz. To the best of our knowledge, it is the highest output spectral purity for reported frequency triplers. The excellent spectral purity, combined with the superhigh carrier mobility of graphene, makes this graphene-based frequency tripler a very promising candidate for the ultrahigh-frequency electronic applications.

Graphene Field-Effect Transistor Current Source With Double Top-Gates and Double Feedback

This letter proposes a graphene field-effect transistor (GFET) device with double top-gates and double feedback. An intuitive explanation of the device is provided and its performance is verified by numerical solution of the GFET large signal model in the p- and n-type regions. Simulation shows that the device can provide full current saturation within a large voltage range using a typical GFETs structure. The saturation current can be adjusted using a control voltage and other circuit parameters, which makes it a voltage-controlled current source suitable for analog and flexible circuit applications.

Enhanced Fluorescence Emission from Silicon-Rich Nitride Guided Mode Resonance Gratings

A one-dimensional subwavelength grating structure is used to enhance the emission from Si nanocrystals embedded in silicon-rich nitride film. The enhancement is attributed to guided-mode resonance effects supported by subwavelength gratings. Both the reflection and photoluminescence experiments are presented aimed at acquiring guided mode spectrum and the emission enhancement factor. A ten-fold enhancement of the fluorescence emission by Si nanocrystals is obtained at the low group velocity band edge of subwavelength gratings. Our results indicate that silicon-rich nitride is a suitable material for the fabrication of high efficiency light emitting structures. The versatility of silicon-rich nitride material system has a far-reaching significance for the realization of optically active complementary metal oxide semiconductor devices.

Microwave photonic filter with reconfigurable and tunable bandpass response using integrated optical signal processor based on microring resonator

Abstract. A bandpass microwave photonic filter based on an integrated optical signal processor is proposed and demonstrated by numerical simulation. The optical signal processor consisting of double-bus-coupled and series-cascaded silicon microrings (MRs) is used to produce two bandpass responses to process optical carrier signal and sideband signal separately. Because of the tunability of MRs, variable −3  dB bandwidth and tunable operating frequency are achieved. The −3  dB bandwidth and operating frequency can be tuned from 1.5 to 12 GHz and from 15 to 34 GHz, respectively. The loss impact, tuning method, and fabrication error tolerance are also discussed.

100Gb/s Ultra wide misalignment tolerance WDM transmitter with novel vertical grating coupler

A 4×25Gb/s ultra wide misalignment tolerance WDM transmitter with novel vertical grating coupler has been demonstrated on CMOS-compatible SOI Platform. The misalignment tolerance along the horizontal direction of grating coupler is as high as ±4μm.