Lanlan Gu

80-micron interaction length silicon photonic crystal waveguide modulator

An ultracompact silicon electro-optic modulator was experimentally demonstrated based on silicon photonic crystal (PhC) waveguides for the first time to our knowledge. Modulation operation was demonstrated by carrier injection into an 80μm-long silicon PhC waveguide of a Mach-Zehnder interferometer (MZI) structure. The π phase shift driving current, Iπ, across the active region is as low as 0.15mA, which is equivalent to a Vπ of 7.5mV when a 50Ω impedance-matched structure is applied. The modulation depth is 92% operating at 1567nm.

High speed silicon photonic crystal waveguide modulator for low voltage operation

A high speed compact silicon modulator is experimentally demonstrated to work at a low driving voltage desirable for on-chip applications. As carrier injection is the only practical option for optical modulation in silicon, a lower limit of current density (∼104A∕cm2) exists for achieving gigahertz modulation in the p-i-n diode configuration. Exploiting the slow group velocity of light in photonic crystal waveguides, the interaction length of this Mach-Zehnder interferometer-type silicon modulator is reduced significantly compared to conventional modulators. The required high current density is achieved with a low voltage (2V) by scaling down the interaction length to 80μm.

Thermooptically Tuned Photonic Crystal Waveguide Silicon-on-Insulator Mach–Zehnder Interferometers

Ultracompact thermooptically tuned photonic crystal waveguide (PCW) silicon-on-insulator Mach-Zehnder interferometers (MZIs) have been proposed and fabricated. A novel thermal design was employed to improve the device switching performance. Both steady-state and transient thermal analyses were performed to evaluate the thermal performance of the thermooptic MZIs. A switching time less than 20 mus has been experimentally achieved, which clearly demonstrated the speed advantage using the new heating approach. The active length of the PCW-based MZIs was 80 mum, nearly one order of magnitude shorter than the conventional silicon waveguide-based MZIs. A maximum modulation depth of 84% for a switching power of 78 mW was obtained at a wavelength of 1548 nm

20dB-enhanced coupling to slot photonic crystal waveguide using multimode interference coupler

The authors experimentally demonstrate a slot photonic crystal structure for guiding light in a sub-100-nm-wide low-index region. A multimode interference-based coupling structure is introduced to couple light into such a narrow slot photonic crystal waveguide. A coupler of 1.26μm long enhances the coupling efficiency by 20dB for the quasi-transverse-electric mode over 35nm optical bandwidth centered at 1562nm. The measured transmission spectra are in good agreement with the simulated band diagram.

Fringing-field minimization in liquid-crystal-based high-resolution switchable gratings

A liquid-crystal (LC)-based high-resolution switchable grating is proposed by using a double-sided structure, where striped electrodes are patterned on both sides of the LC cell. A unique biasing configuration is employed to successfully minimize the distortion of the LC director profile due to the fringing-field effects under two-dimensional electric fields. A first order diffraction angle of 14.5° with a diffraction efficiency of 33% for transmission light at 1.55μm is experimentally achieved. This result approaches the theoretical upper limit of 33.8% for a sinusoidal phase grating. The device efficiency is enhanced 80 times compared to a conventional single-sided device. Experimental results indicate the tolerance of electrode misalignment is 2μm.

80-micron Interaction Length Silicon Nano-Photonic Crystal Waveguide Modulator

An ultra-compact silicon electro-optic modulator is experimentally demonstrated based on silicon photonic crystal (PhC) waveguides for the first time to our knowledge. Modulation operation was demonstrated by carrier injection into an 80mum-long silicon PhC waveguide of a Mach-Zehnder interferometer (MZI) structure. The modulation depth operating at 1567 nm is 92%. The pi phase shift driving current, Ipi, across the active region is as low as 0.15 mA, which is equivalent to a Vpi of 7.5 mV when a 50Omega impedance-mate structure is applied

Physical Mechanism of p-i-n-Diode-Based Photonic Crystal Silicon Electrooptic Modulators for Gigahertz Operation

In this paper, the physical mechanism governing the optical modulation in a p-i-n-diode-embedded photonic crystal (PC) silicon Mach-Zehnder interferometer modulator is examined. Optical simulations have been performed to study how the slow group velocity of the photonic crystal waveguides enables a significant reduction of device size. The theoretical speed limitation in a PC-based silicon modulator is also explored. The 2-D semiconductor device simulator MEDICI has been employed to analyze the transient behavior of the p-i-n-diode-embedded silicon modulator. Electrical simulations have revealed a significant improvement in modulation speed upon the enhancement of current density in a downscaled PC device. High-speed optical modulation at 1 Gmiddots-1 has been experimentally demonstrated. The performance degradation in optical modulation at the low-frequency operation region attributed to the thermooptic effect is identified and discussed. Simulations have also revealed that the modulation speed of our device can be improved up to 10 GHz by further reducing the device dimensions with little penalty of the increased optical loss.

Fabrication of polymer photonic crystal superprism structures using polydimethylsiloxane soft molds

We presented a soft lithography technique of fabricating polymer photonic crystal superprism structures using elastomeric polydimethylsiloxane templates. Dense two-dimensional photonic crystal superprism structures with feature sizes of 150–500nm and aspect ratios of up to 1.25 were replicated. Large field size and easy fabrication are two major advantages when compared with other imprint technology. Atomic force microscopy images showed that the molded structures had high fidelity to the masters. Less than 3% reduction of the depth in the molded structures was achieved with respect to the master. The increase of the surface roughness from the master to the molded structures is minimal. The issue of pattern collapse during pattern transfer of submicron structures was analyzed against the pattern dimensions and aspect ratios; and the experimental results were found in agreement with a prior theory. We also experimentally demonstrated the superprism effect in two-dimensional photonic crystal structure at ne...

Nano-photonic crystal waveguides for ultra-compact tunable true time delay lines

Nanophotonics including photonic crystals promises to have a revolutionary impact on the landscape of photonics technology. Photonic crystal line defect waveguides show high group velocity dispersion and slow photon effect near transmission band edge. By using photonic crystal waveguides to build true time delay based phased array antenna or other optical signal processing systems, the length of the tunable true time delay lines can be dramatically reduced inversely proportional to group velocity dispersion in dispersion enhanced system architecture or reduced inversely proportional to group index in slow photon enhanced system architecture. The group index of the fabricated silicon photonic crystal line defect waveguide is experimentally demonstrated as high as 40 at optical wavelength around 1569 nm. The group velocity dispersion of the fabricated silicon photonic crystal line defect waveguide is as high as 50 ps/nm∙mm at wavelength around 1569 nm, which is more than 107 times the dispersion of the standard telecom fiber (D = 3 ps/nm∙km). Due to the integration nature of photonic crystals, system-on-chip integration of the true time delay modules can be easily achieved.

True-time-delay modules based on a single tunable laser in conjunction with a waveguide hologram for phased array antenna application

A wavelength-controlled continuous beam-steering four-element X-band (8- to 12-GHz) phased array antenna system is presented. The system is based on the continuously tunable optical true-time-delay technique. Dispersion-enhanced waveguide holograms were proposed and used to fabricate the optical true-time-delay devices. The devices are characterized both theoretically and experimentally. The wavelength of a laser was tuned within the system to get continuously tunable true time delay. The time delay was measured for a wavelength tuning range from 1537 to 1547 nm in 10-nm steps. The far-field radiation patterns of the antenna system were measured at 9 and 10.3 GHz, and they showed no beam squint. The true-time-delay formation idea presented here is suitable for not only X-band, but also for higher microwave frequencies, such as K-band.