Harish Subbaraman

All ink-jet-printed carbon nanotube thin-film transistor on a polyimide substrate with an ultrahigh operating frequency of over 5 GHz

We report a flexible carbon nanotube (CNT) thin-film transistor (TFT) fabricated solely by ink-jet printing technology. The TFT is top gate configured, consisting of source and drain electrodes, a carrier transport layer based on an ultrapure, high-density (>1000 CNTs/μm2) CNT thin film, an ion-gel gate dielectric layer, and a poly(3,4-ethylenedioxythiophene) top gate electrode. All the TFT elements are ink-jet printed at room temperature on a polyimide substrate without involving any photolithography patterning or surface pretreatment steps. This CNT-TFT exhibits a high operating frequency of over 5 GHz and an on-off ratio of over 100. Such an all-ink-jet-printed process eliminates the need for lithography, vacuum processing, and metallization procedures and thus provides a promising technology for low-cost, high-throughput fabrication of large-area high-speed flexible electronic circuits on virtually any desired flexible substrate.

Photonic Crystal Fiber-Based True-Time-Delay Beamformer for Multiple RF Beam Transmission and Reception of an X-Band Phased-Array Antenna

We report multiple beam transmission and reception of an X-band phased antenna array utilizing highly dispersive photonic crystal fiber (PCF), which has a dispersion value of - 600 ps/nm/km at 1550 nm, as true-time-delay (TTD) elements. In the transmission mode, two RF signals with frequencies 8.4 and 12 GHz are simultaneously steered at angles 7.4 and 21.2 degrees, respectively. In the receiving mode experiment, two RF signals with frequencies 8.4 and 12 GHz impinging upon an X-band antenna array from angles -7.4 and -21.2 degrees, respectively, are detected and the angles of arrival are determined accurately. Many RF beams can be simultaneously transmitted or received. The demonstration is only limited by the hardware availability and the bandwidth of the wavelength differentiation capability of the system.

Design of a broadband highly dispersive pure silica photonic crystal fiber

A highly dispersive dual-concentric-core pure silica photonic crystal fiber is designed with a maximum chromatic dispersion value of about -9500 ps/(nm km) around the 1.56 microm wavelength region and a full width at half-maximum (FWHM) of 55 nm. The change in the dispersion-bandwidth product as a function of period is carefully studied by using the plane wave expansion method. The coupled mode theory matches well with the plane wave expansion method that was used to simulate the chromatic dispersion. This kind of a photonic crystal fiber structure is suitable for high-dispersion application in phased array antenna systems based on photonic crystal fiber arrays.

Fully printed phased-array antenna for space communications

We present a flexible active 2-bit 2-element phased-array antenna (PAA) fully fabricated using ink-jet printing technology. High speed carbon nanotube (CNT) based field effect transistors (FETs) function as switch in the true-time delay line of the PAA. The 2-bit 2-element active PAA is printed out at room temperature on 100μm thick Kapton substrate. The FET switch works well for 5GHz RF signals. An ON-OFF ratio of over 100 is obtained at a low Vds bias of 1.8V. The measured azimuth beamsteering angles of PAA agree well with simulation values.

Photonic dual RF beam reception of an X band phased array antenna using a photonic crystal fiber-based true-time-delay beamformer

We report dual RF beam reception of an X band phased array antenna using a photonic crystal fiber (PCF)-based delay network. Each incoming RF signal can be independently received, and the angle of arrival can be determined based on the delay time-dependent wavelength. Two RF signals with frequencies 8.4 and 12 GHz impinge upon an X-band antenna array from -7.4 degrees and -21.2 degrees . These signals are detected, and the angle of arrival is determined with a very good degree of accuracy using PCF-based true-time delay. The total number of RF beams that can be simultaneously detected is limited by the hardware availability and the bandwidth of the wavelength differentiation capability of the system.

Low dispersion slow light in silicon-on-insulator photonic crystal waveguide

We present a design methodology for silicon-on-insulator photonic crystal waveguides to achieve wideband lowdispersion slow light with only tuning the position of the first three inner rows. We aim to maximize the group index - bandwidth product or the slowdown factor. Our design achieves a constant group index of 39.3 over 12 nm bandwidth around 1550 nm, corresponding to a slow down factor of 0.3.

Reply to Comment on "Design of a broadband highly dispersive pure silica photonic crystal fiber"

A reply to the Comment by Mortensen et al. [Appl. Opt. 47, xxxx-xxxx (2007)] on our previous paper on on our earlier paper published in [Appl. Opt.46, 3263-3268 (2007)APOPAI0003-6935 is presented. In the theoretical study of a highly dispersive dual concentric-core pure silica photonic crystal fiber, we presented a design wherein the high value of the dispersion coefficient was due to coupling of modes between two asymmetric cores at a phase-matched wavelength of 1.55 μm. In the condition in which both the supermodes are excited simultaneously, the dispersion effects of the two modes do not cancel each other, but the final dispersion coefficient depends on the input launching condition. We also predict the feasibility of manufacturing low-loss pure silica dual concentric-core photonic crystal fibers in future with advancement in the current fabrication techniques.

Packaging and system demonstration of an X-band phased array antenna utilizing highly dispersive photonic crystal fiber based true- time-delay

We demonstrate single RF beam transmission and reception of an X-band phased array antenna using highly dispersive photonic crystal fiber (PCF) based true-time-delay (TTD) lines. The dispersion coefficient of the fabricated fiber is as high as -600ps/nm/km at a wavelength of 1545nm. Coupling between a dispersion shifted fiber (DSF) and the fabricated PCF is performed by using an ultra high numerical aperture (UHNA) intermediate fiber, which helps in achieving a good coupling efficiency and keeping the insertion loss of the delay lines to under 3.5dB. Using the PCF-TTD network, we report the transmission of 8.4GHz signal at 7.40 and 12GHz signal at 21.20 by tuning the laser wavelength to 1547.72nm and 1552.52nm respectively. Single beam receiving capability is also demonstrated by accurately detecting 8.4GHz signal coming from -7.40 and 12GHz signal coming from -21.20 by tuning the wavelengths to 1547.72nm and 1552.52nm respectively..

Silicon nano- and micro-photonic devices

Silicon VLSI plays a key role in information technology. Recent progresses in silicon photonics have significantly moved the conventional silicon VLSI to high bandwidth photonics with lower power consumption for switching and interconnects. These devices include novel waveguides, modulators and detectors that are compatible with Si CMOS fabrication process. There are two major obstacles to build a monolithic nano-photonic system on a silicon chip:

Modified slab photonic crystal structure for delay time enhancement using capsule shaped holes

A slow group velocity photonic crystal slab waveguide is designed using capsule shaped air holes in hexagonal lattice. The presented design achieves nearly flat band waveguiding with group-index of 21–36 over the normalized bandwidth (Δω/ω) of 1.38%∼0.4%.