Ziyuan Li

Coupling of light from microdisk lasers into plasmonic nano-antennas

An optical dipole nano-antenna can be constructed by placing a sub-wavelength dielectric (e.g., air) gap between two metallic regions. For typical applications using light in the infrared region, the gap width is generally in the range between 50 and 100 nm. Owing to the close proximity of the electrodes, these antennas can generate very intense electric fields that can be used to excite nonlinear effects. For example, it is possible to trigger surface Raman scattering on molecules placed in the vicinity of the nano-antenna, allowing the fabrication of biological sensors and imaging systems in the nanometric scale. However, since nano-antennas are passive devices, they need to receive light from external sources that are generally much larger than the antennas. In this article, we numerically study the coupling of light from microdisk lasers into plasmonic nano-antennas. We show that, by using micro-cavities, we can further enhance the electric fields inside the nano-antennas.

Spatially Resolved Doping Concentration and Nonradiative Lifetime Profiles in Single Si-Doped InP Nanowires Using Photoluminescence Mapping

We report an analysis method that combines microphotoluminescence mapping and lifetime mapping data of single semiconductor nanowires to extract the doping concentration, nonradiative lifetime, and internal quantum efficiency along the length of the nanowires. Using this method, the doping concentration of single Si-doped wurtzite InP nanowires are mapped out and confirmed by the electrical measurements of single nanowire devices. Our method has important implication for single nanowire detectors and LEDs and nanowire solar cells applications.

A plasmonic staircase nano-antenna device with strong electric field enhancement for surface enhanced Raman scattering (SERS) applications

In this paper, a staircase plasmonic nano-antenna device is analysed both theoretically and experimentally. The tapered nano-antenna cavity with a grating leads to electric field enhancement factor (EF) as high as 31 close to 830nm. The integration of a metallic grating aids the coupling of light coming from the vertical direction to the nano-antenna, increasing the electric field in the nano-antenna by a factor of 3. The smallest air gap width between the metallic regions of the fabricated nano-antenna is about 35nm, fabricated using focused ion beam system. The small air gaps in the nano-antennas can generate very high intensity electric fields which can be used in applications in biological sensing and imaging, nanoparticle manipulations and enhancement of nonlinear effects. In this paper, to experimentally demonstrate that with the integration of a well designed grating and reflectors, the resonance inside the nano-antenna cavity is increased significantly, we exploit one application of this device: the enhancement of surface enhanced Raman scattering (SERS). The present structure can lead to SERS EFs above 1 million. (Some figures may appear in colour only in the online journal)

Room temperature GaAsSb single nanowire infrared photodetectors

Antimonide-based ternary III-V nanowires (NWs) allow for a tunable bandgap over a wide range, which is highly interesting for optoelectronics applications, and in particular for infrared photodetection. Here we demonstrate room temperature operation of GaAs0.56Sb0.44 NW infrared photodetectors grown by metal organic vapor phase epitaxy. These GaAs0.56Sb0.44 NWs have uniform axial composition and show p-type conductivity with a peak field-effect mobility of ∼12 cm(2) V(-1) s(-1)). Under light illumination, single GaAs0.56Sb0.44 NW photodetectors exhibited typical photoconductor behavior with an increased photocurrent observed with the increase of temperature owing to thermal activation of carrier trap states. A broadband infrared photoresponse with a long wavelength cutoff at ∼1.66 μm was obtained at room temperature. At a low operating bias voltage of 0.15 V a responsivity of 2.37 (1.44) A/W with corresponding detectivity of 1.08 × 10(9) (6.55 × 10(8)) cm√Hz/W were achieved at the wavelength of 1.3 (1.55) μm, indicating that ternary GaAs0.56Sb0.44 NWs are promising photodetector candidates for small footprint integrated optical telecommunication systems.

Broadband Phase-Sensitive Single InP Nanowire Photoconductive Terahertz Detectors.

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a powerful tool for materials characterization and imaging. A trend toward size reduction, higher component integration, and performance improvement for advanced THz-TDS systems is of increasing interest. The use of single semiconducting nanowires for terahertz (THz) detection is a nascent field that has great potential to realize future highly integrated THz systems. In order to develop such components, optimized material optoelectronic properties and careful device design are necessary. Here, we present antenna-optimized photoconductive detectors based on single InP nanowires with superior properties of high carrier mobility (∼1260 cm(2) V(-1) s(-1)) and low dark current (∼10 pA), which exhibit excellent sensitivity and broadband performance. We demonstrate that these nanowire THz detectors can provide high quality time-domain spectra for materials characterization in a THz-TDS system, a critical step toward future application in advanced THz-TDS system with high spectral and spatial resolution.

Tailoring the Properties of Photonic Nanojets by Changing the Material and Geometry of the Concentrator

Some microobjects can concentrate an incoming incident plane wave and produce the so- called photonic nanojets. The highly focused emerging beams have a high intensity and can be used in applications in microscopy, beam manipulation and imaging. In this article, it is shown that an adequate choice of geometric shape and material can lead to an improvement of the electric field enhancement capacity of nanojets by a factor of 40%.

Driving plasmonic nanoantennas with triangular lasers and slot waveguides

Plasmonic nanoantennas can generate high-intensity electric fields in a very small area. However, being passive devices, they need to be excited by external laser sources. The excitation of nanoantennas by semiconductor lasers can be inefficient and a significant amount of light may return back to the laser source after being scattered by the nanoantenna. In this paper, it is shown that the amount of light being returned to the semiconductor laser can be reduced by using dielectric slot waveguides. These waveguides can transport the incident light to the nanoantennas, but the amount of nondirectional back-scattered light is reduced after propagation through the slot waveguide.

Titanium Nano-Antenna for High-Power Pulsed Operation

While plasmonic nano-antennas can produce intense electric fields in a very small area, in general, these devices cannot handle high power, because of their small footprints. In order to increase the maximum peak power that these devices can withstand, they can be driven by nano-second pulses from a larger diameter Q-switched laser, which reduces the fluence reaching the devices, thus avoiding their destruction. Furthermore, we show that an increase in the power density capacity of the nano-antennas can be achieved by replacing gold with titanium: more than 18 dB greater power density can be handled by titanium based nano-antennas without significant reduction in their electric field enhancement capabilities.

Merging Photonic Wire Lasers and Nanoantennas

One of the main goals of photonic integration is to combine different components that are capable of executing different functions. One of these functions is the generation of light: in this sense, photonic wire lasers may become a key component in future generations of integrated circuits because of their small footprints. Another is the generation of high-intensity electric fields that can be used to excite nonlinear effects, such as surface-enhanced Raman scattering, or to visualize nano-objects, in small regions and can be achieved by using plasmonic nanoantennas. In this paper, the combination of photonic wire lasers and plasmonic nanoantennas is examined. We show that a very compact photonic wire nanoantenna laser, which generates a high-intensity electric field inside the nanoantenna, can be produced.

Charnia-like broadband plasmonic nano-antenna

Charnia was a pre-Cambrian life-form that exhibited a fractal structure to improve the extraction of nutrients from the pre-historic seas. Inspired by its fractal structure, this paper studies the potential application of these self-similarity fractal structures to create a plasmonic nano-antenna that can operate over a large linewidth. These devices are studied both theoretically and experimentally. It is shown that these nano-antennas can produce electric field enhancements above 8 over 200 nm range and surface enhanced Raman scattering (SERS) enhancements higher than 105.