Sarwat A. Baig

Biomimetic Reflectors Fabricated Using Self-Organising, Self-Aligning Liquid Crystal Polymers

The photograph shows a polymer reflector that mimics the colour and underlying molecular structure of a golden beetle. It is formed from self-organizing layers of photopolymerised liquid crystal. These require an aligning layer, but we show that a layer of the material can be used as to self-align subsequent coatings, enabling the construction of complex structures by sequential coating of engineered materials.

A review of the electrical properties of semiconductor nanowires: insights gained from terahertz conductivity spectroscopy

© 2016 IOP Publishing Ltd. Accurately measuring and controlling the electrical properties of semiconductor nanowires is of paramount importance in the development of novel nanowire-based devices. In light of this, terahertz (THz) conductivity spectroscopy has emerged as an ideal non-contact technique for probing nanowire electrical conductivity and is showing tremendous value in the targeted development of nanowire devices. THz spectroscopic measurements of nanowires enable charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities to be measured with high accuracy and high throughput in a contact-free fashion. This review spans seminal and recent studies of the electronic properties of nanowires using THz spectroscopy. A didactic description of THz time-domain spectroscopy, optical pump-THz probe spectroscopy, and their application to nanowires is included. We review a variety of technologically important nanowire materials, including GaAs, InAs, InP, GaN and InN nanowires, Si and Ge nanowires, ZnO nanowires, nanowire heterostructures, doped nanowires and modulation-doped nanowires. Finally, we discuss how THz measurements are guiding the development of nanowire-based devices, with the example of single-nanowire photoconductive THz receivers.

An Ultrafast Switchable Terahertz Polarization Modulator Based on III–V Semiconductor Nanowires

Progress in the terahertz (THz) region of the electromagnetic spectrum is undergoing major advances, with advanced THz sources and detectors being developed at a rapid pace. Yet, ultrafast THz communication is still to be realized, owing to the lack of practical and effective THz modulators. Here, we present a novel ultrafast active THz polarization modulator based on GaAs semiconductor nanowires arranged in a wire-grid configuration. We utilize an optical pump-terahertz probe spectroscopy system and vary the polarization of the optical pump beam to demonstrate ultrafast THz modulation with a switching time of less than 5 ps and a modulation depth of -8 dB. We achieve an extinction of over 13% and a dynamic range of -9 dB, comparable to microsecond-switchable graphene- and metamaterial-based THz modulators, and surpassing the performance of optically switchable carbon nanotube THz polarizers. We show a broad bandwidth for THz modulation between 0.1 and 4 THz. Thus, this work presents the first THz modulator which combines not only a large modulation depth but also a broad bandwidth and picosecond time resolution for THz intensity and phase modulation, making it an ideal candidate for ultrafast THz communication.

Electrowetting pixels with improved transmittance using dye doped liquid crystals

Electrowetting display pixels have been created using a dye doped liquid crystal as the dielectric liquid in a simple electrowetting architecture. In addition to electrowetting, the dye doped liquid crystal reorients, giving two mechanisms to modulate the light. We show that realignment of the liquid crystal, due to the electric field, occurs both before and during electrowetting. The transmission of the pixel has been compared to the transmission of a pixel containing an isotropic liquid, using a simple mathematical model, and we show that electrical realignment of the LC improves the transmission of the pixel. We show a 6.8% gain in the transmission during electrowetting, and before electrowetting occurs.

Engineering III-V nanowires for optoelectronics: from epitaxy to terahertz photonics

Nanowires show unique promise as nanoscale building blocks for a multitude of optoelectronic devices, ranging from solar cells to terahertz photonic devices. We will discuss the epitaxial growth of these nanowires in novel geometries and crystallographic phases, and the use of terahertz conductivity spectroscopy to guide the development of nanowire-based devices. As an example, we will focus on the development of nanowire-based polarization modulators for terahertz communications systems.

Probing the photophysics of semiconductor nanomaterials using optical pump-terahertz probe spectroscopy: from nanowires to perovskites

Optical pump-terahertz probe spectroscopy is a powerful contact-free technique for probing the electronic properties of novel nanomaterials and their response to photoexcitation. This technique can measure charge carrier transport and dynamics with sub-picosecond temporal resolution. Electrical conductivity, charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities can be measured with high accuracy and with considerably higher throughput than achievable with traditional contact-based techniques. We describe how terahertz spectroscopy is revealing the fascinating properties and guiding the development of a number of promising semiconductor materials, with particular emphasis on III-V semiconductor nanowires and devices.

Semiconductor nanowires in terahertz photonics: From spectroscopy to ultrafast nanowire-based devices

Nanowires show unique promise for a multitude of optoelectronic devices, ranging from solar cells to terahertz (THz) photonic devices. Here, we discuss how THz spectroscopy is guiding the development of such nanowire-based devices. As an example, we focus on developing nanowire-based THz polarization modulators.