Modulation of terahertz polarization on picosecond timescales using polymer-encapsulated semiconductor nanowires

© OSA 2017. We exploit the photoconductivity of semiconductor nanowires to achieve ultrafast broad-bandwidth modulation of THz pulses. A modulation depth of -8 dB was exhibited by a polarizer consisting of 14 layers of nanowires encapsulated in polymer

Terahertz nonlinear optics & III-V semiconductor nanowires

Most commercially available terahertz spectrometers make use of resonant and non-resonant methods to generate and detect THz radiation. This allows only the low frequency spectrum to be accessed, usually from 0.1 THz to 8 THz, severely limiting the range over which dynamics can be observed. In this thesis, the physics of non-linear optics is exploited and a state-of-the-art terahertz air-plasma generation and detection setup is showcased. THz radiation is generated through non-linear interaction of a two-colour laser field under a dry nitrogen atmosphere producing a plasma, while the detection is accomplished with an Air Biased Coherent Detection (ABCD) technique. The issue of phonon resonances, damage threshold, etalon effects and dispersion of crystals in the THz region are effectively bypassed. The system was specifically designed and built to ensure a robust experimental setup is obtained with several components being made by our physics workshop. Furthermore, significant improvements have been put in place to increase data acquisition speed and maximising signal-to-noise ratio for better results. III-V semiconductor nanowires are playing an increasingly important role in the field of condensed matter physics. From nanoscale components to highly efficient photovoltaic devices, there is a strong urge to have a solid fundamental understanding of their optoelectronic properties. The air plasma system was used to demonstrate two novel ultrafast THz modulators based on GaAs semiconductor nanowires operating at very high THz bandwidth (up to 40 THz). A significant improvement in modulation depth was achieved, up to 45%, together with picosecond modulation speed. Additionally, to further widen the study of III-V nanowires, this thesis presents a study showing a direct relation between the nanowire length and the frequency of surface plasmon resonance in the terahertz regime. Theoretical calculations have been done to further strengthen this study with the simulations revealing higher electron mobilities than the previously reported values. Finally, two smaller chapters were dedicated to the the study of the temperature-dependence of the refractive index of c-cut sapphire in the terahertz regime and the optoelectronic properties of hexagonal boron nitride (hBN) at equilibrium state. These provide fundamental insights into the potential usage of sapphire substrates as a replacement to quartz substrates as they are more robust and can support higher temperatures during growth and with the hBN laying the foundation for further avenues of research.</p

An Ultrafast Semiconducting Nanowire THz Polarization Modulator

© 2019 IEEE. In this work, we will demonstrate a novel ultrafast THz modulator based on GaAs semiconductor nanowires at very high THz bandwidth. The modulator devices were fabricated using a highly flexible and cheap substrate-parylene-c. Through the use of multiple layers of this substrate, we show that for higher layers, a higher modulation depth is achieved without affecting the bandwidth over which the devices can operate. A terahertz air-plasma setup was built which allowed us to assess the devices over a broad range bandwidth of 0.1 THz-40 THz

Unveiling the ultrafast optoelectronic properties of 3D Dirac semi-metal Cd3As2

We employ ultrafast optical-pump terahertz-probe spectroscopy and ultrafast THz emission spectroscopy to investigate the ultrafast charge carrier dynamics in the 3D Dirac semi-metal CdAs. We extract the temperature-dependent electron mobility (16,000cmVs at 5K) for CdAs nanowire ensemble. We also demonstrate strong THz emission from both CdAs single crystal and nanowires, whose polarity depends strongly on incident angle and pump polarisation

Choice of Polymer Matrix for a Fast Switchable III-V Nanowire Terahertz Modulator

Progress in ultrafast terahertz (THz) communications has been limited due to the lack of picosecond switchable modulators with sufficient modulation depth. Gallium arsenide nanowires are ideal candidates for THz modulators as they absorb THz radiation, only when photoexcited – giving the potential for picosecend speed switching and high modulation depth. By embedding the nanowires in a polymer matrix and laminating together several nanowire–polymer films, we increase the areal density of nanowires, resulting in greater modulation of THz radiation. In this paper, we compare PDMS and Parylene C polymers for nanowire encapsulation and show that a high modulation depth is possible using Parylene C due to its thinness and its ability to be laminated. We characterize the modulator behavior and switching speed using optical pump–THz probe spectroscopy, and demonstrate a parylene–nanowire THz modulator with 13.5% modulation depth and 1ps switching speed.The authors thank the EPSRC (U.K.) for financial support. H. J. Joyce thanks the Royal Commission for the Exhibition of 1851 for her research fellowship

Topological Dirac semi-metals as novel, optically-switchable, helicity-dependent terahertz sources

The generation and control of terahertz pulses is vital for realizing the potential of terahertz radiation in several sectors, including 6G communication, security and imaging. In this work, we present the topological Dirac semimetal cadmium arsenide as a novel helicity-dependent terahertz source. We show both broadband (single-cycle) and narrowband (multi-cycle) terahertz pulses upon near-infrared photoexcitation at oblique incidence. By varying the incident angle of the photoexcitation pulse, control of the emission frequency can also be achieved, providing a candidate for a tuneable narrowband terahertz source

Narrowband, angle-tuneable, helicity-dependent terahertz emission from nanowires of the topological Dirac semimetal Cd3As2

All-optical control of terahertz pulses is essential for the development of optoelectronic devices for next-generation quantum technologies. Despite substantial research in THz generation methods, polarisation control remains difficult. Here, we demonstrate that by exploiting bandstructure topology, both helicity-dependent and helicity-independent THz emission can be generated from nanowires of the topological Dirac semimetal Cd3As2. We show that narrowband THz pulses can be generated at oblique incidence by driving the system with optical (1.55 eV) pulses with circular polarisation. Varying the incident angle also provides control of the peak emission frequency, with peak frequencies spanning 0.21 – 1.40 THz as the angle is tuned from 15° - 45°. We therefore present Cd3As2 nanowires as a promising novel material platform for controllable terahertz emission

Tracking electron and hole dynamics in 3D dirac semimetals

Using ultrafast optical-pump terahertz-probe spectroscopy (OPTP) and ultrafast terahertz emission spectroscopy, we showcase the electron and hole dynamics in Cd3As2 nanowires (NWs), a well-known 3D Dirac semimetal a subgroup of the newly discovered . A temperature-dependent photoconductivity measurement was carried out yielding an incredibly high electron mobility ~ 16,000 cm2/Vs at 5K. Strong THz emission due to helicity-dependent surface photocurrents was also observed for both nanowires and single crystal (SC) which is highly desirable for devices such as THz sources

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

Engineering III-V Nanowires for optoelectronics: From visible to terahertz

We describe how optimized growth processes and contact-free electrical characterization techniques are accelerating the development of III-V nanowire-based optoelectronic devices with new and enhanced performance