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Ultrafast & Terahertz Photonics Group


Warwick Centre for Ultrafast Spectroscopy

We are part of WCUS, a joint activity between the Physics and Chemistry Departments at the University of Warwick.

Warwick Centre for Ultrafast Spectroscopy

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Recent publications from the group

Scalable interdigitated photoconductive emitters for the electrical modulation of terahertz beams with arbitrary linear polarization

C.D.W. Mosley, M. Staniforth, A. I. Hernandez Serrano, E. Pickwell-MacPherson and J. Lloyd-Hughes
AIP Advances 9, 045323 (Apr 2019) [ pdf ] [ ref ]

A multi-element interdigitated photoconductive emitter for broadband THz polarization rotation is proposed and experimentally verified. The device consists of separate pixels for the emission of horizontally and vertically polarized THz radiation. The broadband (0.3–5.0 THz) nature of the device is demonstrated, and the polarization angle of the generated far-field THz radiation is shown to be readily controlled by varying the relative bias voltage applied to the horizontally and vertically emitting pixels. The device is scalable in design, and with its simple method of polarization rotation it allows the modulation of the generated THz polarization at rates significantly faster than those achievable in ellipsometry systems based on mechanically rotating components.

Fri 26 Apr 2019, 19:08 | Tags: THz components, MacPherson, Lloyd-Hughes, 2019

Design and fabrication of 3-D printed conductive polymer structures for THz polarization control

A.I. Hernandez-Serrano, Q. Sun, E.G. Bishop, E.R. Griffiths, C.P. Purssel, S.J. Leigh, J. Lloyd-Hughes and E. Pickwell-MacPherson
Optics Express 27 8 11635 (April 2019) [ pdf ] [ ref ]

arturo2019.jpg

In this paper, we numerically and experimentally demonstrate the inverse polarization effect in three-dimensional (3-D) printed polarizers for the frequency range of 0.5 - 2.7 THz. The polarizers simply consist of 3-D printed strip lines of conductive polylactic acid (CPLA, Proto-Pasta) and do not require a substrate or any further metallic deposition. The experimental and numerical results show that the proposed structure acts as a broadband polarizer between the range of 0.3 THz to 2.7 THz, in which the inverse polarization effect is clearly seen for frequencies above 0.5 THz. In the inverse polarization effect, the transmission of the transverse electric (TE) component exceeds that of the TM component, in contrast to the behavior of a typical wire-grid polarizer. We show how the performance of the polarizers depends on the spacing and thickness of the CPLA structure; extinction ratios higher than 20 dB are achieved. This is the first report using CPLA to fabricate THz polarizers, demonstrating the potential of using conductive polymers to design THz components efficiently and robustly.

Thu 11 Apr 2019, 16:40 | Tags: THz components, MacPherson, Lloyd-Hughes, 2019

Graphene controlled Brewster angle device for ultra broadband terahertz modulation

Z. Chen, X. Chen, L. Tao, K. Chen, M. Long, K. Yan, R.I. Stantchev, E. Pickwell-MacPherson & J.-B. Xu
Nature Communications 9 4909 (November 2018) [ pdf ] [ ref ]

Chen 2018

Terahertz modulators with high tunability of both intensity and phase are essential for effective control of electromagnetic properties. Due to the underlying physics behind existing approaches there is still a lack of broadband devices able to achieve deep modulation. Here, we demonstrate the effect of tunable Brewster angle controlled by graphene, and develop a highly-tunable solid-state graphene/quartz modulator based on this mechanism. The Brewster angle of the device can be tuned by varying the conductivity of the graphene through an electrical gate. In this way, we achieve near perfect intensity modulation with spectrally flat modulation depth of 99.3 to 99.9 percent and phase tunability of up to 140 degree in the frequency range from 0.5 to 1.6 THz. Different from using electromagnetic resonance effects (for example, metamaterials), this principle ensures that our device can operate in ultra-broadband. Thus it is an effective principle for terahertz modulation.

Thu 29 Nov 2018, 07:41 | Tags: THz components, 2018, nanomaterials, MacPherson

Terahertz spectroscopy of anisotropic materials using beams with rotatable polarization

C. D. W. Mosley, M. Failla, D. Prabhakaran and J. Lloyd-Hughes
Scientific Reports 7:12337 (Sept 2017) [ pdf ][ ref ]

WeRotatable terahertz polarisation introduce a polarization-resolved terahertz time-domain spectrometer with a broadband (0.3-2.5THz), rotatable THz polarization state, and which exhibits minimal change in the electric field amplitude and polarization state upon rotation. This was achieved by rotating an interdigitated photoconductive emitter, and by detecting the orthogonal components of the generated THz pulse via electro-optic sampling. The high precision (<0.1°) and accuracy (<1.0°) of this approach is beneficial for the study of anisotropic materials without rotating the sample, which can be impractical, for instance for samples held in a cryostat. The versatility of this method was demonstrated by studying the anisotropic THz optical properties of uniaxial and biaxial oxide crystals. For uniaxial ZnO and LaAlO3, which have minimal THz absorption across the measurement bandwidth, the orientations of the eigenmodes of propagation were conveniently identified as the orientation angles that produced a transmitted THz pulse with zero ellipticity, and the birefringence was quantified. In CuO, a multiferroic with improper ferroelectricity, the anisotropic THz absorption created by an electromagnon was investigated, mapping its selection rule precisely. For this biaxial crystal, which has phonon and electromagnon absorption, the polarization eigenvectors exhibited chromatic dispersion, as a result of the monoclinic crystal structure and the frequency-dependent complex refractive index.

Wed 27 Sep 2017, 12:22 | Tags: THz spectroscopy, THz components, 2017, Lloyd-Hughes