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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

Recent news from WCUS

   

Recent publications from the group

Ionic liquid gated carbon nanotube saturable absorber for switchable pulse generation

Y. Gladush, A. Mkrtchyan, D. Kopylova, A. Ivanenko, B. Nyushkov, S. Kobtsev, A. Kokhanovskiy, A. Khegai, M. Melkumov, M.G. Burdanova, M. Staniforth, J. Lloyd-Hughes and A.G. Nasibulin
Nano Letters just accepted (Aug 2019) [ ref ]

Ionic liquid gated carbon nanotube saturable absorberMaterials with electrically tunable optical properties offer a wide range of opportunities for photonic applications. The optical properties of the single-walled carbon nanotubes (SWCNTs) can be significantly altered in the near infrared region by means of electrochemical doping. The states’ filling, which is responsible for the optical absorption suppression under doping, also alters the nonlinear optical response of the material. Here, for the first time, we report that the electrochemical doping can tailor the nonlinear optical absorption of SWCNT films and demonstrate its application to control pulsed fiber laser generation. With a pump-probe technique we show that under an applied voltage below 2 V the photo-bleaching of the material can be gradually reduced and even turned to photo-induced absorption. Furthermore, we integrated a carbon nanotube electrochemical cell on a side-polished fiber to tune the absorption saturation and implemented it into the fully polarization-maintaining fiber laser. We show that the pulse generation regime can be reversibly switched between femtosecond mode locking and microsecond Q-switching using different gate voltages. This approach paves the road towards carbon nanotube optical devices with tunable nonlinearity.

Fri 02 Aug 2019, 00:10 | Tags: nanomaterials, Lloyd-Hughes, 2019

Giant negative terahertz photoconductivity in controllably doped carbon nanotube networks

M.G. Burdanova, A.P. Tsapenko, D.A. Satco, R.J. Kashtiban, C.D.W. Mosley, M. Monti, M. Staniforth, J. Sloan, Y. Gladush, A.G. Nasibulin and J. Lloyd-Hughes
ACS Photonics 6 1058 (Mar 2019) [ free e-print ] [ preprint pdf ] [ ref ]

Negative photoconductivity in carbon nanotubesA strong negative photoconductivity was identified in thin film networks of single-walled carbon nanotubes using optical pump, THz probe spectroscopy. The films were controllably doped, using either adsorption doping with different p-type dopant concentrations, or ambipolar doping using an ionic gate. While doping enhanced the THz conductivity and increased the momentum scattering rate, interband photoexcitation lowered the spectral weight and reduced the momentum scattering rate. This negative THz photoconductivity was observed for all doping levels, regardless of the chemical potential, and decayed within a few picoseconds. The strong many-body interactions inherent to these 1D conductors led to trion formation under photoexcitation, lowering the overall conductivity of the carbon nanotube network. The large amplitude of negative THz photoconductivity and the tunability of its recovery time with doping offer promise for spectrally wide-band ultrafast devices including THz detectors, polarizers and modulators.

Sun 17 Mar 2019, 07:40 | Tags: THz spectroscopy, nanomaterials, 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

Conduction properties of thin films from a water soluble carbon nanotube/hemicellulose complex

D. Shao, P. Yotprayoonsak, V. Saunajoki, M. Ahlskog, J. Virtanen, V. Kangas, A. Volodin, C. Van Haesendonck, M. Burdanova, C. D. W. Mosley and J. Lloyd-Hughes
Nanotechnology 29 145203 (Feb 2018) [ pdf ] [ ref ]

Complex THz conductivity of a 130nm thick DWNT/xylan compositeWe have examined the conductive properties of a carbon nanotube (CNT) based thin film, which were prepared via dispersion in water by non-covalent functionalization of the nanotubes with xylan, a type of hemicellulose. Measurements of low temperature conductivity, Kelvin Probe Force Microscopy, and high frequency (THz) conductivity elucidated the intra-tube and inter-tube charge transport processes in this material. The measurements show excellent conductive properties of the as prepared thin films, with bulk conductivity up to 2000 S/cm. The transport results demonstrate that the hemicellulose does not seriously interfere with the inter-tube conductance.

Tue 06 Feb 2018, 13:50 | Tags: THz spectroscopy, 2018, nanomaterials, Lloyd-Hughes