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 just accepted (Mar 2019) [ pdf ] [ ref ]

A 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.

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 ]

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.

R. L. Milot, M. T. Klug, C. L. Davies, Z. Wang, H. Kraus, H. J. Snaith, M. B. Johnston, and L. M. Herz
Advanced Materials (Sept 2018) [ pdf ] [ ref ]

Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI₃) thin films, whose background hole doping density is varied through SnF₂ addition during film fabrication. Monomolecular charge‐carrier recombination exhibits both a dopant‐mediated part that grows linearly with hole doping density and remnant contributions that remain under tin‐enriched processing conditions. At hole densities near 10²⁰ cm⁻³, a strong Burstein–Moss effect increases absorption onset energies by ≈300 meV beyond the bandgap energy of undoped FASnI₃ (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (10²⁰ cm⁻³), temperature‐dependent measurements indicate that the effective charge‐carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 10¹⁹ cm⁻³ and below, the charge‐carrier mobility increases with decreasing temperature according to ≈T^−1.2, suggesting that it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2 × 10¹⁸ cm⁻³, charge‐carrier mobilities reach a value of 67 cm² V⁻¹ s⁻¹ at room temperature and 470 cm² V⁻¹ s⁻¹ at 50 K. Intraexcitonic transitions observed in the THz‐frequency photoconductivity spectra at 5 K reveal an exciton binding energy of only 3.1 meV for FASnI₃, in agreement with the low bandgap energy exhibited by this perovskite.

M. Monti, S. Tao, M. Staniforth, A. Crocker, E. Griffin, A. Wijesekara, R.A. Hatton, and J. Lloyd-Hughes
J. Phys. Chem. C 122 20669 (Aug 2018) [ pdf ] [ ref ]

The dynamic increase in THz photoconductivity resulting from energetic intraband relaxation was used to track the formation of highly mobile charges in thin films of the tin iodide perovskite Cs_1-xRb_xSnI₃, with x=0 and x=0.1. Energy relaxation times were found to be around 500fs, comparable to those in the prototypical inorganic semiconductor GaAs. At low excess energies the efficient intraband energy relaxation in the lowest conduction and valence bands of Cs_1-xRb_xSnI₃ can be understood within the context of the Fröhlich electron-phonon interaction, with a strong coupling strength. For higher excess energies the photoconductivity rise time lengthens in accordance with carrier injection into multiple bands, identified by quantitative first-principles bandstructure calculations and photoluminescence spectroscopy. The findings contribute to the development of design rules for photovoltaic devices capable of extracting hot carriers from perovskite semiconductors.