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Electro-optic sampling allows the electric field of THz, mid-infrared and visible light pulses to be measured directly as a function of time, with data analysis often performed in the frequency domain after fast Fourier transform. Here we introduce a version of the discrete Fourier transform that is consistent with the analytical continuous Fourier transform, enabling the quantitative study of the amplitude and phase of light pulses without the phase ambiguity inherent in the conventional definition of the discrete Fourier transform. We then show that a phenomenological time-domain model, based on carrier and envelope waves, that can reproduce the wide variety of narrow-band and broadband multi- to single-cycle THz pulses reported in the literature, such as from spintronic emitters, photoconductive antennae, optical rectification and free electron lasers. We discuss the influence of the pulse envelope's duration, the carrier-envelope phase and the pulse arrival time on the complex spectra. The methodology and results described provide a valuable toolkit that unlocks the full potential of the phase information available in time-domain spectroscopy., Comment: 14 pages, 5 figures

In perovskite solar cells, photovoltaic action is created by charge transport layers (CTLs) either side of the light-absorbing metal halide perovskite semiconductor. Hence, the rates for desirable charge extraction and unwanted interfacial recombination at the perovskite-CTL interfaces play a critical role for device efficiency. Here, the electrical properties of perovskite-CTL bilayer heterostructures are obtained using ultrafast THz and optical studies of the charge carrier dynamics after pulsed photoexcitation, combined with a physical model of charge carrier transport that includes the prominent Coulombic forces that arise after selective charge extraction into a CTL, and cross-interfacial recombination. The charge extraction velocity at the interface and the ambipolar diffusion coefficient within the perovskite are determined from the experimental decay profiles for heterostructures with three of the highest performing CTLs, namely C$_{60}$, PCBM and Spiro-OMeTAD. Definitive targets for the further improvement of devices are deduced: fullerenes deliver fast electron extraction, but suffer from a large rate constant for cross-interface recombination or hole extraction. Conversely, Spiro-OMeTAD exhibits slow hole extraction but does not increase the perovskite's surface recombination rate, likely contributing to its success in solar cell devices., Comment: 33 pages, 6 figures

Off-axis parabolic mirrors (OAPMs) are widely used in the THz and mm-wave communities for spectroscopy and imaging applications, as a result of their broadband, low-loss operation and high numerical apertures. However, the aspherical shape of an OAPM creates significant geometric aberrations that make achieving diffraction-limited performance a challenge, and which lowers the peak electric field strength in the focal plane. Here we quantify the impact of geometric aberrations on the performance of the most widely-used spectrometer designs, by using ray tracing and physical optics calculations to investigate whether diffraction-limited performance can be achieved in both the sample and the detector plane. We identify simple rules, based on marginal ray propagation, that allow spectrometers to be designed that are more robust to misalignment errors, and which have minimal aberrations for THz beams. For a given source this allows the design of optical paths that give the smallest THz beam focal spot, with the highest THz electric field strength possible. This is desirable for improved THz imaging, for better signal-to-noise ratios in linear THz spectroscopy and optical-pump THz-probe spectroscopy, and to achieve higher electric field strengths in non-linear THz spectroscopy, Comment: 12 pages and 8 figures, To appear in the Journal of Infrared, Millimeter, and Terahertz Waves

Measuring terahertz (THz) conductivity on an ultrafast time scale is an excellent way to observe charge-carrier dynamics in semiconductors as a function of time after photoexcitation. However, a conductivity measurement alone cannot separate the effects of charge-carrier recombination from effective mass changes as charges cool and experience different regions of the electronic band structure. Here we present a form of time-resolved magneto-THz spectroscopy which allows us to measure cyclotron effective mass on a picosecond time scale. We demonstrate this technique by observing electron cooling in the technologically-significant narrow-bandgap semiconductor indium antimonide (InSb). A significant reduction of electron effective mass from 0.032$m_\mathrm{e}$ to 0.017$m_\mathrm{e}$ is observed in the first 200ps after injecting hot electrons. Measurement of electron effective mass in InSb as a function of photo-injected electron density agrees well with conduction band non-parabolicity predictions from ab initio calculations of the quasiparticle band structure.

Full control of the ellipticity of broadband pulses of THz radiation, from linear to left- or right-handed circular polarization, was demonstrated via a 4-pixel photoconductive emitter with an integrated achromatic waveplate. Excellent polarization purity and accuracy was achieved, with Stokes parameters exceeding 97% for linear and circular polarization, via a robust scheme that corrected electrically for polarization changes caused by imperfect optical elements. Further, to assess the speed and precision of measurements of the THz polarization we introduced a new figure of merit, the standard error after one second of measurement, found to be 0.047$^{\circ}$ for the polarization angle., Comment: 5 pages, 4 figures, 1 table

Excitons dominate the optics of atomically-thin transition metal dichalcogenides and 2D van der Waals heterostructures. Interlayer 2D excitons, with an electron and a hole residing in different layers, form rapidly in heterostructures either via direct charge transfer or via Coulomb interactions that exchange energy between layers. Here, we report prominent intertube excitonic effects in quasi-1D van der Waals heterostructures consisting of C/BN/MoS$_2$ core/shell/skin nanotubes. Remarkably, under pulsed infrared excitation of excitons in the semiconducting CNTs we observed a rapid (sub-picosecond) excitonic response in the visible range from the MoS$_2$ skin, which we attribute to intertube biexcitons mediated by dipole-dipole Coulomb interactions in the coherent regime. On longer ($>100$ps) timescales hole transfer from the CNT core to the MoS$_2$ skin further modified the MoS$_2$'s absorption. Our direct demonstration of intertube excitonic interactions and charge transfer in 1D van der Waals heterostructures suggests future applications in infrared and visible optoelectronics using these radial heterojunctions., Comment: 4 figures

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 (0.3-5.0THz) 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., Comment: 12 pages, 4 figures

We demonstrate that electromagnons can be used to directly probe the nature of a phase transition between magnetically ordered phases in an improper ferroelectric. The antiferromagnetic/paraelectric to antiferromagnetic/ferroelectric phase transition in Cu$_{1-x}$Zn$_{x}$O ($x=0, 0.05$) alloys was tracked via the electromagnon response using terahertz time-domain spectroscopy, on heating and cooling through the phase transition. The transition was found to exhibit thermal hysteresis, confirming its first-order nature, and to broaden under the influence of spin-disorder upon Zn substitution. The energy of the electromagnon increases upon alloying, as a result of the non-magnetic ions modifying the magnetic interactions that give rise to the multiferroic phase and electromagnons. We describe our findings in the context of recent theoretical work that examined improper ferroelectricity and electromagnons in CuO from phenomenological and first-principles approaches., Comment: 11 pages, 5 figures

We probe ultra-strong light matter coupling between metallic terahertz metasurfaces and Landau-level transitions in high mobility 2D electron and hole gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron cyclotron resonances in InSb quantum wells, both within highly non-parabolic bands, and compare our results to well known parabolic AlGaAs/GaAs quantum well (QW) systems. Tuning the coupling strength of the system by two methods, lithographically and by optical pumping, we observe a novel behavior clearly deviating from the standard Hopfield model previously verified in cavity quantum electrodynamics: an opening of a lower polaritonic gap.

We have investigated the time-resolved vibrational properties of terahertz quantum cascade lasers by means of ultra-fast laser spectroscopy. By the observation of the acoustic folded branches, and by analyzing the involved phonon modes it is possible to extract accurate structural information of these devices, which are essential for their design and performance., Comment: 8 pages, 5 figures

The carrier dynamics of photoexcited electrons in the vicinity of the surface of (NH4)2S-passivated GaAs were studied via terahertz (THz) emission spectroscopy and optical-pump THz-probe spectroscopy. THz emission spectroscopy measurements, coupled with Monte Carlo simulations of THz emission, revealed that the surface electric field of GaAs reverses after passivation. The conductivity of photoexcited electrons was determined via optical-pump THz-probe spectroscopy, and was found to double after passivation. These experiments demonstrate that passivation significantly reduces the surface state density and surface recombination velocity of GaAs. Finally, we have demonstrated that passivation leads to an enhancement in the power radiated by photoconductive switch THz emitters, thereby showing the important influence of surface chemistry on the performance of ultrafast THz photonic devices., Comment: 4 pages, 3 figures, to appear in Applied Physics Letters

Terahertz time-domain spectroscopy and scanning probe potentiometry were used to investigate charge trapping in polymer field-effect transistors fabricated on a silicon gate. The hole density in the transistor channel was determined from the reduction in the transmitted terahertz radiation under an applied gate voltage. Prolonged device operation creates an exponential decay in the differential terahertz transmission, compatible with an increase in the density of trapped holes in the polymer channel. Taken in combination with scanning probe potentionmetry measurements, these results indicate that device degradation is largely a consequence of hole trapping, rather than of changes to the mobility of free holes in the polymer., Comment: 4 pages, 3 figures

We have developed a detector which records the full polarisation state of a terahertz (THz) pulse propagating in free space. The three-electrode photoconductive receiver simultaneously records the electric field of an electromagnetic pulse in two orthogonal directions as a function of time. A prototype device fabricated on Fe+ ion implanted InP exhibited a cross polarised extinction ratio better than 390:1. The design and optimisation of this device are discussed along with its significance for the development of new forms of polarisation sensitive time domain spectroscopy, including THz circular dichroism spectroscopy., Comment: 7 pages, 6 figures, conference

We have used a three-dimensional pseudo-classical Monte Carlo simulation to investigate the effects of As+ ionimplantation on pulsed terahertz radiation emitters. Devices based on surface-field emitters and photoconductive switches have been modelled. Two implantations of As+ ions at 1.0 MeV and 2.4 MeV were found to produce a uniform distribution of vacancies over the volume of GaAs contributing to THz generation in these devices. We calculate that ionimplantation increases the THz bandwidth of the devices with the cost of decreasing the spectral intensity at lower THz frequencies., Comment: 2 pages, 3figures, conference abstract

We have developed a detector of coherent terahertz (THz) radiation that can recover the full polarisation state of a THz transient. The device is three-contact photoconductive receiver, which is capable of recording two time-varying electric field components of a THz pulse simultaneously. Our receiver was fabricated on Fe+ implanted InP and showed a cross-polarised extinction ratio greater than 100:1. The detector will be useful for spectroscopy of birefringent and optically active materials., Comment: 2 pages, 3 figures, conference abstract

To perform polarization sensitive THz-TDS, it is necessary to be able to measure two (preferably orthogonal) electric field components of a terahertz transient. We have developed a polarization sensitive detector by fabricating a three-contact photoconductive receiver., Comment: Conference, 1 page, 1 figure

We have developed a terahertz radiation detector that measures both the amplitude and polarization of the electric field as a function of time. The device is a three-contact photoconductive receiver designed so that two orthogonal electric-field components of an arbitrary polarized electromagnetic wave may be detected simultaneously. The detector was fabricated on Fe+ ion-implanted InP. Polarization-sensitive detection is demonstrated with an extinction ratio better than 100:1. This type of device will have immediate application in studies of birefringent and optically active materials in the far-infrared region of the spectrum., Comment: 3 pages, 3 figures

A semi-classical Monte Carlo model for studying three-dimensional carrier dynamics in photoconductive switches is presented. The model was used to simulate the process of photoexcitation in GaAs-based photoconductive antennas illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed the power and frequency bandwidth of THz radiation emitted from these devices as a function of bias voltage, pump pulse duration and pump pulse location. We show that the mechanisms limiting the THz power emitted from photoconductive switches fall into two regimes: when illuminated with short duration (<40 fs) laser pulses the energy distribution of the Gaussian pulses constrains the emitted power, while for long (>40 fs) pulses, screening is the primary power-limiting mechanism. A discussion of the dynamics of bias field screening in the gap region is presented. The emitted terahertz power was found to be enhanced when the exciting laser pulse was in close proximity to the anode of the photoconductive emitter, in agreement with experimental results. We show that this enhancement arises from the electric field distribution within the emitter combined with a difference in the mobilities of electrons and holes., Comment: 7 pages, 7 figures

We simulate the terahertz emission from laterally-biased InGaAs and InP using a three-dimensional carrier dynamics model in order to optimise the semiconductor material. Incident pump-pulse parameters of current Ti:Sapphire and Er:fibre lasers are chosen, and the simulation models the semiconductor's bandstructure using parabolic Gamma, L and X valleys, and heavy holes. The emitted terahertz radiation is propagated within the semiconductor and into free space using a model based on the Drude-Lorentz dielectric function. As the InGaAs alloy approaches InAs an increase in the emitted power is observed, and this is attributed to a greater electron mobility. Additionally, low-temperature grown and ion-implanted InGaAs are modelled using a finite carrier trapping time. At sub-picosecond trapping times the terahertz bandwidth is found to increase significantly at the cost of a reduced emission power., Comment: 9 pages, 7 figures