Topological materials have rapidly gained interest as contenders for development of coherent, controllable terahertz emitters. Possessing Weyl nodes either at the surface or within the bulk, they host spin-polarised, helicity-dependent currents that offer possibility to control the emitted THz pulse by changing the polarization of the optical pulses generating the radiation. Here, we show that upon near-infrared excitation at oblique incidence, multi-cycle pulses are generated with a narrow bandwidth of ∼0.4 THz for cadmium arsenide bulk crystals and nanowire ensembles. Both the bandwidth and peak emission frequency of the generated THz radiation can be tuned by respectively varying the photon helicity and angle of incidence of the photoexcitation light.
THz polarimetry is an emerging area that has applications in materials imaging and spectroscopy. Thus, there is a growing need for compact sensors with the ability to record complete polarization information from few-cycles pulses of THz radiation. We have exploited the unique geomatical and electrical properties of semiconductor nanowires to develop monolithic polarization-resolving THz sensors. We demonstrate these sensors in a range of applications from the analysis of polarization-manipulating metamaterials to semiconductor characterization and THz imaging. Our nanowire platform also shows promise for other THz photonic devices.
Semiconductor Nanowires (NWs) have revolutionized photonics by providing minimal footprint optoelectronic devices and coherent light sources. However, given their nanoscale dimensions, their integration with nanophotonic systems is a significant challenge. To overcome this issue, we have developed a hybrid nanofabrication technique, known as nanoscale transfer-printing, permitting the accurate integration of individually-selected NWs at target positions onto desired surfaces. Examples of nanophotonic systems enabled by our technique include 1D/2D NW laser arrays and on-chip waveguide-coupled NW laser systems. We have also recently demonstrated a nanophotonic circuit for THz signal detection formed by a 3D semiconductor NW network coupled with a metallic antenna structure.
III–V semiconductor nanowires combine the properties of III–V materials with the unique advantages of the nanowire geometry, allowing efficient room temperature photodetection across a wide range of photon energies, from a few eV down to meV. For example, due to their nanoscale size, these show great promise as sub-wavelength terahertz (THz) detectors for near-field imaging or detecting elements within a highly integrated on-chip THz spectrometer. We discuss recent advances in engineering a number of sensitive photonic devices based on III–V nanowires, including InAs nanowires with tunable photoresponse, THz polarisers and THz detectors.
Optical pump-terahertz probe spectroscopy is a powerful contact-free technique for probing the electronic properties of novel nanomaterials and their response to photoexcitation. This technique can measure charge carrier transport and dynamics with sub-picosecond temporal resolution. Electrical conductivity, charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities can be measured with high accuracy and with considerably higher throughput than achievable with traditional contact-based techniques. We describe how terahertz spectroscopy is revealing the fascinating properties and guiding the development of a number of promising semiconductor materials, with particular emphasis on III-V semiconductor nanowires and devices.
Nanowires show unique promise as nanoscale building blocks for a multitude of optoelectronic devices, ranging from solar cells to terahertz photonic devices. We will discuss the epitaxial growth of these nanowires in novel geometries and crystallographic phases, and the use of terahertz conductivity spectroscopy to guide the development of nanowire-based devices. As an example, we will focus on the development of nanowire-based polarization modulators for terahertz communications systems.
Hybrid metal halide perovskites are attractive components for many optoelectronic applications due to a combination of their superior charge transport properties and relative ease of fabrication.
A complete understanding of the nature of charge transport in these materials is therefore essential for current and future device development. We have evaluated two systems – the standard perovskite methylammonium lead triiodide (CH3NH3PbI3) and a series of mixed-iodide/bromide formamidinium lead perovskites – in an effort to determine what effect structural and chemical composition have on optoelectronic properties including mobility, charge-carrier recombination dynamics, and charge-carrier diffusion length.
The photoconductivity in thin films of CH3NH3PbI3was investigated from 8 K to 370 K across three structural phases [1]. While the monomolecular charge-carrier recombination rate was found to increase with rising temperature indicating a mechanism dominated by ionized impurity mediated recombination, the bimolecular rate constant decreased with rising temperature as charge-carrier mobility declined. The Auger rate constant was highly phase specific, suggesting a strong dependence on electronic band structure.
For the mixed-halide formamidinuim lead bromide-iodide perovskites, HC(NH2)2Pb(BryI1–y)3, bimolecular and Auger charge-carrier recombination rate constants strongly correlated with bromide content, which indicated a link with electronic structure [2]. Although HC(NH2)2PbBr3 and HC(NH2)2PbI3 exhibited high charge-carrier mobilities and diffusion lengths exceeding 1 μm, mobilities for mixed Br/I perovskites were all lower as a result of crystalline phase disorder.
Accurately measuring the electronic properties of nanowires is a crucial step in the development of novel semiconductor nanowire-based devices. With this in mind, optical pump–terahertz probe (OPTP) spectroscopy is ideally suited to studies of nanowires: it provides non-contact measurement of carrier transport and dynamics at room temperature. OPTP spectroscopy has been used to assess key electrical properties, including carrier lifetime and carrier mobility, of GaAs, InAs and InP nanowires. The measurements revealed that InAs nanowires exhibited the highest mobilities and InP nanowires exhibited the lowest surface recombination velocity.
Quantitative measurement based on THz absorption spectrum is of great importance in THz applications. Several
researchers have worked on it and gained some achievements, but most of them explored pure component or no more
than 2-component s samples. In this paper, a mixture sample consisting of Glutamine, Histidine and Threonine is
investigated in the frequency range from 0.3 to 2.6 THz. The quantitative measurement principle is the Lambert-Beer's
Law which have been accepted in infrared and visible spectra. Our experiments show the validation of the law in THz
region. A Least-Mean-Square algorithm is adopted and measurement errors of Glutamine, Histidine and Threonine are
17.60%, 4.44% and 2.59%.
Using terahertz time-domain spectroscopy we investigate how quantum, magnetic and electrostatic confinement
alters the photoconductivity of nanostructured semiconductors. In 2.0 THz and 2.9THz GaAs/AlGaAs quantum
cascade structures under a magnetic field we observe transitions from the 1s to 2p- or 2p+ magneto-exciton
states. The electron cyclotron resonance is prominent at high excitation fluence. Additionally, we report that
the conductivity of photoexcited electrons in nanoporous InP honeycombs obeys the Drude model of free-carrier
absorption, while the dark conductivity does not. This finding can be explained as a result of surface band bending
spatially separating photoexcited electrons and holes, and also accounts for the long electron recombination
lifetime (exceeding 100 ns) at low temperature.
We have developed a detector which records the full polarization 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 polarized extinction ratio better than 390:1. The design and optimization of
this device are discussed along with its significance for the development of new forms of polarization sensitive
time domain spectroscopy, including THz circular dichroism spectroscopy.
We have experimentally measured the terahertz radiation from a series of ion-implanted semiconductors, both from the bare semiconductor surface and from photoconductive switches fabricated on them. GaAs was implanted with arsenic ions, and InGaAs and InP with Fe+ iron ions, and all samples were annealed post implantation. An increase in emission power is observed at high frequencies, which we attribute to the ultrafast trapping of carriers. We use a three-dimensional carrier dynamics simulation to model the emission process. The simulation accurately predicts the experimentally observed bandwidth increase, without resorting to any fitting parameters. Additionally, we discuss intervalley scattering, the influence of space-charge fields, and the relative performance of InP, GaAs and InAs based photoconductive emitters.
A comparison has been made of the shifts induced in the photoluminescence (PL) emission wavelength of a GaAs/AlGaAs multiple quantum well (QW) structure following irradiation with H, He and As ions. Ions energies and fluences were chosen to produce matching numbers and distributions of lattice atom displacements across the structure. Samples were then annealed at 900 degrees C for 30s to intermix the QWs and low temperature photoluminescence as used to measure the shifts in the QW bandgap energies. At common concentration of atomic displacements, the PL blueshift increased with the mass of the implanted ion. For these anneal parameters, saturation of the blueshift from the narrowest QW was observed in all three irradiation at an average vacancy production concentration of approximately 1022 cm-3. No significant difference in PL shifts was found when the irradiations were performed at 200 degrees C sample temperature.
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