Research

The realisation of efficient terahertz radiation sources and detectors is one of the important objectives of modern applied physics.  THz emitters and detectors have potential applications in biology, medicine, security and non-destructive in-depth imaging.  Existing THz emitters do no satisfy the requirements of these applications, and the creation  of cheap, reliable, scalable and portable sources and detectors is crucial.  Research in the Notedev consortioum aims to tackle the following four scientific and technological tasks:
 Polariton-based THz Emission

Under certain conditions, intersubband polaritons have a bosonic character; as a consequence they can undergo stimulated scattering. We propose to use this property to realize THz lasers, working without population inversion. As well, we will explore the possibility of the achievement of superradiant THz emission from the bilayer systems in which polariton states are hybridized with indirect excitons. After post growth processing and characterization, samples will be integrated with cavities for THz radiation. Such cavities are required to enhance the spontaneous emission rate for THz photons. THz cavities will be developed and modeling of realistic polaritonic emitters and THz optical components will be conducted. The experimental spectroscopy data will be analyzed and theoretical feedback will be provided for optimization and design of the proposed structures. A THz device which will be a combination of THz microcavities with quantum cascade active regions will be developed. THz electroluminescence and absorption experiments will be undertaken.




 Efficient THz  Emission


Two groups of materials will be grown by MBE (molecular beam epitaxy) and investigated: dilute bismides for interband absorption and low-temperature grown InGaAs that will be excited by 1550 nm radiation by the electron transitions from deep donor levels. In parallel, THz surface emitters made from narrow-gap InAs and InGaAs will be investigated as an alternative. The developed optoelectronic THz range components will be implemented in TDS systems activated by a femtosecond Er-fibre laser. THz emitters will be developed where the enhance the non-linear optical rectification effect will be enhanced by using MBE grown heterostructures containing InAs and InGaAs layers and by applying an external bias. For increased out- coupling of the THz radiation from the semiconductor structure, several different concepts will be investigated: corrugated surface structures, surface plasmon-polariton coupling, and illumination of the semiconductor with an interference pattern - all having a spatial period corresponding to the THz wavelength. We will also design  photoconductive THz antennae. Different semiconductor materials including GaBiAs, InGaAs, and InGaBiAs will be grown by the molecular-beam-epitaxy at reduced substrate temperatures. Additional measures such as various microstrip antennae designs and growing the photoconductive layer on a Bragg-reflector will be used for optimizing the device performance.

 Carbon Nanotubes and Graphene

Theoretical work will use the transfer matrix approach to study microcavity-embedded CNT (carbon nanotube) arrays and employ various quantum optics techniques for treating light- induced THz band gap opening in graphene as well as for superluminescence and gain calculations. We will study high-frequency Bloch oscillations in such superlattices as well as associated frequency multiplication in semiconductor CNTs with high chirality. Quasi-metallic CNTs, which have curvature-induced band gap in the THz range when electrically-biased should emit THz and mid-infrared radiation with the cut-off frequency defined by the bias voltage. Theoretical investigations of this effect will be undertaken for an array of CNTs embedded into a microcavity, as well as its experimental observation. Another scheme is based on THz transitions across the curvature-induced gap in quasi-metallic CNTs, with the population inversion created by optical excitation. A feasibility study of using graphene for THz applications will proceed by studying sub- picosecond relaxation processes in graphene using pump-probe measurements. THz absorption and the photoresponse of graphene p-n junction structures will be studied both theoretically and experimentally.


 Tunable THz Devices


Phase sensitive time- domain THz experiments in equilibrium and under applied electric bias or magnetic field and at variable temperatures will be undertaken. The objective of the research is to design and investigate tunable 2D and 3D dielectric structures for the THz spectral range based on dielectric micro- or nano-structures, including domain-patterned films and superlattices. The experiments will be supplemented with electromagnetic simulations of the response functions of the designed structures. The technological part of the work involves thin film deposition, realization of interdigitated or nanocrystalline transparent electrodes and laser micro-machining. Strain-induced multiferroics, which should exhibit giant magneto-electric coupling will be addressed. Biaxial strain in epitaxial thin films can induce the ferroelectric and ferromagnetic state in materials that are typically paraelectric and antiferromagnetic in bulk form. We will concentrate on the study of EuO, Ca3Mn2O7 and SrMnO3 thin films, which were recently predicted to be multiferroic using first principle calculations. Strained Ca3Mn2O7 thin film should even allow 180 degree switching of magnetization using an electric field.