Reversible Mode Switching in Y coupled Terahertz Lasers

Electrically independent terahertz (THz) quantum cascade lasers (QCLs) are optically coupled in a Y configuration. Dual frequency, electronically switchable emission is achieved in one QCL using an aperiodic grating, designed using computer-generated hologram techniques, incorporated directly into the QCL waveguide by focussed ion beam milling. Multi-moded emission around 2.9 THz is inhibited, lasing instead occurring at switchable grating-selected frequencies of 2.88 and 2.92 THz. This photonic control and switching behaviour is selectively and reversibly transferred to the second, unmodified QCL via evanescent mode coupling, without the transfer of the inherent grating losses

Fluence and polarisation dependence of GaAs based Lateral Photo-Dember terahertz emitters

We characterise THz output of lateral photo-Dember (LPD) emitters based on semi-insulating (SI), unannealed and annealed low temperature grown (LTG) GaAs. Saturation of THz pulse power with optical fluence is observed, with unannealed LTG GaAs showing highest saturation fluence at 1.1 ± 0.1 mJ cm-2. SI-GaAs LPD emitters show a flip in signal polarity with optical fluence that is attributed to THz emission from the metal-semiconductor contact. Variation in optical polarisation affects THz pulse power that is attributed to a local optical excitation near the metal contact

Distinguishing impurity concentrations in GaAs and AlGaAs, using very shallow undoped heterostructures

We demonstrate a method of making a very shallow, gateable, undoped 2-dimensional electron gas. We have developed a method of making very low resistivity contacts to these structures and systematically studied the evolution of the mobility as a function of the depth of the 2DEG (from 300nm to 30nm). We demonstrate a way of extracting quantitative information about the background impurity concentration in GaAs and AlGaAs, the interface roughness and the charge in the surface states from the data. This information is very useful from the perspective of molecular beam epitaxy (MBE) growth. It is difficult to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to have a large enough spacer layer to reduce scattering and switching noise from remote ionsied dopants.Comment: 4 pages, 5 eps figure

Surface-acoustic-wave driven planar light-emitting device

Electroluminescence emission controlled by means of surface acoustic waves (SAWs) in planar light-emitting diodes (pLEDs) is demonstrated. Interdigital transducers for SAW generation were integrated onto pLEDs fabricated following the scheme which we have recently developed. Current-voltage, light-voltage and photoluminescence characteristics are presented at cryogenic temperatures. We argue that this scheme represents a valuable building block for advanced optoelectronic architectures

Anti-bunched photons from a lateral light-emitting diode

We demonstrate anti-bunched emission from a lateral-light emitting diode. Sub-Poissonian emission statistic, with a g$^{(2)}$(0)=0.7, is achieved at cryogenic temperature in the pulsed low-current regime, by exploiting electron injection through shallow impurities located in the diode depletion region. Thanks to its simple fabrication scheme and to its modulation bandwidth in the GHz range, we believe our devices are an appealing substitute for highly-attenuated lasers in existing quantum-key-distribution systems. Our devices outperform strongly-attenuated lasers in terms of multi-photon emission events and can therefore lead to a significant security improvement in existing quantum key distribution systems

Place, Space, and Motion

(a) Topics and Goals. The Junior Research Group »Place, Space and Motion« investigates the role of spatial concepts in physical theories in the millennium from Plato (4th century BCE) through Philoponus and Simplicius (6th century CE). In particular, we examine the explicit theoretical views of ancient physicists and philosophers concerning space, the spatial features of bodies, and the existence of isomorphisms among space, change, and time. Projects are devoted to issues in Plato’s Timaeus and Aristotle’s Physics, and to the interwoven reception of these texts in Middle Platonism and Late Platonism. We trace the evolving answers given to such central questions as whether space is metaphysically basic or is rather dependent upon bodies or even non-spatial entities (such as souls); the possibility of empty space; the causal role of space in nature; how spatial structures make certain kinds of change possible or necessary. The group aims to produce a series of essays and commentaries examining key texts of Plato and Aristotle and tracing the reception and transformation of their views in Middle- and Late Platonism. (b) Methods. The group engages in close reading and interpretation of ancient texts, with the aim of constructing a history of engagement with the questions indicated above. The main areas of expertise brought to bear on the relevant texts lie in classical philology, history of ideas, history of science, and systematic philosophy. In a weekly research seminar, individual research projects and results are presented in detail and discussed in the light of these varied disciplines and skill sets. (c) State of Discussion. Relevant texts are interpreted both internally and in the light of their relationships with earlier sources and later readings. In this way a narrative is emerging of development and interrelationship among ancient theories of space – a narrative with some shape and coherence, but without the suppression of details and uncertainties. The group is also beginning to pay more attention to epistemological issues, concerning the sources of theoretical knowledge about space, and the evolving standards of argument, justification, and presentation of such knowledge

Y coupled terahertz quantum cascade lasers

Here we demonstrate a Y coupled terahertz (THz) quantum cascade laser (QCL) system. The two THz QCLs working around 2.85 THz are driven by independent electrical pulsers. Total peak THz output power of the Y system, with both arms being driven synchronously, is found to be more than the linear sum of the peak powers from the individual arms; 10.4 mW compared with 9.6 mW (4.7 mW + 4.9 mW). Furthermore, we demonstrate that the emission spectra of this coupled system are significantly different to that of either arm alone, or to the linear combination of their individual spectra.Comment: 9 pages, 3 figure