Metamaterials have emerged as the basis of a novel optoelectronic platform operating in the terahertz (THz) range, due to their versatility and strong light-matter interaction. The necessary design of efficient modulators and detectors requires a detailed investigation of metamaterial resonances and their interplay with an active medium, e.g. graphene. An aperture-SNOM (a-SNOM) system based on picosecond THz pulses was used to investigate the spectral characteristics of a set of lithographically tuned metamaterial coupled resonators. This approach allowed the mapping of the supported E-field of each resonator a few microns from the device plane, yielding bonding and antibonding modes reminiscent of electromagnetic induced transparency.
Ramón Bernardo Gavito, Fernando Jiménez Urbanos, Jonathan Roberts, James Sexton, Benjamin Astbury, Hamzah Shokeir, Thomas McGrath, Yasir Noori, Christopher Woodhead, Mohamed Missous, Utz Roedig, Robert Young
In this work, we show how the hysteretic behaviour of resonant tunnelling diodes (RTDs) can be exploited for new functionalities. In particular, the RTDs exhibit a stochastic 2-state switching mechanism that could be useful for random number generation and cryptographic applications. This behaviour can be scaled to N-bit switching, by connecting various RTDs in series. The InGaAs/AlAs RTDs used in our experiments display very sharp negative differential resistance (NDR) peaks at room temperature which show hysteresis cycles that, rather than having a fixed switching threshold, show a probability distribution about a central value. We propose to use this intrinsic uncertainty emerging from the quantum nature of the RTDs as a source of randomness. We show that a combination of two RTDs in series results in devices with three-state outputs and discuss the possibility of scaling to N-state devices by subsequent series connections of RTDs, which we demonstrate for the up to the 4-state case.
In this work, we suggest using that the intrinsic uncertainty in the conduction paths of resonant tunnelling diodes can behave as a source of randomness that can be integrated into current electronics to produce on-chip true random number generators. The N-shaped I-V characteristic of RTDs results in a two-level random voltage output when driven with current pulse trains. Electrical characterisation and randomness testing of the devices was conducted in order to determine the validity of the true randomness assumption. Based on the results obtained for the single RTD case, we suggest the possibility of using multi-well devices to generate N-state random switching devices for their use in random number generation or multi-valued logic devices.
Three InAs quantum dot (QD) samples with dislocation filter layers (DFLs) are grown on Si substrates with and without in-situ annealing. Comparison is made to a similar structure grown on a GaAs substrate. The three Si grown samples have different dislocation densities in their active region as revealed by structural studies. By determining the integrated emission as a function of laser power it is possible to determine the power dependence of the radiative efficiency and compare this across the four samples. The radiative efficiency increases with decreasing dislocation density; this also results in a decrease in the temperature quenching of the PL. A laser structures grown on Si and implementing the same optimum DFL and annealing procedure exhibits a greater than 3 fold reduction in threshold current as well as a two fold increase in slope efficiency in comparison to a device in which no annealing is applied.
We discuss recent progress using the radiative emission of single quantum dots as a triggered source of both single
photons, and photon pairs displaying polarization entanglement. Excitation of a quantum dot with two electrons and two
holes leads to the emission of a pair of photons. We show that, provided the spin splitting of the intermediate exciton
state in the decay is erased, the photon pair is emitted in an entangled polarization state. Using quantum dots to generate
quantum light has the advantage of allowing a robust and compact source to be realised with contacts for electrical
injection. A cavity may be integrated into the semiconductor structure to enhance the photon collection efficiency and
control the recombination dynamics. We detail a process to form a sub-micron current aperture within the device,
allowing single quantum dots to be addressed electrically.
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