My Milo Swinkels

Ultrafast non-local control of spontaneous emission

The radiative interaction of solid-state emitters with cavity fields is the basis of semiconductor microcavity lasers and cavity quantum electrodynamics (CQED) systems. Its control in real time would open new avenues for the generation of non-classical light states, the control of entanglement and the modulation of lasers. However, unlike atomic CQED or circuit quantum electrodynamics, the real-time control of radiative processes has not yet been achieved in semiconductors because of the ultrafast timescales involved. Here we propose an ultrafast non-local moulding of the vacuum field in a coupled-cavity system as an approach to the control of radiative processes and demonstrate the dynamic control of the spontaneous emission (SE) of quantum dots (QDs) in a photonic crystal (PhC) cavity on a ∼ 200 ps timescale, much faster than their natural SE lifetimes.

Diameter dependence of the thermal conductivity of InAs nanowires

The diameter dependence of the thermal conductivity of InAs nanowires in the range of 40-1500 nm has been measured. We demonstrate a reduction in thermal conductivity of 80% for 40 nm nanowires, opening the way for further design strategies for nanoscaled thermoelectric materials. Furthermore, we investigate the effect of thermal contact in the most common measurement method for nanoscale thermal conductivity. Our study allows for the determination of the thermal contact using existing measurement setups. The thermal contact resistance is found to be comparable to the wire thermal resistance for wires with a diameter of 90 nm and higher.

Assessing the thermoelectric properties of single InSb nanowires:The role of thermal contact resistance

The peculiar shape and dimensions of nanowires (NWs) have opened the way to their exploitation in thermoelectric applications. In general, the parameters entering into the thermoelectric figure of merit are strongly interdependent, which makes it difficult to realize an optimal thermoelectric material. In NWs, instead, the power factor can be increased and the thermal conductivity reduced, thus boosting the thermoelectric efficiency compared to bulk materials. However, the assessment of all the thermoelectric properties of a NW is experimentally very challenging. Here, we focus on InSb NWs, which have proved to be promising thermoelectric materials. The figure of merit is accurately determined by using a novel method based on a combination of Raman spectroscopy and electrical measurements. Remarkably, this type of experiment provides a powerful approach allowing us to neglect the role played by thermal contact resistance. Furthermore, we compare the thermal conductivity determined by this novel method to the one determined on the same sample by the thermal bridge method. In this latter approach, the thermal contact resistance is a non-negligible parameter, especially in NWs with large diameters. We provide experimental evidence of the crucial role played by thermal contact resistance in the assessment of the thermal properties of nanostructures, using two different measurement methods of the thermal conductivity.

Nanowires for heat conversion

This review focuses on the investigation and enhancement of the thermoelectric properties of semiconducting nanowires (NWs). NWs are nanostructures with typical diameters between few to hundreds of nm and length of few to several μm, exhibiting a high surface-to-volume ratio. Nowadays an extraordinary control over their growth has been achieved, enabling also the integration of different type of heterostructures, which can lead to the engineering of the functional properties of the NWs. In this review we discuss all concepts which have been presented and achieved so far for the improvements of the thermoelectric performances of semiconducting NWs. Furthermore, we present a brief survey of the experimental methods which enable the investigation of the thermoelectric properties of these nanostructures. Kewords: Nanowires, core-shell nanowires, superlattice nanowires, thermal conductivity, Seebeck coefficient, figure of merit ZT, thermoelectrics

Controlling spontaneous emission by vacuum field modulation

The spontaneous emission (SE) rate represents a fundamental parameter governing the light emission process. Control over this rate can be achieved by shaping the vacuum field (the quantized electromagnetic field associated to the lowest possible energy in quantum theory) at the position of quantum emitters (e.g., atoms or quantum dots, QDs) within periodic dielectric structures.1, 2 Both the inhibition and acceleration of the SE rate have been demonstrated by applying a static or quasistatic change to the local density of optical states in photonic crystals (PhCs).3 However, the real-time manipulation of radiative processes creates another degree of freedom, essential for various applications in classical and quantum photonics. For instance, to achieve high efficiencies in quantum information systems based on photon exchange between semiconductor artificial atoms (e.g., QDs), photon emission and absorption rates must be controlled.4 Ultrafast control over the local density of optical states in PhCs, based on a local perturbation of the photonic bandgap structure via the transient injection of free carriers, was theoretically proposed one decade ago.5 Unfortunately, this technique leads to a simultaneous perturbation of the carrier population, thereby limiting its applicability. We have proposed and demonstrated an ultrafast, non-local control scheme based on the coupling of a control cavity to a target cavity containing QD emitters.6 By injecting free carriers into the control cavity, the electric field distribution in the coupledcavity system can be modified, resulting in the real-time shaping of the vacuum field in the target cavity. Using this method, Figure 1. Non-local control scheme of the emitter-cavity interaction. The lattice constant in different regions along the photonic crystal (PhC) waveguide is modulated to define the target (red shaded area) and Fabry-Perot (FP) cavity (blue shaded area). The dynamic tuning of the FP cavity modes, in and out of the resonance with the target cavity mode, results in a redistribution of the mode field and a change of Q-factor, as shown in the upper left inset. (Illustration designed and prepared by Yan Liang, L2Molecule.com.)

Optical control of the quality factor using coupled photonic crystal cavities

We have demonstrated optical control of quality-factor (Q-factor) using coupled photonic crystal cavities. A double heterostructure (DHS) cavity has been thermo-optically brought in resonance to an adjacent Febry-Pérot cavity to reduce the optical confinement and hence decrease the Q-factor. Both the strong and weak coupling phenomena between two cavities have been studied. Up to 50% Q-factor change has been experimentally observed. The corresponding modification of the local density of states enables the control of the spontaneous emission rate of the quantum emitters in the DHS cavity.