Semiconductor quantum dots (QDs) combine high optical activity and the possibility of integration in a myriad of devices. Here, we demonstrate the integration of (Al)GaAs QDs in nanophononic strings and show that the excitonic two-level system of the QD couples to the confined phononic modes in string. For the chosen design the coupling is mediated via the valence band deformation potential and shear strain. Finite element modelling (FEM) shows that the optomechanical coupling parameter γom>0.15 meV/nm, exceeding that of vibrating nanowire architectures by one order of magnitude1.
We review1 the fully-scalable fabrication of a large array of hybrid molybdenum disulfide (MoS2) - silicon dioxide (SiO2) one-dimensional (1D), freestanding photonic-crystal cavities (PCCs) capable of enhancement of the MoS2 photoluminescence (PL) at the narrow cavity resonance. As demonstrated in our prior work [S. Hammer et al., Sci. Rep. 7, 7251 (2017)]1, geometric mode tuning over the wide spectral range of MoS2 PL can be achieved by changing the PC period. In this contribution, we provide a step-by-step description of the fabrication process and give additional detailed information on the degradation of MoS2 by XeF2 vapor. We avoid potential damage of the MoS2 monolayer during the crucial XeF2 etch by refraining from stripping the electron beam (e-beam) resist after dry etching of the photonic crystal pattern. The remaining resist on top of the samples encapsulates and protects the MoS2 film during the entire fabrication process. Albeit the thickness of the remaining resists strongly depends on the fabrication process, the resulting encapsulation of the MoS2 layer improves the confinement to the optical modes and gives rise to a potential enhancement of the light-matter interaction.
Nanothermochromic diffraction gratings based on the metal-insulator transition of VO2 are fabricated by site- selective ion-beam implantation in a SiO2 matrix. The studied diffraction gratings were defined (i) directly by spatially selective ion-beam synthesis or (ii) by site-selective deactivation of the metal-insulator transition by ion-beam induced structural defects. The strongest increase of the diffraction efficient was observed at a wavelength of 1550 nm exceeding one order of magnitude for the selectively deactivated gratings. The observed pronounced thermal hysteresis extends down close to room temperature and makes these optical elements well suited for optical memory devices.
We apply ultrafast pump-probe photocurrent spectroscopy to directly probe few Fermion charge and spin dynamics in an
artificial molecule formed by vertically stacking a pair of InGaAs self-assembled quantum dots. As the relative energy of
the orbital states in the two dots are energetically tuned by applying static electric fields, pronounced anticrossings are
observed arising from electron tunnel couplings. Time resolved photocurrent measurements performed in the vicinity of
these anticrossings provide direct information on the comparative roles of elastic and inelastic resonant tunneling
processes between the two quantum dots forming the molecule. Resonant pumping of the neutral exciton in the upper
dot with circularly polarized light facilitates ultrafast initialization of hole spin qubits over timescales limited only by the
laser pulse duration (<5ps) and a near perfect Pauli spin-blockade with a near unity suppression of absorption (>96%) for
spin forbidden transitions. Such a spin selective photocurrent response opens the way to probe spin dynamics in the
system over ultrafast timescales.
We report on recent progress towards single photon sources based on quantum dot and quantum post nanostructures
which are manipulated using surface acoustic waves. For this concept acoustic charge conveyance in a quantum well is
used to spatially separate electron and hole pairs and transport these in the plane of the quantum well. When conveyed to
the location of a quantum dot or quantum post these carriers are sequentially captured into the confined levels. Their
radiative decays gives rise to the emission of a train of single photons. Three different approaches using (i) straininduced
and (ii) self-assembled quantum dots, and (iii) self-assembled quantum posts are discussed and their application
potential is discussed. First devices and initial experiments towards the realization of such an acoustically driven single
photon source are presented and remote acoustically triggered injection into few individual emitters is demonstrated.
We study the influence of the mechanical deformation induced by a surface acoustic wave (SAW) on the resonance
frequency of a defect cavity in a 2D photonic crystal membrane. Using FDTD-simulations we determine the
resonance frequency and quality factor of a nanocavity of a GaAs based structure with embedded InAs quantum
dots. Under the influence of a SAW, we find a periodic modulation of the cavity resonance wavelength of Δλ >2
nm accompanied by only a weak < 0.5× reduction of the Q-factor. Initial experiments for a SAW wavelength of
~ 1.8μm show a pronounced broadening of the time-integrated cavity emission line corresponding to a shift of
≥ 1 nm.
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