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Davydov splitting and singlet fission in excitonically coupled pentacene dimers
Singlet fission (SF) allows two charges to be generated from the absorption of a single photon and is, therefore, potentially transformative toward improving solar energy conversion. Key to the present study of SF is the design of pentacene dimers featuring a xanthene linker that strictly places two pentacene chromophores in a rigid arrangement and, in turn, enforces efficient, intramolecular π-overlap that mimics interactions typically found in condensed state (e.g., solids, films, etc.). Inter-chromophore communication ensures Davydov splitting, which plays an unprecedented role toward achieving SF in pentacene dimers. Transient absorption measurements document that intramolecular SF evolves upon excitation into the lower Davydov bands to form a correlated triplet pair at cryogenic temperature. At room temperature, the two spin-correlated triplets, one per pentacene moiety within the dimers, are electronically coupled to an excimer state. The presented results are transferable to a broad range of acene morphologies including aggregates, crystals, and films. © 2019 The Royal Society of Chemistry.
Photon transport in one-dimensional systems coupled to three-level quantum impurities
We discuss the transport properties of a single photon in a one-dimensional waveguide with an embedded three-level atom and utilize both stationary plane-wave solutions and time-dependent transport calculations to investigate the interaction of a photon with driven and undriven V- and Λ-systems. Specifically, for the case of an undriven V-system, we analyze the phenomenon of long-time occupation of the upper atomic levels in conjunction with almost dark states. For the undriven Λ-system, we find non-stationary dark states and we explain how the photon's transmittance can be controlled by an initial phase difference between the energetically lower-lying atomic states. With regard to the driven three-level systems, we discuss electromagnetically induced transparency in terms of the pulse propagation of a single photon through a Λ-type atom. In addition, we demonstrate how a driven V-type atom can be utilized to control the momentum distribution of the scattered photon.
Surface acoustic wave modulation of single photon emission from GaN/InGaN nanowire quantum dots
On-chip quantum information processing requires controllable quantum light sources that can be operated on-demand at high-speeds and with the possibility of in-situ control of the photon emission wavelength and its optical polarization properties. Here, we report on the dynamic control of the optical emission from core-shell GaN/InGaN nanowire (NW) heterostructures using radio frequency surface acoustic waves (SAWs). The SAWs are excited on the surface of a piezoelectric lithium niobate crystal equipped with a SAW delay line onto which the NWs were mechanically transferred. Luminescent quantum dot (QD)-like exciton localization centers induced by compositional fluctuations within the InGaN nanoshell were identified using stroboscopic micro-photoluminescence (micro-PL) spectroscopy. They exhibit narrow and almost fully linearly polarized emission lines in the micro-PL spectra and a pronounced anti-bunching signature of single photon emission in the photon correlation experiments. When the nanowire is perturbed by the propagating SAW, the embedded QD is periodically strained and its excitonic transitions are modulated by the acousto-mechanical coupling, giving rise to a spectral fine-tuning within a ~1.5 meV bandwidth at the acoustic frequency of ~330 MHz. This outcome can be further combined with spectral detection filtering for temporal control of the emitted photons. The effect of the SAW piezoelectric field on the QD charge population and on the optical polarization degree is also observed. The advantage of the acousto-optoelectric over other control schemes is that it allows in-situ manipulation of the optical emission properties over a wide frequency range (up to GHz frequencies).
Multiharmonic Frequency-Chirped Transducers for Surface-Acoustic-Wave Optomechanics
Wide-passband interdigital transducers are employed to establish a stable phase lock between a train of laser pulses emitted by a mode-locked laser and a surface acoustic wave generated electrically by the transducer. The transducer design is based on a multiharmonic split-finger architecture for the excitation of a fundamental surface acoustic wave and a discrete number of its overtones. Simply by introducing a variation of the transducer's periodicity p, a frequency chirp is added. This combination results in wide frequency bands for each harmonic. The transducer's conversion efficiency from the electrical to the acoustic domain is characterized optomechanically using single quantum dots acting as nanoscale pressure sensors. The ability to generate surface acoustic waves over a wide band of frequencies enables advanced acousto-optic spectroscopy using mode-locked lasers with fixed repetition rate. Stable phase locking between the electrically generated acoustic wave and the train of laser pulses is confirmed by performing stroboscopic spectroscopy on a single quantum dot at a frequency of 320 MHz. Finally, the dynamic spectral modulation of the quantum dot is directly monitored in the time domain combining stable phase-locked optical excitation and time-correlated single-photon counting. The demonstrated scheme will be particularly useful for the experimental implementation of surface-acoustic-wave-driven quantum gates of optically addressable qubits or collective quantum states or for multicomponent Fourier synthesis of tailored nanomechanical waveforms.
Coherent interaction of atoms with a beam of light confined in a light cage
Controlling coherent interaction between optical fields and quantum systems in scalable, integrated platforms is essential for quantum technologies. Miniaturised, warm alkali-vapour cells integrated with on-chip photonic devices represent an attractive system, in particular for delay or storage of a single-photon quantum state. Hollow-core fibres or planar waveguides are widely used to confine light over long distances enhancing light-matter interaction in atomic-vapour cells. However, they suffer from inefficient filling times, enhanced dephasing for atoms near the surfaces, and limited light-matter overlap. We report here on the observation of modified electromagnetically induced transparency for a non-diffractive beam of light in an on-chip, laterally-accessible hollow-core light cage. Atomic layer deposition of an alumina nanofilm onto the light-cage structure was utilised to precisely tune the high-transmission spectral region of the light-cage mode to the operation wavelength of the atomic transition, while additionally protecting the polymer against the corrosive alkali vapour. The experiments show strong, coherent light-matter coupling over lengths substantially exceeding the Rayleigh range. Additionally, the stable non-degrading performance and extreme versatility of the light cage provide an excellent basis for a manifold of quantum-storage and quantum-nonlinear applications, highlighting it as a compelling candidate for all-on-chip, integrable, low-cost, vapour-based photon delay.
A photonic platform for donor spin qubits in silicon
Donor spins in silicon are highly competitive qubits for upcoming quantum technologies, offering complementary metal-oxide semiconductor compatibility, coherence (T2) times of minutes to hours, and simultaneous initialization, manipulation, and readout fidelities near ~99.9%. This allows for many quantum error correction protocols, which will be essential for scale-up. However, a proven method of reliably coupling spatially separated donor qubits has yet to be identified. We present a scalable silicon-based platform using the unique optical properties of “deep” chalcogen donors. For the prototypical 77Se+ donor, we measure lower bounds on the transition dipole moment and excited-state lifetime, enabling access to the strong coupling limit of cavity quantum electrodynamics using known silicon photonic resonator technology and integrated silicon photonics. We also report relatively strong photon emission from this same transition. These results unlock clear pathways for silicon-based quantum computing, spin-to-photon conversion, photonic memories, integrated single-photon sources, and all-optical switches.