Coherent transport of nanowire surface plasmons coupled to quantum dots

Tunable single photon nonreciprocal scattering based on giant atom-waveguide chiral couplings

We theoretically investigate the single photon scattering properties in a waveguide chirally coupling to a giant atom. The single photon transmission spectrum depends on the direction of the single photon incident when the energy loss of the giant atom can not be neglected. The difference between the transmission probabilities corresponding to opposite transport direction ΔT is calculated. It shows that both of the position and width of the ΔT are dependent on the size of the giant atom. Furthermore, the position of the maximum ΔT and the frequency width of ΔT can be modulated by a classical laser beam. Our results will be beneficial to control single photons in quantum devices design involving giant atoms.

Unidirectional reflectionless phenomena in a non-Hermitian quantum system of quantum dots coupled to a plasmonic waveguide

Unidirectional reflectionless phenomena are investigated theoretically in a non-Hermitian quantum system composed of several quantum dots and a plasmonic waveguide. By adjusting the phase shifts between quantum dots, single- and dual-band unidirectional reflectionlessnesses are realized at exceptional points based on two and three quantum dots coupled to a plasmonic waveguide, respectively. In addition, single- and dual-band unidirectional perfect absorptions with high quality factors are obtained at the vicinity of exceptional points.

Coherent frequency down-conversions and entanglement generation in a Sagnac interferometer

A waveguide loop coupled to two external line waveguides by a 50/50 beam splitter forms a Sagnac interferometer. We consider the situation where two Λ-type three-level emitters are symmetrically coupled to the loop of a Sagnac interferometer and a single photon is input through one end of the line waveguides. Since the incoming photon is always in a superposition of the clockwise and counterclockwise modes of the loop and the two emitters are positioned symmetrically with respect to the input port of photon, the processes of photon scattering at the two emitters are symmetric and coherent. When the separation of the emitters and the coupling strengths of the emitters with the waveguide loop take some special values, due to quantum interference, a frequency down-conversion can certainly happen at one of the two emitters during the photon scattering but one cannot know at which emitter the frequency down-conversion takes place. This indistinguishability of the coherent frequency down-conversion processes can result in the generation of the symmetric or antisymmetric two-qubit maximally entangled states of the emitters. In the present scheme, a single photon comes in and goes out of the waveguide loop, and no photon localization modes exists. The entangled states result from the coherent frequency down-conversion processes of the emitters. Thus, the resulting entangled states are stable if the two lower-lying states of the emitters have no decay. We also investigate the influence of the dissipation of the emitters and the finite bandwidth of an input photon wavepacket on the success probability of entanglement generation, and find that the present scheme is robust to these effects and feasible with current available technologies.

Plasmonic bio-sensing for the Fenna-Matthews-Olson complex

We study theoretically the bio-sensing capabilities of metal nanowire surface plasmons. As a specific example, we couple the nanowire to specific sites (bacteriochlorophyll) of the Fenna-Matthews-Olson (FMO) photosynthetic pigment protein complex. In this hybrid system, we find that when certain sites of the FMO complex are subject to either the suppression of inter-site transitions or are entirely disconnected from the complex, the resulting variations in the excitation transfer rates through the complex can be monitored through the corresponding changes in the scattering spectra of the incident nanowire surface plasmons. We also find that these changes can be further enhanced by changing the ratio of plasmon-site couplings. The change of the Fano lineshape in the scattering spectra further reveals that “site 5” in the FMO complex plays a distinct role from other sites. Our results provide a feasible way, using single photons, to detect mutation-induced, or bleaching-induced, local defects or modifications of the FMO complex, and allows access to both the local and global properties of the excitation transfer in such systems.

Scattering of nanowire surface plasmons coupled to quantum dots with azimuthal angle difference

Coherent scatterings of surface plasmons coupled to quantun dots have attracted great attention in plasmonics. Recently, an experiment has shown that the quantum dots located nearby a nanowire can be separated not only in distance, but also an angle ϕ along the cylindrical direction. Here, by using the real-space Hamiltonian and the transfer matrix method, we analytically obtain the transmission/reflection spectra of nanowire surface plasmons coupled to quantum dots with an azimuthal angle difference. We find that the scattering spectra can show completely different features due to different positions and azimuthal angles of the quantum dots. When additionally coupling a cavity to the dots, we obtain the Fano-like line shape in the transmission and reflection spectra due to the interference between the localized and delocalized modes.

Plasmon-controlled excitonic emission from vertically-tapered organic nanowires

Organic molecular nanophotonics has emerged as an important avenue to harness molecular aggregation and crystallization on various functional platforms to obtain nano-optical devices. To this end, there is growing interest to combine organic molecular nanostructures with plasmonic surfaces and interfaces. Motivated by this, herein we introduce a unique geometry: vertically-tapered organic nanowires grown on a plasmonic thin film. A polarization-sensitive plasmon-polariton on a gold thin-film was harnessed to control the exciton-polariton propagation and subsequent photoluminescence from an organic nanowire made of diaminoanthraquinone (DAAQ) molecules. We show that the exciton-polariton emission from individual DAAQ nanowires can be modulated up to a factor of 6 by varying the excitation polarization state of surface plasmons. Our observations were corroborated with full-wave three-dimensional finite-difference time-domain calculations performed on vertically-tapered nanowire geometry. Our work introduces a new optical platform to study coupling between propagating plasmons and propagating excitons, and may have implications in emerging fields such as hybrid-polariton based light emitting devices and vertical-cavity nano-optomechanics.

Quantum entanglement in plasmonic waveguides with near-zero mode indices

We investigate the quantum entanglement between two quantum dots (QDs) in a plasmonic waveguide with a near-zero mode index, considering the dependence of concurrence on interdot distance, QD waveguide frequency detuning, and coupling strength ratio. High concurrence is achieved for a wide range of interdot distances due to the near-zero mode index, which largely relaxes the strict requirement of interdot distance in conventional dielectric waveguides or metal nanowires. The proposed QD waveguide system with near-zero phase variation along the waveguide near the mode cutoff frequency shows very promising potential in quantum optics and quantum information processing.

Quantum entangling gates using the strong coupling between two optical emitters and nanowire surface plasmons

We propose a scheme to generate quantum entangling gate using one-dimensional surface plasmon waveguide. The protocol is based on the detection of the transmission spectrum of the single optical plasmons passing through two separate three-level emitters on metallic nanowire waveguide. It is shown that the low efficiency in direct detection of the single photon can be avoided by repeating the measurement of the transmission spectrum.

Plasmon-mediated resonance energy transfer by metallic nanorods

We investigate the enhancement of the resonance energy transfer rate between donor and acceptor associated by the surface plasmons of the Ag nanorods on a SiO2 substrate. Our results for a single nanorod with different cross sections reveal that the cylinder nanorod has the strongest ability to enhance the resonance energy transfer rate. Moreover, for donor and acceptor with nonparallel polarization directions, we propose simple V-shaped nanorod structures which lead to the remarkable resonance energy transfer enhancement that is ten times larger than that by the single nanorod structure. We demonstrate that these structures have good robustness and controllability. Our work provides a way to improve the resonance energy transfer efficiency in integrated photonic devices. PACS: 78.67.Qa, 73.20.Mf, 42.50.Ex.

Correspondence between entanglement and Fano resonance of surface plasmons

We investigate the Fano resonance of the scattering spectra in a system consisting of a metal nanowire coupled to two colloidal quantum dots. By varying the coupling strengths and the energy spacings of the quantum dot qubits, we find that both the line shapes and the presence of the Fano resonance can be controlled. Furthermore, the degree of two-qubit entanglement can vary from unity to zero when the Fano resonance occurs. This indicates that there exists correlations between the two-qubit entanglement and the Fano resonance.

Fano resonance analysis in a pair of semiconductor quantum dots coupling to a metal nanowire

We investigate theoretically a surface plasmon transport in the metal nanowire coupling to a pair of quantum dots. The Fano-type transmission spectrum is analyzed. The phase shift and group velocity delay of the transmitted surface plasmon are explored. The electromagnetically-induced-transparency-type transmission spectrum is also discussed.

Controlling Fano resonance of nanowire surface plasmons

We theoretically study the effect of the applied magnetic field on the scattering properties of the nanowire surface plasmons coupled to two quantum dots. The dispersion relations of the surface plasmon are found to be upwardly displaced in the presence of an applied magnetic field. The symmetric double peaks in the transmission spectrum resulting from the interference between the localized and delocalized channels of the surface plasmon can combine together and the associated Fano lineshape will be smeared out when increasing the magnitude of the magnetic field.