Optical bistability in a heterodimer composed of a quantum dot and a metallic nanoshell.

We theoretically explore the conditions for generating optical bistability (OB) in a heterodimer comprised of a semiconductor quantum dot (SQD) and a metallic nanoshell (MNS). The MNS is made of a metallic nanosphere as a core and a dielectric material as a shell. For the specific hybrid system considered, the bistable effect appears only if the frequency of the pump field is equal to (or slightly less than) the exciton frequency for a proper shell thickness. Bistability phase diagrams, when plotted, show that the dipole-induced bistable region can be greatly broadened by changing the shell thickness of the MNS in a strong exciton-plasmon coupling regime. In particular, we demonstrate that the multipole polarization not only narrows the bistable zone but also enlarges the corresponding thresholds for a given intermediate scaled pumping intensity. On the other hand, when the SQD couples strongly with the MNS, the multipole polarization can also significantly broaden the bistable region and induce a great suppression of the FWM (four-wave mixing) signal for a fixed shell thickness. These interesting findings offer a fresh understanding of the bistability conditions in an SQD/MNS heterodimer, and may be useful in the fabrication of high-performance and low-threshold optical bistable nanodevices.

Ultra-strong optical four-wave mixing signal induced by strong exciton-phonon and exciton-plasmon couplings.

We propose a scheme to generate ultra-strong four-wave mixing (FWM) signal based on a suspended monolayer graphene nanoribbon nanomechanical resonator (NR) coupled to an Au nanoparticle (NP). It is shown that, the FWM spectrum can switch among two-peaked, three-peaked, four-peaked or five-peaked via the modulation of exciton-phonon and exciton-plasmon couplings. This is mainly attributed to the vibrational properties of NR related to the exciton-phonon coupling, and the energy-level splitting of the localized exciton correlated to three classes of resonances consisting of three-photon resonance, Rayleigh Resonance, and AC-Stark atomic resonance. Especially, in a dual-strong coupling regime, the gains for these peaks can be as high as nine orders of magnitude (∼ 109) around the lower bistable threshold due to a combined effect of two couplings. Our findings not only offer an efficient way to measure the vibrational frequency of NR and the exciton-phonon coupling strength but also provide a possibility to fabricate high-performance optoelectronic nanodevices.

Dual-channel bistable switch based on a monolayer graphene nanoribbon nanoresonator coupled to a metal nanoparticle.

We theoretically propose a dual-channel bistable switch based on a monolayer Z-shaped graphene nanoribbon nanoresonator (NR) coupled to a metal nanoparticle (MNP). We show that the bistable nonlinear absorption response can be realized due to a competition and combination of the exciton-plasmon and exciton-phonon interactions. We map out two-dimensional and three-dimensional bistability phase diagrams, which reveal clearly the dynamical evolution of the roles played by these two interactions in managing optical bistability (OB) at all stages. Specifically, the bistable switch proposed can be controlled via a single channel or dual channels by only adjusting the intensity or frequency of the pump field. In/outside these channels, the switch will be turned on/off. The results obtained here not only can be employed to measure precisely the distance between the MNP and the NR but also provide promising applications in optical switching and optical storage.

Plasmon-modulated bistable four-wave mixing signals from a metal nanoparticle-monolayer MoS2 nanoresonator hybrid system.

We present a study for the impact of exciton-phonon and exciton-plasmon interactions on bistable four-wave mixing (FWM) signals in a metal nanoparticle (MNP)-monolayer MoS2 nanoresonator hybrid system. Via tracing the FWM response we predict that, depending on the excitation conditions and the system parameters, such a system exhibits 'U-shaped' bistable FWM signals. We also map out bistability phase diagrams within the system's parameter space. Especially, we show that compared with the exciton-phonon interaction, a strong exciton-plasmon interaction plays a dominant role in the generation of optical bistability, and the bistable region will be greatly broadened by shortening the distance between the MNP and the monolayer MoS2 nanoresonator. In the weak exciton-plasmon coupling regime, the impact of exciton-phonon interaction on optical bistability will become obvious. The scheme proposed may be used for building optical switches and logic-gate devices for optical computing and quantum information processing.

Bistable four-wave mixing response in a semiconductor quantum dot coupled to a photonic crystal nanocavity.

We perform a theoretical study of the bistable four-wave mixing (FWM) response in a coupled system comprised of a semiconductor quantum dot (SQD) and a photonic crystal (PC) nanocavity in which the SQD is embedded. It is shown that the shape of the FWM spectrum can switch among single-peaked, double-peaked, triple-peaked, and four-peaked arising from the vacuum Rabi splitting and the exciton-nanocavity coupling. Especially, we map out bistability phase diagrams within a parameter subspace of the system, and find that it is easy to turn on or off the bistable FWM response by only adjusting the excitation frequency or the pumping intensity. Our results offer a feasible means for measuring the SQD-PC nanocavity coupling strength and open a new avenue to design optical switches and memories.

Switching freely between superluminal and subluminal light propagation in a monolayer MoS2 nanoresonator.

We theoretically propose a feasible scheme to advance or slow the propagation of light in a monolayer MoS2 nanoresonator (NR). The scheme allows one to easily turn on or off the fast (superluminal) and slow (subluminal) light effects and switch freely between fast and slow light propagation by only adjusting the frequency or intensity of the pump field. As the exciton interacts strongly with the phonons in MoS2, the slow light effect will appear along with a large dispersion with a very steep negative slope and a sharp absorption peak. Especially, the maximal group velocity index of the slow light in the monolayer MoS2 NR can reach two orders of magnitude larger than that in a carbon nanotube resonator. These results provide a new way to measure the exciton-phonon coupling strength and may prove useful in device applications such as optical switching and optical signal processing.

Four-wave mixing signal enhancement and optical bistability of a hybrid metal nanoparticle-quantum dot molecule in a nanomechanical resonator.

We investigate theoretically four-wave mixing (FWM) response and optical bistability (OB) in a hybrid nanosystem composed of a metal nanoparticle (MNP) and a semiconductor quantum dot (SQD) coupled to a nanomechanical resonator (NR). It is shown that the FWM signal is enhanced by more than three orders of magnitude as compared to that of the system without exciton-phonon interaction, and the FWM signal can also be suppressed significantly and broadened due to the exciton-plasmon interaction. As the MNP couples strongly with the SQD, the bistable FWM response can be achieved by adjusting the SQD-MNP distance and the pumping intensity. For a given pumping constant and a fixed SQD-MNP distance, the enhanced exciton-phonon interaction can promote the occurrence of bistability. Our findings not only present a feasible way to detect the spacing between two nanoparticles, but also hold promise for developing quantum switches and nanoscale rulers.

Four-wave parametric amplification in semiconductor quantum dot-metallic nanoparticle hybrid molecules.

We study theoretically four-wave parametric amplification arising from the nonlinear optical response of hybrid molecules composed of semiconductor quantum dots and metallic nanoparticles. It is shown that highly efficient four-wave parametric amplification can be achieved by adjusting the frequency and intensity of the pump field and the distance between the quantum dot and the metallic nanoparticle. Specifically, the induced probe-wave gain is tunable in a large range from 1 to 1.43 × 105. This gain reaches its maximum at the position of three-photon resonance. Our findings hold great promise for developing four-wave parametric oscillators.

Plasmon resonances in a stacked pair of graphene ribbon arrays with a lateral displacement.

We find that a stacked pair of graphene ribbon arrays with a lateral displacement can excite plasmon waveguide mode in the gap between ribbons, as well as surface plasmon mode on graphene ribbon surface. When the resonance wavelengthes of plasmon waveguide mode and surface plasmon mode are close to each other, there is a strong electromagnetic interaction between the two modes, and then they contribute together to transmission dip. The plasmon waveguide mode resonance can be manipulated by the lateral displacement and longitudinal interval between arrays due to their influence on the manner and strength of electromagnetic coupling between two arrays. The findings expand our understanding of electromagnetic resonances in graphene-ribbon array structure and may affect further engineering of nanoplasmonic devices and metamaterials.

Enhanced electrochromic properties of WO3/ITO nanocomposite smart windows.

Tungsten oxide is regarded as the most promising electrochromic material owing to its continuously tunable optical properties, low cost, and high coloration efficiency. Further improving its optical modulation, switching speed, and coloration efficiency is important to electrochromic smart windows and related devices. Here, we demonstrate an enhanced electrochromic film composed of a WO3 nanosheet and ITO nanoparticles developed by an all-solution technology. The WO3 nanosheet is fabricated by an acid-assisted hydrothermal process with high product efficiency. The introduction of an ITO into the WO3 nanosheets significantly improved the electrochemical activity and the conductivity of the composite film. Compared with a reported electrochromic film without ITO doping, our synthesized composite WO3 film exhibited optical modulation up to 88% and a high coloration efficiency of 154.16 cm2 C-1. Particularly, our electrochromic film was based on the dispersant solution and spin-coating technology, which may also be realized with nano-spray coating for large scale applications. The results offer an effective way to develop large-area electrochromic film and devices.

Adjustable phase resonances in a compound metallic grating with perpendicular cuts.

We propose a compound metallic grating with perpendicular cuts in each slit and investigate its optical transmission property theoretically. The odd and even waveguide modes exhibit different behaviors when the cuts are set asymmetrically in the slits. Particularly, it is shown that the transmission dips of transmission spectrum for this compound periodic structure can be realized alternately by shifting the position of cuts in the slit. The effect of cut size on the phase resonances in the proposed metallic grating is also identified, and the underlying physics is discussed by the simulated field and phase maps.

Tailoring optical transmission via the arrangement of compound subwavelength hole arrays.

The transmission properties of light through metal films with compound periodic subwavelength hole arrays is numerically investigated by using the finite-difference time-domain (FDTD) method. The sharp dips in the transmission bands, together with the suppression of surface Plasmon resonance (SPR) (0, 1) peak, are found when two square holes in every unit cell are arranged asymmetrically along the polarization direction of the incident light. However, the shape of transmission spectra is not sensitive to the symmetry if the holes are arranged perpendicular to the propagation direction of surface plasmon polaritons (SPPs). The physics origin of these phenomena is explained qualitatively by the phase resonance of SPPs.

A wide bandgap plasmonic Bragg reflector.

Surface plasmon polaritons (SPPs) Bragg reflector with more excellent optical properties are investigated numerically. By introducing a finite array of periodic grooves on the two surfaces of metal-insulator-metal (MIM) waveguide, we fulfill the periodical changes of effective refractive index, which leads to the photonic band gap (PBG). And it has been further widened by inserting a dielectric material with higher refractive index in the waveguide with narrow slit width. Finite difference time domain (FDTD) simulation confirms the widened bandgap. In addition, a SPP nanocavity is introduced by breaking the periodicity of our proposed structure.

[Electroacupuncture intervention improved changes of skin barrier and color in type 2 diabetes rats].

OBJECTIVE: To observe the effect of electroacupuncture (EA) of auricular concha region (ACR), "Zusanli" (ST 36) and "Sanyinjiao"(SP 6) on abnormal skin barrier and color in streptozotocin (STZ)-induced type 2 diabetes mellitus (T 2 DM) rats so as to find a better therapeutic method. METHODS: Fifty male Sprague Dawley (SD) rats were randomized into control (n = 10), model (n = 13), EA-ST 36-SP 6 (n = 14) and EA-ACR (n = 13) groups. T 2 DM model was established by intraperitoneal injection of 1% STZ (50 mg/kg) and by feeding the animals with high-fat diet for 30 days. EA (15 Hz, 1 mA) was applied to bilateral "Zusanli" (ST 36), "Sanyinjiao" (SP 6) and ACR respectively for 30 min, once daily for 14 days. Multi Probe Adapter 9 skin measurement system (MPA 9) was used to measure the stratum corneum hydration (SCH), transepidermal water loss (TEWL), erythema index (El), melanin index (MI) and chromatic aberration values (L. a. b.) of the abdomenal and back (L1 - L6) skin, respectively. RESULTS: In comparison with the control group and pre-modeling, blood glucose contents of the model, EA-ST 36-SP 6 and EA-ACR groups were increased significantly (P < 0.01) after modeling. Compared with the model group, blood glucose levels of both EA-ST 36-SP 6 and EA-ACR groups were down-regulated apparently (P < 0.01) after treatment. Thirty days after modeling, the SCH, TEWL, EI, MI, L. and a. levels of the back skin, and the SCH and L. levels of the abdominal skin were decreased significantly (P < 0.05), while the level of b. in the back skin, and those of TEWL, EL, a. and b. of the abdominal skin increased considerably in the model group (P < 0.05). After EA, the levels of SCH, TEWL MI, L. a. and b. in the EA-ST 36-SP 6 group, SCH, TEWL, L. and b. in the EA-ACR group of the back skin, and SCH, L. and b. of both EAST 36-SP 6 and EA-ACR groups, as well as EI and a. of the EA-ST 36-SP 6 group of the abdominal skin were reversed remarkably (P < 0.05). No significant differences were found between the EA-ST 36-SP 6 and model groups in the levels of El of the back skin, TEWL and MI of the abdominal skin, and between the EA-ACR and model groups in the levels of SCH, EI, MI and a. of the back skin, and TEWL, EI, MI and a. of the abdominal skin (P > 0.05). In spite of a mildly better therapeutic effect of EA-ST 36-SP 6 in regulating some of the abovementioned indexes, no significant differences were found between the two EA groups in all the indexes (P > 0.05). CONCLUSION: EA of both ST 36-SP 6 and ACR can effectively down-regulate blood glucose level, suppress DM-induced decrease of SCH, TEWL, L. and a. of the back skin, and SCH of the abdominal skin, and down-regulate b. levels of the back and abdominal skin in T 2 DM rats, showing an improvement of the impaired skin barrier and color changes following EA intervention.

Electromagnetic interaction in stacked split ring resonator arrays.

We theoretically demonstrate the coupling between the unit cells and the interaction between constituents within each cell in metamaterials consisting of stacked split ring resonator arrays which are embedded in a homogeneous dielectric. It is found that the resonant frequency due to plasmon hybridization depends on the symmetry of resonance modes. Both for the first and third order plasmon resonances, we show that the resonances at lower frequency are not sensitive to the variation of lattice density, while the resonances at higher frequency rely on the coupling between cells due to the symmetric distribution of current. The underlying physics is qualitatively interpreted according to the quasistatic electric and magnetic dipole coupling model combined by the calculated field distributions.