Inelastic two-wave mixing induced high-efficiency transfer of optical vortices.

A scheme for high-efficiency transfer of optical vortices is proposed by an inelastic two-wave mixing (ITWM) process in an inverted-Y four-level atomic medium, which is originally prepared in a coherent superposition of two ground states. The orbital angular momentum (OAM) information in the incident vortex probe field can be transferred to the generated signal field through the ITWM process. Choosing reasonable experimentally realizable parameters, we find that the presence of the off-resonance control field can greatly improve the conversion efficiency of optical vortices, rather than in the absence of a control field. This is caused by the broken of the destructive interference between two one-photon excitation pathways. Furthermore, we also extend our model to an inelastic multi-wave mixing process and demonstrate that the transfer efficiency between multiple optical vortices strongly depends on the superposition of the ground states. Finally, we explore the composite vortex beam generated by collinear superposition of the incident vortex probe and signal fields. It is obvious that the intensity and phase profiles of the composite vortex can be effectively controlled via adjusting the intensity of the control field. Potential applications of our scheme may exist in OAM-based optical communications and optical information processing.

Simultaneous Incorporation of CsI in the Two-step Deposition Process Boosts the Power Conversion Efficiency and Stability of Perovskite Solar Cells.

Two-step deposition method has been widely exploited to fabricate FA1-x Csx PbI3 perovskite solar cells. However, in previous studies, CsI is mainly added into the PbI2 precursor with DMF/DMSO as solvent. Here in this study, a novel method to fabricate FA1-x Csx PbI3 perovskite has been proposed. The CsI is simultaneously added into the PbI2 precursor and the organic FAI/MACl salts solution in our modified two-step deposition process. The resulting FA1-x Csx PbI3 film exhibits larger perovskite crystals and suppressed defect density (4.05×1015  cm-3 ) compared with the reference perovskite film (9.23×1015  cm-3 ) without CsI. Therefore, the obtained FA1-x Csx PbI3 perovskite solar cells have demonstrated superior power conversion efficiencies (PCE=21.96 %) together with better long-term device stability.

Fast near-infrared photodetectors from p-type SnSe nanoribbons.

Low-dimensional tin selenide nanoribbons (SnSe NRs) show a wide range of applications in optoelectronics fields such as optical switches, photodetectors, and photovoltaic devices due to the suitable band gap, strong light-matter interaction, and high carrier mobility. However, it is still challenging to grow high-quality SnSe NRs for high-performance photodetectors so far. In this work, we successfully synthesized high-quality p-type SnSe NRs by chemical vapor deposition and then fabricated near-infrared photodetectors. The SnSe NR photodetectors show a high responsivity of 376.71 A W-1, external quantum efficiency of 5.65 × 104%, and detectivity of 8.66 × 1011Jones. In addition, the devices show a fast response time with rise and fall time of up to 43µs and 57µs, respectively. Furthermore, the spatially resolved scanning photocurrent mapping shows very strong photocurrent at the metal-semiconductor contact regions, as well as fast generation-recombination photocurrent signals. This work demonstrated that p-type SnSe NRs are promising material candidates for broad-spectrum and fast-response optoelectronic devices.

Optical nonreciprocity and nonreciprocal photonic devices with directional four-wave mixing effect.

A scheme for magnetic-free optical nonreciprocity in an ensemble of four-level cold atoms is proposed by exploiting the directional four-wave mixing effect. Using experimentally achievable parameters, the nonreciprocal optical responses of the system can be observed and the conversion on nonreciprocal transmission and nonreciprocal phase shift can be implemented. These nonreciprocal phenomena originate from the directional phase matching, which breaks the time-reversal symmetry and dynamic reciprocity of the cold atomic system. Moreover, by embedding the cold atoms into a Mach-Zehnder interferometer and choosing proper parameters, a two-port optical isolator with an isolation ratio of 79.70 dB and an insertion loss of 0.35 dB and a four-port optical circulator with a fidelity of 0.9985 and a photon survival probability of 0.9278 can be realized, which shows the high performance of isolation and circulation. The proposal may enable a new class of optically controllable cavity-free nonreciprocal devices in optical signal processing at the low light level.

Coherent control of spatial and angular Goos-Hänchen shifts with spontaneously generated coherence and incoherent pumping.

We propose an efficient scheme to manipulate the Goos-Hänchen (GH) shift of a reflected beam from a metal-clad waveguide, where a coherent atomic medium with a Λ-type configuration is employed as the substrate. Using experimentally achievable parameters, we identify the conditions under which spontaneously generated coherence (SGC) allows us to enhance the spatial and angular GH shifts of the reflected beam. With the help of SGC, the relative phases of the probe and control fields can alter the absorption gain and refractive index of the atomic medium, thereby manipulating the magnitudes, signs, and positions of the spatial and angular shifts. Furthermore, the spatial and angular GH shifts can be coherently controlled via adjusting the incoherent pumping rate and the intensity of the control field. Our proposal provides an avenue for the manipulation of spatial and angular GH shifts and potential applications in optical switching and optical steering.

High-precision three dimensional atom localization via multiphoton quantum destructive interference.

We propose an effective scheme for high-precision three dimensional(3D) atom localization via measuring the population of excited state in a four-level atomic system driven by a probe field and three orthogonal standing-wave fields. In this scheme, the position-dependent multiphoton quantum destructive interference leads to multiphoton excitation of the excited state and enhances the fluorescence emission. We show that adjusting the frequency detuning and phase shifts associated with the standing-wave fields can modify the multiphoton quantum destructive interference and lead to a redistribution of the atoms. The maximal probability of finding the atom at the certain position in one period of the standing-wave fields can be 100% and the highest spatial precision is about 0.02λ.

Force measurement in squeezed dissipative optomechanics in the presence of laser phase noise.

We investigate the force measurement sensitivity in a squeezed dissipative optomechanics within the free-mass regime under the influence of shot noise (SN) from the photon number fluctuations, laser phase noise from the pump laser, thermal noise from the environment, and optical losses from outcoupling and detection inefficiencies. Generally, squeezed light could generate a reduced SN on the squeezed quadrature and an enlarged quantum backaction noise (QBA) due to the antisqueezed conjugate quadrature. With an appropriate choice of phase angle in homodyne detection, QBA is cancellable, leading to an exponentially improved measurement sensitivity for the SN-dominated regime. By now, the effects of laser phase noise that is proportional to laser power emerge. The balance between squeezed SN and phase noise can lead to an sub-SQL sensitivity at an exponentially lower input power. However, the improvement by squeezing is limited by optical losses because high sensitivity is delicate and easily destroyed by optical losses.

Perfectly asymmetric Raman-Nath diffraction in disordered atomic gratings.

We investigate the effects of geometrical and structural disorders on perfectly asymmetric diffraction (PAD) in Raman-Nath regime. The two types of disorders are realized by introducing random fluctuations in the position and width of one-dimensional (1D) driven atomic lattices. Raman-Nath diffraction is modified differently with respect to the geometrical and structural disorders. It is shown that the PAD is observed with a certain strength range of geometrical disorder, exceeding which it can be destroyed, while the PAD is rather robust against structural disorder. The different behaviors originate from the disorder-induced random variations of the spatial phase shifts of the standing-wave (SW) coupling field and atomic lattices with Gaussian profile. Furthermore, we find that, in the presence of geometrical disorder, the PAD is more susceptible to correlated disorder than to uncorrelated disorder. Our scheme may be useful for understanding the effects of disorder on the diffraction of light and matter waves in disordered potentials..

Highly sensitive mass detection based on nonlinear sum-sideband in a dispersive optomechanical system.

We present a theoretical scheme to realize high-sensitive mass detection in a dispersive optomechanical system (DOMS) via nonlinear sum-sideband. In this scheme, DOMS assisted by a degenerate parametric amplifier (DPA) provides a well-established optomechanical circumstance, where nonlinear optomechanical interaction between cavity mode and mechanical mode of dielectric membrane is expected for creating the frequency components at optical sum-sideband. Such a scheme for mass detection mainly relies on monitoring the conversion efficiency of generated sum-sideband after the added mass is absorbed on the dielectric membrane. Using experimentally achievable parameters, we find that the conversion efficiency of sum-sideband and the sensitivity of mass detection can be simultaneously improved when the nonlinear gain of DPA increases. Furthermore, our results also demonstrate that this mass detection of DOMS can reach femtogram (fg) level resolution, when the method of mass detection relies on a direct relationship between maximum efficiency of sum-sideband and mass-change of membrane.

Propagation of toroidal localized spoof surface plasmons using conductive coupling.

Here, on a platform of two split-ring resonator (SRR) disks in the microwave regime, we have numerically and experimentally investigated the coupling of toroidal localized spoof surface plasmons (LSSPs). The coupling effect is investigated both theoretically and experimentally. We observe that magnetic dipole coupling exists in the toroidal LSSPs coupling and causes a rearrangement of the toroidal LSSPs, which suppresses the propagation of toroidal LSSPs. To realize the propagation of toroidal LSSPs, we introduce conductive coupling into the SRR disks. The conductive coupling can correct magnetic dipole coupling and enhance toroidal LSSPs coupling. Both numerical simulations and experiments are in good agreement. The toroidal LSSPs can be effectively propagated, even in the three right-angle-bent SRR disks. This study paves the way toward a better understanding of toroidal LSSPs coupling and finds many applications in the transfer of electromagnetic energy using toroidal moments.

Lop-sided Raman-Nath diffraction in PT-antisymmetric atomic lattices.

Two-dimensional (2D) optical lattices of driven cold atoms can provide a useful platform to construct 2D electromagnetically induced grating (EIG) with parity-time (PT) antisymmetry. This atomic grating is achieved by the spatial modulations of the atomic density and frequency detunings in the four-level double-Λ atomic system. Gain-assisted PT antisymmetry allows us to realize lop-sided Raman-Nath diffraction with high diffraction efficiency at the exception point. It is shown that the nontrivial phenomenon originates from non-Hermitian degeneracy of PT antisymmetry. Our scheme may provide the possibility for active all-optical control and conversion of the spatial beam in optics.

Quadrature squeezing of a higher-order sideband spectrum in cavity optomechanics.

We propose an efficient scheme to generate quadrature squeezing of a higher-order sideband spectrum in an optomechanical system. This is achieved by exploiting a well-established optomechanical circumstance, where a second-order nonlinearity is embedded into the optomechanical cavity driven by a strong control field and a weak probe pulse. Using experimentally achievable parameters, we demonstrate that the second-order nonlinearity intensity and the frequency detuning of a control field allow us to modify the amplitude of higher-order sidebands and improve the amount of squeezing of a higher-order sideband spectrum. Furthermore, in the presence of a strong second-order nonlinearity, an optimizing quadrature squeezing of a higher-order sideband spectrum can be achieved, which provides a practical opportunity to design the squeezed frequency combs and other precision measurements.

Effective hyper-Raman scattering via inhibiting electromagnetically induced transparency in monolayer graphene under an external magnetic field.

We propose and analyze an effective scheme to generate hyper-Raman scattering via inhibiting electromagnetically induced transparency (EIT) in a monolayer graphene under a magnetic field. By solving the Schrödinger-Maxwell formalism, we derive explicitly analytical expressions for linear susceptibility, nonlinear susceptibility, and generated Raman electric field under the steady-state condition. Based on dressed-state theory, our results show a competition between EIT and hyper-Raman scattering, and the hyper-Raman process is totally dominant when multiphoton destructive interference is completely suppressed.

Ultrafast optical switching in quantum dot-metallic nanoparticle hybrid systems.

We study ultrafast excitonic population inversion resulting from the interaction of a semiconductor quantum dot (SQD) with localized surface plasmons. The plasmonic enhanced fields are generated when a metallic nanoparticle (MNP) is subject to a nonlinear chirped few-cycle pulse train. By numerically solving the time-dependent Bloch equations beyond the rotating-wave approximation, we show that the complete population inversion can be achieved for small interparticle distance and the dynamic in population inversion exhibits a steplike transition between absorption and amplifying. This phenomenon can be exploited as an all-optical ultrafast switching device. Moreover, the final state of population inversion is shown to be modified significantly with the interparticle distances, which is not only robust against the variation of probe pulse parameters but also suggests a straightforward method for measuring the interparticle distances via probing the final populations.

High-order harmonics in a quantum dot and metallic nanorod complex.

We investigate the high-order harmonic generation (HHG) in a semiconductor quantum dot (SQD) and metallic nanorod (MNR) complex driven by a moderate intensity (<10(12) W/cm(2)) frequency-chirped Gaussian few-cycle pulse. Our numerical results indicate that the cutoff energy of the HHG can be controlled by optimizing the shape of the MNR and surface-to-surface distance between the SQD and the MNR. We also show that the extreme ultraviolet supercontinuum harmonics (25 eV maximal photon energy) and isolated ultrashort pulses (2.67-4.36 fs FWHM) are achievable.

Tunneling-induced giant Goos-Hänchen shift in quantum wells.

Tunneling-induced quantum interference experienced by an incident probe in the asymmetric double AlGaAs/GaAs quantum well (QW) structure can be modulated by means of an external control light beam and the tunable coupling strengths of resonant tunneling. These phenomena can be exploited to devise a novel intracavity medium to control Goos-Hänchen (GH) shifts of a mid-infrared probe beam incident on a cavity. For a suitably designed QW structure, our results show that maximum negative shift of 2.62 mm and positive shift of 0.56 mm are achievable for GH shifts in the reflected and transmitted light.

Enhanced four-wave mixing efficiency in four-subband semiconductor quantum wells via Fano-type interference.

We propose and analyze an efficient way to enhance four-wave mixing (FWM) signals in a four-subband semiconductor quantum well via Fano-type interference. By using Schrödinger-Maxwell formalism, we derive explicitly analytical expressions for the input probe pulse and the generated FWM field in linear regime under the steady-state condition. With the aid of interference between two excited subbands tunneling to the common continuum, the efficiency to generate FWM field is found to be significantly enhanced, up to 35%. More interestingly, a linear growth rate in the FWM efficiency is demonstrated as the strength of Fano-type interference increases in presence of the continuum states, which can be maintained for a certain propagation distance (i.e., 50µm).

Lasing on surface states in vertical-cavity surface-emission lasers.

We report experimental observation of lasing on surface states, in the form of standing waves at the termination of a defect-free photonic crystal on top of vertical-cavity surface-emission lasers. Direct images of lasing modes at the truncated periodic potential, along one side of a square lattice, are demonstrated by collecting near-field radiation patterns, as well as in numerical simulations. Our results provide a step toward realizing surface and edge states in optical cavities.

Surface Passivation of Perovskite Solar Cells with Oxalic Acid: Increased Efficiency and Device Stability.

Solution processed perovskite films usually exhibit numerous defect states on the surfaces of the films. Here in this work, oxalic acid (H2 C2 O4 ), which has two C=O groups, is selected and used to passivate the surface defects of the two-step deposited perovskite films via post-treatment. Strong interaction between H2 C2 O4 molecule and the Pb2+ ions located on the surface of perovskite film has been confirmed via Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, which can result in an effective suppress of the surface defects. Furthermore, time-resolved PL spectrum indicates that carrier lifetime is prolonged in the H2 C2 O4 passivated perovskite film. After optimizing the H2 C2 O4 concentration, the target perovskite solar cells can demonstrate superior power conversion efficiencies (21.67 % from reverse measurement and 21.54 % from forward measurement) and superior device-stability.

Nonlinear localized modes in bandgap microcavities.

We study experimentally an electrically pumped GaAs-based bandgap structure based on a vertical cavity surface emitting laser (VCSEL). We demonstrate that a microcavity embedded into this bandgap VCSEL structure supports localized optical modes without any holding beam. We propose a model of surface-structured VCSELs based on a reduced dissipative wave equation for describing electromagnetic modes in such semiconductor cavities and analyze a crossover between linear and nonlinear solitonlike cavity modes.