Giant transmission Goos–Hnchen shift in surface plasmon polaritons excitation and its physical origin

Excitation of surface plasmon polaritons(SPPs) propagating at the interface between a dielectric medium and a silver thin film by a focused Gaussian beam in a classical Kretschmann prism setup is studied theoretically. We find that the center of the transmitted Gaussian evanescent wave has a giant lateral shift relative to the incident Gaussian beam center for a wide range of incident angle and Gaussian beam wavelength to excite SPPs, which can be more than two orders of magnitude larger than the silver film thickness. The phenomenon is closely related with the conventional Goos–Hnchen effect for total internal reflection of light beam, and it is called the transmission Goos–Hnchen shift. We find that this lateral shift depends heavily on the excitation wavelength, incident angle, and the silver layer thickness. Finite-difference time-domain simulations show that this transmission Goos–Hnchen shift is induced by a unique dynamical process of excitation, transport, and leakage of SPPs.     

Tunable Bistability in the Goos–H?nchen Effect with Nonlinear Graphene

We present a planar model system of a silica covered with a monolayer of nonlinear graphene to achieve a tunable Goos–H?nchen(GH) shift in the terahertz range. It is theoretically found that the transition between a negative shift and a large positive one can be realized by altering the intensity of incident light. Moreover, by controlling the chemical potential of graphene and the incident angle of light, we can further control the tunable GH shift dynamically. Numerical simulations for GH shifts based on Gaussian waves are in good agreement with our theoretical calculations.     

Modulation and enhancement of photonic spin Hall effect with graphene in broadband regions

The photonic spin Hall effect(SHE)holds great potential applications in manipulating spin-polarized photons.However,the SHE is generally very weak,and previous studies of amplifying photonic SHE were limited to the incident light in a specific wavelength range.In this paper,we propose a four-layered nanostructure of prism-graphene-air-substrate,and the enhanced photonic SHE of reflected light in broadband range of 0 THz–500 THz is investigated theoretically.The spin shift can be dynamically modulated by adjusting the thickness of air gap,Fermi energy of graphene,and also the incident angle.By optimizing the structural parameter of this structure,the giant spin shift(almost equal to its upper limit,half of the incident beam waist)in broadband range is achieved,covering the terahertz,infrared,and visible range.The difference is that in the terahertz region,the Brewster angle corresponding to the giant spin shift is larger than that of infrared range and visible range.These findings provide us with a convenient and effective way to tune the photonic SHE,and may offer an opportunity for developing new tunable photonic devices in broadband range.     

Goos–H?nchen-like shift related to spin and valley polarization in ferromagnetic silicene

We study the Goos–H?nchen-like shift of single silicene barrier under the external perpendicular electric field, offresonant circularly polarized light and the exchange field modulation using the stationary-phase method. The results show that the Goos–H?nchen-like shift of silicene resulting from the external perpendicular electric field does not have the characteristics of spin or valley polarization, while that from off-resonant circularly polarized light or the exchange field is spin-polarized. More importantly, the combined effect of the external perpendicular electric field and the exchange field or off-resonant circularly polarized light can cause the Goos–H?nchen-like shift of the system to be spin and valley polarized.It is particularly worth noting that when the three modulations are considered at the same time, as the exchange field changes, the system will have a positive or negative Goos–H?nchen-like shift.     

Polarization-independent terahertz wave modulator based on graphene-silicon hybrid structure

In this study, we propose and demonstrate a broadband polarization-independent terahertz modulator based on graphene/silicon hybrid structure through a combination of continuous wave optical illumination and electrical gating.Under a pump power of 400 mW and the voltages ranging from-1.8 V to 1.4 V, modulation depths in a range of-23%–62% are achieved in a frequency range from 0.25 THz to 0.65 THz. The modulator is also found to have a transition from unidirectional modulation to bidirectional modulation with the increase of pump power. Combining the Raman spectra and Schottky current–voltage characteristics of the device, it is found that the large amplitude modulation is ascribed to the electric-field controlled carrier concentration in silicon with assistance of the graphene electrode and Schottky junction.     

Tunable terahertz plasmon in grating-gate coupled graphene with a resonant cavity

Plasmon modes in graphene can be tuned into resonance with an incident terahertz electromagnetic wave in the range of 1–4 THz by setting a proper gate voltage.By using the finite-difference-time-domain(FDTD)method,we simulate a graphene plasmon device comprising a single-layer graphene,a metallic grating,and a terahertz cavity.The simulations suggest that the terahertz electric field can be enhanced by several times due to the grating–cavity configuration.Due to this near-field enhancement,the maximal absorption of the incident terahertz wave reaches up to about 45%.     

Far-field superlens for nanolithography

A far-field optical lithography is developed in this paper. By designing the structure of a far-field optical superlens, lithographical resolution can be improved by using a conventional UV light source. The finite different time domain numerical studies indicate that the lithographic resolution at 50~nm line width is achievable with the structure shown in this paper by using 365~nm wavelength light, and the light can be transferred to a far distance in the photoresist.     

Experimental investigation on wake profile detection based on laser scattering by bubbles

The optical system for detecting wake profiles based on laser backscattering by bubbles at 180°is reported, in which the monostatic optical geometry is adopted and the power density estimation is used to process bubble scattering signal.The profiles of wakes produced by a two-blade propeller with a diameter of 46 mm at 6000 and 8000 rpm are measured using this system.It is shown that the wake region can be identified,the wake with different shapes can be distinguished,and the fine structure within wakes can be detected.Also,the repeatability of the results is tested experimentally.Results show the feasibility of this system in wake profile detection.     

Optical response of tunable terahertz plasmon in a grating-gated graphene transistor

Tunable terahertz plasmon in a graphene-based device with a grating serving as a top gate is studied. Transmission spectra exhibit a distinct peak in the terahertz region when the terahertz electric field is perpendicular to the grating fingers.Our results show that the extinction in the transmission of single-layer graphene shields beyond 80%. Electronic results further show that the graphene plasmon can be weakly adjusted by tuning the gate voltage. Theoretical calculation also implies that the plasmon frequency of graphene can fall into the terahertz region of 1–2 THz by improving the sustaining ability and capacitance of the top gate.     

Phase-controlled gain, phase shift, and group velocity using a room-temperature active-Raman-gain medium

A four-level tripod active-Raman-gain scheme is analyzed for obtaining phase-controlled gain, phase shift, and group velocity at room temperature. The scheme can be used to eliminate significant probe field atten- uation or distortion which is unavoidable in the scheme based on electromagnetically induced transparency. It is shown that the intensity gain, phase shift, and group velocity of a probe field can be simultaneously manipulated by changing the relative phase of two pump fields. The scheme is also different from that proposed recently by Deng et al. where a probe-field gain always exists. New features of the scheme presented here raise the possibility of designing rapidly responding optical switches and gates for optical information processing.     

Thermally tunable microfiber knot resonator with flexible graphene heater

Efficiently tuning the output intensity of an optical device is of vital importance for the establishment of optical interconnects and networks. Thermo-optical modulation is an easily implemented and convenient approach and has been widely employed in photonic devices. In this paper, we proposed a novel thermo-optical modulator based on a microfiber knot resonator(MKR) and graphene heater. Upon applying voltage to graphene, the resonant property of the MKR could be thermally tuned with a maximum phase shift of 2.1π. Intensity modulation shows a fast optical response time thanks to the high thermal conductivity of graphene and the thin microfiber diameter of the MKR.     

Broadband tunability of surface plasmon resonance in graphene-coating silica nanoparticles

Graphene decorated nanomaterials and nanostructures can potentially be used in military and medical science applications. In this article, we study the optical properties of a graphene wrapping silica core–shell spherical nanoparticle under illumination of external light by using the Mie theory. We find that the nanoparticle can exhibit surface plasmon resonance(SPR) that can be broadly tuned from mid infrared to near infrared via simply changing the geometric parameters. A simplified equivalent dielectric permittivity model is developed to better understand the physics of SPR, and the calculation results agree well qualitatively with the rigorous Mie theory. Both calculations suggest that a small radius of graphene wrapping nanoparticle with high Fermi level could move the SPR wavelength of graphene into the near infrared regime.     

Asymmetric light propagation based on semi-circular photonic crystals

A new structure based on a semi-circular photonic crystal is proposed to achieve asymmetric light propagation. The semi-circular photonic crystal structure proposed in this paper is a deformation of a two-dimensional conventional square photonic crystal. Through the directional bandgap of the semi-circular photonic crystal, the light from one direction can transfer to the other side, but the light from the opposite direction cannot. A high contrast ratio is obtained by designing the constitutive parameters of the photonic crystal and choosing the suitable light frequency. This structure promises a significant potential in optical integration and other areas.     

Photonic crystal structures:Beam deflector and beam router

We investigate the optical characteristic,transverse magnetic(TM) and transverse electric(TE) band of twodimensional(2 D) square lattice photonic crystal structure,which is composed of cylindrical air regions positioned at the corners of the square shaped dielectric rods.We obtain the wide photonic bandwidths between TM1–TM2 and TM3–TM4 bands.According to the results,we demonstrate the band gaps close to each other in the TM and TE frequencies for proposed structures.The resulting photonic gaps are formed to be about 8% at the higher frequencies of TE modes(TE4–TE5)and TM modes(TM7–TM8 and TM9–TM10).In addition,we examine isotropically generated structures for light guiding properties and observe that the light is directed in a particular route without using any deflection.We also investigate the self-collimation effect with the designed structure.The obtained results reveal the influences of the radius of cylindrical air holes and the angle between these air holes on absolute and partial photonic band gaps.Moreover,we observe the TM and TE band gaps that overlap.It is thought that the obtained band overlap will provide an easy way to produce the photonic crystals in practical applications like photonic insensitive waveguide.It is also believed that these results can provide the photonic crystal structures to work as a beam deflecting and beam router in integrated optical circuit applications.     

Pulse compression and supercontinuum at different powers of femtosecond pulses in water

Propagation of femtosecond light pulses in water is investigated.By changing input power from slightly above to highly above threshold for self-focusing,it is shown that either group velocity dispersion(GVD) or multi-photon ionization(MPI)and multi-photon absorption(MPA)may arrest the collapse.When MPI and MPA dominate the propagation,the pulse can be self-compressed to a few optical cycles.In spectral domain,MPI alone induces a strong blue shift.MPA can restrain this strong blue shift.     

Supercontinuous high harmonic generation from asymmetric molecules in the presence of a terahertz field

We have investigated high-order harmonic generation from asymmetric molecules. It is found that supercontinuous high harmonics,which are produced from asymmetric molecules by significantly steering the ionization,are broken down when the electric field of the 5-fs driving laser pulse is increased to 0.16 a.u. The high harmonic generation from asymmetric molecules with the presence of a terahertz field is also investigated. This reveals that the terahertz controlled laser pulse significantly increases the energy difference between photons,emitted from the ejected electrons,in the first and second halves of the optical cycle at the centre of the driving laser pulse. In this way,a 200-eV broadband supercontinuum can be produced in the plateau,from which a 60-as pulse with a bandwidth of 60 eV can be directly obtained with a minor post-pulse.     

Engineering Photonic Crystal Impurity Bands for Multiple Channelled Optical Switches

We report theoretically on the engineering of photonic crystal impurity bands to realize multiple channelled optical switches. The mechanism is based on the confinement of the impurity band in a photonic quantum-well structure, leading to N-quantized confined states coming from N-coupled defects. Due to the strong localization of electromagnetic wave at defect regions, the transmission of confined states are greatly dependent on the defects and then multiple channelled optical switches can be realized by slightly changing the defects by a control light. The dependence of the threshold of such a switch on the layer number of photonic barriers is also given.     

Photodynamics of Ga_(Zn)–V_(Zn) complex defect in Ga-doped ZnO

The wide-band-gap II–VI compound semiconductor ZnO is regarded as a promising single-photon emission(SPE)host material. In this work, we demonstrate that a(GaZn–VZn)-complex defect can readily be obtained and the density can be controlled in a certain range. In analogy to nitrogen vacancy centers, such a defect in ZnO is expected to be a new single photon source. The optical properties of the(GaZn–VZn)-complex defect are further studied by photoluminescence and time-resolved photoluminescence spectra measurements. The electron transitions between the defect levels emit light at ～ 650 nm with a lifetime of 10–20 nanoseconds, indicating a good coherent length for SPE. Finally, a two-level emitter structure is proposed to explain the carrier dynamics. We believe that the photodynamics study of the(GaZn–VZn)-complex defect in this work is important for ZnO-based quantum emitters.     

The design and preparation of a Fabry–Pérot polarizing filter

A novel single-cavity narrowband Fabry–Pe′rot(FP) polarizing filter at normal incidence,constructed from a sandwich structure with sculptured anisotropic space layer and symmetric isotropic HR mirrors,is designed and prepared.The optical performances of transmittance,phase shift,central wavelength,and bandwidth for two polarized states are analyzed with the characteristic matrix.The numerical studies accord reasonably well with the experimental results.It is demonstrated that the polarization state of the electromagnetic wave and phase shift can be modulated by employing an anisotropic space layer in the polarizing beam splitter system.The birefringence of the anisotropic space layer provides a sophisticated phase modulation by varying the incidence angles over a broad range to have a wide-angle phase shift.     

Manipulating optical Tamm state in the terahertz frequency range with graphene

The optical Tamm state(OTS), which exists generally at the interface between metal and a dielectric Bragg mirror, has been studied extensively in the visible and near infrared spectra. Nevertheless, OTS in the terahertz(THz) region normally receives far less attention. In this Letter, we demonstrate the physical mechanism of OTS at the interface between graphene and a dielectric Bragg mirror in the THz frequency band by applying the transfer matrix method and dispersion characteristics. Based on such mechanisms, we propose an efficient method that can precisely generate and control OTS at a desired angle and frequency. Moreover, we show that the OTS is dependent on the optical conductivity of graphene, making the graphene–dielectric-Bragg-mirror a good candidate for dynamic tunable OTS device in the THz frequency range.