Nonlocal Metasurface with Chiral Exceptional Points in the Telecom-Band.

The exceptional point (EP) is the critical phase transition point in parity-time (PT) symmetry systems, offering many unique physical phenomena, such as a chiral response. Achieving chiral EP in practical applications has been challenging due to the delicate balance required between gain and loss and complicated fabrication, limiting both working band and device miniaturization. Here, we proposed a nonlocal metasurface featuring orthogonal gold nanorods, where loss modulation is achieved through rod size and lattice pitch. By tuning the coupling strength, we experimentally observed the PT symmetry phase transition and chiral EP in the telecom-band. The experimental and simulated circular conversion dichroism at EP reach 0.79 and 0.99, respectively. We also demonstrated an abrupt phase flip of a specific component near EP theoretically. This work provides a feasible scheme for exploring EP in polarized space within the telecom-band, which may find applications in polarization control, wavelength division multiplexing, ultrasensitive sensing, imaging, etc.

Parity-time symmetry breaking optical nanocircuit.

Gain and loss balanced parity-time (PT) inversion symmetry has been achieved across multiple platforms including acoustics, electronics, and photonics. Tunable subwavelength asymmetric transmission based on PT symmetry breaking has attracted great interest. However, due to the diffraction limit, the geometric size of an optical PT symmetric system is much larger than the resonant wavelength, which limits the device miniaturization. Here, we theoretically studied a subwavelength optical PT symmetry breaking nanocircuit based on the similarity between a plasmonic system and an RLC circuit. Firstly, the asymmetric coupling of an input signal is observed by varying the coupling strength and gain-loss ratio between the nanocircuits. Furthermore, a subwavelength modulator is proposed by modulating the gain of the amplified nanocircuit. Notably, the modulation effect near the exceptional point is remarkable. Finally, we introduce a four-level atomic model modified by the Pauli exclusion principle to simulate the nonlinear dynamics of a PT symmetry broken laser. The asymmetric emission of a coherent laser is realized by full-wave simulation with a contrast of about 50. This subwavelength optical nanocircuit with broken PT symmetry is of great significance for realizing directional guided light, modulator and asymmetric-emission laser at subwavelength scales.

Efficient four-wave mixing based on multiple plasmonic resonance.

Plasmonic nanostructures provide a new way to improve nonlinear optical effects. As a mode of surface plasmons (SP), localized SPs can highly localize and enhance electromagnetic fields within a subwavelength volume. In this work, we developed a one-dimensional V-groove Ag nanograting. Through simulation, we realized triple-resonance enhanced four-wave mixing (FWM), in which both the excitation and signal waves are in resonance with LSPRs modified by propagating SPs, and can perfectly overlap with each other in each single nanogroove. Compared with that from a flat Ag plate, the FWM enhancement factor can be over six orders of magnitude. Next, we filled the Ag V-groove with nonlinear polymer 2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene and further improved the enhancement factor to eight orders of magnitude, together with a conversion efficiency of 1.02×10-2. Finally, by changing the water filling ratio, the FWM signal is tuned over 180 nm, while keeping the enhancement factor over seven orders of magnitude.

Chromaticity-tunable white random lasing based on a microfluidic channel.

The color and/or chromaticity controllability of random lasing is a key factor to promote practical applications of random lasers as high luminance sources for speckle-free imaging. Here, white coherent random lasing with tunable chromaticity is obtained by using broadband enhancement Au-Ag nanowires as scatterers and the resonance energy transfer process between different dyes in the capillary microfluidic channel. Red, green and blue random lasers are separately fabricated with low thresholds, benefiting from the plasmonic resonance of the nanogaps and/or nanotips with random distribution and sizes within Au-Ag nanowires and positive optical feedback provided by the capillary wall. A white random laser system is then designed through reorganizing the three random lasers. And, the chromaticity of the white random laser is flexibly tunable by adjusting pump power density. In addition, the white random laser has anisotropic spectra due to the coupling role between the lasers. This characteristic is then utilized to obtain different random lasing with different chromaticity over a broad visible range. The results may provide a basis for applying random laser in the field of high brightness illumination, biomedical imaging, and sensors.

Temporal profiles for measuring threshold of random lasers pumped by ns pulses.

The working threshold is an important parameter to assess the performance of cavity-free random lasers. Here, the temporal profile measurement is proposed as an alternative method to determine the thresholds of the surface plasmon based random lasers pumped by ns pulses based on analyzing the delay time (t Delay) and rising time (t R) of the emission signal. The obvious and slight inflection points of the curves of t Delay and t R varying with the pump power density are observed as indicators for the thresholds of random lasing and for the transition of lasing mode, respectively. The proposed method supplies consistent values to those supplied by traditional methods in frequency-domain for the random systems with different gain length. The demonstrated temporal profile approaches are free from the spectrometers and may be as a candidate for measuring the threshold of random lasers in ultrafast optics, nonlinear optics and bio-compatible optoelectronic probes.

Broadband high efficiency asymmetric transmission of achiral metamaterials.

Asymmetric transmission (AT) effect has attracted great interest in recent years, due to its potential application in integrated photonics from GHz to optical frequency. To realize AT effect, numerous metamaterials have been proposed, mainly based on the chirality of the structure. In this paper, we demonstrate that achiral metamaterials can also have AT effect. Furthermore, it is shown that modal conversion is more essential than chirality to achieve AT effect. In particular, we have proposed a mirror symmetric metamaterial with broadband high efficiency AT effect for circular polarization wave operating at THz region. With further optimization of the unit cell, >80% of the central frequency bandwidth and average 74.05 (maximum150) transmission ratio can be obtained. The idea demonstrated here can also be applied to other frequency regions.

Single-excitation dual-color coherent lasing by tuning resonance energy transfer processes in porous structured nanowires.

Single-excitation dual-color coherent lasing was achieved in a mixed random system of a binary dye and the suspension of gold-silver porous nanowires with plenty of nanogaps. This greatly enhanced the local electromagnetic field in the visible range and guaranteed a low threshold and high Q factor (>10 000) operator for simultaneous dual-color lasing. By tuning the resonance energy transfer process in the stimulated emission, triple output modes (single chartreuse lasing, chartreuse and red dual-color lasing, and single red coherent lasing) were easily obtained. This triple-mode coherent random lasing introduces a new approach to designing multi-functional micro-optoelectronic devices for multi-color speckle-free imaging and interference.

A plasmonic random laser tunable through stretching silver nanowires embedded in a flexible substrate.

A mechanically-tunable random laser based on a waveguide-plasmonic scheme has been investigated. This laser can be constructed by spin coating a solution of polydimethylsiloxane doped with the rhodamine 6G organic dye and silver nanowires onto a silicone rubber slab. The excellent overlap of the plasmon resonance peak of the silver nanowires with both the pump wavelength and the photoluminescence spectrum provides the low threshold and tuning properties of the random laser. The random laser wavelength can be tuned from 558 to 565 nm by stretching the soft substrate, which causes reorientation and breakage of the silver nanowires. The polarization state of the random laser can also be changed from random polarization to partial polarization by stretching. The laser performance remains unchanged after the stretching and restoration experiments. These results not only enable easy realization of an ultrathin flexible plasmonic random laser but also provide insights into the mechanisms of three-dimensional plasmonic feedback random lasers.

Cascade-pumped random lasers with coherent emission formed by Ag-Au porous nanowires.

A series of sequentially cascade-pumped random lasers is reported. It consists of three random lasers in which the Ag-Au bimetallic porous nanowires play the role of scatterers, and the gain materials are coumarin 440 (C440), coumarin 153 (C153), and rhodamine 6G (R6G), respectively. The random laser with C440 is first pumped by a 355 nm pulsed laser. The emission of C440 pumps the C153, and the emission of C153 pumps the R6G sequentially. Low-threshold coherent emissions from the three random lasers are observed. The cascade-pumped random lasers can be achieved easily with low cost and can be used in applications conveniently.

Localization of electromagnetic wave with continuous eigenmodes in free space cavities of cylindrical or arbitrary shapes.

A scheme for constructing the electromagnetic localization structure (ELS) is proposed based on the transformation optics. The ELS may have a free space cavity of cylindrical or arbitrary shapes enclosed by a metamaterials layer. The electromagnetic field can be localized in the free space cavity with no energy leaked in the metamaterials layer and the eigenmodes of the cavity is continuous, which are novel properties that the reported metamaterials ELSs could not realize. The principle and feasibility of the scheme are described in detail through the cylindrical ELS. It is shown that all the material parameters of the designed cylindrical ELS change smoothly with finite values. Therefore it is more practical than the reported metamaterials ELS. In the designing of ELS, the space transformation function was solved via solving the Laplace equation with the Dirichlet boundary condition, which makes it possible to design the ELS of arbitrary shape. The viability of the ELS with arbitrary shape is analyzed and demonstrated by the full-wave numerical simulations.

Achieving laser ignition using zero index metamaterials.

The possibility of laser ignition using zero index metamaterials (ZIM) is investigated theoretically. Using this method, multiple laser beams can be focused automatically regardless of the number and location of electromagnetic sources in the ZIM, and it is not necessary to aim all beams at the focus. Our calculation shows that although the size of the focus is dependent on the wavelength, the beams can be focused to a tiny spot. Methods for achieving practical performance are proposed.

Controlling magnetic dipole transition with magnetic plasmonic structures.

A plasmonic structure with double gold patches is proposed for enhancing the spontaneous emission of a magnetic dipole transition through a magnetic hot area. A Purcell factor of nearly 2000 can be obtained at optical frequencies together with a low sensitivity in spatial and spectral mismatches between the light emitter and the resonance mode. The associated resonance can be tuned from the visible to the IR frequencies, enabling efficient control of forbidden transitions using plasmonic structures.

Band-edge oscillations of the diffraction spectrum of a volume hologram investigated by the air-doping model.

Oscillation exists at the high-frequency band edge in the diffraction spectrum of a volume hologram. An air-doping model of a volume hologram is proposed to explain the phenomenon. The numerical results show good agreement with the experimental results, which cannot be explained by the conventional models. The results show that the position of air impurity is the key factor to control the number and width of the oscillations. The present work gives a new approach to generate and control the defect mode in a holographic photonic crystal.

[Spectroscopic properties of Pr3+ doped transparent oxyfluoride vitroceramics].

In the present paper, the room-temperature absorption spectrum of Pr+ -doped transparent oxyfluoride vitroceramics (Pr(0.2):FOV) was studied systematically. The optical characterisation of Pr(0.2):FOV was performed. The standard and modified Judd-Ofelt theories were used to determine the J-O intensity parameters. The problems with standard Judd-Ofelt theorie for Pr3+ were discussed. Based on the intensity parameters, some predicted optical parameters, such as the spontaneous radiative transition probabilities, radiative lifetimes, branching ratios and integrated emission cross section were calculated. And the application of Pr:FOV was analyzed. Especially there are large oscillator strength and large integrated emission cross section in the transitions of (3)P0-->(3)H4, (3)P1-->(3)H5 and (3)P0-->(3)H6, (3)P0-->(3)F2. So, they are more worthy of attention. The obtained spectroscopic results show the potential application of the Pr3+ -doped oxyfluoride vitroceramics for solid-state lasers.

[Optical parameters of Er3+ in Er3+ : YVO4].

In the present paper the authors firstly measured the absorption spectra of Er3+ in the sample Er3+ : YVO4 (0.5%), then calculated the intensity parameters are calculated by using the Judd-Ofelt theory. After that the authors dealed with some predicted spectroscopic parameters, such as the oscillator strength, spontaneous radiative transition rate, branching ratio and integrated emission cross section. And Er : YVO4 crystal application value has been analyzed with the optical parameters. Especially there are large oscillator strengths and large integrated emission cross sections in the transitions of 4 I1/2 --> 4 I15/2, 2 H11/2 --> 4I15/2, 4S3/2 --> 4 I15/2, and 4F9/2 --> 4 I15/2. So, they are more worth of attention. Moreover, by comparing the Er-doped yttrium vanadate crystal and other Er-doped crystal optical properties, the authors can see the advantages of YVO4 as laser crystal. Finally, the authors discussed the splitting of the energy levels of Er3+ in the crystal YVO4 based on the group theory.

Electromagnetic localization based on transformation optics.

Localization of an electromagnetic field can be achieved by transformation optics using metamaterials. A coordinate transformation structure different from traditional resonator is proposed. Wherein, arbitrary frequency of the whole band of electromagnetic wave can be localized without energy loss, i.e., the modes in this structure are continuous. Theoretical analysis and numerical simulation show that the material parameter variations at the outer boundary of the structure have little influence on the localization property. When realizable physical structure is considered, multi-layer approximation should be applied. The calculated results show that the estimated localization time is about 100 ns for an 8-layer inhomogeneous approximation, and it could reach several seconds for a 30-layer homogeneous approximation. The present work may present a new application of transformation optics.

Polarization controller based on embedded optical transformation.

A universal 2D transformation formula for embedded transformation optics is suggested. Linear and nonlinear transformation results using the suggested formula under different conditions reveal several interesting phenomena and potential applications, and designs for various polarization controllers can be achieved based on this idea. For example, an incident Gaussian beam can be transformed into a typical spherical wave, and a "cloak" based on a polarization splitter is proposed for the first time.

Amplification of stimulated Brillouin scattering of two collinear pulsed laser beams with orthogonal polarizations.

A polarization-controlling device was developed based on the fact that there can be a time delay between the seeder and the pumping beams during the amplification of a stimulated Brillouin scattering signal. The device causes two coaxially transmitted pulsed beams with orthogonal polarizations to have the same polarization in order to implement amplification by the pumping effect. An experiment showed that good pumping amplification can be achieved by using this technique.

Stimulated Raman scattering enhanced by stimulated Brillouin scattering.

A phenomenon of stimulated Raman scattering enhanced by stimulated Brillouin scattering is investigated experimentally. Also, this phenomenon is analyzed based on physical mechanisms. It is shown that the pumping effect of stimulated Brillouin scattering on backstimulated Raman scattering is a commonly existing rule regardless of the experiment conditions.

[The upconversion luminescence of Tm(0.35)Yb(5):FOV nanophase oxyfluoride vitroceramics].

The upconversion luminescence of nanophase oxyfluoride vitroceramics Tm(0.35)Yb(5) : FOV when excited by a 975 nm diode laser was studied in the present paper. Several ultraviolet upconversion luminescence lines positioned at 363.6 nm, (462.6 nm, 477.0 nm), 648.7 nm, (699.7 nm, 680.7 nm) and (777.6 nm, 800.7 nm) were found. They can be attributed to the fluorescence transitions of 1 D2-->3 H6, 1 G4-->3 H6, 1 G4-->3 F4, 3 F3-->3 H6 and 3 H4-->3 H6 of Tm3+ ion. The careful measurement and analysis of the variation of upconversion luminescence intensity F as a function of the 975 nm pumping laser power P has proven that the upconversion luminescence of 1 D2 state is partly a five-photon upconversion luminescence, while the upconversion luminescence of 1 G4 and 3 H4 state is the three-photon and two-photon upconversion luminescence respectively.