Gain-assisted indented plasmonic waveguide for low-threshold nanolaser applications

A subwavelength plasmonic indented waveguide with an active InGaAsP core is proposed.The characteristics of the gap plasmon mode and gain required for lossless propagation are investigated and analyzed by the finite element method.We numerically calculate the normalized mode areas and percentages of energy confined in InGaAsP and metal for plasmonic nanolaser applications.It is shown that the indentation of the sidewalls has an optimal value for which the lasing threshold gain is minimal.The structure could enable low-threshold subwavelength lasing and applications for optoelectronic integrated circuits.     

深亚波长约束的表面等离子体纳米激光器研究

本文提出了一种新颖的基于半导体纳米线/空气间隙/金属薄膜 复合结构的表面等离子体纳米激光器, 并给出了理论研究和仿真分析. 这种结构通过金属界面的表面等离子体模式与高增益介质纳米线波导模式耦合, 从而使场增强效应得到显著提高. 同时通过数值仿真研究, 得到该混合波导结构的模式特性和增益阈值随空气槽宽度、纳米线半径的变化规律, 表明它可以实现对输出光场的深亚波长约束, 同时保持低损耗传输和高场强限制能力. 通过最优化选择, 最终得到纳米等离子体激光器的最优结构尺寸. 关键词： 表面等离子体 混合等离子体波导 纳米激光器     

亚波长尺度下混合等离子泄漏模式激光

由于光存在衍射极限,因此传统方法不能实现亚波长尺度下的激光激射.为了打破这一衍射极限,本文设计了金属-介电层-半导体堆叠结构来实现深亚波长尺度下的激光激射,并讨论了相关结构对模式传播的影响.结构设计上,采用低介电常数金属银作为衬底、10?nm厚的LiF作为介电层、具有六边形截面的半导体纳米线ZnO作为高介电常数层,采用...     

Highly directional spaser array for the red wavelength region

The spaser offers an opportunity to achieve coherent optical sources at nanometer scales due to the extreme confinement of optical fields. However, achievement of spasers with directional propagation in the visible wavelength region remains a challenge thus far, owing to the unique optical feedback mechanism and large dissipative losses of the metal cavity. Here, we experimentally demonstrate for the first time a spaser showing highly directional emission in the visible by using a periodic subwavelength hole array perforated in a metal film, which function as plasmonic nanocavities, along with an organic laser dye to supply gain. The lasing occurs in the red wavelength region and shows a single mode. It is suggested that the optical feedback for spasing is provided by the SPP–Bloch wave, which is supported by the fact that no spasing was attained in aperiodic holes as well as in periodic holes that do not support the SPP–Bloch wave at the spasing wavelength.     

A low-threshold nanolaser based on hybrid plasmonic waveguides at the deep subwavelength scale

A novel nanolaser structure based on a hybrid plasmonic waveguide is proposed and investigated. The coupling between the metal nanowire and the high-index semiconductor nanowire with optical gain leads to a strong field enhancement in the air gap region and low propagation loss, which enables the realization of lasing at the deep subwavelength scale.By optimizing the geometric parameters of the structure, a minimal lasing threshold is achieved while maintaining the capacity of ultra-deep subwavelength mode confinement. Compared with the previous coupled nanowire pair based hybrid plasmonic structure, a lower threshold can be obtained with the same geometric parameters. The proposed nanolaser can be integrated into a miniature chip as a nanoscale light source and has the potential to be widely used in optical communication and optical sensing technology.     

Dynamics of amplification in a nanoplasmonic metamaterial

Plasmonic metamaterials form an exciting new class of engineered media that promise a range of important applications, such as subwavelength focusing, cloaking and slowing/stopping of light. Recently it has been shown that the internal losses due to the natural absorption of metals at optical frequencies can be compensated by gain. Here, we employ a Maxwell–Bloch methodology which allows us to study the dynamics of the coherent plasmon-gain interaction, nonlinear saturation, field enhancement and radiative damping. Using numerical pump-probe experiments on a double-fishnet metamaterial with dye-molecule inclusions we investigate the buildup of the inversion and the formation of the plasmonic modes in the low-Q fishnet cavity. We find that loss compensation occurs in the negative-refractive-index regime and that, due to the loss compensation and the associated sharpening of the resonance, the real part of the refractive index of the metamaterial becomes more negative compared to the passive case. Furthermore, we investigate the behavior of the metamaterial above the lasing threshold, and we identify the occurrence of a far-field lasing burst and gain depletion. Our results provide deep insight into the internal processes that affect the macroscopic properties of active metamaterials. This could guide the development of amplifying and lasing plasmonic nanostructures.     

Nano‐opto‐mechanical effects in plasmonic waveguides

In order to achieve interaction between light beams, a mediating material object is required. Nonlinear materials are commonly used for this purpose. Here a new approach to control light with light, based on a nano‐opto‐mechanical system integrated in a plasmonic waveguide is proposed. Optomechanics of a free‐floating resonant nanoparticle in a subwavelength plasmonic V‐groove waveguide is studied. It is shown that nanoparticle auto‐oscillations in the waveguide induced by a control light result in the periodic modulation of a transmitted plasmonic signal. The modulation depth of 10% per single nanoparticle of 25 nm diameter with the clock frequencies of tens of MHz and the record low energy‐per‐bit energies of 10⁻¹⁸ J is observed. The frequency of auto‐oscillations depends on the intensity of the continuous control light. The efficient modulation and deep‐subwavelength dimensions make this nano‐optomechanical system of significant interest for opto‐electronic and opto‐fluidic technologies.     

Subwavelength Hyperlens Resolution With Perfect Contrast Function

Recently it has been shown that plasmonic effects in hyperbolic metamaterials may facilitate overcoming the diffraction limit and enhance the contrast function of an image by filtering background radiation. Unfortunately, the contrast function of such a dark‐field hyperlens degrades in the deep‐subwavelength regime. We push forward the concept of the contrast function revival in the subwavelength imaging by introduction of the proper phase difference between coherent sources. To study this effect we develop a simplified theory of the wave propagation through a hyperbolic metamaterial and show that, in principle, two sources standing apart at any subwavelength distance can be distinguished. We suggest two feasible designs, the first of which employs the obliquely incident light, while the second one is based on a properly designed metasurface. The concept can be used in high‐contrast subwavelength microscopy.     

An eigenvalue method to study the threshold behaviors of plasmonic nano-lasers

An eigenvalue method is proposed to study the threshold behaviors of plasmonic nano-lasers. The medium gain and dispersion are taken into consideration based on semi-classical laser dynamics, and therefore the lasing threshold, mode pattern, and lasing frequency can be theoretically predicted. The lasing properties of dielectric, plasmonic core, and plasmonic shell nano-lasers are investigated in details. It is found that the lasing thresholds of nano-lasers can be reduced by two orders of magnitude when introducing localized surface plasmon modes.     

Subwavelength propagation and localization of light using surface plasmons: A brief perspective

Surface plasmons at the metal–dielectric interface have emerged as an important candidate to propagate and localize light at subwavelength scales. By tailoring the geometry and arrangement of metallic nanoarchitectures, propagating and localized surface plasmons can be obtained. In this brief perspective, we discuss: (1) how surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) can be optically excited in metallic nanoarchitectures by employing a variety of optical microscopy methods; (2) how SPPs and LSPs in plasmonic nanowires can be utilized for subwavelength polarization optics and single-molecule surface-enhanced Raman scattering (SERS) on a photonic chip; and (3) how individual plasmonic nanowire can be optically manipulated using optical trapping methods.     

Terahertz near-field imaging using subwavelength plasmonic apertures and a quantum cascade laser source

The first demonstration, to our knowledge, of near-field imaging using subwavelength plasmonic apertures with a terahertz quantum cascade laser source is presented. "Bull's-eye" apertures, featuring subwavelength circular apertures flanked by periodic annular corrugations were created using a novel fabrication method. A fivefold increase in intensity was observed for plasmonic apertures over plain apertures of the same diameter. Detailed studies of the transmitted beam profiles were undertaken for apertures with both planarized and corrugated exit facets, with the former producing spatially uniform intensity profiles and subwavelength spatial resolution. Finally, a proof-of-concept imaging experiment is presented, where an inhomogeneous pharmaceutical drug coating is investigated.     

Proposed electrically-pumped,III–V-metal hybrid plasmonic lasers

We propose an electrically-pumped hybrid plasmonic laser through the integration of a semiconductor quantum well laser with a thin metal film. Due to the coupling between the TM waveguide mode in the III–V active layer and the LRSPP mode around the metal thin film, light can be confined in both regions and optical gain can be provided by the active layer. We have shown that the quasi-odd supermode is the preferred lasing mode because of its larger confinement factor and lower modal loss compared to the quasi-even supermode. Through optimizing the gap distance between the active region and the metal film, we can obtain low threshold and a large amount of optical energy confined around the metal film for the hybrid plasmonic laser.     

金属微结构纳米线中等离激元传播和分光特性

本文从理论和实验两方面探讨了具有微结构的金属纳米线系统中表面等离激元传播规律和分光特性. 我们由麦克斯韦方程组出发, 利用严格耦合波近似和有限元差分等方法首先从理论上给出了金属纳米线系统中等离激元的色散关系和能带特征, 然后基于微结构的银纳米线及其等离激元能带结构, 设计并制备出等离激元分光原型器件, 实验展示其将不同频率的光在微小空间分离的特性. 该研究结果是我们前期相关工作的延续和补充, 可应用于构造多功能集成的光子芯片和新型亚波长光电材料和器件.     

Theoretical study on the lasing plasmon of a split ring for label-free detection of single molecules and single nanoparticles

This paper reports the plasmonic lasing of a split ring filled with gain material in water. The lasing mode(1500 nm)is far from the pump mode(980 nm), which can depress the detection noise from the pump light. The laser intensities of the two modes simultaneously increase by more than 10~3 in amplitude, which can intensify the absorption efficiency of the pumping light and enhance the plasmonic lasing. The plasmonic lasing is a sensitive sensor. When a single protein nanoparticle(n = 1.5, r = 1.25 nm) is trapped in the gap of the split ring, the lasing spectrum moves by 0.031 nm, which is much larger than the detection limit of 10~(-5) nm. Moreover, the lasing intensity is also very sensitive to the trapped nanoparticle. It reduces to less than 1/600 when a protein nanoparticle(n = 1.5, r = 1.25 nm) is trapped in the gap.     

Broadband amplification of spoof surface plasmon polaritons at microwave frequencies

Efficient amplification of spoof surface plasmon polaritons (SPPs) is proposed at microwave frequencies by using a subwavelength‐scale amplifier. For this purpose, a special plasmonic waveguide composed of two ultrathin corrugated metallic strips on top and bottom surfaces of a dielectric substrate with mirror symmetry is presented, which is easy to integrate with the amplifier. It is shown that spoof SPPs are able to propagate on the plasmonic waveguide in broadband with low loss and strong subwavelength effect. By loading a low‐noise amplifier chip produced by the semiconductor technology, the first experiment is demonstrated to amplify spoof SPPs at microwave frequencies (from 6 to 20GHz) with high gain (around 20dB), which can be directly used as a SPP amplifier device. The features of strong field confinement, high efficiency, broadband operation, and significant amplification of the spoof SPPs may advance a big step towards other active SPP components and integrated circuits.

     

Nanolasers Enabled by Metallic Nanoparticles: From Spasers to Random Lasers

Owing to exotic optical responses, metallic nanoparticles and nanostructures are finding broad applications in laser science, leading to numerous design variations of plasmonic nanolasers. Nowadays, two of the most intriguing plasmonic nanolasing devices are spasers and random lasers. While a spaser is based on a single metallic nanoparticle resonator with the optical feedback provided by the localized surface plasmon resonance, the operation of a random laser relies on multiple light scattering within randomly distributed metallic nanoparticles. In this paper, an up‐to‐date review on the applications of metallic nanoparticles in spasers and random lasers is provided. Principles of a random spaser, a device combining the features of a spaser and a random laser, are briefly discussed as well. The paper is focused on major theoretical and experimental approaches to control the core metrics of lasing performance, including threshold, resonant wavelength, and emission directionality. The applications of spasers and random lasers in the fields of sensing and imaging are also mentioned. Finally, the challenges and future perspectives in this area of research are discussed.     

Ultrasmall subwavelength nanorod plasmonic cavity

We propose an ultrasmall plasmonic cavity consisting of a high-index/low-index dielectric nanorod covered with silver. Full three-dimensional subwavelength confinement of the surface-plasmon polaritons was achieved at the high-index dielectric-silver interface without propagating to the low-index dielectric-silver interface. The numerical simulations showed that the plasmonic mode excited in this cavity has a deep subwavelength mode volume of 0.0038(λ/2n)(3) and a quality factor of 1500 at 40 K, and consequently a large Purcell factor of ～2×10(5). Therefore, this plasmonic cavity is expected to be useful for the demonstration of high-efficiency single photon sources or low-threshold lasers in an ultracompact nanophotonic circuit.     

Plasmonic leaky‐mode lasing in active semiconductor nanowires

Leaky modes are below‐cutoff waveguide modes that lose part of their energy to the continuum of radiation modes during propagation. In photonic nanowire lasers, leaky modes have to compete with almost lossless above‐cutoff modes and are therefore usually prevented from crossing the lasing threshold. The situation is drastically different in plasmonic nanowire systems where the above‐cutoff plasmonic modes are very lossy because of their strong confinement to the metal surface. Due to gain guiding, the threshold gain of the hybrid electric leaky mode does not increase strongly with reduced wire diameter and stays below that of all other modes, making it possible to observe leaky‐mode lasing. Plasmonic ZnO nanowire lasers operating in the gain‐guided regime could be used as coherent sources of surface plasmon polaritons at the nanoscale or as surface plasmon emitting diodes with an emission angle that depends on the nanowire diameter and the color of the surface plasmon polariton.

     

Transverse localization of Tamm plasmon in metal-DBR structure with disordered layer

Transverse localization of the optical Tamm plasmon(OTP) is studied in a metal-distributed Bragg reflector(DBR)structure with a one-dimensional disordered layer embedded at the interface between the metal and the DBR. The embedded disordered layer induces multiple scattering and interference of light, forming the light localization in the transverse direction. This together with the formation of Tamm plasmonic modes at the metal-DBR interface(i.e., the confinement of light in the longitudinal direction), gives birth to the so called transverse-localized Tamm plasmon. It is shown that for both transverse electric(TE) and transverse magnetic(TM) polarized light injection, the excited transverse-localized Tamm plasmon broadens and splits the dispersion curve due to spatial incoherence in the transverse direction, thus proving the stronger light confinement especially in the TE polarized injection. By adding the gain medium, specific random lasing modes are observed. The proposed study could be an efficient way of trapping and locally enhancing light on a subwavelength scale,which is useful in applications of random lasers, optical sensing, and imaging.     

基于多形貌银纳米颗粒的多波长相干随机激光研究

采用溶剂热法分别制备了球形银纳米颗粒和多形貌银纳米颗粒,其中球形银纳米颗粒具有400 nm的窄带等离激元共振峰,而多形貌银纳米颗粒的共振区间在400～700 nm之间,将它们分别掺入R6G与PVP的混合溶液中,利用旋涂法在玻璃基板上制备银纳米颗粒嵌入染料掺杂聚合物薄膜随机激光器。采用纳秒脉冲激光进行随机激光泵浦实验,实验结果表明球形银纳米颗粒染料掺杂聚合物薄膜只有自发辐射峰,而多形貌银纳米颗粒染料掺杂聚合物薄膜具有线宽<0.8 nm的相干随机激光发射光谱,其阈值为1.9 mJ·cm-2, 这可能是由于银纳米颗粒的等离激元共振区间与R6G的发射光谱重叠,支持局域等离激元效应的形成,明显的局域场增强有效地改善了与附近分子的相互作用,从而激发了更多的辐射光子,促进了高增益的形成。进一步,利用多形貌银纳米颗粒在银纳米颗粒染料掺杂聚合物薄膜中随机分布的特性,通过改变泵浦位置,实现了20 nm范围内的随机激光输出波长的调控,具体输出范围为590.1～610.4 nm。认为这是由于多形貌银纳米颗粒在不同位置的组成和分布不同,改变了表面等离激元的相互作用和光子的散射能力,从而形成不同的增益效应和不同的封闭光振荡路径。此外,考虑到多形貌银纳米颗粒的共振波长较宽,探究了其用于输出其他颜色光的可能性。以与上述银纳米颗粒R6G染料掺杂聚合物薄膜相似的制备方法,制备了多形貌银纳米颗粒掺杂DCJTB染料聚合物薄膜,并且进行随机激光泵浦实验。结果表明,可以有效的产生波长为675 nm,半高宽<0.8 nm的相干红光随机激光,并且阈值仅为0.98 mJ·cm-2。研究结果在宽带可调谐随机激光器研究以及多色随机激光器研究领域具有重要的参考价值。