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.     

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,采用有限差分本征模和时域有限差分方法对所设计的结构进行光学仿真模拟。首先,通过改变ZnO纳米线的直径,使用有限本征模方法分析介电层中的光学模式,得到4种模式分布。然后,通过这4种光学模式在不同纳米线直径下的有效折射率和损耗计算了对应的波导传输距离以及激射阈值增益。最后,采用三维时域有限差分方法仿真分析纳米线稳态激光发射过程中各模式的电场分布。结果表明:在纳米线和金属衬底之间的介电层上存在混合等离子体模式和混合电模式,对于直径低于75 nm的ZnO纳米线,没有有效的物理光学模式,即混合等离子体模式和混合电模式都被切断,当ZnO纳米线的直径大于75 nm时,混合等离子体模式可以有效存在,而混合电模式在ZnO纳米线的直径达到120 nm之后才出现。虽然混合等离子体模式可以更好地限制在介电层中,但是它们的模式损耗太大,传播距离相对较小。此外,与混合等离子体模式相比,混合电模式的传播距离更长。在给定微米线的直径(D=240μm)下,混合电模式传播距离超过50μm。综上可知,在深亚波长尺度下利用混合泄漏模式可以打破光学衍射极限并实现激光激射。     

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.     

Low-threshold plasmonic lasing based on high-Q dipole void mode in a metallic nanoshell

We propose a novel type of plasmonic lasing nanostructure consisting of a metallic shell and a gain core. We demonstrate numerically that highly localized void modes of such metallodielectric core-shell nanoparticles have a very high quality factor. We found that the dipole void mode has a lasing threshold as low as 128 cm(-1) at 800 nm as a result of the unique mode distribution within the shell, due to a maximum field enhancement around the void center. The lasing condition for a symmetry-reduced silver nanocup is also investigated and the low plasmonic lasing threshold is sustained provided that the opening angle of the nanocup is smaller than 10°. Our proposal presents a new path toward plasmonic lasers with low gain threshold.     

Lasing action assisted by long‐range surface plasmons

It is shown that lasing action at subwavelength scales can be achieved in realistic plasmonic systems supporting long‐range surface plasmons (LRSPPs). To this end, a general numerical framework has been developed that is able to accurately account for the full spatio‐temporal lasing dynamics and the vastly different length‐ and time‐scales featured by this class of systems. Starting from a loss compensation regime for propagating LRSPPs, it is shown how the introduction of an optical feedback mechanism induces the formation of a self‐sustained laser oscillation at moderate pump intensities. The simplicity of the proposed subwavelength scale laser offers significant potential as a novel class of planar light sources in complex plasmonic circuits.     

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.     

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     

Hybrid plasmonic waveguide with gain medium for lossless propagation with nanoscale confinement

In this Letter, we propose a hybrid plasmonic nanosystem consisting of a silver cladding layer with a semicylinder bump on top of InGaAsP nanowire. Because of the coupling between the dielectric waveguide mode and surface plasmon polariton mode, the hybrid plasmonic mode can exhibit low loss with strong field localization. The finite element method numerical simulations are employed to evaluate the performances of the hybrid mode. In order to achieve the lossless propagation of the hybrid mode with the mode area of 0.0058(λ2/4) at 1.55 μm, the material gain of 200 nm × 300 nm InGaAsP nanowire should reach 1223 cm?1.     

A high-quality factor hybrid plasmonic nanocavity based on distributed Bragg reflectors

Herein,we propose a high-quality(Q) factor hybrid plasmonic nanocavity based on distributed Bragg reflectors(DBRs) with low propagation loss and extremely strong mode confinement.This hybrid plasmonic nanocavity is composed of a high-index cylindrical nanowire separated from a metal surface possessing shallow DBRs gratings by a sufficiently thin low-index dielectric layer.The hybrid plasmonic nanocavity possesses advantages such as a high Purcell factor(Fp) of up to nearly 20000 and a gain threshold approaching 266 cm~(-1)at 1550 nm,promising a greater potential in deep sub-wavelength lasing applications.     

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.     

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.     

Negative index materials with gain media for fast modulation

Negative index materials (NIM) enable subwavelength resolution and are promising for applications in integrated optical systems, since the mode volume is small. Most promising NIM systems essentially use noble metals (Ag, Au) with material losses much lower than in other metals, but still rather hefty, like in metal–dielectric–metal “fishnets”. Therefore, we perform extensive finite-difference time-domain modeling of NIM “fishnets” in combination with gain medium, InGaAsP multiple quantum wells in the present work. The signal recovery is weak, which is related to weak overlap between the radiation field and the gain medium. The signal modulation speed may be very large, in a picosecond range.     

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.     

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.     

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.     

Metal-dielectric-metal plasmonic waveguide devices for manipulating light at the nanoscale Invited Paper

We review some of the recent advances in the development of subwavelength plasmonic devices for ma- nipulating light at the nanoscale, drawing examples from our own work in metal-dielectric-metal (MDM) plasmonic waveguide devices. We introduce bends, splitters, and mode converters for MDM waveguides with no additional loss. We also demonstrate that optical gain provides a mechanism for on/off switch- ing in MDM plasmonie waveguides. Highly efficient compact couplers between dielectric waveguides and MDM waveguides are also introduced.     

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.     

Room-Temperature Photonic Crystal Laser in H3 Cavity on InGaAsP/InP Slab

We fabricate and investigate two-dimensional photonic crystal H3 microcavities in an InGaAsP slab. The lasing action at room temperature is observed. The lasering threshold is 7mW under the pulsed pump of 0.75% duty cycle. The Q factor and the lasing mode characteristics are simulated by three-dimensional finite difference time domain method. The simulation result matches well with the experiment.     

基于耦合介质纳米线的深亚波长局域波导

提出了一种基于耦合介质纳米线的深亚波长局域波导,通过两根紧邻的高折射率介质纳米线的耦合,该波导可以将光场有效束缚在纳米线之间的低折射率纳米缝隙中. 计算模拟的结果表明,该波导的有效模场面积达到Λ²₀/200,比单根纳米线波导小一个数量级,这种深亚波长的模场束缚能力可以与表面等离激元混合波导相比拟. 计算模拟的结果还表明,纳米线可能带有的低折射率氧化膜、低折射率衬底的存在、以及纳米线间尺寸存在的一定差异对于该波导结构的实际应用都不会产生很大 关键词： 介质波导 亚波长局域 表面等离激元波导 纳米线     

Terahertz superconducting plasmonic hole array

We demonstrate a superconductor array of subwavelength holes with active thermal control over the resonant transmission induced by surface plasmon polaritons. The array was lithographically fabricated on a high-temperature yttrium barium copper oxide superconductor and characterized by terahertz time-domain spectroscopy. We observe a clear transition from a virtual excitation of the surface plasmon mode to a real surface plasmon mode. The highly controllable superconducting plasmonic crystals may find promising applications in the design of low-loss, large- dynamic-range amplitude modulation and surface-plasmon-based terahertz devices.