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.     

Electronic transport in a long wavelength infrared quantum cascade detector under dark condition

We present a joint experimental and theoretical investigation on a long wavelength infrared quantum cascade detector to reveal its dark current paths. The temperature dependence of the dark current is measured. It is shown that there are two different transport mechanisms, namely resonant tunneling at low temperatures and thermal excitation at higher temperature, dominate the carrier flow, respectively. Moreover, the experimental intersubband transition energies obtained by the magneto-transport measurements matches the theoretical predictions well. With the aid of the calculated band structures, we can explain the observed oscillation phenomena of the dark current under the magnetic field very well. The obtained results provide insight into the transport properties of quantum cascade detectors thus providing a useful tool for device optimization.     

External quantum efficiency-enhanced Pt Si Schottky-barrier detector utilizing plasmonic ZnO:Al nanoparticles and subwavelength gratings

A infrared light trapping structure combining front subwavelength gratings and rear Zn O:Al nanoparticles for a Pt Si Schottky-barrier detector over a 3–5 μm waveband is theoretically investigated. By selecting the proper plasmonic material and optimizing the parameters for the proposed structure, the absorption of the Pt Si layer is dramatically improved. The theoretical results show that this improvement eventually translates into an equivalent external quantum efficiency(EQE) enhancement of 2.46 times at 3–3.6 μm and 2.38 times at 3.6–5 μm compared to conventional structures. This improvement in the EQE mainly lies in the increase of light path lengths within the Pt Si layer by the subwavelength grating diffraction and nanoparticle-scattering effects.     

Numerical analysis of two-color HgCdTe infrared photovoltaic heterostructure detector

We report on 2D numerical simulations of spectral photoresponse characteristic for two-color HgCdTe infrared photovoltaic detector. Effects of thickness of absorption layer and doping profiles on the photoresponse, quantum efficiency and crosstalk have been investigated. Optimal thickness of absorption layers and doping profiles are numerically calculated.     

Engineering optical gradient force from coupled surface plasmon polariton modes in nanoscale plasmonic waveguides

We explore the dispersion properties and optical gradient forces from mutual coupling of surface plasmon polariton(SPP) modes at two interfaces of nanoscale plasmonic waveguides with hyperbolic metamaterial cladding.With Maxwell's equations and Maxwell stress tensor,we calculate and compare the dispersion relation and optical gradient force for symmetric and antisymmetric SPP modes in two kinds of nanoscale plasmonic waveguides.The numerical results show that the optical gradient force between two coupled hyperbolic metamaterial waveguides can be engineered flexibly by adjusting the waveguide structure parameters.Importantly,an alternative way to boost the optical gradient force is provided through engineering the hyperbolic metamaterial cladding of suitable orientation.These special optical properties will open the door for potential optomechanical applications,such as optical tweezers and actuators.     

Terahertz generation in quantum cascade structures by two-color deriving fields: An approach to reach inversion population via optical pumping

In this paper, a quantum cascade laser (QCL) design is proposed based on GaAs/AlGaAs material system, which simultaneously operates at three widely separated wavelengths (${\lambda }_1=11.1\mu m,{\lambda }_2=14.1\mu m$ and ${\lambda }_{THz}=60\mu m$). In the design, all the wavelength radiations are achieved by the engineering of the electronic spectrum via the quantum-well widths and the applied electric field in a single active region within a same waveguide. The mid-infrared (mid-IR) wavelengths are obtained by adoption a dual-upper-state active region, and the proposed design aims to use both the mid-IR radiations as the coherent deriving fields to populate the upper THz lasing state to aid the THz-laser population inversion via optical pumping instead of direct electrical injection. A detailed analysis of electronic transport in the structure is carried out using a multi-level rate-equation model. The results show that the proposed structure offers an alternative approach to room temperature THz generation in QCLs.     

太赫兹量子级联激光器研究进展

太赫兹技术涉及电磁学、光电子学、半导体物理学、材料科学以及微加工技术等多个学科,它在信息科学、生物学、医学、天文学、环境科学等领域有重要的应用价值.太赫兹辐射源是太赫兹频段应用的关键器件.本文简要介绍了太赫兹电磁波的研究背景、重要特点以及潜在应用,重点讨论了太赫兹半导体量子级联激光器的工作原理和研究进展等.     

A plasmonic waveguide end-coupled to two plasmonic cavities in a non-Hermitian quantum system for dual-band unidirectional reflectionlessness

We demonstrate the possibility of dual-band unidirectional reflectionlessness in a non-Hermitian quantum system composed of a plasmonic waveguide and two end-coupled plasmonic cavities(PCs).Our scheme exhibits dual-band unidirectional reflectionlessness can be obtained at exceptional points by properly adjusting the coupling strength between two PCs,the ratio of decay rates of two PCs,and the ratio of plasmonic cavity-waveguide coupling strengths.As a valuable feature,the quality factor reaches to~175.4 in forward direction,while the backward quality factor is close to~188.2.     

Band-pass plasmonic filter based on periodic cascade resonant cavities

A band-pass plasmonic filter based on periodic cascade resonant cavities is theoretically designed and analyzed. The transmission modes of surface plasmon polariton are well studied by the finite-difference time-domain method. The structure indicates the band-pass selection capability with several types of modes in the transmission spectra. The results show that the number of the transmission modes depends on the number of the cascade cavities period. These modes exhibit a shift with varying incident intensity involving Kerr medium in cascade cavities. The structure provides flexibility in design for pass-bands filter.     

太赫兹量子级联激光器研究进展

太赫兹技术涉及电磁学、光电子学、半导体物理学、材料科学以及微加工技术等多个学科，它在信息科学、生物学、医学、天文学、环境科学等领域有重要的应用价值．太赫兹辐射源是太赫兹频段应用的关键器件．本文简要介绍了太赫兹电磁波的研究背景、重要特点以及潜在应用，重点讨论了太赫兹半导体量子级联激光器的工作原理和研究进展等．     

周期耦合量子阱中的输运问题

采用Gurvitz等人直接求解薛定谔方程的方法并结合数值计算,分析了驱动频率对周期耦合量子阱体系的电流的影响.结果表明：当驱动频率小于耦合量子阱间的能级差时,随着驱动频率的增大,系统平衡时的电流增加,当驱动频率大于耦合量子阱间能级差时,随着驱动频率的增大,平衡时的电流减小.这样,通过控制外场驱动频率来达到控制电流的目的. 关键词： 量子阱 驱动频率 电流     

Free space optical beam coupled to surface plasmonic polariton waves via designed grooves in metal film

Surface plasmonic polariton(SPP) waves with complicated wavefronts have important implications in nanophotonic sciences and applications. The surface electromagnetic wave holography method is applied to designed grooves on a metal surface for coupling a plane wave in free space to complicated wavefront SPP waves.The grooves illuminated by the plane wave incident from free space serve as secondary SPP waves sources, that radiate cylindrical SPP waves. New controllable wavefronts originate from these secondary SPP waves interfering with each other, based on the Huygens–Fresnel principle. Several applications of the method are demonstrated, such as converting coupling waves in free space into focusing SPP waves on a metal surface.     

Performances of quantum cascade detectors

In this paper, we present an idealised model of quantum cascade detectors that calculates the highest performances reachable with actual technology. It is based on an ideal vision of the electronic behavior of the transport and a Fabry–Pérot approach for the light-coupling scheme. This simple but complete picture of the device is a proficient tool to understand the detectors physics. In particular, the variation of the performances with regards to different parameters is carefully discussed and guidelines for the detector design are sketched out. This approach gives also some insight into the potentiality of quantum cascade detector technology.     

Microcavity THz quantum cascade laser

We report operation of disk and ring shaped terahertz (THz) quantum-cascade lasers (QCLs) emitting in the THz region between 3.0 and 3.4 THz. The GaAs/Al_0.15Ga_0.85As heterostructure is based on longitudinal-optical phonon scattering for depopulation of the lower radiative state. A double metal waveguide is used to confine the whispering gallery modes in the gain medium. The threshold current density is at 5 K. 3D Finite-Difference Time-Domain (FDTD) simulations were performed to obtain the field distributions within a THz QCl resonator at different frequencies.     

ZnO-based terahertz quantum cascade lasers

High-power terahertz sources operating at room-temperature are promising for many applications such as explosive materials detection, non-invasive medical imaging, and high speed telecommunication. Here we report the results of a simulation study, which shows the significantly improved performance of room-temperature terahertz quantum cascade lasers (THz QCLs) based on a ZnMgO/ZnO material system employing a 2-well design scheme with variable barrier heights and a delta-doped injector well. We found that by varying and optimizing constituent layer widths and doping level of the injector well, high power performance of THz QCLs can be achieved at room temperature: optical gain and radiation frequency is varied from 108 cm^?1 @ 2.18 THz to 300 cm^?1 @ 4.96 THz. These results show that among II–VI compounds the ZnMgO/ZnO material system is optimally suited for high-performance room-temperature THz QCLs.     

Low-temperature characteristics of two-color InAs/InP quantum dots laser

We report on the lasing characteristics of a two-color InAs/InP quantum dots(QDs)laser at a low tem-perature.Two lasing peaks with a tunable gap are simultaneously observed.At a low temperature of 80 K,a tunable range greater than a 20-nm wavelength is demonstrated by varying the injection current from 30 to 500 mA.Under a special condition,we even observe three lasing peaks,which are in contrast to those observed at room temperature.The temperature coefficient of the lasing wavelength was obtained for the two colors in the 80?280 K temperature range,which is lower than that of the reference quantum well(QW)laser working in the same wavelength region.     

Tuning of plasmonic behaviours in coupled metallic nanotube arrays

We theoretically investigate the influence of the shape of nanoholes on plasmonic behaviours in coupled elliptical metallic nanotube arrays by the finite-difference time-domain (FDTD) method.We study the structure in two cases:one for the array aligned along the minor axis and the other for the array aligned along the major axis.It is found that the optical properties and plasmonic effects can be tuned by the effective surface charges as a result of the variation in the minor axis length.Based on the localized nature of electric field distributions,we also clearly show that the presence of localized plasmon resonant modes originates from multipolar plasmon polaritons and a large magnitude of opposing surface charges build up in the gap between adjacent nanotubes.     

Multifold enhancement of quantum dot luminescence in plasmonic metamaterials

We report that hybridizing semiconductor quantum dots with plasmonic metamaterial leads to a multifold intensity increase and narrowing of their photoluminescence spectrum. The luminescence enhancement is a clear manifestation of the cavity quantum electrodynamics Purcell effect and can be controlled by the metamaterial's design. This observation is an essential step towards understanding loss compensation in plasmonic metamaterials with gain media and for developing metamaterial-enhanced gain media.     

Nonlinear optics with quantum cascade lasers

We discuss new approaches to monolithic integration of quantum cascade lasers with resonant intersubband nonlinearities. We show that the proposed approaches can greatly enhance the performance of quantum cascade lasers and give rise to new functionalities. Examples considered include extreme frequency up-or down-conversion and wide-range electric tuning.     

Third-order photonic-crystal distributed-feedback quantum cascade lasers

We demonstrate room-temperature operation of broad-area edge-emitting photonic-crystal distributed-feedback quantum cascade lasers at λ ～ 4.6 μm. The lasers use a weak-index perturbed third-order photonic-crystal lattice to control the optical mode in the wafer plane. Utilizing this coupling mechanism, the near-diffraction-limited beam quality with a far-field profile normal to the facet can be obtained. Single-mode operation with a signal-to-noise ratio of about 20 dB is achieved in the temperature range of 85–290 K. The single-facet output power is above 1 W for a 55 μm × 2.5 mm laser bar at 85 K in pulsed mode.