Sensitivity‐enhancement methods for surface plasmon sensors

Surface plasmon resonance (SPR) sensors have been a mature technology for more than two decades now, however, recent investigations show continuous enhancement of their sensitivity and their lower detection limit. Together with the recent investigations in localized SPR phenomena, extraordinary optical transmission through nanoapertures in metals, and surface‐enhanced spectroscopies, drastic developments are expected to revolutionize the field of optical biosensing. Sensitivity‐enhancement (SE) techniques are reviewed focusing both on the physical transduction mechanisms and the system performance. In the majority of cases the SE is associated with the enhancement of the electromagnetic field overlap integral describing the interaction energy within the analyte. Other important mechanisms are the interaction between plasmons and excitons and between the analyte molecules and the metal surface. The lower detection limit can be reduced significantly if systems with high signal‐to‐noise ratio are used such as common‐path interferometry, ellipsometry or polarimetry systems.     

不同密度银纳米粒子对氧化锌基发光二极管发光的增强

采用分子束外延法制备不同密度的银纳米粒子(AgNPs)修饰的局域表面等离子体共振增强n-ZnO/iZnO/MgO/p-GaN异质结发光二极管(LEDs),并对其电学及光学性质进行表征。结果显示:LEDs中引入适当浓度的AgNPs有利于AgNPs局域表面等离子体激元与ZnO激子相耦合,可以显著提高器件的电致发光性能;随着AgNPs浓度的增加,LEDs发光增强倍数先增大后减小,分析认为这是AgNPs局域表面等离子体共振耦合增强过程和AgNPs的消光过程两者之间相互博弈而导致的结果。     

Optical susceptibility of metal nanoparticle-double quantum dot hybrid system

This work is modeling the linear optical susceptibility of a double quantum dot (DQD)–metal nanoparticle (MNP) hybrid system using density matrix equations that consider the interaction between excitons and surface plasmons. The wetting layer (WL)-QD interaction is considered.The absorption of the hybrid DQD-MNP system increases by five times compared to the bare DQD system. The absorption peak is reducing in a similar ratio of increasing MNP radius. A Mollow triplet with asymmetric sidebands appears and decreases with increasing MNP radius, which is compatible with recent experimental evidence. The susceptibility peaks at resonance for a small distance between the hybrid MNP-DQD. The susceptibility is increased by one order for the DQD system compared to a single QD. A similar result is obtained under WL-QD detuning compared to the case of the probe between DQD states. Small DQDs give high absorption and dispersion, offering more shifts from resonance.     

电场调谐InAs量子点荷电激子光学跃迁

在低温5 K下, 采用光致发光光谱及外加偏压调谐量子点电荷组态研究了InAs单量子点的精细结构和对应发光光谱的偏振性、不同带电荷激子的圆偏振特性. 得出如下结果: 1) 指认InAs单量子点中不同荷电激子的发光光谱和对应的激子本征态的偏振特性; 2) 外加偏压可以调谐量子点的荷电激子的发光光谱; 3) 伴随着电子、空穴的能量弛豫, 电子的自旋弛豫时间远大于空穴的自旋弛豫时间. 关键词： InAs量子点 激子 荧光光谱 电场调谐     

Surface plasmons on a quasiperiodic grating

A method is presented for calculating the frequencies of non-retarded surface plasmons propagating on a semi-infinite medium with a surface profile described by a one-dimension quasiperiodic function. The profiles are generated, in analogy with previous work on quasiperiodic superlattices, by repeating unitary cells constructed according to an inflation rule. Dispersion relations are obtained for a semi-infinite free-electron metal as the active medium, with surface profiles obeying the Fibonacci and Thue-Morse sequences.Received: 24 June 2003, Published online: 19 November 2003PACS: 73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations) - 71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons     

Surface polaritons in semi-infinite crystals

Recent progress in the field of optical surface modes in semi-infinite crystals is reviewed. The basic equations for surface polaritons originating from phonons, plasmons, excitons and magnons are given. Their observation by ATR, the resulting information and possible applications are discussed.     

Theoretical study of photon emission from single quantum dot emitter coupled to surface plasmons

Motivated by the recent pioneering advances on nanoscale plasmonics and also nanophotonics technology based on the surface plasmons (SPs), in this work, we give a master equation model in the Lindblad form and investigate the quantum optical properties of single quantum dot (QD) emitter coupled to the SPs of a metallic nanowire. Our main results demonstrate the QD luminescence results of photon emission show three distinctive regimes depending on the distance between QD and metallic nanowire, which elucidates a crossover passing from being metallic dissipative for much smaller emitter-nanowire distances to surface plasmon (SP) emission for larger separations at the vicinity of plasmonic metallic nanowire. Besides, our results also indicate that, for both the resonant case and the detuning case, through measuring QD emitter luminescence spectra and second-order correlation functions, the information about the QD emitter coupling to the SPs of the dissipative metallic nanowire can be extracted. This theoretical study will serve as an introduction to understanding the nanoplasmonic imaging spectroscopy and pave a new way to realize the quantum information devices.     

Few-particle effects in semiconductor quantum dots: observation of multicharged excitons

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photodepletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QD-impurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons.     

The interaction between an electron and the polarization modes of a metal-insulator interface

We study the interaction between an electron and the polarization modes of a metal-insulator interface by employing a self-energy approach. The coupling between the metal surface plasmons and the insulator surface excitons is explicitly considered. The surface optical phonons are shown to be screened out by the plasmon field. We derive expressions for the charge-coupling Hamiltonians of the bulk and interface modes and calculate a self-energy profile that includes recoil effects due to the quantum nature of the electron's motion.     

Formation of plasmon pulses in the cooperative decay of excitons of quantum dots near a metal surface

The formation of pulses of surface electromagnetic waves at a metal–dielectric boundary is considered in the process of cooperative decay of excitons of quantum dots distributed near a metal surface in a dielectric layer. It is shown that the efficiency of exciton energy transfer to excited plasmons can, in principle, be increased by selecting the dielectric material with specified values of the complex permittivity. It is found that in the mean field approximation, the semiclassical model of formation of plasmon pulses in the system under study is reduced to the pendulum equation with the additional term of nonlinear losses.     

Charged and neutral biexciton–exciton cascade in a single quantum dot within a photonic bandgap

We investigate the recombination dynamics of positively charged and neutral biexcitons and excitons in a single InAs/GaAs quantum dot (QD) within a two-dimensional (2D) photonic bandgap (PBG). The 2D PBG makes the exciton lifetime four times longer and enhances photon-extraction efficiency compared to those without the PBG. Photon cross-correlation measurements demonstrate the cascade emissions of both charged and neutral biexcitons–excitons from the same QD. In the charged case, a hole in the p-shell relaxes into the s-shell between the cascade, and the corresponding transition is confirmed based on the spin configuration. The long exciton lifetime with the PBG helps us to reveal the spin dynamics that did not clearly appear in intrinsic QDs.     

Strong coupling between localized plasmons and organic excitons in metal nanovoids

Hybrid emitting exciton-plasmonic composites are constructed by coating arrays of spherical nanovoids embedded in a gold film with organic semiconducting molecular J-aggregate films. In such plasmonic crystals, localized plasmons confined inside the voids can be excited. We report the first observation of polaritonic spectral narrowing and strong coupling between localized plasmons and J-aggregate excitons with Rabi splittings of 230 meV at room temperature.     

Exciton-plasmon interaction in hybrid quantum dot/metal cluster structures fabricated by molecular-beam epitaxy

Hybrid structures consisting of InAs/AlGaAs quantum dots and In clusters on the surface separated by an AlGaAs spacer with a specified thickness are investigated. The enhancement of the photoluminescence signal in the long-wavelength region manifesting itself in the appearance of a new narrow emission line is observed. This line is absent in the control structure with a quantum-dot array and no metal clusters. It also disappears with increasing thickness of the dielectric spacer between the layers. These results are explained in terms of a model taking into account the resonance interaction of excitons in quantum dots with localized surface plasmons in metal clusters.     

Coupling of spontaneous emission from GaN-AlN quantum dots into silver surface plasmons

We have demonstrated the decay of spontaneous emission (SE) from AlN-GaN quantum dots (QDs) into silver surface plasmon (SP) modes in the ultraviolet at approximately 375-380 nm. Using time-resolved photoluminescence (PL), we show that the electron-hole recombination rate in AlN-GaN QDs is enhanced when SE is resonantly coupled to a metal SP mode, corresponding to the dip in the continuous-wave PL spectrum. Exciton recombination by means of silver SP modes is as much as 3-7 times faster than in normal QD SE and depends strongly on emission wavelength and thickness of the silver.     

Effect of the plasmon-exciton interaction on optical properties of core-shell nanoparticles

The plasmon-exciton interaction in composite nanoparticles consisting of a metal (Au) core coated with two layers of organic materials, i.e., a monomolecular dielectric interlayer and a layer of cyanine dye J-aggregate was studied. In contrast to previously studied systems, metal in nanoobjects under consideration is at a distance of 1.2 nm from the J-aggregate, which allows maintaining the interaction of metal plasmons and J-aggregate excitons and weakening luminescence quenching at the same time. The particle morphology was studied by transmission electron and atomic-force microscopy. Bands caused by the dye J-aggregate formation on the surface were detected in the absorption spectrum of such particles. Fast dynamics (several picoseconds) of optical density variation was detected by the pump-probe method. The photoluminescence spectrum contains a narrowband characteristic of J-aggregates, which is excited due to a locally strengthened field of metal core plasmons.     

Lifetime of quasiparticles in a hybrid electron-exciton system

The lifetimes (damping) of one-body and collective excitations in a hybrid system consisting of spatially separated layers of a two-dimensional electron gas and a gas of indirect excitons have been calculated at zero temperature in the presence of the Bose-Einstein condensate of excitons. It has been shown that the electron-exciton interaction leads to a considerable shortening of the lifetime of electrons as compared to the electron-electron interaction and to the appearance of a nonzero (linear in the wave vector) damping of plasmons. The interaction of the exciton Bose gas with the electron layer induces damping of Bogoliubov phonons in the exciton Bose gas, which is, however, much lower than their intrinsic (Belyaev) damping.     

Resonant mechanisms of inelastic light scattering by low-dimensional electron gases

The contribution of elementary excitations in low-dimensional electron gases to resonant inelastic light scattering is found to be determined by interband transitions involving states at specific wave vectors. In modulation-doped GaAs/GaAlAs quantum wells, we detect only the single-particle excitations (SPE) at resonances with electron-hole transitions at the Fermi wave vector, and only plasmons at resonances with zone-center excitons. The plasmon cross section is comparable to the SPE when double electronic resonance is achieved by tuning the plasmon energy to a valence subband separation.     

Coupling to a microdisk cavity containing a three-level quantum-dot with two orthogonal modes

The dynamics of a composite system containing two orthogonal degenerate whispering-gallery cavity modes coupling to a quantum dot (QD) is presented by a full quantum approach. The energy levels of the quantum dot are modeled as a V-type three-level system, which consist of the ground state, right- and left-polarized excitons. The counterclockwise mode a and the clockwise mode b are coupled with the transitions corresponding to the right- and left-polarized excitons with coupling rates g_R and g_L, respectively. An exact solution is proposed in a real-space approach. We majorly discuss the effects of the backscattering rate β on the spectra of the transmission and reflection in a strong coupling regime. A new insight is that one can overcome the excitons' fine structure splitting of a real QD with appropriate backscattering rate β by fine designing the cavity, which would be possible for applications to produce the degenerate entangled photon pairs in a real QD system.     

Emission from neutral and charged excitons in a single quantum dot in a magnetic field

We report on optical spectroscopy of self-assembled InAs quantum dots in a magnetic field. We describe how we measure the emission characteristics of a single quantum dot (QD) in high magnetic fields at low temperature using a miniature, fiber-based confocal microscope. Example results are presented on a QD whose charge can be controlled using a field-effect device. For the uncharged, singly and doubly charged excitons we find a diamagnetism and the spin Zeeman effect. In contrast, for the triply-charged exciton we find a fundamentally different behavior. Anti-crossings in magnetic field imply that confined states of the QD are hybridized with Landau-like levels associated with the two-dimensional continuum.     

Photoluminescence and time-resolved photoluminescence of star-shaped ZnO nanostructures

Optical properties of star-shaped ZnO nanostructures were studied. The temperature-dependent photoluminescence (PL) was examined up to fourth-order longitudinal optical (LO) phonon assisted emissions of free excitons and confirmed that the nature of the room temperature PL in ZnO is 1-LO phonon assisted emission of free excitons. Low threshold ultraviolet stimulated emissions (SE) were obtained for our powder samples at room temperature. Picosecond time-resolved PL measurements detected a bi-exponential decay behavior which is strongly dependent on the excitation intensity: the slow decay term decreased faster than the fast decay term as the excitation intensity increased and the emission decays were dominated by the fast one. We also found that the emission decays decreased super-linearly before the appearance of the SE. This behavior may be used to deduce the threshold of SE or lasing.