有机吸附物对多孔硅微腔发光的影响

理论上,采用Bruggeman有效介质近似,研究了有机吸附物对多孔硅微腔的折射率及其光致发光谱的影响.实验上,采用计算机控制的电化学腐蚀法制备了多孔硅微腔样品,并利用机械泵油的蒸气分子与该微腔样品进行相互作用.研究发现,多孔硅微腔发射的窄化光致发光谱对泵油蒸气分子的吸附与脱附很敏感,与之伴随的是该窄化光致发光谱发生明显的峰位移动(可达71nm)和强度变化.结合Bruggeman近似和表面态对多孔硅发光的影响,对实验结果进行了定性解释.实验结果与理论模拟结果符合较好. 关键词： Bruggeman近似 吸附物 多孔硅微腔 光致发光谱     

Photoluminescence studies of porous silicon microcavities

Narrow photoluminescence peaks with a full-width at half-maximum of 14–20 nm are obtained from porous silicon microcavities (PSM) fabricated by the electrochemical etching of a Si multilayer grown by molecular beam epitaxy. The microcavity structure contains an active porous silicon layer sandwiched between two distributed porous silicon Bragg reflectors; the latter were fabricated by etching a Si multilayer doped alternatively with high and low boron concentrations. The structural and optical properties of the PSMs are characterised by scanning electron microscopy and photoluminescence (PL). The wavelength of the narrow PL peaks could be tuned in the range of 700–810 nm by altering the optical constants.     

Enhanced emission of single quantum dot formed by interface fluctuations in photonic-crystal microcavities

We fabricated photonic-crystal (PhC) microcavities tuned to GaAs quantum dots (QDs) formed by interface fluctuation for the first time and observed the spontaneous emission enhancement in a weak coupling regime. A QD is a very thin GaAs quantum well (QW), and its interface steps exhibit quantum dot-like behavior. The emission intensity from the PhC cavity was stronger than that from the area where no PhC pattern was fabricated and the overall shape of the photoluminescence (PL) agreed with the cavity mode calculated with the three-dimensional (3D) finite-difference time domain (FDTD) method. The spontaneous emission enhancement factor was 10.     

Isotropic photonic bandgap in Penrose textured metallic mircocavity

Photonic microcavity has modified photonic modes that have intense localized electric field, which can couple strongly with the embedded emission centers. In this work, we fabricated 2D photonic microcavities with Penrose quasicrystal pattern with 10-fold symmetry. Organic luminescence material tris(8-hydroxyquinoline) aluminum (Alq₃) was embedded into the microcavity and the angle resolved transmission (ART) and photoluminescence (ARPL) spectra of the microcavity were measured. The results showed that the normal Gaussian photoluminescence spectrum of Alq₃ has been strongly modified by the microcavity dispersion characteristic. In addition, omni-directional photonic band gap exists in the microcavity. The higher symmetry of Penrose quasicrystal pattern means that there was minimal difference in the directional dispersion characteristics.     

Effects of F2 Color Center Spatial Distribution on the Emission Properties of Optical Microcavities Based on LiF Films

Highly stable F₂ color centers are very efficiently produced in lithium fluoride (LiF) by electron beam irradiation at room temperature. We have fabricated optical microcavities in which the active medium is a low-energy electron beam irradiated LiF film, whose optical thickness is comparable with the peak wavelength (～668nm) of the F₂ broad photoluminescence band. By selecting the proper electron beam energy, one can control the F₂ color center depth distribution. This distribution influences the photoemission angular distribution of the microcavity, whose resonance properties are determined by the coupling of the depth profile of the defects with the pump electromagnetic field and microcavity modes.     

Design and electro-optical characterization of Si-based resonant cavity light emitting devices at 850 nm

We report on the fabrication and characterization of Si/SiO₂ Fabry-Perot microcavities. These structures are used to enhance the external quantum efficiency along the cavity axis and the spectral purity of emission from silicon rich oxide films that are used as active media to fabricate a Si based RCLED (resonant cavity light emitting devices). A new structure to electrically pump the active media in the resonant cavity has been designed. These structures are fabricated by chemical vapour deposition on a silicon substrate. The microcavities are tuned at 850 nm and present a quality factor ranging from 17 to 150 depending on the number of pairs constituting the dielectric mirrors. An enhancement of the electro and photoluminescence (PL) signal of 20 times is achieved for the selected emission wavelength. These cavities are characterized by TEM analysis to evaluate film uniformity, thicknesses and the densification after annealing processes for temperature ranging from 800 to 1100 °C. The electrical properties of the active media are analyzed. The electroluminescence spectral features are compared with PL spectra correlated with the quality factor of the cavities. The photometric diagram shows also a high directionality of the emitted light within a 30° cone from the sample normal.     

Optical properties of microcavity polaritons

This work contains a theoretical analysis of the optical properties of semiconductor quantum wells embedded in planar Fabry-Perot microcavities. In particular, the properties of the system in correspondence to the excitonic transition are studied by means of the polariton formalism. The polariton states in microcavities are derived and the polar-iton dispersion is presented. Particular emphasis is put on the existence of two well distinct regimes depending on the exciton and cavity parameters: strong coupling and weak coupling regime. The main experimental results are reviewed and compared with the prediction of the theory. After the polariton states have been characterized, the optical response of the system is discussed, with particular attention to the photoluminescence measurements. The polariton formation and relaxation through phonon scattering and the effect of the exciton inhomogeneous broadening are considered and, finally, a phe-nomenological model for the polariton photoluminescence spectra is presented.     

Purcell effect of GaAs quantum dots by photonic crystal microcavities

We fabricate photonic crystal slab microcavities embedded with GaAs quantum dots by electron beam lithography and droplet epitaxy. The Purcell effect of exciton emission of the quantum dots is confirmed by the micro photoluminescence measurement. The resonance wavelengths, widths, and polarization are consistent with numerical simulation results.     

Photonic quantum dots based on Bragg reflectors grown by conformal deposition on patterned substrates

A new approach, that combines the photolithography and conformal deposition techniques, was proposed to fabricate Si-based three-dimensional optical microcavities on patterned substrates. Different from the lateral optical confinement of 3D microcavities by using total internal reflection, Bragg reflectors are used for all three-dimensional optical confinement. From the room temperature photoluminescence spectra, discrete optical modes with obvious side-dependence were observed. With the lateral size decreased from 4.5 μm to 1.5 μm, the modes shift to higher energies and the mode splitting increases, which indicates that 3D optical microcavities act like photonic quantum dots. The numerical calculations of quantized photon states in photonic quantum dots show a quantitative agreement with these observed discrete optical eigenmodes.     

Conformal coverage for two-dimensional arrays of microcavites with quasi-three dimensional confinement by distributed Bragg reflectors

Different from conventional three-dimensional confined microcavity fabrication method in which micropillar microcavities were obtained through the etching of planar semicoductor microcavities, we adopted the conformal coverage to fabricate two-dimensional arrays of quasi three-dimensional confined optical microcavities providing both vertical and lateral optical confinement by the distributed Bragg reflectors (DBRs). Our microcavity samples were directly deposited on the patterned substrates with two-dimensional arrays of air holes. The SEM and cross-section TEM images show that the periodicity of the patterned substrate was still kept after deposition while the growth of DBRs along the sidewalls occurred simultaneously, which provided the transverse optical confinement. In order to probe the optical modes of this kind of microcavities, room temperature photoluminescence signals from prepared microcavities were detected. Three resonant modes were presented and exhibited obvious angular dependence. We attributed these phenomena to quantization of the in-plane wave vector components confined by lateral DBRs.     

Improvement of the luminescence in p-type as-prepared or dye impregnated porous silicon microcavities

We have realized distributed Bragg reflectors and microcavities with a remarkable optical quality (R_max.=99.5% at 850 nm, FWHM=5 nm at 772 nm) with low doped p-type silicon. This is due to a strong decrease of the porous Si/bulk Si interface roughness that was obtained by low-temperature anodization. The properties of porous silicon microcavities are investigated by photoluminescence and reflection measurements. We also have filled porous silicon with Rhodamine 800 dye. The spontaneous emission spectrum of the optically excited Rhodamine 800 is drastically modified by microcavity effect: the peak emission intensity is increased, the line width is narrowed. The results demonstrate that using all porous silicon or dye-filled microcavities provides new possibilities to improve the properties of photonic devices.     

Simultaneous broadening and enhancement of the 1.5 μm photoluminescence peak of Er3+ ions embedded in a 1-D photonic crystal microcavity

It is proposed for the first time that the 1.5 μm Er³⁺ photoluminescence peak may be significantly broadened by a photonic crystal microcavity structure. Two coupled microcavities have been designed and prepared by sol-gel processing based on alternating layers of silicate glass and titania materials. The 1.5 μm emission spectra of the Er³⁺ ions embedded in these microcavity structures are measured and discussed. It is found that the spontaneous emission spectra are effectively broadened to a point where they become approximately square in shape. The full width at half maximum (FWHM) reached a value as large as 183 nm, which is the best result reported so far. The measured photoluminescence profiles agree well with simulated curves and an intensity enhancement by the microcavities has also been observed. These results may be of significance for applications requiring broadened or otherwise modified spontaneous emission spectra, such as application-specific LEDs and other non-coherent light sources. The present technique can easily be applied to other active materials.     

酞菁铜掺杂TiO2微腔的光谱特性和精细结构

化学方法合成了酞菁铜(CuPc)掺杂TiO2微腔,用傅里叶变换红外光谱、拉曼光谱研究了其的光谱特性,用X射线吸收精细结构(XAFS)谱分析了其的精细结构.结果显示,TiO2微腔被CuPc掺杂后,CuPc和TiO2之间发生了相互作用,使红外光谱出现了900.76 cm-1的振动吸收峰;同时,在3 392.75 cm-1的OH振动和2 848.83 cm-1的CH振动发生了"红移";酞菁大环平面的C-C或C-N振动、苯环上C-H面内和C-N面外的弯曲振动也有一定的峰位移动和强度变化.在拉曼光谱图上,CuPc掺杂TiO2微腔中出现的403.4,592.1和679.1 cm-1的TiO2的特征振动峰,但它们发生了波数移动,而在1 586.8和1 525.6cm-1出现的振动峰说明CuPc和TiO2形成了复合体,这些变化与酞菁铜的大环分子结构的平面取向有关.在XAFS上,CuPc掺杂TiO2微腔中的Ti呈现四面体TiO4的结构形式,TiO2的内层的中间距离和表面结构发生了变化.     

Exciton-photon interaction in low-dimensional semiconductor microcavities

The structure of the photon states and dispersion of cavity polaritons in semiconductor microcavities with two-dimensional optical confinement (photon wires), fabricated from planar Bragg structures with a quantum well in the active layer, are investigated by measuring the angular dependence of the photoluminescence spectra. The size quantization of light due to the wavelength-commensurate lateral dimension of the cavity causes additional photon modes to appear. The dispersion of polaritons in photon wires is found to agree qualitatively with the prediction for wires having an ideal quantum well, for which the spectrum is formed by pairwise interaction between exciton and photon modes of like spatial symmetry. The weak influence of the exciton symmetry-breaking random potential in the quantum well indicates a mechanism of polariton production through light-induced collective exciton states. This phenomenon is possible because the light wavelength is large in comparison with the exciton radius and the dephasing time of the collective exciton state is long. Zh. éksp. Teor. Fiz. 114, 1329–1345 (October 1998)     

Quantum theory of spin dynamics of exciton-polaritons in microcavities

We present the quantum theory of momentum and spin relaxation of exciton-polaritons in microcavities. We show that giant longitudinal-transverse splitting of the polaritons mixes their spin states, which results in beats between right- and left-circularly polarized photoluminescence of microcavities, as was recently experimentally observed [Phys. Rev. Lett. 89, 077402 (2002)]]. This effect is strongly sensitive to the bosonic stimulation of polariton scattering.     

Standing-wave nonlinear optics in an integrated semiconductor microcavity

We present a concept of standing-wave optical frequency conversion in dispersive microcavities theoretically and experimentally, allowing efficient ultracompact nonlinear photonics. We developed a time-dependent model, incorporating the dispersion into the structure of the spatial cavity modes, where the conversion efficiency is enhanced by the optimization of a nonlinear cavity mode overlap. We designed and fabricated integrated double-resonance semiconductor microcavities for standing-wave second-harmonic generation. The measured efficiency exhibits a significant maximum near the cavity resonance owing to the intracavity power enhancement and the dispersion-induced wavelength detuning effect on the mode overlap, in good agreement with our theoretical predictions.     

Inhibition and enhancement of the spontaneous emission of quantum dots in structured microresonators

Spontaneous emission of quantum dot systems in laterally structured microcavities that exhibit photon confinement in all three directions has been studied by time-resolved photoluminescence spectroscopy. For on-resonance conditions, we find that the dot emission rate is increased substantially over that of the unstructured planar cavity. For off-resonance conditions, we are able to suppress the emission rate by an order of magnitude by using cavities with metal coatings, which we attribute to the suppression of leaky optical modes in these structures.     

Magnetic Circular Polarization of Exciton Photoluminescence

Experimental and theoretical studies of circular polarization of photoluminescence of excitons (MCPL) in semiconductors placed in an external magnetic field are reviewed. The advantage of the MCPL method is its relative simplicity. In particular, it does not require spectral resolution of the Zeeman sublevels of an exciton and may be applied to a wide class of objects having broad photoluminescence spectral lines or bands: in bulk semiconductors with excitons localized on the defects of the crystal lattice and composition fluctuations, in structures with quantum wells and quantum dots of types I and II, in two-dimensional transition metals dichalcogenides and quantum microcavities. The basic mechanisms of the magnetic circular polarization of luminescence are considered. It is shown that either known mechanisms should be modified or additional mechanisms of the MCPL should be developed to describe the polarized photoluminescence in newly invented nanosystems.     

Planar defects in three-dimensional chalcogenide glass photonic crystals

Here we report on the direct laser writing fabrication of Fabry-Perot-type planar microcavities in a three-dimensional (3D) photonic crystal (PhC) embedded within a high-refractive nonlinear chalcogenide glass (ChG) film. The fabricated planar microcavities in a nonlinear ChG 3D PhC facilitate the observation of resonant modes inside the stop gap. The experimental results show that the length of the planar cavity can be well controlled by the fabrication power and thus be used to tune the defect modes. The tunability of the observed defect modes is confirmed by the theoretical prediction.     

Controlled emission from dye saturated single and coupled microcavities

Modified photoluminescence is demonstrated from the dye saturated porous silicon based single and coupled microcavities. When photonic cavity mode is weakly coupled to the emission states of the dye, photoluminescence line narrowing and intensity enhancement have been observed. Our experimental work and transfer matrix simulations and cavity modelling convincingly explain the tunability and optical field confinement within the microcavity. We also show that the photoluminescence enhancement is due to one-dimensional microcavity effect. These optically active hybrid materials from inexpensive fabrication may become an important consideration for many photonic applications.