光子晶体中四能级系统的量子相干效应

研究了处于光子带隙材料中的四能级原子系统的电磁感应透明、自发辐射和光子开关效应，分析了其稳态与瞬态特性, 发现特殊的模密度能够导致反常的吸收、色散、自发辐射及瞬态无反转增益, 并可以通过外加调制场进行控制.详细讨论了特殊频率处模密度的变化对透明窗口和光子开关效应的影响. 关键词： 光子带隙材料 电磁感应透明 模密度 光子开关     

Benzotriazole-based dyes containing a low band gap for dye-sensitised solar cells: a theoretical study

In this study, we investigated a series of metal-free benzotriazole-based organic dyes. The geometries, electronic properties, light harvesting efficiency, and electronic absorption spectra of these dyes were studied using the density functional theory and time-dependent density functional theory. The optimised geometries indicate that these dyes are non-planar and thereby effectively inhibit close intermolecular π–π aggregation. The band gap of these dyes ensures a positive effect on the process of electron injection and dye regeneration. The band gap trend corroborates well with the predicted spectra data. Our theoretical calculations reveal that the designed metal-free organic dyes can be used as potential sensitisers for solar cells compared to the best known organic sensitiser (Y123) to date.     

Tailored nanoheterojunctions for optimized light emission

We present coupled classical and quantum simulations of 1 to 2 nm Si nanocrystals (NCs) embedded in amorphous SiO(2) and we show that by tuning the density of the oxide matrix one may change the relative alignment of Si NC and SiO(2) electronic states at the interface. We find that interfacial strain plays a key role in determining the variation of the nanaoparticle gap as a function of size, as well as of conduction band offsets with the oxide. In particular, our results show that it is the variation of the valence band offset with size that is responsible for the gap change. Our findings suggest that the elastic properties of the embedding matrix may be tuned to tailor the energy levels of small Si NCs so as to optimize their performance in optoelectronic devices and solar cells.     

The rapid preparation of large-scale CdS opal photonic crystals and study of the optical properties

Monodispersed silica microspheres of 270 nm are synthesized by a colloidal solution method. Larger scale perfect three-dimensional photonic crystals (PCs) are rapidly prepared using the evaporation of acetone to self-assemble the microspheres on quartz substrates by vertical deposition methods. We find that the pseudo-photonic band gap (PBG) of the PC structure changes with increasing annealing temperature; it drastically shifts from 450 nm for as-grown crystals to 409 nm for annealing at 800 °C. CdS photonic crystals are formed by infiltrating CdS nanocrystals of 6 nm into the SiO₂ PC structure. The transmission and spontaneous emission characteristics of CdS PCs have been investigated. The clear dip in the spontaneous emission spectrum relates to the photonic band gap of CdS PCs, indicating that the spontaneous emission is inhibited in the region of the PBG. The emission band of CdS PCs becomes narrower and sharper than that of CdS nanocrystals; this demonstrates that the emission band and intensity of the luminescent devices will be tuned by controlling the position of the PBG. PACS 42.70.Qs; 42.25.Bs; 78.20.-e; 78.55.Et     

Atomic engineering of photon emission with a scanning tunneling microscope

We present measurements of photon emission from individual several-atom silver chains on the NiAl(110) surface, excited by tunneling electrons in a scanning tunneling microscope (STM). The chains were assembled by manipulating single silver atoms on the NiAl(110) surface with the STM. The photon energy of this emission can be tuned by appending a single atom to the chain. These changes in photon emission result from changes in the electronic structure of the silver chain, each electronic state inside the chain being associated with a distinct channel of emission.     

Silicon nitride PhC nanocavities as versatile platform for visible spectral range devices

We propose silicon nitride two-dimensional photonic crystal resonators as flexible platform to realize photonic devices based on spontaneous emission engineering of nanoemitters in the visible spectral range. The versatility of our approach is demonstrated by coupling the two dipole-like modes of a closed band gap H1 nanocavity with: (i) DNA strands marked with Cyanine 3 organic dyes, (ii) antibodies bounded to fluorescent proteins and (iii) colloidal semiconductor nanocrystals localized in the maximum of the resonant electric field. The experimental results are in good agreement with the numerical simulations, highlighting the good coupling of the nanocavities with both organic and inorganic light emitters.     

Fusion in prospect

In the first part of this introductory review we outline the developments in photonic band gap materials from the physics of photonic band gap formation to the fabrication and potential applications of photonic crystals. We briefly describe the analogies between electron and photon localization, present a simple model of a band structure calculation and describe some of the techniques used for fabricating photonic crystals. Also some applications in the field of photonics and optical circuitry are briefly presented. In the second part, we discuss the consequences for the interaction between an atom and the light field when the former is embedded in photonic crystals of a specific type, exhibiting a specific form of a gap in the density of states. We first briefly review the standard treatment (Weisskopf?–?Wigner theory) in describing the dynamics of spontaneous emission in free space from first principles, and then proceed by explaining the alterations needed to properly treat the case of a two-level atom embedded in a photonic band gap material.     

Spontaneous emission from a microwave-driven four-level atom in an anisotropic photonic crystal

The spontaneous emission from a microwave-driven four-level atom embedded in an anisotropic photonic crystal is studied. Due to the modified density of state(DOS) in the anisotropic photonic band gap(PBG) and the coherent control induced by the coupling fields, spontaneous emission can be significantly enhanced when the position of the spontaneous emission peak gets close to the band gap edge. As a result of the closed-loop interaction between the fields and the atom,the spontaneous emission depends on the dynamically induced Autler–Townes splitting and its position relative to the PBG.Interesting phenomena, such as spectral-line suppression, enhancement and narrowing, and fluorescence quenching, appear in the spontaneous emission spectra, which are modulated by amplitudes and phases of the coherently driven fields and the effect of PBG. This theoretical study can provide us with more efficient methods to manipulate the atomic spontaneous emission.     

Position Dependent Spontaneous Emission Spectra of a Λ-Type Atomic System Embedded in a Defective Photonic Crystal

We investigate the position dependent spontaneous emission spectra of a Λ-type three-level atom with one transition coupled to the free vacuum reservoir and the other one coupled to a double-band photonic band gap reservoir with a defect mode in the band gap.It is shown that,for the atom at the defect location,we have a two-peak spectrum with a wide dark line due to the strong coupling between the atom and the defect mode.While,when the atom is far from the defect location(or in the absence of the defect mode),the spectrum has three peaks with two dark lines due to the coupling between the atom and the photonic band gap reservoir with the largest density of states near the band edges.On the other hand,we have a four-peak spectrum for the atom at the space in between.Moreover,the average spontaneous emission spectra of the atoms uniformly embedded in high dielectric or low dielectric regions are described.It is shown that the atoms embedded in high(low) dielectric regions far from the defect location,effectively couple to the modes of the lower(upper) photonic band.However,the atoms embedded in high dielectric or low dielectric regions at the defect location,are coupled mainly to the defect modes.While,the atoms uniformly embedded in high(low) dielectric regions with a normal distance from the defect location,are coupled to both of defect and lower(upper) photonic band modes.     

掺钛化学腐蚀法制备发光稳定的多孔硅

本文采用掺钛化学腐蚀法制备出了发光稳定性和均匀性都较好的多孔硅 (PS)。存放和退火试验表明 ,光致发光 (PL)谱峰位不蓝移 ,强度不衰减。多孔硅发光稳定性的提高归因于制备过程中样品表面原位氧和钛的钝化。钛的浓度和腐蚀时间对PL强度有明显影响 ,最佳钛浓度约为 0 0 8mol·L- 1 ,最佳腐蚀时间约为 2 0min。掺钛化学腐蚀法制备的PS的光激发过程符合量子限域机制 ,而其PL过程却与量子限域模型不符 ,这表明其光发射过程不是带 带直接跃迁产生的 ,即存在一个表面态