A mid-infrared tunable filter in a semiconductor-dielectric photonic crystal containing doped semiconductor defect

In this work, we theoretically analyze tunable filtering properties in a semiconductor-dielectric photonic crystal (SDPC) containing doped semiconductor defect in the mid-infrared frequency region. We consider two possible configurations of filter structures, the symmetric and asymmetric ones. With a defect of the doped n-type semiconductor, n-Si, the resonant transmission peak can be tuned by varying the doping concentration, that is, the peak wavelength will be shifted to the position of lower wavelength for both structures. Additionally, by increasing the defect thickness, it is also possible to have a filter with multiple resonant peaks, leading to a multichannel filter. The results provide another type of tunable filter in the defective SDPC that could be of technical use for semiconductor applications in optical electronics.     

用紧束缚方法描述光子晶体缺陷耦合的共振频率移动

通常采用含两个耦合参数的紧束缚近似，就能很好地描述光子晶体缺陷因耦合而导致的共振频率分裂.然而，缺陷耦合造成的共振频率移动，即包含奇数个缺陷的耦合系统的中央共振频率位置与原来单缺陷时的共振频率位置存在差异，则只有采用含三个耦合参数的严格紧束缚方法才能正确描述.根据耦合参数与共振频率的关系，利用三缺陷耦合系统的模拟计算结果确定了三个耦合参数的具体值，从而在理论上能够预言由任意个缺陷构成的耦合系统的共振频率的移动和分裂.理论预言与基于有限时域差分法的数值计算完全相符.     

Dynamical behavior of electron transport in AlGaAs/GaAs double-barrier structures under a high-frequency radiation field

We investigate the time-dependent dynamical behavior of electron transport in AlGaAs/GaAs double-barrier structures under a high-frequency radiation field. The effects of the radiation field with different amplitude and frequency on the real-time and mean current-voltage curves are taken into account. We find that the amplitude and frequency of the radiation field affect the final stable state current-voltage (I-V) behaviors, which leads to the switching between different current states at a smaller bias than that of the absence of the radiation field, and both current hysteresis and resonant peaks are suppressed by the external radiation field. The high radiation field strength can make the resonant peak of current split and the hysteresis of current disappear. This effect provides the potential to use double-barrier structure as a THz photoelectric switch.     

二维负磁导率材料中的缺陷效应

研究了以金属铜六边形开口谐振环为基元的二维负磁导率材料的缺陷效应.利用电路板刻蚀 技术制备了二维负磁导率材料样品.采用波导法测量了点缺陷和线缺陷对二维负磁导率材料X 波段(8—12GHz)微波透射行为的影响.实验发现，无缺陷的二维负磁导率材料样品存在一个 谐振频率，在稍大于该谐振频率的极窄区域内表现为负磁导率.点缺陷和线缺陷SRR的引入导 致材料主谐振峰的强度下降、谐振频率发生移动，品质因数Q显著下降.缺陷的存在破坏 了材料的周期性结构，从而引起其谐振峰的谐振强度和谐振频率发生变化.缺陷效应的研究 不 关键词： 负磁导率 缺陷效应 开口谐振环     

Spatial and frequency domain effects of defects in 1D photonic crystal

The aim of this paper is to present the analysis of influence of defects in 1D photonic crystal (PC) on the density of states and simultaneously spontaneous emission, in both spatial and frequency domains. In our investigations we use an analytic model of 1D PC with defects. Our analysis reveals how presence of a defect causes a defect mode to appear. We show that a defect in 1D PC has local character, being negligible in regions of PC situated far from the defected elementary cell. We also analyze the effect of multiple defects, which lead to photonic band gap splitting.     

Defect-induced enhancement of absorption coefficient and electroabsorption properties in GaN/AlGaN centered defect quantum box (CDQB) nanocrystal

In this paper, effect of an introduced cubic defect on electrical and optical properties of cubic quantum dot is studied. Self-consistent solution of the Schrödinger-Poisson equations for evaluation of the proposed complex quantum dot is used. Optical properties (absorption and electroabsorption properties associated with intersublevel transition) of the proposed structure are also investigated using density matrix method. Effects of defect size on energy levels, carrier density, matrix element and optical linear absorption coefficient of centered defect quantum box (CDQB) are examined. It is shown that with increasing the defect size a considerable enhancement in magnitude of the absorption coefficient and also red-shift in resonance frequency are achievable. We show that the CDQB has higher absorption peak (at least 80 times) and tunable absorption spectra, due to increase of the matrix element and modified energy sublevels, compared quantum box structure without defect. Also, it is shown that the defect enhances electroabsorption properties (modulation bandwidth and the maximum variation of absorption peak with external field) of the quantum box structure.     

Resonant tunneling properties of inverted Morse double quantum barrier

We study resonant tunneling characteristics of inverted Morse double quantum barrier structures. The effect of electric bias and structure parameters is calculated by using non-equilibrium Green's function method. Results for the transmission coefficients are compared with the structure parameters. Our results show that the widths of the wells and heights of barriers have a significant effect on the transmission properties. We found that the resonant peak of the transmission coefficient decreases with increasing electric field bias. Moreover, resonant energy level increases with increasing barrier height and increasing width parameters.     

Controllable Photonic Band Gap and Defect Mode in a 1D CO2-Laser Optical Lattice

We propose a new method to form a novel controfiable photonic crystal with cold atoms and study the photonic band gap （PBG） of an infinite 1D CO2-laser optical lattice of SSRb atoms under the condition of quantum coherence. A significant gap generated near the resonant frequency of the atom is founded and its dependence on physical parameters is also discussed. Using the eigenquation of defect mode, we calculate the defect mode when a defect is introduced into such a lattice. Our study shows that the proposed new method can be used to optically probe optical lattice in situ and to design some novel and controllable photonic crystals.     

Study of resonant modes in a 700 nm pitch macroporous silicon photonic crystal

In this study the modes produced by a defect inserted in a macroporous silicon (MP) photonic crystal (PC) have been studied theoretical and experimentally. In particular, the transmitted and reflected spectra have been analyzed for variations in the defect’s length and width. The performed simulations show that the resonant frequency is more easily adjusted for the fabricated samples by length tuning rather than width. The optimum resonance peak results when centered in the PC bandgap. The changes in the defect geometry result in small variations of the optical response of the PC. The resonance frequency is most sensitive to length variations, while the mode linewidth shows greater change with the defect width variation. Several MPS photonic crystals were fabricated by the electrochemical etching (EE) process with optical response in the range of 5.8 μm to 6.5 μm. Results of the characterization are in good agreement with simulations. Further samples were fabricated consisting of ordered modulated pores with a pitch of 700 nm. This allowed to reduce the vertical periodicity and therefore to have the optical response in the range of 4.4 μm to 4.8 μm. To our knowledge, modes working in this range of wavelengths have not been previously reported in 3-d MPS structures. Experimental results match with simulations, showing a linear relationship between the defect’s length and working frequency inside the bandgap. We demonstrate the possibility of tailoring the resonance peak in both ranges of wavelengths, where the principal absorption lines of different gases in the mid infrared are placed. This makes these structures very promising for their application to compact gas sensors.     

Study on maximum band gap of two-dimensional photonic crystal with elliptical holes

In this paper, the maximum photonic band gap (PBG) of two-dimensional (2D) photonic crystal (PC) with elliptical air holes was studied by the finite-difference time-domain (FDTD) method based on changing the ratio (semi-major axis length of elliptical hole to the filling ratio) and azimuth angle of elliptical holes, respectively. It is shown that the PBG exhibits a peak value when the ratio of semi-major axis length to the filling ratio is equal to 0.86 approximately by increasing the filling ratio, and central frequency and the low boundary frequency of PBG decrease linearly with the increasing of semi-major axis length. In the aspect of the influence of azimuth angle from 0 to 90°, the PBG presents a minimum value, and central frequency and the low boundary frequency of PBG become high non-linearly by the increasing of azimuth angle to any filling ratio.     

Andreev reflection in a quantum dot coupled to ferromagnetic and superconductor leads under magnetic fields

By employing the nonequilibrium Green's function, we investigate the spin-dependent linear Andreev reflection (AR) resonant tunneling through a quantum dot connected to a ferromagnetic lead and a superconducting lead, where the magnetization direction in the ferromagnetic lead can be tuned by one. We focus our attention on the effects of the magnetic fields on the AR conductance. One high conductance peak and one low conductance peak are developed in the linear AR conductance when a stronger magnetic field is considered. The interplay between the spin-flip scattering and the magnetic fields on the AR conductance are also studied.     

Influence of random errors on the characteristics of typical 2D photonic crystal microcavity

We studied theoretically how random errors which may arise during fabrication, including radius and position Errors, can affect the most fundamental properties of typical 2D photonic crystal microcavities (single defect modes in a 2D square lattice). It is shown that the disorder caused by radius and position errors has little influence on the quality factor but has a large influence on the resonance frequency, given the gain width of conventional semiconductors. In addition, the resonant mode distribution is tolerant to large radius errors but sensitive to position errors. PACS 42.70.Qs; 42.79.Ci     

Analysis of photonic band gap structure for the design of photonic devices

A detailed analysis, based on Kronig–Penney model and finite-difference time-domain (FDTD) method, is used to explain the air-filling factor effect on the optical properties of defect-free photonic crystals. By the use of the Kronig–Penney model, we calculated the photonic band structure for electromagnetic waves in a structure consisting of a periodic square array of dielectric rods of lattice constant a separated by air holes. Gaps in the resulting band structures are found for waves of both polarisations. We analysed the air-filling factor effect on both polarisations in low and high frequency regions. It is shown that the frequency of the lower TE (transverse-electric) band edge is independent of the air-filling factor in the low frequency region. The opposite behaviour holds for the upper band edge, growing rapidly with the air-filling factor. Using the FDTD we simulated the electric field as the pulse propagates through the structure. The results of both approaches are compared, and the operation characteristics of the measuring air-filling factor device are described. We investigate the optical properties of a single and two defects incorporated in the PC, which can be potentially applied to ultra small surface-emitting-type channel drop filter. It is shown that the frequency and polarisation of the dropped light can be controlled by changing the size and/or shape of the defect. The electric field distribution calculations show that the electric field for a given frequency is located only at the defect, which means that each defect can detect only its corresponding wavelength. To cite this article: F. Ouerghi et al., C. R. Physique 5 (2004).     

Level width of a quasibound state in a double-barrier parabolic-well resonant tunneling structure

Taking exact Airy functions and Hermitian functions as envelope functions, we investigate in detail the level width of a quasibound state for electrons coherent resonant tunneling through symmetric and asymmetric double-barrier parabolic-well resonant tunneling structures (DBRT) with the transfer-matrix formalism. It is found that for the symmetric structure and the asymmetric structure with left barrier thicker than the right one, both the level width and the peak value vary monotonously with increasing applied bias, but for the asymmetric DBRT structure with left barrier thinner than the right one, they change nonmonotonously. The nonmonotonous variations of the level width and the peak value reflect the transition of tunneling type (i.e. first from incompletely resonant tunneling to completely resonant tunneling, and then from completely resonant tunneling back to incompletely resonant tunneling). The effects of well width, barrier thickness and barrier height on the level width and the peak value are also inspected.     

Polarization- and direction-independent defect modes in a wide incident-angle range within Bragg gaps by photonic heterostructures containing negative-index materials

We propose a type of photonic heterostructure by combining dielectric one-dimensional (1D) defective photonic crystals (PCs) and magnetic 1D defective PCs. Both of the two PCs consist of alternating positive-index-material (PIM) layers with a negative-index-material (NIM) defect layer. It is demonstrated by transfer matrix method that there is a polarization- and direction-independent defect mode in a wide incident-angle range within Bragg gaps in the heterostructure. The field distributions prove that the dielectric 1D defective PC benefits to achieve the approximately omnidirectional defect mode for TE waves while the magnetic 1D defective PC benefits for TM waves. Such a structure is useful for designing polarization-independent and omnidirectional or large incident angle narrow-passband filters in optical devices.     

A computational study of the optical response of strongly coupled metal nanoparticle chains

We study the optical response of strongly coupled metal nanoparticle chains using rigorous multiple scattering calculations. The collective resonant frequency of silver nanosphere chains and the coupling between chains are considered. The coupling between silver nanoparticle chains are understood by the transmission and reflection calculations of 2D periodic arrays of nanospheres. The results are in agreement with recent experiments. The splitting of plasmon resonance modes for different polarizations of the incident light are explored. Our results show that the transverse mode resonant wavelength is very sensitive to the inter-chain distance. Results on the effect of disorder are also presented.     

Ultra-narrow bandwidth resonant reflection grating filters using the second diffracted orders

In this paper, we design resonant reflection grating filters employing the second diffracted orders as the leaky modes, then analyze the bandwidth of the reflection peak and the electric field distributions inside the waveguide under resonance. The numeric calculation confirms that ultra-narrow resonant reflection peaks can be observed in these structures. At the same time, strong electric field enhancement appears under resonance. It provides a new approach to diversify the resonant reflection filters and may open a new way to the realization of ultra-narrow bandwidth filters.     

Transfer matrix methods for sound attenuation in resonators with perforated intruding inlets

A resonator with perforated intruding inlet (PII) is a superior silencer element, since the use of perforated inlet extensions can dramatically improve the acoustic performance. In this work, both a one-dimensional (1D) and a two-dimensional (2D) transfer matrix methods are developed to predict the transmission loss of the resonator without considering the mean flow. Based on the two groups of comparisons with tests, it is found out that the applicability of 1D method is limited by the resonator geometry even when the frequency is below the cut-off value of plane wave. Whereas the 2D approach is much more accurate while predicting the transmission losses within entire frequency range. Subsequently, five groups of resonators are chosen to determine the effects of structure parameters to transmission loss based on the 2D approach. The resonant frequency decreases and more resonant peaks appear when the length of inlet extension increases. A higher perforation rate leads to a shift of resonant peak towards higher frequencies. Besides, better acoustic performance could be obtained with the perforation being properly designed. Reducing the inlet/outlet radius can obviously improve the transmission loss without changing the frequency of resonant peak. The theories and conclusions in this study can be used for the design and optimization of resonators in various engineering applications.     

Broadening of Non-Transmission Band of Ultra-Narrow Band-Pass Filters Using Heterostructures

Transmission characteristics are studied for the hybrid structures combining defect and multiple heterostructures. It is shown that the non-transmission frequency range can be substantially enlarged and the phenomenon of narrow band-pass filter can be realized by adjusting the number, position and size of the defect. The theoretical and experimental results on heterostructures containing Ta2O5/SiO2 multilayer films are presented. With perfect non-transmission frequency range and high peak transmissivity, this structure opens a promising way to fabricate ultra-narrow band-pass filters with wide non-transmission frequency range.     

Dynamical method for studying localization-delocalization transition in two dimensional percolating systems

We study quantum percolation which is described by a tight-binding Hamiltonian containing only off-diagonal hopping terms that are generally in quenched binary disorder (zero or one). In such a system, transmission of a quantum particle is determined by the disorder and interference effects, leading to interesting sharp features in conductance as the energy, disorder, and boundary conditions are varied. To aid understanding of this phenomenon, we develop a visualization method whereby the progression of a wave packet entering the cluster through a lead on one side and exiting from another lead on the other side can be tracked dynamically. Using this method, we investigate the localization-delocalization transition in a 2D system for various boundary conditions. Our results indicate the existence of two different kinds of localized regimes, namely exponential and power law localization, depending on the amount of disorder. Our study further suggests that there may be a delocalized state in the 2D quantum percolation system at very low disorder. These results are based on a finite size scaling analysis of the systems of size up to 70 × 70 (containing 4900 sites) on the square lattice.