Fabrication of semiconductor-and polymer-based photonic crystals using nanoimprint lithography

The technology of fabricating photonic crystals with the use of nanoimprint lithography is described. One-and two-dimensional photonic crystals are produced by direct extrusion of polymethyl methacrylate by Si moulds obtained via interference lithography and reactive ion etching. The period of 2D photonic crystals, which present a square array of holes, ranges from 270 to 700 nm; the aperture diameter amounts to the half-period of the structure. The holes are round-shaped with even edges. One-dimensional GaAs-based photonic crystals are fabricated by reactive ion etching of GaAs to a depth of 1 μm through a mask formed using nanoimprint lithography. The resulting crystals have a period of 800 nm, a ridge width of 200 nm, and smooth nearly vertical side walls.     

Process of fabricating semiconductor microcavities and photon crystals

A process of fabricating microcavities and photon crystals in GaAs structures by means of electron lithography and reactive ion etching is described. Two types of structures, with micropillars and with photon crystals, are considered. The latter structures have the form of a square or hexagonal array of holes in a planar waveguiding structure. The minimal diameter of the micropillars is 100 nm, and their height is 700 nm. The size of the holes in the photon crystals and the photon crystal period are controllably varied from 140 to 500 nm and from 400 to 1000 nm, respectively. The etch depth of the crystals is more than 350 nm.     

椭圆空气孔微结构光纤限制损耗的分析

以完美匹配层为边界条件,用复有限元法分析了椭圆空气孔三角形微结构光纤中基模的限制损耗.数值结果表明: 当孔间距,空气填充率和空气孔环数增大时,基模的限制损耗减小.从计算结果还可以得出:限制损耗与基模的偏振有关,随着孔间距,空气填充率和孔的椭圆率的增大,y偏振模与x偏振模的限制损耗比也增大.最后,提出了两种可实现光单偏振,单模传输的高非线性微结构光纤. 关键词： 微结构光纤 限制损耗 复有限元法 完美匹配层     

Dispersion management with microstructured optical fibers: ultraflattened chromatic dispersion with low losses

We numerically demonstrate ultraflattened chromatic dispersion with low losses in microstructured optical fibers (MOFs). We propose using two different MOF structures to get this result. Both structures are based on a subset of a triangular array of cylindrical air holes; the cross sections of these inclusions are circular, and a missing hole in the fiber's middle forms the core. In this MOF structure the diameters of the inclusions increase with distance from the fiber axis until the diameters reach a maximum. With this new design and with three different hole diameters, it requires only seven rings to reach the 0.2-dB/km level at lambda = 1.55 microm with a variation amplitude of dispersion below 3.0 x 10(-2) ps nm(-1) km(-1) of lambda = 1.5-1.6 microm. With the usual MOF (made from holes of identical diameter), we show that at least 18 hole rings are required for losses to decrease to < 1 dB/km at lambda = 1.55 microm.     

Fluorescence-excitation and hole-burning spectroscopy of heavy-dose neutron-irradiated α-Al2O3

We report on persistent spectral hole-burning and fluorescence-excitation spectroscopy in heavy-dose (3 × 10²⁰ n/cm²) neutron-irradiated and annealed sapphire using a Ti:Sapphire ring laser in single-frequency and broad-band operation. The optimum conditions for hole burning were obtained after annealing the crystals to 400 °C. Holes have been detected in the near-infrared spectral range between 745 and 795 nm. At 1.5 K, the narrowest hole widths were approximately 2 GHz, so that about 10⁴ holes can be burnt in this spectral region. Besides the storage density in the wavelength dimension, the coefficient describing the electric-field-induced filling of a spectral hole also rises by more than one order of magnitude as compared to crystals with low neutron-irradiation dose.     

Size parameter effect of dielectric small particle mediated nano-hole patterning on silicon wafer by femtosecond laser

By use of a polystyrene particle with a fundamental (800 nm) and a second-harmonic (400 nm) wave of a femtosecond Ti:sapphire laser, nano-hole patterning properties on a silicon wafer were experimentally compared by keeping the size parameter constant. With the 800-nm wave, the patterned hole diameter ranged from 100 to 250 nm and the depth ranged from 20 to 100 nm. With the 400-nm wave, the hole diameter ranged from 50 to 200 nm while the depth ranged from 10 to 60 nm. The patterned diameter and the depth of patterned nano-holes were also controllable by the laser fluence. By the 3D finite-difference time-domain method it is numerically predicted that if the size parameter is kept at π approximately, the nano-hole patterning is efficiently performed even in the ultraviolet region of the spectrum.     

Out-of-plane scattering in 1-D photonic crystal slabs

We present a detailed study of out-of-plane scattering losses in a 1D approximation of 2D photonic crystal slabs. In 2D photonic crystals with a waveguide structure in the third dimension, the periodic structure (in a lot of applications a 2D arrangement of holes etched through the core and cladding) will cause light to scatter out of the waveguide plane. We studied the out-of-plane scattering losses of these holes using a 2D approximation of this 3D structure, with etched slots instead of holes. Our simulation techniques included mode expansion with PML and FDTD. We will present the influence of the refractive index contrast between core and cladding of the layered structure. We show that the losses increase with higher index contrast between core and cladding, but that with very high index contrasts and under the right circumstances light can be coupled into lossless Bloch modes.     

Nanohole 3D-size tailoring through polystyrene bead combustion during thin film deposition

A novel approach is presented for nanohole 3D-size tailoring. The process starts with a monolayer of polystyrene (PS) beads spun coat on silicon wafer as a template. The holes can be directly prepared through combustion of PS beads by oxygen plasma during metal or oxide thin film deposition. The incoming particles are prevented from adhering on PS beads by H₂O and CO₂ generated from the combustion of the PS beads. The hole depth generally depends on the film thickness. The hole diameter can be tailored by the PS bead size, film deposition rate, and also the combustion speed of the PS beads. In this work, a series of holes with depth of 4-24 nm and diameter of 10-36 nm has been successfully prepared. The hole wall materials can be selected from metals such as Au or Pt and oxides such as SiO₂ or Al₂O₃. These templates could be suitable for the preparation and characterization of novel nanodevices based on single quantum dots or single molecules, and could be extended to the studies of a wide range of coating materials and substrates with controlled hole depth and diameters.     

悬浮二维晶体材料反射光谱和光致发光光谱的周期性振荡现象

研究了在二氧化硅/硅衬底上制备的悬浮石墨烯以及二硫化钼的反射光谱以及悬浮二硫化钼的光致发光光谱.研究发现:悬浮多层石墨烯的反射光谱表现出明显的振荡现象,并且该振荡具有一定的周期性;振荡周期的大小不依赖于悬浮多层石墨烯的层数,而是随着衬底上沉孔深度(空气层厚度)的增加而减小.利用多重光学干涉模型可以解释这种振荡现象以及振荡周期随沉孔深度改变的变化趋势.该模型计算结果表明,只有当沉孔深度达到微米量级时这种振荡现象才会显著出现;并且可由振荡周期定量地确定出沉孔深度.对于悬浮的二硫化钼样品,其反射光谱和光致发光光谱也出现了类似的振荡现象.这表明这种振荡现象是在各种衬底上悬浮二维材料反射光谱和光致发光光谱的一种普遍性结果,也预示悬浮二维材料器件的电致发光光谱也会出现类似的振荡现象,对悬浮二维晶体材料的物理性质和器件性能研究具有一定的参考价值.     

Opposite spin asymmetry of elastic and inelastic scattering of nonequilibrium holes injected into a ferromagnet

The spin asymmetry of elastic and inelastic scattering of nonequilibrium holes injected into Co thin films is examined using a p-type magnetic tunnel transistor. Spin-dependent transmission yields a positive or negative magnetocurrent depending on Co thickness and hole energy. Up to a critical thickness of about 3 nm, (quasi)elastic scattering dominates with a short attenuation length (<1 nm) and preferential attenuation of holes in the majority spin bands, consistent with spin-wave emission. At a larger Co thickness, inelastic scattering dominates with a larger attenuation length ( approximately 4 nm) and opposite spin asymmetry.     

Comparison of the sensitivity of extraordinary transmittance peaks with the surrounding media for hole and slit arrays

In this paper we compare the sensitivity of the Extraordinary Transmittance (ET) peak position with the surrounding media for two types of metallic structures: arrays of holes and arrays of slits recorded in Au films. Both types of array were fabricated using Interference Lithography (IL) with a period of 700 nm and an Au thickness of 150 nm. The transmission spectra measurements were performed at normal incidence using a spectrophotometer. The results show that an array of slits presents a higher sensitivity for the surrounding media than the array of holes. Theoretical TE and TM simulations of the transmission spectra for the slit arrays agree very well with the experimental results, confirming the better sensitivity of the slit arrays.     

Luminescence in x-irradiated CaF2:Co

Photoluminescence of X-irradiated CaF₂:Co single crystals is reported. The emission spectrum shows four peaks at 505, 550, 640 and 685 nm, all of them with an excitation band at 275 nm. The same emission spectrum, plus a band at 280 nm, is found in X-ray excited luminescence measurements. Thermoluminescence of 80 K X-irradiated crystals gives a glow curve with five peaks at 100, 125, 145, 190 and 225 K. The spectral distribution of these glow peaks is similar to that of the X-ray excited luminescence. The 280 nm band is associated with electron—hole recombination. The other four bands are associated with electron transitions among excited states of Co²⁺ produced by recombination of holes and Co⁺-ions created by X-irradiation.     

Influence of ambient pressure on the hole formation in laser deep drilling

We investigate the temporal evolution of the hole depth and shape for percussion drilling at different ambient pressure conditions. Deep drilling is performed in silicon as target material by ultrashort laser pulses at 1030 nm and a duration of 8 ps. Simultaneously, the backlit silhouette of the hole is imaged perpendicular to the drilling direction. While typical process phases like depth development and shape evolution are very similar for atmospheric pressure down to vacuum conditions (10^?2 mbar), the ablation rate in the initial process phase is significantly increased for reduced pressure. The number of pulses till the stop of the drilling process also increases by a pressure reduction and exceeds drilling at atmospheric conditions by two orders of magnitude for a pressure of ca. 10^?2 mbar. Accordingly, the maximum achievable hole depth is more than doubled. We attribute this behavior to an enlarged mean free path for ablation products at reduced pressure and therefore lower or no deposition of particles inside the hole capillary under vacuum conditions while debris fills the hole already after a few thousand pulses at atmospheric pressure. This is supported by scanning electron cross section images of the holes.     

Photonic band gap engineering in 2D photonic crystals

The polarization-dependent photonic band gaps (TM and TE polarizations) in two-dimensional photonic crystals with square lattices composed of air holes in dielectric and vice versa i.e., dielectric rods in air, using the plane-wave expansion method are investigated. We then study, how the photonic band gap size is affected by the changing ellipticity of the constituent air holes/dielectric rods. It is observed that the size of the photonic band gap changes with changing ellipticity of the constituent air holes/dielectric rods. Further, it is reported, how the photonic band gap size is affected by the change in the orientation of the constituent elliptical air holes/dielectric rods in 2D photonic crystals.     

Double nanohole apex-enhanced transmission in metal films

We first present experimental results of enhanced transmission through nanofabricated double-hole arrays in a gold film. An increase in the transmission is observed when the holes are overlapping to produce two apexes, with the transmission more than doubling when the apexes are nearly touching. When the holes are non-overlapping, the transmission maximum drops. The measured spectra through these arrays showed a red-shift in the peak transmission wavelength around 770 nm of nearly 30 nm. These experimental results agree well with our finite-difference time-domain simulations of the double-hole arrays. PACS 78.66.Bz; 42.79.Ag; 42.79.Dj     

Origin of superenhanced light transmission through two-dimensional subwavelength rectangular hole arrays

Superenhanced light transmission through subwavelength rectangular hole arrays have been reported and some investigations have been made into the physical origin of this phenomenon [K.J. Klein Koerkamp et al., Phys. Rev. Lett. 92, 183901 (2004)]. In our current work, by performing FDTD (finite difference in the time domain) numerical simulations, we demonstrate that mechanism that is different from surface plasmon polaritons set up by the periodicity at the in-plane metal surfaces may account for this superenhanced light transmission. We suggest that for arrays of rectangular holes with small enough width in comparison to the wavelength of the incident light, standing electromagnetic fields can be set up inside the cavity by the surface plasmons on the hole walls with its intensity being substantially enhanced inside the cavity. So resonant cavity-enhanced light transmission is predominant and responsible for its superenhanced light transmission. Rectangular holes behave as Fabry-Pérot resonance cavities except that the frequency of their fundamental modes is restricted by their TM cutoff frequency. However we believe that both localized surface plasmon modes and surface plasmon polaritons set up by the periodicity at the in-plane metal surfaces have their shares in extraordinary optical transmission of rectangular hole arrays especially when the width of the rectangular hole is not small enough and the metal film is not thick enough.     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

Discrete tunneling of holes in porous silicon

It is pointed out that in the partial oxidation of porous silicon (PS) formed on heavily doped crystals, the topology of the pores can result in the formation of an anisotropic material with strings of nanometersized silicon granules embedded in insulating silicon dioxide SiO₂. In this range of granule sizes the correlation effects in the tunneling of electrons (holes) are strong on account of their Coulomb interaction. This should be manifested as discrete electron and hole tunneling at temperatures comparable to room temperature. The room-temperature current-voltage characteristics of n ⁺-PSp ⁺-p ⁺ diode structures with a PS interlayer on p ⁺-Si, which exhibit current steps on the forward and reverse branches, are presented. The current steps are attributed to discrete hole tunneling along the silicon strings in SiO₂. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 794–797 (25 May 1998)