1.4 ps rise‐time high‐voltage photoconductive switching

We report on the generation of 825 V electrical pulses with 1.4 ps rise time and 4.0 ps duration using a pulse‐biased low‐temperature‐grown GaAs photoconductive switch triggered by an amplified femtosecond dye laser. Dependence of the pulse shape on both electric field and optical energy is observed and discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70925/2/APPLAB-59-12-1455-1.pd

Dirty black holes: Quasinormal modes for "squeezed" horizons

We consider the quasinormal modes for a class of black hole spacetimes that, informally speaking, contain a closely ``squeezed'' pair of horizons. (This scenario, where the relevant observer is presumed to be ``trapped'' between the horizons, is operationally distinct from near-extremal black holes with an external observer.) It is shown, by analytical means, that the spacing of the quasinormal frequencies equals the surface gravity at the squeezed horizons. Moreover, we can calculate the real part of these frequencies provided that the horizons are sufficiently close together (but not necessarily degenerate or even ``nearly degenerate''). The novelty of our analysis (which extends a model-specific treatment by Cardoso and Lemos) is that we consider ``dirty'' black holes; that is, the observable portion of the (static and spherically symmetric) spacetime is allowed to contain an arbitrary distribution of matter.Comment: 15 pages, uses iopart.cls and setstack.sty V2: Two references added. Also, the appendix now relates our computation of the Regge-Wheeler potential for gravity in a generic "dirty" black hole to the results of Karlovini [gr-qc/0111066

Hyperbolic planforms in relation to visual edges and textures perception

We propose to use bifurcation theory and pattern formation as theoretical probes for various hypotheses about the neural organization of the brain. This allows us to make predictions about the kinds of patterns that should be observed in the activity of real brains through, e.g. optical imaging, and opens the door to the design of experiments to test these hypotheses. We study the specific problem of visual edges and textures perception and suggest that these features may be represented at the population level in the visual cortex as a specific second-order tensor, the structure tensor, perhaps within a hypercolumn. We then extend the classical ring model to this case and show that its natural framework is the non-Euclidean hyperbolic geometry. This brings in the beautiful structure of its group of isometries and certain of its subgroups which have a direct interpretation in terms of the organization of the neural populations that are assumed to encode the structure tensor. By studying the bifurcations of the solutions of the structure tensor equations, the analog of the classical Wilson and Cowan equations, under the assumption of invariance with respect to the action of these subgroups, we predict the appearance of characteristic patterns. These patterns can be described by what we call hyperbolic or H-planforms that are reminiscent of Euclidean planar waves and of the planforms that were used in [1, 2] to account for some visual hallucinations. If these patterns could be observed through brain imaging techniques they would reveal the built-in or acquired invariance of the neural organization to the action of the corresponding subgroups.Comment: 34 pages, 11 figures, 2 table

Dirty black holes: Quasinormal modes

In this paper, we investigate the asymptotic nature of the quasinormal modes for "dirty" black holes -- generic static and spherically symmetric spacetimes for which a central black hole is surrounded by arbitrary "matter" fields. We demonstrate that, to the leading asymptotic order, the [imaginary] spacing between modes is precisely equal to the surface gravity, independent of the specifics of the black hole system. Our analytical method is based on locating the complex poles in the first Born approximation for the scattering amplitude. We first verify that our formalism agrees, asymptotically, with previous studies on the Schwarzschild black hole. The analysis is then generalized to more exotic black hole geometries. We also extend considerations to spacetimes with two horizons and briefly discuss the degenerate-horizon scenario.Comment: 15 pages; uses iopart.cls setstack.sty; V2: one additional reference added, no physics changes; V3: two extra references, minor changes in response to referee comment

Binocular rivalry in children with schizophrenia: the conscious and unconscious cognitive processing of interpersonal information

背景: 儿童期精神分裂是一种严重的精神障碍，有理论认为其认知功能无论是意识水平还是潜意识水平都存在异常。但目前尚无针对儿童期精神分裂的潜意识认知功能研究。 目的: 开发新的双眼竞争测验版本，用于评估个体在意识和潜意识状态下对人际交往信息的认知加工过程，并依此判断儿童期精神分裂症患者的社会认知功能是否受损。 方法: 选取3种不同类型的图片（图片中无人物、有2~3个人物、有4个及以上的人物）共30张，这些图片在双眼竞争测验不可见模式（存在双眼竞争性抑制，反映潜意识状态下认知加工）和可见模式（无双眼竞争性抑制，反映意识状态下）中展示。对15名年龄&le;16岁、以妄想为主要症状的精神分裂症患者及15名健康儿童进行双眼竞争测验，通过对目标图片出现后立即在其左侧或右侧出现光栅的方向的判别来比较两组儿童的正确率，并比较两组间的反应时间。 结果: 患者组对所有类型图片中光栅方向判断的正确率均低于对照组，但是12对比较中仅2对的差异有统计学意义。与对照组相比，无论是在可见模式还是非可见模式下，患者对人物图片的注意要比对无人图片的注意有所增加，但无显著性意义。我们并未发现精神病性症状的严重程度与对图像认知加工的受损程度之间存在任何关联。当要求被试对三组图片进行评分时，患者组对存在2~3个人物的图片报告的高兴程度明显高于对照组的评分，差异有统计学意义。 结论: 儿童期精神分裂症患者对描绘人际关系的图片注意一定程度的增加，提示该病与社会信息的认知处理过程受损相关，但目前结果尚不能证实这一关系。我们将双眼竞争范式应用于认知功能差异的研究，只是取得部分的成功，其主要原因是该测验中对不同类型图片的反应注意量的关键指标存在较大的变异。</p