Photonic crystals have been an object of interest because of their properties to inhibit specific wavelengths and allow the transmission of others. Using these properties, we have designed a microcavity of Porous Silicon using two one-dimensional photonic crystals with an air defect between them. When we illuminate the microcavity with the appropriate light (laser with a wavelength of 633 nm) allows us to generate electromagnetic forces within the structure. These electromagnetic forces allow the microcavity to oscillate mechanically and we have named such a device Photodyne.
Experimentally, we have characterized the maximum displacement of several photodynes by using different driven frequencies and light powers. The displacements were put in evidence using a commercial vibrometer and by interferometry. From these measurements, it is possible to estimate the generated forces. Finally, we induced mechanical self-oscillations. The electromagnetic force generated within the whole photonic structure, by light is enough to overcome energy losses and sustain self- oscillations at two different frequencies. From these mechano-optical measurements, we estimated the stiffness and Young's modulus of porous silicon and compared the results with values reported elsewhere and with values estimated herein by a mechanical method.
The theoretical and experimental study of porous silicon-based UV microcavities is discussed in this work. The obtaining of CMs in the Ultraviolet range expands the field of research of porous silicon photonic structures. The porous silicon microcavities (PSM) consisted of two Bragg reflectors (BRs) with a defect between them. It was fabricated by electrochemical etching. Microcavities (MCs) were subjected to dry oxidation process (DOP). In this way we obtained an oxidized porous silicon (OPS) that induces a shift of the response to the ultraviolet (UV) region on both, the minimum peak of the reflectance spectrum and the maximum peak of the transmittance spectrum; two UV microcavities showed maximum transparency in the UV of 67 %. The shift is explained as due to the formation of silicon dioxide (SiO₂); this wavelength shift shows a logarithm-like function of oxidation times. It was used a theoretical model to predict the refractive index of the MCs that contains two components (Si and air) and tree component (Si, SiO₂, and air). Moreover, a photonic model was used to obtain the photonic band gap structure and the defect modes of different MCs in the UV-Visible range. The theoretical results showed that the experimental peaks within the UV photonic bandgap are indeed defect modes.
Characterization of MCs was performed by SEM, FTIR and UV-Vis-NIR spectroscopy before and after the DOP. These results open the possibility to create silicon-based photonic structures within the UV range where usually silicon or porous silicon either strongly absorb or scatter light.
We induced forced and auto-oscillations in one-dimensional photonic crystals (1-D-PCs) with localized defects when light impinges transversally to the defect layer. The photonic structure used consists of a microcavity-like structure formed of two 1-D-PCs made of free-standing porous silicon, separated by a variable air gap (the defect) and the working wavelength is 633 nm. The force generation was made evident by driving a laser light by means of a chopper; the light hit the photonic structure and induced a vibration and the vibration was characterized by using a very sensitive vibrometer. For example, we measured peak displacements and velocities ranging from 2 to 167 μm and 0.4 to 2.1 mm/s with a power light level from 2.6 to 13 mW. In comparison, recent evidence showed that giant resonant light forces could induce average velocity values of 0.45 mm/s in microspheres embedded in water with a 43-mW light power.
The perception of blur in humans is intrinsic to our visual system, and dioptric power can improve clarity in many cases. This was evaluated experimentally to establish the best correction with dioptric power shifts. We used Near Infrared Spectroscopy (NIRS) to measure Oxy-, Deoxy- and Total-hemoglobin concentration changes in the brain while viewing images and reading a Snellen chart. Participants were tested with their usual correction (no diopter power shift (0 D)), with a 0.25 diopter power shift (0.25 D), and with a 0.5 diopter power shift (0.5 D). The concept of Approximate Entropy (AE) was applied to quantify the regularity of these hemoglobin time series of finite length. AE computations are based on the likelihood that similar templates in a time series remain similar on the next incremental comparison, so that time series with large AE have high irregular fluctuation. We found that the dioptric power shift eliciting the highest AE indicates the clearest visual condition for subjects. This technique may impact the current way in which ophthalmic lenses are prescribed.
We induced forced and auto oscillations in one-dimensional photonic crystals with localized defects when light impinges transversally to the defect layer. The photonic structure consists of a microcavity like structure formed of two onedimensional photonic crystals made of free-standing porous silicon, separated by variable air gap and the working wavelength is 633 nm. The force generation is made evident by driving a laser light by means of a chopper; the light hits the photonic structure and induces a vibration and the vibration is characterized by using a very sensitive vibrometer. Moreover we measured peak displacements and velocities ranging from 2 up to 35 microns and 0.4 up to 2.1 mm/s with a power of 13 mW. Recent evidence showed that giant resonant light forces could induce average velocity values of 0.45 mm/s in microspheres embedded in water with 43 mW light power.
We studied theoretically and experimentally the induction of electromagnetic forces in one-dimensional photonic crystals with localized defects when light impinges transversally to the defect layer. The theoretical calculations indicate that the electromagnetic forces increases at a certain frequency that coincide with a defect photonic state. The photonic structure consists of a microcavity like structure formed of two one-dimensional photonic crystals made of free-standing porous silicon, separated by variable air gap and the working wavelength is 633 nm. The force generation is made evident by driving a laser light by means of a chopper; the light hits the photonic structure and induces a vibration and the vibration is characterized by using a very sensitive vibrometer.
The objective was to analyze visually driven postural responses and characterize any non-linear behaviour. We recorded physiological responses for two adults, 260 trials each. The subjects maintained quite stance while fixating for four seconds within an immersive room, EON Icube, where the reference to the visual stimuli, i.e., the virtual platform, randomly oscillated in Gaussian orientation 90° and 270° for antero-posterior (AP), and, 0° and 180° for medio-lateral (ML) at three different frequencies (0.125, 0.25, and 0.5 Hz). We accomplished stationary derivatives of posture time series by taking the intrinsic mode functions (IMFs). The phase space plot of IMF shows evidence of the existence of non-linear attractors in both ML and AP. Correlation integral slope with increasing embedding dimension is similar to random white noise for ML, and similar to non-linear chaotic series for AP. Next, recurrence plots indicate the existence of more non-linearity for AP than that for ML. The patterns of the dots after 200th time stamp (near onset) appears to be aperodic in AP. At higher temporal windows, AP entropy tends more toward chaotic series, than that of ML. There are stronger non-linear components in AP than that in ML regardless of the speed conditions.
In this work we present two experiments as evidence of negative refraction in one dimensional photonics crystals (1D
PC). Particularly the porous silicon (p-Si) multilayer structure is used as 1D PC since this structure presents periodic
dielectric components with specific refraction indexes and under certain conditions it can abnormally refract the light. In
the first experiment we show the negative refraction for two different wavelengths, one in the visible, and the other in the
infrared regions of the spectrum. In this experiment we use a fixed incidence angle for a conditioned white light beam
and we look for the emerging negative refracted beam. In the second experiment we characterize de negative refraction
observed for the same material by varying the incidence angle in a wide range. The obtained results are compared with a
theoretic prediction according a model proposed by the authors . We present a brief description of the material
production and its properties, as well.
Photonic crystals are artificial structures that have periodic dielectric components with different refractive indices.
Under certain conditions, they abnormally refract the light, a phenomenon called negative refraction. Here, we
discuss recent theoretical and simulation results that showed that negative refraction could be present near the low
frequency edge of at least the second, fourth and sixth bandgaps of a lossless one-dimensional photonic crystals (1DPC)
structure. That is, negative refraction is a multiband phenomenon. We also discuss the negative refraction
correctness condition that gives the angular region where negative refraction occurs. We compare two current
negative refraction theoretical models with recent experimental results. In order to succeed, an output refraction
correction is utilized. The correction uses Snell’s law and an effective refractive index based on two effective
dielectric constants. We found good agreement between experiment and both theoretical models in the negative
In a previous study, a new adaptive method (AM) was developed to adjust the learning rate in artificial neural networks:
the generalized no-decrease adaptive method (GNDAM). The GNDAM is fundamentally different from other traditional
AMs. Instead of using the derivative sign of a given weight to adjust its learning rate, this AM is based on a trial and
error heuristic where global learning rates are adjusted according to the error rates produced by two identical networks
using different learning rates. This AM was developed to solve a particular task: the orientation detection of an image
defined by texture (the texture task). This new task is also fundamentally different from other traditional ones since its
data set is infinite, each pattern is a template used to generate stimuli that the network learns to classify. In the previous
study, the GNDAM showed its strength over standard backpropagation for this particular task. The present study
compares this new AM to other traditional AMs on the texture task and other benchmark tasks. The results showed that
some AMs work well for some tasks while others work better for other tasks. However, all of them failed to achieve a
good performance on all tasks.
This study attempted to determine the influence of non-linear visual movements on our capacity to maintain postural control. An 8x8x8 foot CAVE immersive virtual environment was used. Body sway recordings were obtained for both head and lower back (lumbar 2-3) positions. The subjects were presented with visual stimuli for periods of 62.5 seconds. Subjects were asked to stand still on one foot while viewing stimuli consisting of multiplied sine waves generating movement undulation of a textured surface (waves moving in checkerboard pattern). Three wave amplitudes were tested: 4 feet, 2 feet, and 1 foot. Two viewing conditions were also used; observers looking at 36 inches in front of their feet; observers looking at a distance near the horizon. The results were compiled using an instability index and the data showed a profound and consistent effect of visual disturbances on postural balance in particular for the x (side-to-side) movement. We have demonstrated that non-linear visual distortions similar to those generated by progressive ophthalmic lenses of the kind used for presbyopia corrections, can generate significant postural instability. This instability is particularly evident for the side-to-side body movement and is most evident for the near viewing condition.
This paper deals with practical and theoretical issues related to motion parallax. Motion parallax implies that the perception of depth can be extracted from a temporal sequence of images that contain different perspectives. The present paper will focus on the relative effectiveness of motion parallax as compared to stereoscopic depth perception. It will be argued that motion parallax alone will generate a strong sense of depth, even in the absence of stereoscopic cues. Two studies directly comparing motion parallax and stereoscopy will be presented showing that, under certain conditions, these cues can be equally efficient and that there can be an additive effect when both cues are present. A theoretical discussion on the effect of optical distortions and how such distortions can influence motion parallax from a viewer's perspective will follow. Particular emphasis will be placed on the optical distortions produced by progressive addition lenses used to correct for presbyopia. Finally, research avenues will be proposed to answer some of the theoretical and practical issues related to motion parallax in our daily activities.