We study the ultrafast switching-on and -off of planar GaAs/AlAs microcavities. Up to 0.8% refractive index changes are achieved by optically exciting free carriers at λ = 1720 nm and pulse energy EPump = 1.8±0.18 µJ. The cavity resonance is dynamically tracked by measuring reflectivity versus time delay with tunable laser pulses, and is found to shift by as much as 3.3 linewidths within a few ps. The switching-off occurs with a decay time of ∼ 50 ps. We derive the dynamic behavior of the carrier density, and of the complex refractive index. We propose that our inferred 10 GHz switching rate may be tenfold improved by optimized sample growth.

Past studies of shape-coding in V4 have demonstrated that neurons can accurately represent isolated shapes in terms of their component contour features. However, rich natural scenes contain many partially occluded objects, which have "accidental" contours at the junction between the occluded and occluding objects. These contours do not represent the true shape of the occluded object and are known to be perceptually discounted. To discover whether V4 neurons differentially encode accidental contours, we studied the responses of single neurons in fixating monkeys to complex shapes and contextual stimuli presented either in isolation or adjoining each other to provide a percept of partial occlusion. Responses to preferred contours were suppressed when the adjoining context rendered those contours accidental. The observed suppression was reversed when the partial occlusion percept was compromised by introducing a small gap between the component stimuli. Control experiments demonstrated that these results likely depend on contour geometry at T-junctions and cannot be attributed to mechanisms based solely on local color/luminance contrast, spatial proximity of stimuli or the spatial frequency content of images. Our findings provide novel insights into how occluded objects, which are fundamental to complex visual scenes, are encoded in area V4. They also raise the possibility that the weakened encoding of accidental contours at the junction between objects could mark the first step of image segmentation along the ventral visual pathway.

We report a novel class of V4 neuron in the macaque monkey that responds selectively to equiluminant colored form. These "equiluminance" cells stand apart because they violate the well established trend throughout the visual system that responses are minimal at low luminance contrast and grow and saturate as contrast increases. Equiluminance cells, which compose about 22% of V4, exhibit the opposite behavior: responses are greatest near zero contrast and decrease as contrast increases. While equilumiance cells respond preferentially to equiluminant colored stimuli, strong hue tuning is not their distinguishing feature—some equilumiance cells do exhibit strong unimodal hue tuning but many show little or no tuning for hue. We find that equiluminance cells are color and shape selective to a degree comparable to other classes of V4 cells with more conventional contrast response functions. Those more conventional cells respond equally well to achromatic luminance and equiluminant color stimuli, analogous to color-luminance cells described in V1. The existence of equiluminance cells, which have not been reported in V1 or V2, suggests that chromatically-defined boundaries and shapes are given special status in V4 and raises the possibility that form at equiluminance and form at higher contrasts are processed in separate channels in V4.

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We performed non-degenerate pump-probe experiments on a GaAs/AlAs photonic structure. We switched the photonic properties using the optical Kerr effect and free carriers excited by three photon absorption. From these measurements we extracted the non-degenerate Kerr coefficients over a broad wavelength range and we extracted the three photon absorption coefficient for GaAs at three wavelengths in the near infrared. We show that the optical Kerr effect is large enough to switch a cavity resonance with a high Q (Q > 1000) photonic cavity.