3 research outputs found
Strong group delay dispersion in 3D photonic band gap crystals and in planar microcavities.
No full textWe discovered a small mistake in the dataprocessing. Please see or our next output for the corrected version.We have developed an interferometric optical reflectivity microscope to observe the phase-sensitive reflectivity of nanophotonic structures with high spatial and spectral resolution over broad frequency ranges from 4000 to 13300 cm-1, corresponding to wavelengths 750 < λ < 2500 nm. From the frequency-resolved phases we obtain the (group) time delay. We study planar microcavities made from GaAs/AlAs, and three-dimensional (3D) photonic band gap crystals made from silicon with the woodpile structure. Measurements on planar microcavities have been compared with analytic transfer matrix theory, where an excellent agreement is found. The mean difference between measured and calculated reflectivity is better than 4 percent-points. For a planar microcavity with a stopband centred at 1331 nm and a relative bandwidth of 16% we observe time delays exceeding 4 ps at the edge of the stopband. For the 3D woodpile structure we observe time delays exceeding 550 fs at the edge of the 3D bandgap. Combined with the very thin metasurface like structure, this yields a lower bound for the group index of n_g≥210, much more than previously observed in photonic crystal waveguides. Current studies include developing a model for the 3D woodpile crystal to understand the large group delay
Wavefront Shaping with varying degrees of freedom
No full textOptical WaveFront Shaping (WFS) uses the physical feature that whereas light scattering is complex, it is a linear process, thus deterministic. The incident wavefront is controlled to focus light through a scattering sample, by spatially dividing an incoming wavefront and modulating the resulting segments with Spatial Light Modulators (SLMs) or Digital Micromirror Devices (DMDs) paired with a holography system.The main criterion for such a process is the enhancement of the intensity at the target, defined as the ratio of the optimized intensity at the target, and the average intensity at the target for many realizations of the scattering sample. We focus on the effect of restricting the degrees of freedom of the phase modulating devices on the optimization performance. By turning off certain segments, which contribute very little to the optimization, it is possible to greatly shorten optimizations without a significant loss in enhancement. By shrinking the active area of segments, issues with holography systems occur, as small segments and phase transitions negatively affect performance.Our results lead to better choices regarding the areas of interest and limits of such optimizations to improve speed and efficiency, which are relevant for WFS applications
