arXiv subjects

Willem L. Vos

Publications and source records attributed to Willem L. Vos.

At least 73 records · Page 4Linked to original sources

arXiv2013

Optimal control of light propagation through multiple-scattering media in the presence of noise

We study the control of coherent light propagation through multiple-scattering media in the presence of measurement noise. In our experiments, we use a two-step optimization procedure to find the optimal incident wavefront. We conclude that the degree of optimal control of coherent light propagation through a multiple-scattering medium is only determined by the number of photoelectrons detected per single speckle spot. The prediction of our model agrees well with the experimental results. Our results offer opportunities for imaging applications through scattering media such as biological tissue in the shot noise limit.

physics.optics↗

arXiv2012

All-optical Switching of a Microcavity Repeated at Terahertz Rates

We have performed ultrafast pump-probe experiments on a GaAs-AlAs microcavity with a resonance near 1300 nm in the "original" telecom band. We exploit the virtually instantaneous electronic Kerr effect to repeatedly and reproducibly switch a GaAs-AlAs planar microcavity. We achieve repetition times as fast as 300 fs, thereby breaking the THz modulation barrier. The rate of the switching in our experiments is only determined by optics and not by material related relaxation. Our results offer novel opportunities for fundamental studies of cavity-QED and optical information processing in sub-picosecond time scale.

physics.optics↗

arXiv2011

Observation of a stronger-than-adiabatic change of light trapped in an ultrafast switched GaAs-AlAs microcavity

We study the time-resolved reflectivity spectrum of a switched planar GaAs-AlAs microcavity. Between 5 and 40 ps after the switching (pump) pulse we observe a strong excess probe reflectivity and a change of the frequency of light trapped in the cavity up to 5 linewidths away from the cavity resonance. This frequency change does not adiabatically follow the fast-changing cavity resonance. The frequency change is attributed to an accumulated phase change due to the time-dependent refractive index. An analytical model predicts dynamics in qualitative agreement with the experiments, and points to crucial parameters that control future applications.

physics.optics↗

arXiv2011

Observation of sub-Bragg diffraction of waves in crystals

We investigate the diffraction conditions and associated formation of stopgaps for waves in crystals with different Bravais lattices. We identify a prominent stopgap in high-symmetry directions that occurs at a frequency below the ubiquitous first-order Bragg condition. This sub-Bragg diffraction condition is demonstrated by reflectance spectroscopy on two-dimensional photonic crystals with a centred rectangular lattice, revealing prominent diffraction peaks for both the sub-Bragg and first-order Bragg condition. These results have implications for wave propagation in 2 of the 5 two-dimensional Bravais lattices and 7 out of 14 three-dimensional Bravais lattices, such as centred rectangular, triangular, hexagonal and body-centred cubic.

physics.optics↗

arXiv2011

Nanophotonic control of Forster Resonance Energy Transfer

Here we report on the experimental details of a study on the influence of the photonic environment on the emission of a FRET system. We modified the local density of optical states (LDOS) by placing the FRET system at precisely defined distances to a metallic mirror. We measured the energy donor lifetime in the presence of the FRET acceptor and the lifetime of an identical sample lacking an acceptor fluorophore for different LDOS. From the lifetimes we determined the FRET rate as well as the FRET efficiency for each sampled LDOS.

physics.chem-ph↗

arXiv2011

Signature of a three-dimensional photonic band gap observed on silicon inverse woodpile photonic crystals

We have studied the reflectivity of CMOS-compatible three-dimensional silicon inverse woodpile photonic crystals at near-infrared frequencies. Polarization-resolved reflectivity spectra were obtained from two orthogonal crystal surfaces corresponding to 1.88 pi sr solid angle. The spectra reveal broad peaks with high reflectivity up to 67 % that are independent of the spatial position on the crystals. The spectrally overlapping reflectivity peaks for all directions and polarizations form the signature of a broad photonic band gap with a relative bandwidth up to 16 %. This signature is supported with stopgaps in plane wave bandstructure calculations and with the frequency region of the expected band gap.

physics.optics↗

arXiv2011

Controlling the quality factor of a tuning-fork resonance between 9 K and 300 K for scanning-probe microscopy

We study the dynamic response of a mechanical quartz tuning fork in the temperature range from 9 K to 300 K. Since the quality factor Q of the resonance strongly depends on temperature, we implement a procedure to control the quality factor of the resonance. We show that we are able to dynamically change the quality factor and keep it constant over the whole temperature range. This procedure is suitable for applications in scanning probe microscopy.

physics.optics↗

arXiv2011

Ultimate fast optical switching of a planar microcavity in the telecom wavelength range

We have studied a GaAs-AlAs planar microcavity with a resonance near 1300 nm in the telecom range by ultrafast pump-probe reflectivity. By the judicious choice of pump frequency, we observe a ultimate fast and reversible decrease of the resonance frequency by more than half a linewidth due to the instantaneous electronic Kerr effect. The switch-on and switch-off? of the cavity is only limited by the cavity storage time of ?tcav = 0.3ps and not by intrinsic material parameters. Our results pave the way to supra-THz switching rates for on-chip data modulation and real-time cavity quantum electrodynamics.

physics.optics↗

arXiv2011

Focusing Light through Random Photonic Media by Binary Amplitude Modulation

We study the focusing of light through random photonic materials using wavefront shaping. We explore a novel approach namely binary amplitude modulation. To this end, the light incident to a random photonic medium is spatially divided into a number of segments. We identify the segments that give rise to fields that are out of phase with the total field at the intended focus and assign these a zero amplitude, whereas the remaining segments maintain their original amplitude. Using 812 independently controlled segments of light, we find the intensity at the target to be 75 +/- 6 times enhanced over the average intensity behind the sample. We experimentally demonstrate focusing of light through random photonic media using both an amplitude only mode liquid crystal spatial light modulator and a MEMS-based spatial light modulator. Our use of Micro Electro-Mechanical System (MEMS)-based digital micromirror devices for the control of the incident light field opens an avenue to high speed implementations of wavefront shaping.

physics.optics↗

arXiv2011

Broad-band coherent backscattering spectroscopy of the interplay between order and disorder in 3D opal photonic crystals

We present an investigation of coherent backscattering of light that is multiple scattered by a photonic crystal by using a broad-band technique. The results significantly extend on previous backscattering measurements on photonic crystals by simultaneously accessing a large frequency and angular range. Backscatter cones around the stop gap are successfully modelled with diffusion theory for a random medium. Strong variations of the apparent mean free path and the cone enhancement are observed around the stop band. The variations of the mean free path are described by a semi-empirical three-gap model including band structure effects on the internal reflection and penetration depth. A good match between theory and experiment is obtained without the need of additional contributions of group velocity or density of states. We argue that the cone enhancement reveals additional information on directional transport properties that are otherwise averaged out in diffuse multiple scattering.

cond-mat.mes-hall↗

arXiv2010

Optical characterization and selective addressing of the resonant modes of a micropillar cavity with a white light beam

We have performed white-light reflectivity measurements on GaAs/AlAs micropillar cavities with diameters ranging from 1 μm up to 20 μm. We are able to resolve the spatial field distribution of each cavity mode in real space by scanning a small-sized beam across the top facet of each micropillar. We spectrally resolve distinct transverse optical cavity modes in reflectivity. Using this procedure we can selectively address a single mode in the multimode micropillar cavity. Calculations for the coupling efficiency of a small-diameter beam to each mode are in very good agreement with our reflectivity measurements.

physics.optics↗

arXiv2009

Orientation-dependent spontaneous emission rates of a two-level quantum emitter in any nanophotonic environment

We study theoretically the rate of spontaneous emission of a two-level quantum emitter embedded in realistic systems: near a mirror, near a plasmonic sphere, or in a 3D photonic bandgap crystal. At constant frequency and position, we find striking (up to $10^{2}\times$) variations in emission rate by varying the orientation of the transition dipole moment. We present a general representation of rate versus orientation that only invokes symmetry of the Green function. The concomitant 3-dimensional geometric surfaces provide much insight in how preferred orientations for enhancement (or inhibition) depend on the emitter's frequency and location, and on optimal strategies to actively switch emission rates by controlling oriented dipoles.

physics.optics↗

arXiv2009

Time-resolved resonance and linewidth of an ultrafast switched GaAs/AlAs microcavity

We explore a planar GaAs/AlAs photonic microcavity using pump-probe spectroscopy. Free carriers are excited in the GaAs with short pump pulses. The time-resolved reflectivity is spectrally resolved short probe pulses. We show experimentally that the cavity resonance and its width depend on the dynamic refractive index of both the lambda-slab and the lambda/4 GaAs mirrors. We clearly observe a double exponential relaxation of both the the cavity resonance and its width, which is due to the different recombination timescales in the lambda-slab and the mirrors. In particular, the relaxation time due to the GaAs mirrors approaches the photon storage time of the cavity, a regime for which nonlinear effects have been predicted. The strongly non-single exponential behavior of the resonance and the width is in excellent agreement to a transfer-matrix model taking into account two recombination times. The change in width leads to a change in reflectivity modulation depth. The model predicts an optimal cavity Q for any given induced carrier density, if the modulation depth is to be maximized.

physics.optics↗

arXiv2009

Measuring the true quality factor of an ultrafast photonic microcavity: homogeneous versus inhomogeneous broadening

We have measured time-resolved the photon storage time and the quality factor of an ultrafast photonic cavity using an autocorrelator. The cavity consists of a $λ$-thick GaAs layer sandwiched between GaAs/AlAs distributed Bragg reflectors and resonates at 985 nm wavelength. The inverse relative linewidth measured with white light reflectivity is 830, while the quality factor obtained from the time resolved measurements is 1500. The photon storage time in the cavity is 0.78 ps. We show that the difference between the quality factor and the inverse relative linewidth results from inhomogeneous broadening of the microcavity resonance due to a spatial gradient in the cavity layer.

physics.optics↗

arXiv2009

Identification of competing ultrafast all-optical switching mechanisms in Si woodpile photonic crystals

We present a systematic study of ultrafast all-optical switching of Si photonic band gap woodpile crystals using broadband tunable nondegenerate pump-probe spectroscopy. At pump-probe coincidence, we investigate the behavior the differential reflectivity at the blue edge of the stopband for a wide range of pump- and probe frequencies. Both dispersive and absorptive features are observed from the probe spectra at coincidence. As the pump frequency is tuned through half the electronic bandgap of Si, the magnitude of both these features increases. For the first time we unambiguously identify this dispersive effect with the electronic Kerr effect in photonic crystals, and attribute the the absorptive features to nondegenerate two photon absorption. The dispersive and absorptive nonlinear coefficients are extracted, and are found to agree well with literature. Finally, we propose a nondegenerate figure of merit (NFOM), which defines the quality of switching for all nondegenerate optical switching processes.

physics.optics↗

arXiv2009

Broadband sensitive pump-probe setup for ultrafast optical switching of photonic nanostructures and semiconductors

We describe an ultrafast time resolved pump-probe spectroscopy setup aimed at studying the switching of nanophotonic structures. Both fs pump and probe pulses can be independently tuned over broad frequency range between 3850 and 21050 cm$^{-1}$. A broad pump scan range allows a large optical penetration depth, while a broad probe scan range is crucial to study strongly photonic crystals. A new data acquisition method allows for sensitive pump-probe measurements, and corrects for fluctuations in probe intensity and pump stray light. We observe a tenfold improvement of the precision of the setup compared to laser fluctuations, allowing a measurement accuracy of better than $Δ$R= 0.07% in a 1 s measurement time. Demonstrations of the improved technique are presented for a bulk Si wafer, a 3D Si inverse opal photonic bandgap crystal, and z-scan measurements of the two-photon absorption coefficient of Si, GaAs, and the three-photon absorption coefficient of GaP in the infrared wavelength range.

physics.optics↗

arXiv2009

Accurate calculation of the local density of optical states in inverse-opal photonic crystals

We have investigated the local density of optical states (LDOS) in titania and silicon inverse opals -- three-dimensional photonic crystals that have been realized experimentally. We used the H-field plane-wave expansion method to calculate the density of states and the projected local optical density of states, which are directly relevant for spontaneous emission dynamics and strong coupling. We present the first quantitative analysis of the frequency resolution and of the accuracy of the calculated local density of states. We have calculated the projected LDOS for many different emitter positions in inverse opals in order to supply a theoretical interpretation for recent emission experiments and as reference results for future experiments and theory by other workers. The results show that the LDOS in inverse opals strongly depends on the crystal lattice parameter as well as on the position and orientation of emitting dipoles.

physics.optics↗

arXiv2009

The role of fabrication deviations on the photonic band gap of 3D inverse woodpile nanostructures

In this report the effects of unintended deviations from ideal inverse woodpile photonic crystals on the band gap are discussed. These deviations occur during the nanofabrication of the crystal. By computational analyses it is shown that the band gap of this type of crystal is robust to most types of deviations that relate to the radii, position and angular alignment of the pores. However, the photonic band gap is very sensitive to tapering of the pores, i.e., conically shaped pores instead of cylindrical pores. To obtain three-dimensional inverse woodpile photonic crystals with a large volume, our work shows that with modern fabrication performances, tapering contributes most significantly to a reduction in the photonic strength of inverse woodpile photonic crystals.

physics.optics↗