arXiv subjects

Allard P. Mosk

Publications and source records attributed to Allard P. Mosk.

69 records · Page 4Linked to original sources

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

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↗

arXiv2010

Control of light transmission through opaque scattering media in space and time

We report the first experimental demonstration of combined spatial and temporal control of light trajectories through opaque media. This control is achieved by solely manipulating spatial degrees of freedom of the incident wavefront. As an application, we demonstrate that the present approach is capable to form bandwidth-limited ultrashort pulses from the otherwise randomly transmitted light with a controllable interaction time of the pulses with the medium. Our approach provides a new tool for fundamental studies of light propagation in complex media and has potential for applications for coherent control, sensing and imaging in nano- and biophotonics.

physics.optics↗

arXiv2010

Digital Plasmonics

The field of plasmonics offers a route to control light fields with metallic nanostructures through the excitation of Surface Plasmon Polaritons (SPPs). These surface waves, bound to a metal dielectric interface, tightly confine electromagnetic energy. Active control over SPPs has potential for applications in sensing, photovoltaics, quantum communication, nano circuitry, metamaterials and super-resolution microscopy. We achieve here a new level of control of plasmonic fields using a digital spatial light modulator. Optimizing the plasmonic phases via feedback we focus SPPs at a freely pre-chosen point on the surface of a nanohole array with high resolution. Digital addressing and scanning of SPPs without mechanical motion will enable novel interdisciplinary applications of advanced plasmonic devices in cell microscopy, optical data storage and sensing.

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

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↗

arXiv2007

Trapping of Rb atoms by ac electric fields

We demonstrate trapping of an ultracold gas of neutral atoms in a macroscopic ac electric trap. Three-dimensional confinement is obtained by switching between two saddle-point configurations of the electric field. Stable trapping is observed in a narrow range of switching frequencies around 60 Hz. The dynamic confinement of the atoms is directly visualized at different phases of the ac switching cycle. We observe about 10^5 Rb atoms in the 1 mm^3 large and several microkelvins deep trap with a lifetime of approximately 5 s.

physics.atom-ph↗

arXiv2007

Relaxation oscillations in long-pulsed random lasers

We have measured the evolution of the light intensity of a random laser during a nanosecond pump pulse. Relaxation oscillations in a titania random laser were observed in the time trace of the total emitted intensity. We compare our experimental results with a simple model, based on the four-level rate equations for a single mode laser.

physics.optics↗

arXiv2007

Spatial Extent of Random Laser Modes

We have experimentally studied the distribution of the spatial extent of modes, and the crossover from essentially single-mode to distinctly multi-mode behavior inside a porous gallium phosphide random laser. This system serves as a paragon for random lasers due to its exemplary high index contrast. In the multi-mode regime we observed mode competition. We have measured the distribution of spectral mode spacings in our emission spectra, and found level repulsion that is well described by the Gaussian orthogonal ensemble of random-matrix theory.

cond-mat.dis-nn↗

arXiv2006

Quantitative analysis of several random lasers

We prescribe the minimal set of experimental data and parameters that should be reported for random-laser experiments and models. This prescript allows for a quantitative comparison between different experiments, and for a criterion whether a model predicts the outcome of an experiment correctly. In none of more than 150 papers on random lasers that we found these requirements were fulfilled. We have nevertheless been able to analyze a number of published experimental results and recent experiments of our own. Using our method we determined that the most intriguing property of the random laser (spikes) is in fact remarkably similar for different random lasers.

cond-mat.dis-nn↗

arXiv2006

Intrinsic fluctuations in random lasers

We present a quantitative experimental and theoretical study of shot-to-shot intensity fluctuations in the emitted light of a random laser. A model that clarifies these intrinsic fluctuations is developed. We describe the output versus input power graphs of the random laser with an effective spontaneous emission factor (beta factor).

cond-mat.dis-nn↗

arXiv2006

Gain narrowing in few-atom systems

Using a density matrix approach, we study the simplest systems that display both gain and feedback: clusters of 2 to 5 atoms, one of which is pumped. The other atoms supply feedback through multiple scattering of light. We show that, if the atoms are in each other's near-field, the system exhibits large gain narrowing and spectral mode redistribution. The observed phenomena are more pronounced if the feedback is enhanced. Our system is to our knowledge the simplest exactly solvable microscopic system which shows the approach to laser oscillation.

quant-ph↗

arXiv2005

The independent scattering approximation in the saturated regime

We show that a saturable single-frequency elastic T-matrix approach to scattering of light by atoms agrees remarkably well with a master equation description in the regime of unsaturated atoms, or for large separation between the atoms. If the atoms are in each other's near-field and the saturation of the atoms is high enough, the two approaches yield different results.

cond-mat.dis-nn↗

arXiv2005

Dynamics of a two-level system in a broadband light field

We provide analytic solutions of the optical Bloch equations for a two-level system interacting with a broadband light field. We will focus on two special examples: incident light with a Lorentzian spectral density and the limit of incident monochromatic light. The results are used to obtain an expression for the nonlinear polarizability of a two-level atom irradiated by a wave with a Lorentzian spectral density.

cond-mat.dis-nn↗