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Multimode fibers (MMFs) show great promise as miniature probes for sensing, imaging and spectroscopy applications. Different parameters of the fibers, such as numerical aperture, refractive index profile and length, have been already optimized for better performance. Here we investigate the role of the core shape, in particular for wavefront shaping applications where a focus is formed at the output of the MMF. We demonstrate that in contrast to a conventional round-core MMF, a square-core design doesn’t suffer from focus aberrations. Moreover, we find that how the interference pattern behind a square-core fiber decorrelates with the input frequency is largely independent of the input light coupling. Finally, we demonstrate that a square core shape provides an on-average uniform distribution of the output intensity, free from the input-output correlations seen in round fibers, showing great promise for imaging and spectroscopy applications.

It is well-known that fruitful analogies exist between the physics of photons at the nanoscale, and electrons, spins, and phonons in condensed matter. A famous example is the analogy between a 3D photonic band gap in a photonic crystal and the electronic band gap in a semiconductor like silicon. Here, we perform the first ever study of light hopping in a 3D superlattice of coupled cavities in a photonic band gap, whose defect bands are analogous to the impurity bands describing (Anderson) electronic transport in a semiconductor [1,2]. Our samples consist of 3D photonic band gap crystals with evenly spaced cavities, made from silicon by deep reactive ion-etching. We developed a setup to probe reflectivity and scattering, including position and polarization-resolved response [3]. Bright and broad reflectivity peaks appear at frequencies matching the 3D band gap, while sharp scattering peaks represent cavity-confined light leaking in the 3rd dimension. The sharp scattering peaks correlate positively with the focus being on or off the defects, while the maximum reflectivity and the gap width anti-correlate, both as expected. We discuss our observational evidence of impurity bands in a band gap, and interpret these with calculated band structures and extensive simulations [4]. [1] P.W. Anderson, Phys. Rev. 109 (1958) 1492 [2] S.A. Hack, et al., Phys. Rev. B 99 (2019) 115308 [3] M. Adhikary, et al., Opt. Express 28 (2020) 2683 [4] D. Devashish, et al., Phys. Rev. B 99 (2019) 075112

An optical fiber is a promising tool for remote imaging [1]. A multimode fiber (MMF) offers multiple advantages: compactness, flexibility as well as the ability to transmit a large amount of information via multiple spatial modes. There are several methods to do optical imaging through a MMF. One can use wavefront shaping (WFS) to compensate for the modal dispersion and mode mixing so that the incident light can converge to a desired pattern at the fiber output [2]. By using WFS, we can sequentially generate focal spots at the distal fiber facet to scan the sample. Another imaging method through a MMF is compressive imaging (CI). The interfering fiber modes create a speckle pattern, which varies with wavelength and input light position, which. can be used as a basis for CI [3]. CI allows to reconstruct the image with a resolution beating the Abbe limit with fewer measurements than the number of pixels [4]. Finally, the decorrelation of the speckle patterns with wavelength encodes the spectral information of input light, which enables MMF spectroscopy applications [5].

Multimode fibers (MMFs) show great promise in imaging applications where space is limited, due to their small diameter, yet high NA [1] , [2]. One such application is endoscopy, where a multimode fiber can be used as a thin and flexible probe. Unfortunately, the perturbation sensitive mode mixing in a multimode fiber makes it difficult to reconstruct an image that is transmitted through the fiber [3]. Methods to overcome this difficulty, such as spatial wavefront shaping, still need re-optimalization after significant fiber perturbations. Such methods are also usually based on linear imaging. However, nonlinear imaging can provide increased resolution, reduced background and the ability for 3D imaging [4]. Combining ultrashort pulses with MMFs is challenging because of the complex spatiotemporal response of an MMF. So far, only methods based on spatial domain wavefront shaping have been used to selectively focus an ultrashort pulse through an MMF [5] , which are still perturbation-sensitive.

Imaging and transport light through scattering opaque media is a hot topic pursued in multiple fields, ranging from nanotechnology to life sciences. A promising technique to do this is wavefront shaping (WFS), where the light propagation through a scattering medium is controlled by interference [1][2]. Recently, the potential of WFS was even extended to, for instance, time-varying samples [3][4]. In most cases to date, WFS has been done on the quintessential scattering sample geometry, namely in slabs. Real-world applications, however, require samples to have any shape – “free-form scattering optics” – that defies current theories. Here, we present the study of an opaque sample of TiO2 particles suspended in silicone. Exploiting the flexibility of silicone, we are able to modify the geometry of the sample and measure the enhancement of the intensity η in a point of the speckle pattern. Using this opportunity, we compare the performance of a flat and a free form sample. These experimental measurements will be compared with a newly formulated theory of light transport in free form scattering media.

With the recent claim of a quantum advantage demonstration in photonics by Zhong et al, the question of the computation of lower-order approximations of boson sampling with arbitrary quantum states at arbitrary distinguishability has come to the fore. In this work, we present results in this direction, building on the results of Clifford and Clifford. In particular, we show: 1) How to compute marginal detection probabilities (i.e. probabilities of the detection of some but not all photons) for arbitrary quantum states. 2) Using the first result, how to generalize the sampling algorithm of Clifford and Clifford to arbitrary photon distinguishabilities and arbitrary input quantum states. 3) How to incorporate truncations of the quantum interference into a sampling algorithm. 4) A remark considering maximum likelihood verification of the recent photonic quantum advantage experiment.

Photonic scattering materials, such as biological tissue and white paper, are made of randomly positioned nanoscale inhomogeneities in refractive index that lead to multiple scattering of light. Typically these materials, both naturally-occurring or man-made, are formed through self assembly of the scattering inhomogeneities, which imposes challenges in tailoring the disorder and hence the optical properties. Here, We report on the nanofabrication of photonic scattering media using direct laser writing with deterministic design. These deterministic scattering media consist of submicron thick polymer nanorods that are randomly oriented within a cubic volume. We study the total transmission of light as a function of the number density of rods and of the sample thickness to extract the scattering and transport mean free paths using radiative transfer theory. Such photonic scattering media with deterministic and controllable properties are model systems for fundamental light scattering in particular with strong anisotropy and offer new applications in solid-state lighting and photovoltaics., 18 pages, 9 figures

Photonic processors are pivotal for both quantum and classical information processing tasks using light. In particular, linear optical quantum information processing requires both largescale and low-loss programmable photonic processors. In this paper, we report the demonstration of the largest universal quantum photonic processor to date: a low-loss, 12-mode fully tunable linear interferometer with all-to-all coupling based on stoichiometric silicon nitride waveguides.

Control over the interaction between light and matter will have far-ranging consequences in chemistry, material science, physics, and biology. An important tool for the advanced control of light is offered by 3D photonic crystals with a photonic band gap, a range of frequencies for which light is forbidden to exist. We have developed methods to precisely control the location of lead sulfide quantum dots on silicon surfaces by attaching the dots to functionalized polymer brushes. Encouraged by successes on planar silicon wafers, we introduced quantum dots with polymer brushes in the nanopores of 2D and even 3D photonic band gap crystals. By synchrotron X-ray fluorescence tomography, we obtain detailed information on the positioning of the brushes and the dots. Notably, the quantum dot signal correlates very strongly with the brush signal, validating our approach. In optical experiments we find evidence for considerable emission inhibition in the 3D photonic band gap.

Mechanical elements with dimensions in the nanometer range, at least in one direction, have been successfully employed as sensors in various devices. Their mechanical properties must be known with maximum precision in order to quantify the sensor response to external excitation. This often poses a significant challenge due to the mechanical fragility of the sensor elements. Here we present a measurement of the mechanical response of a 100 nm thick silicon nitride membrane. The external excitation force is provided by a laser beam modulated in amplitude, while the displacement of the membrane is measured by a Michelson interferometer with a homodyne readout.

We demonstrate grid scanning of an ultrashort pulse at the output of a multi-mode fiber by temporally shaping the input pulse in a single spatial mode. Our method has applications in nonlinear endoscopic imaging.

Hierarchical plasmonic–photonic microspheres (PPMs) with high controllability in their structures and optical properties have been explored toward surface-enhanced Raman spectroscopy. The PPMs consist of gold nanocrystal (AuNC) arrays (3rd-tier) anchored on a hexagonal nanopattern (2nd-tier) assembled from silica nanoparticles (SiO2NPs) where the uniform microsphere backbone is termed the 1st-tier. The PPMs sustain both photonic stop band (PSB) properties, resulting from periodic SiO2NP arrangements of the 2nd-tier, and a surface plasmon resonance (SPR), resulting from AuNC arrays of the 3rd-tier. Thanks to the synergistic effects of the photonic crystal (PC) structure and the AuNC array, the electromagnetic (EM) field in such a multiscale composite structure can tremendously be enhanced at certain wavelengths. These effects are demonstrated by experimentally evaluating the Raman enhancement of benzenethiol (BT) as a probe molecule and are confirmed via numerical simulations. We achieve a maximum SERS enhancement factor of up to ∼108 when the resonances are tailored to coincide with the excitation wavelength by suitable structural modifications.

It is well known that waves incident upon a crystal are transported only over a limited distance - the Bragg length - before being reflected by Bragg interference. Here, we demonstrate how to send waves much deeper into crystals, by studying light in exemplary two-dimensional silicon photonic crystals. By spatially shaping the optical wavefronts, we observe that the intensity of laterally scattered light, that probes the internal energy density, is enhanced at a tunable distance away from the front surface. The intensity is up to $100 \times$ enhanced compared to random wavefronts and extends as far as $8 \times$ the Bragg length. Our novel steering of waves inside a forbidden gap exploits the transport channels induced by unavoidable deviations from perfect periodicity, here unavoidable fabrication deviations.

International audience; Philip W. Anderson is one of the founding fathers of modern condensed-matter physics. With his death on 29 March 2020, we have lost one of the most influential physicists of the twentieth century.

We have developed a computationally efficient simulation model for the optimization of redirecting electrical front contacts for multijunction solar cells under concentration, and we present its validation by comparison with experimental literature results. The model allows for fast determination of the maximum achievable efficiency under a wide range of operating conditions and design parameters such as the contact finger redirecting capability, period and width of the fingers, the light concentration, and the metal and emitter sheet resistivity. At the example of a state-of-the-art four-junction concentrator solar cell, we apply our model to determine ideal operating conditions for front contacts with different light redirection capabilities. We find a 7% relative efficiency increase when enhancing the redirecting capabilities from 0% to 100%.

We study the hardness of classically simulating boson sampling with superposition and Gaussian input states at nonzero photon indistinguishability. We find that, similar to regular boson sampling, distinguishability causes exponential attenuation of the many-photon interference terms in both these boson sampling variants. For superposition sampling, we find that it is not simulable with out method at zero indistinguishability, which is evidence for the computational hardness of this problem, and we find that it is simulable at any level of particle distinguishability, similar to regular boson sampling. If an efficient classical algorithm to approximate a given sum over permanents is found, this approach also leads to an efficient classical algorithm to simulate Gaussian boson sampling in the presence of distinguishability.

A three-dimensional (3D) photonic band gap crystal is an ideal tool to completely inhibit the local density of optical states (LDOS) at every position in the crystal throughout the band gap. This notion, however, pertains to ideal infinite crystals, whereas any real crystal device is necessarily finite. This raises the question as to how the LDOS in the gap depends on the position and orientation inside a finite-size crystal. Therefore, we employ rigorous numerical calculations using finite-difference time domain simulations of 3D silicon inverse woodpile crystals filled with air or with toluene, as previously studied in experiments. We find that the LDOS versus position decreases exponentially into the bulk of the crystal. From the dependence on dipole orientation, we infer that the characteristic LDOS decay length ${\ensuremath{\ell}}_{\ensuremath{\rho}}$ is mostly related to far-field dipolar radiation effects, whereas the prefactor is mostly related to near-field dipolar effects. The LDOS decay length has a remarkably similar magnitude to the Bragg length for directional transport, which suggests that the LDOS in the crystal is dominated by vacuum states that tunnel from the closest interface toward the position of interest. Our work leads to design rules for applications of 3D photonic band gaps in emission control and lighting, quantum information processing, and in photovoltaics.

We study the optical reflectivity of real three-dimensional (3D) photonic band-gap crystals with increasing thickness. The crystals consist of GaAs plates with nanorod arrays that are assembled by an advanced stacking method into high-quality 3D woodpile structures. We observe intense and broad reflectivity peaks with stop bands that correspond to a broad gap in the photonic band structures. The maximum reflectivity quickly reaches high values, even for a few crystal layers. Remarkably, the bandwidth of the stop bands hardly decreases with increasing crystal thickness, in good agreement with finite-difference time domain (FDTD) simulations. This behavior differs remarkably from the large changes observed earlier in weakly interacting 3D photonic crystals. The nearly constant bandwidth and high reflectivity are rationalized by multiple Bragg interference that occurs in strongly interacting photonic band-gap crystals, whereby the incident light scatters from multiple reciprocal lattice vectors simultaneously, in particular, from oblique ones that are parallel to a longer crystal dimension and thus experience hardly any finite-size effects. Our insights have favorable consequences for the application of 3D photonic band-gap crystals, notably since even thin structures reveal the full band-gap functionality, including devices that shield quantum bits from vacuum fluctuations.

In this thesis the preparation and characterization of opal-based photonic crystals are described with the aim to control the properties of internal light sources.

When a wave is incident on a complex scattering medium, the transmitted intensity differs from the incident one due to extinction. In the absence of absorption, the extinguished power is equal to the total scattered power, a well-known conservation law termed the optical theorem. Here, we extend the case of a single incident wave to the situation of scattering and extinction by multiple incoming waves. The emerging generalized optical theorem has the exciting consequence that multiple incident waves show mutual extinction and mutual transparency, phenomena that do not exist in common forward scattering or self-extinction. Based on both exact calculations of realistic three-dimensional (3D) samples containing many (up to 104) scatterers and on approximate Fraunhofer diffraction theory we make the striking observation that the total extinction of two incident waves is greatly enhanced, called mutual extinction, or greatly reduced, mutual transparency, by up to 100% of the usual single-beam extinction. In view of the surprisingly strong mutual extinction and transparency, we propose new experiments to observe mutual extinction and transparency, namely in two-beam experiments with either elastic and absorbing scatterers, in optical wavefront shaping, in dynamic light scattering, and we discuss possible applications.

Spherical photonic crystals (SPCs) with tailorable multiscale structure, versatile surface morphology, and controllable optical properties of both photonic stop band (PSB) and surface plasmon resonance (SPR), have been fabricated using a robust and facile method. The fabricated SPCs consist of well-spaced gold nanocrystals (AuNCs) (3rd-tier) anchored on silica nanopatterns (2nd-tier) confined in microspherical templates (1st-tier). Droplet microfluidics is used to produce microdroplets containing silica nanoparticles (SiO2NPs) which assemble to form two-tier SPCs. Subsequently, three-tier SPCs are obtained by thermal dewetting and evaporation of metal films deposited on the two-tier SPCs, with the 3rd-tier morphology being controlled by the deposited film morphology and programmed thermal annealing. Optical PSB and SPR properties of the prepared SPCs can be on-demand tailored by the 2nd and 3rd-tier morphology and their corresponding constituent materials. It is found that the scattering from AuNC arrays on the SPCs can be amplified by tailoring the PSB properties. The hierarchical SPCs manufactured by this method take advantages of low-cost, high controllability, and further processability. The manufacturing of flexible film encapsulated well-assembled SPCs as anticounterfeiting stamps, which are easy to be identified using the mobile phone with a flash, is demonstrated.

Quantum communication aims to provide absolutely secure transmission of secret information. State-of-the-art methods encode symbols into single photons or coherent light with much less than one photon on average. For long distance communication, typically a single-mode fiber is used and significant effort has been devoted already to increase the data carrying capacity of a single optical line. Here we propose and demonstrate a fundamentally new concept for remote key establishment. Our method allows high-dimensional alphabets using spatial degrees of freedom by transmitting information through a light-scrambling multimode fiber and exploiting the no-cloning theorem. Eavesdropper attacks can be detected without using randomly switched mutually unbiased bases. We prove the security against a common class of intercept-resend and beam-splitting attacks with single-photon Fock states and with weak coherent light. Since it is optical fiber based, our method allows to naturally extend secure communication to larger distances. We experimentally demonstrate this new type of key exchange method by encoding information into a few-photon light pulse decomposed over guided modes of an easily available multimode fiber., 16 pages, 3 figures

Surface-initiated atom transfer radical polymerization (SI-ATRP) was used to graft poly(N-isopropylacrylamide) (PNIPAM) brush layers with a controllable thickness in the 10-nm range from silicon substrates. The rate of polymerization of N-isopropylacrylamide was tuned by the [Cu(II)]0/[Cu(I)]0 ratio between the deactivating and activating species. The polymer layer thickness was characterized by atomic force microscopy (AFM) and ellipsometry. PNIPAM layers with a dry thickness between 5.5 and 16 nm were obtained. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirmed that the chemical structure is PNIPAM brushes. Analysis of the AFM data showed that our procedure leads to polymer grafts in the "mushroom-to-brush" transition regime.

Surfactant-stabilized emulsion droplets of liquid crystals (LCs) suspended in water and labeled with a fluorescent dye form active, anisotropic optofluidic microresonators. These microresonators can host whispering gallery modes (WGMs), high-quality morphology-dependent optical resonances that are supported due to the contrast of refractive index between the LC droplets and the surrounding aqueous medium. In addition, owing to the refractive index contrast, such LC emulsion droplets can be stably trapped in three dimensions using optical tweezers, enabling long-term investigation of their spectral characteristics. We explore various combinations of fluorescently dyed LC droplets and host liquid-surfactant systems and show that the WGM emission spectra of optical resonators based on optically trapped LC emulsion droplets can be largely and (almost) reversibly tuned by controlled changes of the ambient temperature. Depending on the actual range of temperature modulation and LC phase of the studied droplet, thermally induced effects can either lead to phase transitions in the LC droplets or cause modifications of their refractive index profile without changing their LC phase. Our results indicate feasibility of this approach for creating miniature thermally tunable sources of coherent light that can be manipulated and stabilized by optical forces. (C) 2017 Optical Society of America

Recently, we applied wavefront shaping techniques to implement quantum-secure readout of a physical unclonable key for authentication [1]. Unfortunately, spatial wavefront shaping is unsuitable for long-distance use. We therefore propose to use wavefront shaping in the time domain using a single spatial mode. We show spatiotemporal control of a pulse through a complex medium. Furthermore, we investigate a secure communication method based on physical unclonable functions in the time domain. We will report on the progress.

no. 56

An important open challenge in quantum information processing is to demonstrate a quantum device outperforming a classical computer. One of the most promising ways is by sending single photons through a passive linear-optical network, and sampling their output distribution [1]. However, this requires the nontrivial generation of many spectrally pure (indistinguishable) single photons. Most importantly, -spectral distinguishability reduces the visibility of quantum interference of the photons, but can be mitigated by filtering at the cost of optical losses. Unfortunately, losses are also detrimental to quantum interference. We recently quantified the combined role of spectral imperfections and loss in a multiphoton interference experiment [2]. We now apply these results to the problem of constructing spontaneous parametric down-conversion sources. In particular, we investigate the optimal filter bandwidth for different sources and optimize the down-conversion crystal parameters. Our results show that demonstrating a quantum advantage using photonics is difficult, but possible

We study the optical reflectivity of three-dimensional (3D) photonic band gap crystals with increasing thickness. The crystals consist of GaAs plates with nanorod arrays that are assembled by an advanced stacking method into high-quality 3D woodpile structures. We observe intense and broad reflectivity peak with stop bands that correspond to a broad gap in the photonic band structures. The maximum reflectivity quickly reaches high values even for a few crystal layers. Remarkably, the bandwidth of the stop bands hardly decreases with increasing crystal thickness, in good agreement with FDTD simulations. This behavior differs remarkably from the large changes observed earlier in weakly interacting 3D photonic crystals. The nearly constant bandwidth and high reflectivity are rationalized by multiple Bragg interference that occurs in strongly interacting photonic band gap crystals, whereby the incident light scatters from multiple reciprocal lattice vectors simultaneously, in particular from oblique ones that are parallel to a longer crystal dimension and thus experience hardly any finite size effects. Our new insights have favorable consequences for the application of 3D photonic band gap crystals, notably since even thin structures reveal the full band gap functionality, including devices that shield quantum bits from vacuum fluctuations.

An important step for photonic quantum technologies is the demonstration of a quantum advantage through boson sampling. In order to prevent classical simulability of boson sampling, the photons need to be almost perfectly identical and almost without losses. These two requirements are connected through spectral filtering, improving one leads to a decrease of the other. A proven method of generating single photons is spontaneous parametric downconversion (SPDC). We show that an optimal trade-off between indistinguishability and losses can always be found for SPDC. We conclude that a 50-photon scattershot boson-sampling experiment using SPDC sources is possible from a computational complexity point of view. To this end, we numerically optimize SPDC sources under the regime of weak pumping and with a single spatial mode.

We show a method for assessing the quality of photonic quantum interference (boson sampling) in the regime of fewer samples than outcomes. We present an error budget for a state of the art boson sampler.

In this work, we report the feasibility of the silver nanowire (AgNW) foils as highly-sensitive, reproducible and facile detection tools for surface-enhanced Raman spectroscopy (SERS) applications. These flexible and free-standing AgNW foils, fabricated by vacuum filtration method following a modified polyol synthesis of AgNWs, are adequately structured for both biological specimen filtering and trace amount of molecule detection simultaneously. The compatibility of AgNW foil in SERS is investigated by using a Raman active molecule of different steric volumes across the filter cross-section. We have shown that AgNW foils exhibit extremely strong SERS activity with detection limit up to 10-9 M of crystal violet (CV) molecule with 20% variation over ∼cm2 , revealing reliable homogeneity of the acquired signal. While naturally occurring polyvinylpyrrolidone (PVP) layer during polyol synthesis contribute to controlled aggregation, oxidation prevention, and size - shape control purposes, it also creates a major challenge for obtaining enormous enhancement factors. However, controlled thickness of aluminum oxide (Al2O3) coating on PVP@AgNW foils affords to achieve higher enhancement factors than the uncoated ones. What is interesting is that the maximum intensity is achieved from two cycles of Al2O3 deposited on AgNW foils. This is attributed to the two different origins: first, a higher adsorption affinity of CV molecules to Al2O3 layer than PVP layer; second, tunneling barrier formation against quantum tunneling effects.

Full control over light propagation and emission in 3D is an outstanding goal in nanophotonics and relevant to applications of 3D waveguiding, quantum information processing and Anderson localization of light. For this purpose, 3D photonic band gap crystals offer promising possibilities. Due to the vanishing density of states, there are no modes in the band gap and light propagation is prohibited in all directions. To realize fully controlled unconventional light propagation in the forbidden gap, 3D nanocavities are formed by tailoring defects in the photonic crystal. Theory predicts that coupled cavity resonances give rise to new, allowed bands. In such a system, photons hop predominantly in x,y,z directions, which is known as “Cartesian light”. We fabricated 3D cavity superlattices in 3D photonic band gap crystals made from silicon that operate at telecom wavelengths. We have developed a near-infrared optical microscope to collect scattering and reflection from the sample from two orthogonal directions. We collect broadband reflectivity to identify the gap and simultaneously collect spectra of the scattered light from the lateral side of the crystal. We find that intensity peaks in the lateral scattering spectra reproduce at different locations on the crystal that distinguishes them from speckle, and identify them as superlattice modes. The observations of Cartesian light open the avenue to 3D cavity superlattice physics, including 3D Anderson localization.

Lighting is a crucial technology that is used in our daily lives. The introduction of the white light emitting diode (LED), which consists of a blue LED combined with a phosphor layer, greatly reduces the energy consumption for lighting. Despite the fast-growing market, white LEDs are still being designed with slow, numerical, trial-and-error methods. Here we introduce a radically new design principle that is based on an analytical model instead of a numerical approach. Our model predicts the white LED's color point for any combination of design parameters. In addition, our model provides the reflection and transmission coefficients of the scattered and re-emitted light intensities, as well as the energy density distribution inside the LED. To validate our model, we have performed extensive experiments on an emblematic white LED and found excellent agreement. Our model provides for a fast and efficient design, resulting in reductions of both design and production costs.

In this PhD Thesis, we theoretically and numerically investigate novel aspects of light propagation in three-dimensional (3D) photonic band gap crystals that open up new opportunities to tune the interaction between light and matter. We also develop highly accurate nonreflecting boundary conditions for a mixed DG discretization of the Maxwell equations on finite size computational domains. Photonic crystals are a class of nanostructures with a periodically varying refractive index in space. Three-dimensional photonic crystals can have a 3D photonic band gap: due to wave interference, light is not allowed to exist within the crystal. A 3D band gap offers new opportunities to tailor the interaction between light and matter. We explore for the first time the propagation of light in a 3D cavity superlattice inside a 3D photonic band gap crystal. Each cavity acts as a "cage" for light due to the surrounding band gap. In a superlattice with closely spaced cavities, the light sometimes "escapes" and hops to one of the neighboring cavities. We calculate the coupling coefficients that describe the hopping transport. We observe for one of the modes of light propagation that light hops only in the Cartesian directions. This cannot be readily inferred from radiation patterns, due to higher-dimensional wave interference. We also study time-resolved propagation patterns in a 3D cavity superlattice, and the effects of disorder on the density of states (DOS) in 3D photonic band gap crystals. Mixed discontinuous Galerkin (DG) finite element methods are well suited to solve the Maxwell equations in complex structures, as they allow to efficiently capture singularities at dielectric corners and interfaces and also provide a spectrum of electric field solutions that is free of spurious modes. In this PhD Thesis, we combine a mixed DG discretization with nonreflecting boundary conditions. The boundary conditions are obtained by applying the well-known Hagstrom-Warburton nonreflecting boundary conditions separately to the tangential components of the electric field. The discretized Hagstrom-Warburton boundary conditions successfully describe unboundedness with only small spurious reflections that are accurately predicted by an estimate. These new nonreflecting boundary conditions will allow accurate computations of the behavior of light in finite structures.

Boson Sampling is the problem of sampling from the same distribution as indistinguishable single photons at the output of a linear optical interferometer. It is an example of a non-universal quantum computation which is believed to be feasible in the near term and cannot be simulated on a classical machine. Like all purported demonstrations of "quantum supremacy", this motivates optimizing classical simulation schemes for a realistic model of the problem, in this case Boson Sampling when the implementations experience lost or distinguishable photons. Although current simulation schemes for sufficiently imperfect boson sampling are classically efficient, in principle the polynomial runtime can be infeasibly large. In this work, we develop a scheme for classical simulation of Boson Sampling under uniform distinguishability and loss, based on the idea of sampling from distributions where at most k photons are indistinguishable. We show that asymptotically this scheme can provide a polynomial improvement in the runtime compared to classically simulating idealised Boson Sampling. More significantly, we show that in the regime considered experimentally relevant, our approach gives an substantial improvement in runtime over other classical simulation approaches., Comment: 15 pages, 5 figures, comments welcome

Universal linear optical networks made of on-chip tunable beam splitters and phase shifters form a very promising platform for quantum information processing (QIP). Thanks to their phase stability and reconfigurability, they are robust and enable a variety of quantum information and communication protocols such as quantum teleportation [1], quantum key distribution [2], photonic qubit gate protocols [3] and boson sampling [4]. Two known materials for on-chip platforms are silicon-on-insulator (SOI) and doped silica, where SOI allows for a high component density due to its high index contrast and silica has a low loss.

Secure communication is of paramount importance in modern society. Asymmetric cryptography methods such as the widely used RSA method allow secure exchange of information between parties who have not shared secret keys. However, the existing asymmetric cryptographic schemes rely on unproven mathematical assumptions for security. Further, the digital keys used in their implementation are susceptible to copying that might remain unnoticed. Here we introduce a secure communication method that overcomes these two limitations by employing Physical Unclonable Keys (PUKs). Using optical PUKs realized in opaque scattering materials and employing off-the-shelf equipment, we transmit messages in an error-corrected way. Information is transmitted as patterned wavefronts of few-photon wavepackets which can be successfully decrypted only with the receiver's PUK. The security of PUK-Enabled Asymmetric Communication (PEAC) is not based on any stored secret but on the hardness of distinguishing between different few-photon wavefronts., Comment: 20 pages, 8 figures