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Periodic arrays of deep nanopores etched in silicon by deep reactive ion etching are desirable structures for photonic crystals and other nanostructures for silicon nanophotonics. Previous studies focused on realizing as deep as possible nanopores with as high as possible aspect ratios. The resulting nanopores suffered from structural imperfections of the nanopores, such as mask undercut, uneven and large scallops, depth dependent pore radii and tapering. Therefore, our present focus is to realize nanopores that have as cylindrical as possible shapes, in order to obtain a better comparison of nanophotonic observations with theory and simulations. To this end in our 2-step Bosch process we have improved the mask undercut, the uneven scallops, pore widening and positive tapering by optimizing a plethora of parameters such as the etch step time, capacitively coupled plasma (ion energy) and pressure. To add further degrees of control, we implemented a 3-step DREM (deposit, remove, etch, multistep) process. Optimization of the etching process results in cylindrical nanopores with a diameter in the range between 280 and 500 nm and a depth around 7 μm, corresponding to high depth-to-diameter aspect ratios between 14 and 25, that are very well suited for the realization of silicon nanophotonic structures.

In this PhD thesis, we study the propagation and behavior of waves in crystalline systems, with the emphasis on electromagnetic waves in photonic crystals and their confinement. In Chapter 1, we introduce the topic of wave manipulation by crystals and the basics of the mathematical description of electromagnetic waves in photonic crystals. Chapter 2 introduces a general scaling theory of identification and classification of wave confinement in crystal superlattices. Starting from the confinement energy and the mode volume, we use finite-size scaling to find that ratios of these quantities raised to certain powers yield the confinement dimensionality of each band. In Chapter 3, we investigate the possibility of improving the accuracy of our scaling method for classification of wave confinement in very small supercells by means of machine learning. We therefore slightly reformulate the scaling theory of Chapter 2 and use it to implement a physics- based clustering algorithm to classify wave confinement in photonic crystals. In Chapter 4, we employ the results of the previous chapters to analyze the light-confining properties of inverse woodpile photonic superlattices with respect to their structural parameters, namely the regular and defect pore radii. We create so-called confinement maps, depicting the dependence of the number and energy concentration of 3D confined bands on the crystal structural parameters. In Chapter 5 we investigate the problem of light propagation through realistically large photonic crystals - a notoriously computationally difficult task. To this end, we develop a size-robust discontinuous Galerkin finite element method, where we approximate the solution of a large but finite crystal by a set of Bloch modes, representing the solutions of an infinite crystal. We conclude in Chapter 6 by summarizing the results of this PhD thesis and providing suggestions for future research.

Since thin-film silicon solar cells have limited optical absorption, we explore the effect of a nanostructured back reflector to recycle the unabsorbed light. As a back reflector we investigate a 3D photonic band gap crystal made from silicon that is readily integrated with the thin films. We numerically obtain the optical properties by solving the 3D time-harmonic Maxwell equations using the finite-element method, and model silicon with experimentally determined optical constants. The absorption enhancement relevant for photovoltaics is obtained by weighting the absorption spectra with the AM 1.5 standard solar spectrum. We study thin films either thicker ($L_{Si} = 2400$ nm) or much thinner ($L_{Si} = 80$ nm) than the wavelength of light. At $L_{Si} = 2400$ nm, the 3D photonic band gap crystal enhances the spectrally averaged ($\lambda = 680$ nm to $880$ nm) silicon absorption by $2.22$x (s-pol.) to $2.45$x (p-pol.), which exceeds the enhancement of a perfect metal back reflector ($1.47$ to $1.56$x). The absorption is enhanced by the (i) broadband angle and polarization-independent reflectivity in the 3D photonic band gap, and (ii) the excitation of many guided modes in the film by the crystal's surface diffraction leading to enhanced path lengths. At $L_{Si} = 80$ nm, the photonic crystal back reflector yields a striking average absorption enhancement of $9.15$x, much more than $0.83$x for a perfect metal, which is due to a remarkable guided mode confined within the combined thickness of the thin film and the photonic crystal's Bragg attenuation length. The broad bandwidth of the 3D photonic band gap leads to the back reflector's Bragg attenuation length being much shorter than the silicon absorption length. Consequently, light is confined inside the thin film and the absorption enhancements are not due to the additional thickness of the photonic crystal back reflector., Comment: 23 pages, 15 figures

Controlling light scattering by nanoparticles is fundamentally important for the understanding and the control of light with photonic nanostructures, as well as for nanoparticle scattering itself, including Mie scattering. Here, we theoretically and numerically investigate the possibility to manipulate nanoparticle scattering by wavefront shaping that was initially developed to control light scattered by large numbers of nanoparticles in nanophotonic media. By employing a scattering matrix analysis, we find that even a single nanoparticle supports multiple strongly scattering eigenchannels, suggesting wavefront shaping as a promising tool to manipulate scattered light of a single nanoparticle. By sending in shaped wavefronts, we selectively excite eigenchannels, as is apparent from the distinct field distributions. These scattering eigenchannels are related to different resonant leaky modes of the scatterer, that reveal remarkable localized "hot spots" where the field is substantially enhanced. Moreover, we investigate the backscattered spectra; to this send in wavefronts relevant for a particular eigenchannel, and observe that the backscattered spectrum reveals not only the excited channel but also several others. This result points to the existence of short and long-range spectral correlations for an eigenchannel. Our work offers a flexible tool to manipulate light scattering of a single nanoparticle, and thus opens new possibilities to control field patterns and light-matter interactions in a nanoparticle, as well as to explore new features of nanoparticle scattering such as the spectral correlation and temporal response of light scattered by nano scatterers, including Mie spheres., 8 pages, 2 tables, 4 figures

Bose-Einstein condensation (BEC) is the massive occupation of a single quantum state in a gas of bosons at a finite temperature and is one of the few phenomena demonstrating quantum behaviour at the macroscopic scale. BEC of photons can be relised in quantum confined structures such as optical microcavities, where photons can thermalise to the temperature of the optical medium through repeated absorption and emission cycles. The critical particle number for such a system can be as low as 10 photons at room temperature, making photon BEC experimentally easily accessible. In an optical microcavity, photons behave like massive 2-dimensional particles confined in a potential defined by the mirror geometry and the optical medium refractive index. Controlling this potential is essential for exploring the physics of the condensation process. In this thesis, we develop, characterise and employ a number of flexible methods which rely on modified dielectric mirrors that have an additional optically absorptive silicon layer between the substrate and the dielectric stack. This layer allows us to locally heat the mirror using laser light. The first method is the permanent nanostructuring, resulting from pore formation inside the dielectric stack layers. The second method involves thermal expansion. Finally, introducing a thermoresponsive polymer into the optical medium allows us to control the refractive index. We experimentally investigate BEC under non-equilibrium conditions in potentials resembling a Mach-Zehnder interferometer. The switching behaviour of the interferometer in various configurations provides insight into the formation process of the condensate. We show that by adjusting their frequency, BECs naturally try to avoid particle loss and destructive interference in their environment. As a first step towards spin-glass simulators, an optical analogue of a 0,pi Josephson junction is implemented and characterised. Our experiments show a high degree of control over the junction state. Alternative types of condensate couplings and their possible implementations are investigated as well. In particular, we show that couplings can range from dispersive to dissipative, and show that the loss channel in dissipative couplings may be realised by drilling holes using focused ion beam milling.

Open quantum systems can be systematically controlled by making changes to their environment. A well-known example is the spontaneous radiative decay of an electronically excited emitter, such as an atom or a molecule, which is significantly influenced by the feedback from the emitter’s environment, for example, by the presence of reflecting surfaces. A prerequisite for a deliberate control of an open quantum system is to reveal the physical mechanisms that determine its state. Here, we investigate the Bose-Einstein condensation of a photonic Bose gas in an environment with controlled dissipation and feedback. Our measurements offer a highly systematic picture of Bose-Einstein condensation under non-equilibrium conditions. We show that by adjusting their frequency Bose-Einstein condensates naturally try to avoid particle loss and destructive interference in their environment. In this way our experiments reveal physical mechanisms involved in the formation of a Bose-Einstein condensate, which typically remain hidden when the system is close to thermal equilibrium., Non-equilibrium Bose-Einstein condensates exist in different systems like polaritons, photons. Here the authors demonstrate photonic BECs in an excited or a non-equilibrium state and explore the flow of the photons coupled to the interferometer in order to minimize the loss.

Solar photovoltaic (PV) energy plays a pivotal role in the quest for achieving a sustainable and greener future. Increasing the yield of PV modules will not only help in meeting the everincreasing global electricity demands, but also make the PV more economically lucrative, as compared to other sources. Bifacial modules offer an effective way to increase the power generation by accepting the solar irradiance from both faces, i.e., front and rear. The yield of the module depends on the spectral [1] and angular [2] composition of the light incident on it. The spectro-angular composition of the irradiance in outdoor conditions depends on numerous factors, such as, the geographical location [1], time of the day [4] and year [5], climate [6], weather [7], cloud coverage [8], atmospheric conditions, and the reflectors [1] or surroundings. This leads to serious complications in irradiance modeling and, consequently, in PV yield modeling as the input irradiance influences the PV output. Measuring the spectro-angular irradiance helps in addressing this issue. A high-quality hour- or minute-resolved year-long spectro angular irradiance data will help in verifying and training existing and emerging models. Finally, to boost bifacial yield, understanding and optimizing the reflectors become vital. This thesis offers solutions for the above two challenges.

We discuss the design and fabrication of deterministic photonic scattering media using direct laser writing. Static measurements and light transport calculations validate that the light scattering parameters match the design model.

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., Comment: 9 pages, 5 figures

The synchronization of coherent states of light has long been an important subject of basic research and technology. Recently, a new concept for analog computers has emerged where this synchronization process can be exploited to solve computationally hard problems - potentially faster and more energy-efficient than what can be achieved with conventional computer technology today. The unit cell of such systems consists of two coherent centers that are coupled to one another in a controlled manner. Here, we experimentally characterize and analyze the synchronization process of two photon Bose-Einstein condensates, which are coupled to one another, either dispersively or dissipatively. We show that both types of coupling are robust against a detuning of the condensate frequencies and show similar time constants in establishing mutual coherence. Significant differences between these couplings arise in the behaviour of the condensate populations under imbalanced optical pumping. The combination of these two types of coupling extends the class of physical models that can be investigated using analog simulations.

We have uploaded all data to the Zenodo database, enabling everyone to reuse our data and to reproduce all the figures of our paper. The upload contains the file "README.txt" explaining the content of the upload., This work was supported by the NWO-TTW program P15-36 "Free-Form Scattering Optics" (FFSO) and the MESA+ Institute section Applied Nanophotonics (ANP).

The fluorescence quantum yield of four representative red fluorescent proteins mCherry, mKate2, mRuby2, and the recently introduced mScarlet was investigated. The excited state lifetimes were measured as a function of the distance to a gold mirror in order to control the local density of optical states (LDOS). By analyzing the total emission rates as a function of the LDOS, we obtain separately the emission rate and the nonradiative rate of the bright states. We thus obtain for the first time the bright state quantum yield of the proteins without interference from dark, nonemitting states. The bright state quantum yields are considerably higher than previously reported quantum yields that average over both bright and dark states. We determine that mCherry, mKate2, and mRuby2 have a considerable fraction of dark chromophores up to 45%, which explains both the low measured quantum yields of red emitting proteins reported in the literature and the difficulties in developing high quantum yield variants of such proteins. For the recently developed bright mScarlet, we find a much smaller dark fraction of 14%, accompanied by a very high quantum yield of the bright state of 81%. The presence of a considerable fraction of dark chromophores has implications for numerous applications of fluorescent proteins, ranging from quantitative fluorescence microscopy to FRET studies to monitoring protein expression levels. We recommend that future optimization of red fluorescent proteins should pay more attention to minimizing the fraction of dark proteins.

We propose a size-robust multiscale FEM to compute electromagnetic propagation through locally periodic media. Our multiscale finite elements exploit the local periodicity to alleviate the computational complexity of the problem.

We propose an integrated network of ring resonators on a silicon-nitride chip for use as an all-optical single-spatial-mode physical unclonable key, enabling secret-free optical authentication with standard communication channels such as telecom fibres.

In most previous studies about light scattering, no geometry is used besides a slab. In this work, we study how macroscopic changes in the geometry affects light scattering using the Wavefront Shaping technique.

To investigate the performance of three-dimensional (3D) nanostructures, it is vital to study their internal structure with a methodology that keeps the device fully functional and ready for further integration. To this aim, we introduce here traceless X-ray tomography (TXT) that combines synchrotron X-ray holographic tomography with high X-ray photon energies (17 keV) in order to study nanostructures "as is" on massive silicon substrates. The combined strengths of TXT are a large total sample size to field-of-view ratio and a large penetration depth. We study exemplary 3D photonic band gap crystals made by CMOS-compatible means and obtain real space 3D density distributions with 55 nm spatial resolution. TXT identifies why nanostructures that look similar in electron microscopy have vastly different nanophotonic functionality: one "good" crystal with a broad photonic gap reveals 3D periodicity as designed; a second "bad" structure without a gap reveals a buried void, and a third "ugly" one without gap is shallow due to fabrication errors. Thus, TXT serves to nondestructively differentiate between the possible reasons of not finding the designed and expected performance and is therefore a powerful tool to critically assess 3D functional nanostructures.

A density-functional-theory based relativistic scattering formalism is used to study charge transport through thin Pt films with room temperature lattice disorder. A Fuchs-Sondheimer specularity coefficient $p \sim 0.5$ is needed to describe the suppression of the charge current at the surface even in the absence of surface roughness. The charge current drives a spin Hall current perpendicular to the surface. Analysing the latter with a model that is universally used to interpret the spin Hall effect in thin films and layered materials, we are unable to recover values of the spin-flip diffusion length $l_{\rm sf}$ and spin Hall angle $\Theta_{\rm sH}$ that we obtain for bulk Pt using the same approximations. We trace this to the boundary conditions used and develop a generalized model that takes surface effects into account. A reduced value of $\Theta_{\rm sH}$ at the surface is then found to describe the first-principles transport results extremely well. The in-plane spin Hall effect is substantially enhanced at the surface.

Photonic crystals are powerful tools to control light by a photonic band gap, analogous to the band gap in semiconductors. A functional class of photonic crystals can be fabricated by etching nanopores in silicon with controlled shape, size and reproducibility. These nanopores are created by deep reactive ion etching. Due to the decreased effective concentration of reactive ions with increasing aspect ratio, it is challenging to produce nanopores with the same diameter for the whole pore length. A 3-step (deposition, remove, etch) Bosch process has been developed to control the etching process and create straight walled nanopores. Nanopores with a diameter of 320 nm and high aspect ratios, > 20, have been achieved. The process was optimised to achieve as close to cylindrical pore profile as possible, minimising scallops, tapering and other deviations.

Photonic crystals are powerful tools to control light by a photonic band gap, analogous to the band gap in semiconductors. A functional class of photonic crystals can be fabricated by etching nanopores in silicon with controlled shape, size and reproducibility. These nanopores are created by deep reactive ion etching. Due to the decreased effective concentration of reactive ions with increasing aspect ratio, it is challenging to produce nanopores with the same diameter for the whole pore length. A 3-step (deposition, remove, etch) Bosch process has been developed to control the etching process and create straight walled nanopores. Nanopores with a diameter of 320 nm and high aspect ratios, > 20, have been achieved. The process was optimised to achieve as close to cylindrical pore profile as possible, minimising scallops, tapering and other deviations.

Interference of scattered waves is fundamental for modern light-scattering techniques, such as optical wavefront shaping. Recently, a new type of wavefront shaping was introduced where the extinction is manipulated instead of the scattered intensity. The underlying idea is that upon changing the phases or the amplitudes of incident beams, the total extinction will change due to interference described by the cross terms between different incident beams. Here, we experimentally demonstrate the mutual extinction and transparency effects in scattering media, in particular, a human hair and a silicon bar. To this end, we send two light beams with a variable mutual angle on the sample. Depending on the relative phase of the incident beams, we observe either nearly zero extinction, mutual transparency, or almost twice the single-beam extinction, mutual extinction, in agreement with theory. We use an analytical approximation for the scattering amplitude, starting from a completely opaque object, and we discuss the limitations of our approximation. We discuss the applications of the mutual extinction and transparency effects in various fields such as non-line-of-sight communications, microscopy, and biomedical imaging.

The goal of this thesis is to steer light deep inside otherwise opaque media. Opaque media are those that strongly interact with light, leading to low transmission and high scattering or reflection. We perform experimental studies on samples that interact with light in different manners. The opaque samples range from randomly distributed nanoparticles that scatter light in all directions to 3D periodically ordered photonic crystals with a forbidden range of light frequencies, a full photonic band gap, and even superperiodic structures, namely 3D arrays of coupled resonating cavities in a 3D band gap. In presence of multiple scattering, the wavefront shaping phase modulation technique is used to focus light behind or inside the medium. We apply this technique to photonic crystals with a forbidden energy gap for light that have intrinsic fabrication disorder that results in multiple scattering. By adding periodically repeated cavities in 3D band gap crystals, we finally present a novel controlled wave transport in superperiodic media, where light hops from cavity to cavity within an otherwise forbidden photonic band gap.

Multiphoton quantum interference is a one of the two technology platforms in which a quantum advantage has been demonstrated [1]. In these systems, a large-scale quantum state forms due to repeated effective interactions of photons (through the generalized Hong-Ou-Mandel effect) at the nodes of a photonic processor, resulting in a state that intrinsically possesses quantum complexity. A key question is how to validate the formation of a large-scale quantum state. One aspect of this is to measure whether the output quantum state is entangled – a necessary but not sufficient criterion for complexity. In this work, we derive and demonstrate an entanglement witness for large-scale optical quantum interference, and experimentally use this to demonstrate the presence of entanglement in our optical processor [2]. [1] Zhong et al., Science 370, 1460 – 1463 (2020) [2] Taballione et al., ArXiv 2012.05673 (2021)

The concept of the local density of optical states (widely referred to as "LDOS") is ubiquitous in nanophotonics as it serves to explain a wide range of electromagnetic phenomena, varying from antenna physics [1] , fluorescence [2] - [4] and emission control [5] , [6] , inter-molecular energy transfer [7] , [8] , and strong coupling [9] . Recently, we explored the fundamentals of this concept and its consequences in a broad didactic overview [10] . Here, we focus in greater detail on one of the wide-ranging implications from our study, namely our newly gained insights of how one can view the local density of states in several different ways. Each of these different approaches offers an alternative conceptual insight into the local density of optical states. Importantly, by making a detailed comparison between the different approaches we show the equivalence of rather different research fields, namely electrical engineering, quantum optics, and nanophotonics viewpoints.

We study the propagation of light through a three-dimensional (3D) superlattice of cavities that is embedded in a 3D photonic band gap. In such a superlattice of cavities, light propagation takes place as a result of inter-cavity coupling. Light supported by the cavity modes takes certain preferred high-symmetry paths inside the structure, hence known as ‘Cartesian light’ [1] .

Three dimensional photonic crystals offer the possibility to completely inhibit the local density of states (LDOS) for emitters inside the crystal [1] allowing the control of the LDOS which is crucial for emission control, quantum information science, photovoltaics and many more. However, total inhibition is completely true only for crystals of infinite size, whereas real devices are finite. Therefore, it is important to investigate the role of thr position and the dipole orientation of sources inside finite three-dimensional photonic crystals.

Recently, quantum advantage over classical computation has been claimed using photons [1] . To control photonic quantum computations such as Boson sampling, a non-universal approach on quantum computation, large-scale quantum processors are needed [2] , [3] . To realize scalable and robust photonic quantum processors integrated photonics is a key technology. In our approach we use the integrated waveguide platform based on silicon nitride [4] , due to its low intrinsic losses and technological maturity. The low losses result in a high fidelity of the quantum photonics processor which is mandatory for reliable quantum computation while maturity of the platform allows for scaling up of the dimensions of the processor. Here, we present a universal 12-mode quantum photonics processor which is the largest of its kind to date.

Nanomesh is a Python library that allows users to quickly and easily create 2D and 3D meshes directly from images of the object they wish to mesh. The automated workflow can preprocess and segment the picture to extract different regions and create conforming meshes of the objects. Analysis tools allow evaluating the quality of the resuting mesh and the detection of problematic regions. The resulting meshes can be exported to a variety of popular formats so that they can be used in finite element simulations. Nanomesh can be used as python library for example in Jupyter notebooks, or through dedicated online dashboards.

It is a major outstanding goal in nanotechnology to precisely position functional nanoparticles, such as quantum dots, inside a three-dimensional (3D) nanostructure in order to realize novel functions. Once the 3D positioning is performed, the challenge arises how to non-destructively verify where the nanoparticles reside in the 3D nanostructure. Here, we study 3D photonic band gap crystals made of Si that are infiltrated with PbS nanocrystal quantum dots. The nanocrystals are covalently bonded to polymer brush layers that are grafted to the Si-air interfaces inside the 3D nanostructure using surface-initiated atom transfer radical polymerization (SI-ATRP). The functionalized 3D nanostructures are probed by synchrotron X-ray fluorescence (SXRF) tomography that is performed at 17 keV photon energy to obtain large penetration depths and efficient excitation of the elements of interest. Spatial projection maps were obtained followed by tomographic reconstruction to obtain the 3D atom density distribution with 50 nm voxel size for all chemical elements probed: Cl, Cr, Cu, Ga, Br, Pb. The quantum dots are found to be positioned inside the 3D nanostructure, and their positions correlate with the positions of elements characteristic of the polymer brush layer and the ATRP initiator. We conclude that X-ray fluorescence tomography is very well suited to non-destructively characterize 3D nanomaterials with photonic and other functionalities.

Computational methods have proven to be essential in the design of three-dimensional (3D) photonic crystals [1] . They have allowed the prediction of the properties of the photonic crystal design without expensive manu-facturing steps. Additionally, computations are used to interpret experimental results from real crystals. Interest-ingly, the fabrication of photonic crystals is necessarily never perfect as structural differences from the design are inevitable [2] , [3] . This structural mismatch between design and realization means that experimental results (trans-mission, reflection) are not faithfully interpreted, as the physical model (plane-wave expansion) does not have the true structural realization as input.

Josephson junctions are the basis for the most sensitive magnetic flux detectors, the definition of the unit volt by the Josephson voltage standard, and superconducting digital and quantum computing. They result from the coupling of two coherent quantum states, as they occur in superconductors, superfluids, atomic Bose-Einstein condensates, and exciton-polariton condensates. In their ground state, Josephson junctions are characterised by an intrinsic phase jump. Controlling this phase jump is fundamental for applications in computing. Here, we experimentally demonstrate controllable phase relations between photon Bose-Einstein condensates resulting from particle exchange in a thermo-optically tunable potential landscape. Our experiment realises an optical analogue of a controllable 0,$\pi$-Josephson junction. By connecting several junctions, we can study a reconfigurable 4-condensate system demonstrating the potential of our approach for analog spin glass simulation. More generally, the combination of static and dynamic nanostructuring techniques introduced in our work offers a powerful platform for the implementation of adaptive optical systems for paraxial light in and outside of thermal equilibrium., Comment: 21 pages, 5 figures

The well-known optical theorem describes that extinction energy lost from a light beam is equal to scattering in all directions, and possible absorption [1] . Recently, our group has generalized the theorem to multiple incident beams [2] , with which we discovered it is possible to either enhanced transparency or enhanced extinction in a scattering system with multiple beams, controlling the relative phase and angle between them. This effect is called mutual transparency or extinction. Here, we present an experimental study of this effect. We use a strongly scattering sample made from a strip of silicon (see figure 1.b ). A liquid crystal phase retarder is used to control the phase difference between two incident beams and using an unbalanced March-Zehnder interferometer, we can control the relative angle with a movable mirror.

It is a major challenge in nanotechnology to precisely position active nanoparticles, like quantum dots, inside a three-dimensional (3D) nanostructure in order to realize novel functions. This is notably relevant to tune and control spontaneous emission and lasing of embedded quantum emitters [1] , [2] . Here, we study 3D photonic band gap crystals made from silicon that are infiltrated with PbS nanocrystal quantum dots that emit in the near infrared including telecom. Our crystals have the inverse woodpile structure and exhibit a broad full 3D band gap. Such crystals strongly inhibit emission of semiconductor quantum dots [3] . The material distribution in the crystals is defined by two perpendicular arrays of pores ( Fig. 1(A) ), running in the Z and X-directions. The crystal are made by CMOS-compatible means by deep reactive ion-etching through tailored masks. The PbS nanocrystals are covalently bonded to polymer brush layers that are grafted to the Si-air interfaces inside the 3D nanostructure using surface-initiated atom transfer radical polymerization (SI-ATRP) [4] .

S-shaped current–voltage ( I – V ) characteristics are a frequently occurring hurdle in the development of new solar cell material combinations and device architectures. Their presence points to the existence of a charge transport bottleneck that needs to be removed in order to unlock high fill factors and power conversion efficiencies. In this review, examples of studies in which s-shaped I – V curves have appeared are presented, and the cause and mitigation are discussed. Different solar cell material systems are often treated by separate communities, thereby, also the physics of s-shaped I – V curves have been treated separately. This review covers the main solar cell technologies—silicon, thin film, organic, hybrid—with the aim to provide an overarching picture of the common mechanisms and universal guidelines for mitigation of s-shaped I – V characteristics in emerging solar cell technologies. Except for a few studies on organic solar cells, s-shaped I – V curves are reported to result from charge transport barriers at one of the (selective) contact layers that can be overcome by interface engineering and doping.

The development of large-scale optical quantum information processing circuits ground on the stability and reconfigurability enabled by integrated photonics. We demonstrate a reconfigurable 8x8 integrated linear optical network based on silicon nitride waveguides for quantum information processing. Our processor implements a novel optical architecture enabling any arbitrary linear transformation and constitutes the largest programmable circuit reported so far on this platform. We validate a variety of photonic quantum information processing primitives, in the form of Hong-Ou-Mandel interference, bosonic coalescence/anticoalescence and high-dimensional single-photon quantum gates. We achieve fidelities that clearly demonstrate the promising future for large-scale photonic quantum information processing using low-loss silicon nitride., Comment: Added supplementary materials, extended introduction, new figures, results unchanged

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., Comment: 7 pages, 7 figures