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While integrated photonics is a robust platform for quantum information processing, architectures for photonic quantum computing place stringent demands on high quality information carriers. Sources of single photons that are highly indistinguishable and pure, that are either near-deterministic or heralded with high efficiency, and that are suitable for mass-manufacture, have been elusive. Here, we demonstrate on-chip photon sources that simultaneously meet each of these requirements. Our photon sources are fabricated in silicon using mature processes, and exploit a dual-mode pump-delayed excitation scheme to engineer the emission of spectrally pure photon pairs through inter-modal spontaneous four-wave mixing in low-loss spiralled multi-mode waveguides. We simultaneously measure a spectral purity of 0.9904 ± 0.0006, a mutual indistinguishability of 0.987 ± 0.002, and >90% intrinsic heralding efficiency. We measure on-chip quantum interference with a visibility of 0.96 ± 0.02 between heralded photons from different sources., Suitability for large-scale quantum computation imposes severe requirements on single-photon sources in terms of purity, indistinguishability and heralding efficiency. Here, the authors boost all these figures of merit through a dual-mode pump-delayed four-wave mixing scheme in low-loss silicon waveguides.

Silicon Photonics, the technology where optical devices are fabricated by the mainstream microelectronic processing technology, was proposed almost 30 years ago. I joined this research field at its start. Initially, I concentrated on the main issue of the lack of a silicon laser. Room temperature visible emission from porous silicon first, and from silicon nanocrystals then, showed that optical gain is possible in low-dimensional silicon, but it is severely counterbalanced by nonlinear losses due to free carriers. Then, most of my research focus was on systems where photons show novel features such as Zener tunneling or Anderson localization. Here, the game was to engineer suitable dielectric environments (e.g., one-dimensional photonic crystals or waveguide-based microring resonators) to control photon propagation. Applications of low-dimensional silicon raised up in sensing (e.g., gas-sensing or bio-sensing) and photovoltaics. Interestingly, microring resonators emerged as the fundamental device for integrated photonic circuit since they allow studying the hermitian and non-hermitian physics of light propagation as well as demonstrating on-chip heavily integrated optical networks for reconfigurable switching applications or neural networks for optical signal processing. Finally, I witnessed the emergence of quantum photonic devices, where linear and nonlinear optical effects generate quantum states of light. Here, quantum random number generators or heralded single-photon sources are enabled by silicon photonics. All these developments are discussed in this review by following my own research path.

Single photons for quantum computing should be pure, indistinguishable and available in a large amount on the same chip. By using pump-delayed intermodal spontaneous four wave mixing we demonstrated the first source of heralded single photons meeting all these requirements on a silicon chip.

Compared to bulk silicon, silicon nanocrystals (Si-NCs) show modified properties, such as tunable emission and enhanced radiative rate, as a result of the quantum confinement, surface chemistry and environment. While the effect of quantum confinement is well understood and experimentally confirmed on the hydrogen-capped Si-NCs, the surface effects in Si-NC with other types of ligands can be very complex and hard to predict. In our work, we argue that the surface chemistry, be it ligands and/or shell, can be designed to further improve the radiative rate of the Si-NCs, beyond what is achievable by the quantum confinement alone. Our experimental work shows a number of effects that indicate that in many instances, the core and surface capping cannot be separated, and optical properties cannot be clearly interpreted as “extrinsic” (related to the surface capping agent) or “intrinsic” (related to the core only). To this end, we performed also a detailed theoretical analysis of a number of surface ligands, to identify the role of chemistry and how that improves the optical properties of Si-NCs. Based on these investigations and findings, we realized two main things. Firstly, we argue that one cannot derive a simple rule to predict which type of element or molecule will improve or deteriorate the optical properties, because every individual element added (covalently) to the surface of Si-NC contributes to the electronic density via several mutually dependent effects, such as (i) orbital displacement, (ii) direct contribution of surface species into the density of states close to the bandgap, (iii) charge transfer due to the relative polarity of the surface capping element and Si, or (iv) ligand/matrix induced strain. Secondly, we realized that the \(\textbf{k}\)-space projections of the molecular orbitals, i.e., the band structure of the nanocrystal, are an essential and critical tool for investigations of the electronic and optical properties in materials with an originally indirect bandgap, since the surface chemistry in our simulations affects strongly the whole band structure.

Microring resonators made of materials with a zinc-blend or diamond lattice allow exploiting their 4-bar symmetry to achieve quasi-phase matching condition for second-order optical nonlinearities. However, fabrication tolerances impose severe limits on the quasi-phase matching condition, which in turn degrades the generation efficiency. Here, we present a method to mitigate these limitations. As an example, we studied the geometry and the pump wavelength conditions to induce the second-harmonic generation in silicon-based microrings with a second-order susceptibility &chi, zxy(2)&ne, 0. We found the best compromises between performances and experimental requirements, and we unveil a strategy to minimize the impacts of fabrication defects. The method can be easily transferred to other material systems.

Electric-field-induced second-harmonic generation is demonstrated in silicon waveguides with reverse biased lateral p-i-n junctions. Phase matching is achieved by periodically poling the applied electric field. Two different poling configurations are compared: in the first, the p- and n-type doped regions of the junctions are on different sides of the waveguide (simple configuration), while in the second, they are alternated periodically across the waveguide sides (interdigitated configuration). Both simulations and experiments show that the generation efficiency is increased by 10 times comparing the interdigitated and simple configurations. The effective second-order susceptibility modulation obtained at a reverse bias voltage of 3.5 V is Δ χ e f f , S ( 2 ) ≃ 0.14 p m / V for the simple configuration and Δ χ e f f , I ( 2 ) ≃ 0.64 p m / V for the interdigitated one.

Second order optical nonlinearities induced in silicon waveguides by the application of an electric field are evidenced by measuring second harmonic generation (SHG) in the mid infrared. The electric field is produced by lateral p-i-n junctions which are periodically disposed across the waveguide to reach a quasi-phase matching condition. Here, we report on the modeling of the experimental results by using stochastic variations of waveguide and junction geometries which are compatible with the fabrication technique. These variations lead to a broad band multiple peaked spectrum of the SHG efficiency around the nominal phase matched wavelength. Agreement between experiments and simulations is found.

We develop integrated spontaneous photon sources in silicon photonics simultaneously satisfying all requirements for large-scale photonic quantum computing: 0:9904 ± 0:0006 spectral purity, 0:985 ± 0:002 indistinguishability, > 90% heralding efficiency, and 0:96 ± 0:02 on-chip heralded quantum interference visibility.

Silicon oxynitride (SiON) microring resonators are used to develop a biosensor device to detect Aflatoxin M1 (AFM1), a carcinogenic mycotoxin present in milk. Two different surface functionalization strategies are tested to realize an affinity biosensing device. Both strategies are based on the formation of a silane surface layer able to bind either specific DNA-aptamers or antigen binding fragments (Fab’). The sensing device consists of an array of four SiON microring resonators, fiber-coupled to a vertical cavity surface emitting laser operating at 850 nm. Detection is realized with off-chip silicon photodetectors. With both functionalizations, we demonstrate a specific detection of AFM1 in buffer, while the nonspecific detection of Ochratoxin is not occurring. Repeatability and reproducibility of both functionalization strategies are discussed.

Silicon waveguides embedded in lateral p-n junctions show field-induced optical nonlinearities. By properly polarizing the junction, these can be used to achieve electro-optic modulation through the Direct Current Kerr effect. In addition, these enable second-order nonlinear processes such as the electric-field-induced second harmonic generation (EFISHG). In this work, we study in detail electro-optic effects in integrated silicon microresonators and demonstrate experimentally a field-induced resonance wavelength shift. This process is due to both the DC Kerr effect and the plasma-dispersion effect. By means of finite element method simulations, these effects are properly modeled and their contributions are accurately disentangled. The strength of the equivalent second-order nonlinear coefficient that would have provided the same electro-optic effect is about 16 pm/V. This result is comparable with that of materials possessing an intrinsic second order nonlinearity, and is one order of magnitude stronger than the most recent measurements of strain-induced Pockels effect in silicon.

Second order nonlinearities are inhibited in centrosymmetric crystals, like silicon. However, in the last ten years many attempts have been carried out to induce second order nonlinear susceptibility applying a stressing layer of silicon nitride on the top of a silicon waveguide. Succesful experiments showed both Second Harmonic Generation (SHG) or electro-optic modulation in strained silicon waveguide. In order to develop new devices, a full comprehension of the origins of such a nonlinearity is needed. In fact, a lot of estimations of the second order nonlinear coefficient have been given, all different from each other and, in some cases, even contradictory. In this work, we perform SHG in multimodal phase-matched silicon waveguides. We propose a way to individuate the origin of the nonlinearity, discriminating among the break of the centrosymmetry, the presence of charged states at the interfaces between silicon and silicon nitride and the overlap of the optical mode with the silicon nitride. We estimated a value of the second order nonlinear coefficient of 0.5 pm/V, demonstrating that it results from the coupling of the silicon third order nonlinear coefficient with the electric field induced by the presence of the trapped charges at the core/cladding interface. We also show preliminary results on SHG in strained silicon microring resonators. Our results open the door to interesting applications, going from broad frequency conversion, to generation of quantum states of light, up to the generation of octave spanning frequency comb based on second order nonlinearities.

We generate correlated photons with 600 nm spectral distance via intermodal four wave mixing in silicon. We want to demonstrate on-silicon-chip heralded single photons, beyond 2 µm, without post spectral filtering.

Four Wave Mixing (FWM) is the main nonlinear interaction in integrated silicon devices, which finds diffuse use in all-optical signal processing and wavelength conversion. Despite the numerous works on coupled resonator devices, which showed record conversion efficiencies and broadband operation, the possibility to coherently control the strength of the stimulated FWM interaction on a chip has received very limited attention. Here, we demonstrate both theoretically and experimentally, the manipulation of FWM in a photonic molecule based on two side coupled silicon microring resonators. The active tuning of the inter-resonator phase and of their eigenfrequencies allows setting the molecule in a sub-radiant state, where FWM is enhanced with respect to the isolated resonators. On the other hand, we can reconfigure the state of the photonic molecule to have energy equipartition among the resonators, and suppress FWM by making the two Signal waves to interfere destructively in the side coupled waveguides. This work constitutes an experimental demonstration of the control of a nonlinear parametric interaction via coherent oscillation phenomena in an integrated optical device.

Silicon is the most common semiconductor in technology. Its main limit is the lacking of efficient light emission properties. Here quantum effects are exploited to fabricate optically active silicon photonics elements. The manuscript reports recent results obtained in different research fields spanning from bio- to integrated-photonics and nonlinear optics.

This publisher’s note contains corrections to Opt. Lett. 45, 3188 (2020)OPLEDP0146-959210.1364/OL.391988.

Strained silicon waveguides have been proposed to break the silicon centrosymmetry, which inhibits second-order nonlinearities. Even if electro-optic effect and second harmonic generation (SHG) were measured, the published results presented plenty of ambiguities due to the concurrence of different effects affecting the process. In this work, the origin of SHG in a silicon waveguide strained by a silicon nitride cladding is investigated in detail. From the measured SHG efficiencies, an effective second-order nonlinear susceptibility of ~0.5 pmV−1 is extracted. To evidence the role of strain, SHG is studied under an external mechanical load, demonstrating no significant dependence on the applied stress. On the contrary, a 254 nm ultraviolet (UV) exposure of the strained silicon waveguide suppresses completely the SHG signal. Since UV irradiation is known to passivate charged defects accumulated in the silicon nitride cladding, this measurement evidences the crucial role of charged centers. In fact, charged defects cause an electric field in the waveguide that via the third order silicon nonlinearity induces the SHG. This conclusion is supported by numerical simulations, which accurately model the experimental results.

On-chip heralded single photon sources are of key importance in the development of chip-scale devices exploiting the quantum properties of light. Single photon states can be produced as single photons heralded from correlated photon pairs generated through spontaneous four wave mixing. On-chip heralded single photon sources based on spontaneous four wave mixing have been already demonstrated. However, the heralded and herald photons are usually generated with wavelengths very close to the pump one, limiting the pump rejection efficiency and the application to the mid infrared. Moreover, the common sources of heralded photons based on spontaneous four wave mixing require spectral post filtering to achieve high purity, limiting the brightness and the integration of these sources. A solution to these problems can be provided by intermodal four wave mixing. In this work, we demonstrate the generation of photon pairs through intermodal four wave mixing in silicon waveguides, measuring the coincidences between the idler at 1.281 μm and the signal at 1.952 μm. We then discuss the application of intermodal four wave mixing to on-chip heralded single photon sources.

In this work, we report on the modeling and the experimental characterization of a 6×400 GHz silicon Arrayed Waveguide Grating (AWG). The design of the device is based on the reduction of the background noise. The good characteristics of the AWG demonstrate that unwanted reflections have a detrimental role on its performance. We demonstrate a smoothing of the output channel shape of the AWG, as well as a reduction of the crosstalk level from −20.6(1) dB to −24.4(1) dB.

Silicon nanocrystals (Si-nc) are reviewed here for their interesting optical applications. On the one hand, they do exhibit quantum confinement effects. This allows turning silicon into a light-emitting material where luminescence can be excited by electrical injection. On the other hand, small sizes, large surfaces, and dielectric mismatch between the core and the surrounding matrix increase dramatically the nonlinear optical coefficients. This allows using Si-nc as a nonlinear material in different waveguide configurations. In this paper, we discuss specifically two different applications of Si-nc: (i) as a nonlinear material in various devices, e.g., in bistable optical cavities, in waveguide optical mode monitors that are based on two-photon excited luminescence detection, and in wavelength shifters by using four-wave mixing (FWM); (ii) as an entropy source for quantum random number generation, the key device for cryptography. (a) Top view optical image of a Si-nc-based whispering-gallery mode (WGM) microdisk resonator, vertically coupled with a bus waveguide. (b) Detail of the coupling zone. (c) Normalized waveguide transmission spectrum over a wide spectral range. The inset shows a blow up of a resonance with the radial and azimuthal mode orders and the Q factor value indicated.

The centrosymmetric crystalline structure of Silicon inhibits second order nonlinear optical processes in this material. We report here that, by breaking the silicon symmetry with a stressing silicon nitride over-layer, Second Harmonic Generation (SHG) is obtained in suitably designed waveguides where multi-modal phase-matching is achieved. The modeling of the generated signal provides an effective strain-induced second order nonlinear coefficient of χ(2) = (0.30 ± 0.02) pm/V. Our work opens also interesting perspectives on the reverse process, the Spontaneous Parametric Down Conversion (SPDC), through which it is possible to generate mid-infrared entangled photon pairs.

In this contribution we give an overview of our development of size-controlled multilayered ensembles of silicon nanocrystals (Si-NCs) for efficient light emitting devices (LEDs). The ensembles of Si-NCs embedded in silicon dioxide are grown by chemical vapour deposition methods and by using standard microelectronic processes. The formation of nanocrystals is monitored by several analytical techniques. Novel LED architectures of nanocrystal ensembles which improve electrical injection and light emission are described. Charge tunnelling in nanosilicon superlattices, light emission efficiency under direct and alternating current injection, and device stability are discussed. Visible light emission from an electroluminescence (EL) device with Si-NCs.

A random number generation approach comprising a silicon nanocrystals LED (Si-NCs LED), silicon single photon avalanche photodiode (Si SPAD) and a field-programmable gate array (FPGA) is introduced. The Si-NCs LED is the source of entropy with photon emission in the visible range detectable by silicon detectors allowing the fabrication of an all-silicon-based device. The proposed quantum random number generator (QRNG) is robust against variations of the internal and external parameters such as aging of the components, changing temperature, the ambient interferences and the silicon detector artifacts. The raw data show high quality of randomness and passed all the statistical tests in National Institute of Standards and Technology (NIST) tests suite without the application of a post-processing algorithm. The efficiency of random number generation is 4-bits per detected photon.

In this paper, we report on time resolved electro-optic measurements in strained silicon resonators. Strain is induced by applying a mechanical deformation to the device. It is demonstrated that the linear electro-optic effect vanishes when the applied voltage modulation varies much faster than the free carrier lifetime, and that this occurs independently on the level of the applied stress. This demonstrates that, at frequencies which lie below the free carrier recombination rate, the electro-optic modulation is caused by plasma carrier dispersion. After normalizing out free carrier effects, it is found an upper limit of χ(2) = (8 ± 3) pm/V to the value of the strain induced χ eff,zzz (2) tensor component. This is an order of magnitude lower than the previously reported values for static electro-optic measurements.

Silicon photonics has produced the first all-optical components intergrated into systems available already on the market. Nanosilicon, where low dimensional silicon allows new phenomena and devices, is a way to expand the reach of silicon photonics. In this article we review some of our past research activities focused on the demonstration of innovative photonic integrated circuits building blocks. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Porous silicon solid supports with pore diameter 0.5–1 μm, infiltrated with Ag nanostructures for surface enhanced Raman scattering (SERS) were prepared according to two procedures: spontaneous Ag+ reduction on the surface of freshly etched porous silicon immersed in Ag+ aqueous solutions, or anchoring colloidal Ag nanoparticles on the surface previously functionalized by aminosilane. Using Rhodamine 6G (RH6G) as analyte the detection limits were of the order of 20 μM and 20 nM with porous silicon metalized by the first and second procedure, respectively. This large increase of sensitivity notwithstanding a reduced surface density of Rhodamine 6G obtained on porous silicon metalized by the second procedure is discussed in terms of better hot spot efficiency and reduced aspecific binding out of the hot regions obtained depositing the colloids on the aminosilane functionalized surface.

Polymer pyrolysis to nanoceramics is employed for the production of photo-luminescent thin film samples, consisting of Si-nanocrystals dispersed in silica matrix. Thin films were deposited on Si wafer by spin-coating technique from a triethoxysilane-derived sol–gel solution. This is a fast process and a low cost technique. Samples were annealed at different temperatures between 600 °C and 1300 °C, under N 2 gas atmosphere. Structural evolution and luminescence from Si-ncs vs. temperature were investigated by FT-IR absorption spectroscopy and photo-luminescence. Absorption spectrum of As-Prep sample shows several vibrational bands due to Si–H, C–H x , and Si–O–Si structural units, without any appreciable luminescence. Silica phase segregation and intense PL (790 nm) due to Si-ncs were observed on annealing at T > 1000 °C.

Silicon the material per excellence for electronics is not used for sourcing light due to the lack of efficient light emitters and lasers. In this review, after having introduced the basics on lasing, I will discuss the physical reasons why silicon is not a laser material and the approaches to make it lasing. I will start with bulk silicon, then I will discuss silicon nanocrystals and Er3+ coupled silicon nanocrystals where significant advances have been done in the past and can be expected in the near future. I will conclude with an optimistic note on silicon lasing.

Noise properties of porous silicon/single-crystalline silicon structures under exposure to active gases are investigated for the first time. Current-voltage and noise characteristics of samples are measured. The Al/porous silicon/single-crystalline Si/Al sandwich-structures were placed in a special chamber and the influence of adsorption of various gases on low-frequency noise of the samples was revealed. The adsorption changes the dynamics of the interaction of charge carriers with traps, i.e. the noise characteristics of the samples. The results are encouraging for practical applications of porous silicon structures as gas sensors using measurements of low-frequency noise characteristics. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We show a vapor controlled one-dimensional photonic crystal built with a silicon-based dielectric mesoporous material, in which the refractive index can be continuously tuned by flowing organic vapors through the pores. Exposure of the crystal to vapors induces an inhomogeneous change in the refractive index through the depth and results in a tilted photonic band structure. We drive the photonic crystal to the optical Zener breakdown condition, introducing an enhanced transmission channel through the crystal. This phenomenon closely resembles the electronic Zener breakdown in a reverse-biased p-n junction. The effect is reversible and opens new perspectives for physics in photonic crystals and novel device concepts based on sensing, switching and processing of light. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Current–voltage and noise characteristics of porous silicon (PS)/single crystalline silicon (SCS) samples were measured under exposure to dry air, air +0.4% CO, dry air +1.7% CO, and dry air+ethyl alcohol vapor. The samples have a sandwich structure comprising Al/PS/SCS/Al. For the dry air +CO mixtures, the noise level was sensitive not only to the presence of CO but also to its percentage, and an increase of the CO concentration led to a change in the spectral density function of the low-frequency noise.

The lack of a dipolar second order susceptibility (F (2) ) in silicon due to its centro-symmetric diamond lattice usually inhibits efficient second order nonlinear optical processes in the silicon bulk. Depositing stressed silicon nitride layers or growing a thermal oxide layer introduces an inhomogeneous st rain into the silicon lattice and breaks the centro-symmetry of its crystal structure thereby creating a F (2) . This causes enhanced second harmonic generation and was observed in reflection and transmission measurements for wavelengths in th e infrared. However strain is not the only means to break the structures symmetry. Fixed charges at the silicon nitride/silicon interface cause a high electric field close to the silico n interface which causes electric-field-induced-second-harmonic (EFISH) contributions too. The combination of both effects leads to F (2) values which are estimated to be of the order as classic F materials like KDP or LiNiO 3 . This paves the way for the exploitation of other second order nonlinear processes in the area of silicon photonics and is an example how fundamental optical properties of materials can be altered by strain. Keywords: susceptibility, second harmonic generation, inhomogenenous strain, silicon, nonlinear optics

We present a post-processing free quantum random number generator (QRNG) based on silicon nanocrystals (Si-NCs) LEDs as a source of randomness. The relatively simple setup for data extraction, a negligible bias measured from the datasets and applying no post-processing operations to the raw data are the main advantages of this QRNG . The obtained bit sequences pass all the NIST tests and the highest bit-rate achieved is 0.6 Mbps.