Characterization of second-harmonic generation in silver nanoparticles for spontaneous parametric down-conversion

Entangled photon sources play a pivotal role in most of the optical systems that utilize quantum effects. Those include quantum key distribution, quantum computing, metrology, sensing and imaging. Photon pair generation using spontaneous parametric down conversion (SPDC) is one of the most popular routes for producing entangled photons. While SPDC has become more popular, the creation of entangled photon pairs in a degree of freedom, such as polarization, is more challenging. The difficulty is related in no small part to the birefringence of the structure where the generation takes place. Birefringence results in distinguishing information that hinders the production of entanglement in the platforms where SPDC has been most popular including ferroelectric crystals.Despite this limitation, entangled sources have been realized in bulk crystals via well designed interference techniques, which often include additional compensating optics. As the boundaries of optical quantum information science continue to be extended, multi-photon entanglement is now required. Solutions based on SPDC have been attempted, and while some outstanding results have been achieved, the interferometers and compensation procedures become increasingly complex as the size of the entangled state grows. Such scalability and stability issues are directly addressed by integrated entangled photon sources, and while still in their infancy, they promise to help a great deal towards the practical utilization and the production of entanglement on a larger scale.Semiconductors are an ideal platform to fabricate such integrated entangled photon sources. Recently, it was demonstrated that the gallium-arsenide (GaAs) based Bragg refection waveguide (BRW), could efficiently produce photon pairs via SPDC. This platfrom was shown to have a distinct advantage over other semiconductor sources, due largely to its monolithic architecture and its layered epitaxy, which underpins many photonic devices. In this work we demonstrate yet another advantage: the intrinsic capability of the BRW to directly produce polarization entangled photon pairs, without any additional interferometry, spectral filtering, compensation or post-selection. Not only do we show that the BRW can produce entangled photons, but we wish to emphasize that the traditional compensation and interferometric methods used to create entanglement, which presumably would have occurred on chip, may no longer be necessary. Our experimental results confirm that the BRW has the potential to be one of the very first self-contained integrated room temperature resources of entanglement.In this talk various approaches for utilizing SPDC processes in various integration platforms will be discussed. The root cause of entanglement generation is the lack of birefringence in these material systems, which otherwise makes exact phase matching impossible in bulk crystals. Because of this, cross-polarized photons propagate at almost identical group velocities, making off-chip path compensation unnecessary.

Sources of entangled photons are cornerstones of photonic quantum technologies, providing the basic resources for a wide range of quantum protocols for computing, simulation and sensing. A typical technique to generate entangled photon pairs exploits spontaneous parametric down conversion (SPDC) in non-linear crystals. In this context, an integrated-optics approach based on non-linear waveguides [1] can boost source brightness due to the enhanced mode confinement, and can take great advantage from the unique compactness and stability of on-chip devices. However, integrated devices are often specialized to produce a given quantum state.

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.

This paper describes a 2D detection method of single metal nanoparticles based on photothermal effect. Recently, it has been recognized that in order to utilize metal nanoparticles for optical labels, a real-time 2D detection method of individual metal nanoparticles is desirable. Metal nanoparticles can be detected by the photothermal effect, which is converting light energy to thermal energy, and localized surface plasmon resonance (LSPR). The photothermal effect induced by LSPR makes it possible to magnify an image of individual nanoparticles apparently. Its image can be detected using a single element interferometer for robust detection and performing phase analysis using the Fourier transform method for real-time detection. This paper reports that the phase of interference fringe shifted by the photothermal effect and LSPR was obtained by use of single element interferometer and the Fourier transform method. Furthermore, measuring phase shift induced by photothermal effect and LSPR, locations of single gold nanoparticles were detected by time-resolved phase measurement.

a‐Si:H thin films‐based solar cells offer the benefit of reducing material consumption and fabrication costs. However, such thin absorbing layer reduces the photovoltaic efficiency, due to their poor light absorption at red and near‐infrared wavelengths. Metal NPs such as silver (Ag) can exhibit strong localized surface plasmon (LSP) resonances at different wavelength ranges, allowing increasing the light absorption in the active layer. The goal of this study is to perform an optimal structure of Ag NPs allowing to optimize the light absorption within the a‐Si:H solar cells by means of scattering. Ag layers are deposited on two different SnO2 substrates (with different roughness). After annealing, quasi‐spherical shaped of Ag NPs with diameters ranging from 10 to 130 nm are obtained. UV‐visible spectroscopy displays one LSP resonance around 560 nm with the less rough substrate, while three LSP resonances around 560, 700, and 850 nm are observed with roughest substrate. These differences are mainly due to the size distribution of NPs and underline the strong influence of the surface roughness and the coupling effect on the NPs behavior. a‐Si:H thin film solar cells with Ag NPs are subsequently elaborated. Spectral response shows an improvement of about 14–15%.

Introduction Light incident on metal thin films or nanoparticles (NPs) may induce a collective oscillation of conduction electrons that can lead to generation of strong electromagnetic fields in the vicinity of the metal surfaces. This phenomenon is commonly referred to as surface plasmon resonance (SPR) for metal films and localized surface plasmon resonance (LSPR) for metal NPs. These plasmon resonances have been applied to affinity-based sensing due to their sharp response to refractive index changes in the vicinity of the metal surface. Although LSPR sensors can be smaller and less expensive than SPR sensors, they are not suitable for colored or turbid sample solutions in general because light must pass through the sample to an optical detector. In this study, LSPR sensors that directly output potential changes were developed (Figure 1).1 The sensors are based on plasmon-induced charge separation (PICS),2in which excited conduction electrons in metal NPs transfer to the semiconductor in direct contact with the NPs. Since light need not pass through the sample solution, the sensor can be applied to colored and turbid sample liquids. Experimental TiO2 thin films (~70 nm thick) were prepared on ITO-coated glass plates by a spray pyrolysis method from 2-propanol containing titanium diisopropoxide bis(acetylacetonate) followed by annealing at 500 °C for 30 min. Citrate-protected Au NPs (40 nm) were adsorbed onto the TiO2 surface by immersing the substrate in a Au NP dispersion at pH 2.7 for 6 h. Au@TiO2 core-shell NPs were also used instead of Au NPs. The Au NPs were covered with 5–7 nm-thick TiO2shells. The open-circuit potential (OCP) measurements were conducted with the Au NP/TiO2/ITO working electrode in water (refractive index n = 1.33) and a Ag|AgCl reference electrode. The working electrode was irradiated with light of different wavelengths, and a negative potential shift upon the light irradiation was measured. Refractive index of the solution was changed by adding glycerol (n= 1.47) to water and peak shifts in the photopotential action spectra were detected. Results and Discussion a) Sensors with Au NPs. The Au NP/TiO2/ITO electrode was irradiated with light and a negative shift of the OCP was observed. The photoresponse is ascribed to injection of electrons from resonant Au NPs to the conduction band of TiO2 due to PICS. In Figure 2, typical potential shifts –ΔE at t = 60 s (light irradiation was started at t = 0 s) are plotted against the energy of irradiated photons. The photopotential action spectrum is in good agreement with the LSPR absorption spectrum in shape. Furthermore, the peak in the action spectrum was redshifted in response to an increase in the refractive index of the sample solution. This peak wavelength shift was plotted against the refractive index of solutions, and a linear relationship was obtained. The refractive index sensitivity was calculated to be 33 ± 4 nm RIU-1 (refractive index unit). The sensor was applied to coffee as a colored and turbid sample. The Au/TiO2film in coffee did not show any extinction peak but a clear photopotential peak. b) Sensors with Au@TiO2 NPs. Since the refractive index sensitivity was not sufficiently high, we replaced the Au NPs with Au@TiO2 NPs. As a result, the sensitivity was increased to 72 nm RIU-1. In the case of the bare Au NPs, electron oscillation is somewhat localized at the bottom of each NP, at which the NP is in contact with TiO2, so that certain volume of the sensing region is occupied by the TiO2 substrate. On the other hand, the electron oscillation is delocalized over the whole NP surface for the Au@TiO2 NPs because the Au core is fully covered with the TiO2 shell, and therefore the masking by the TiO2substrate is less significant. Conclusion and Future Plan The LSPR sensor which directly output electrical signals from TiO2loaded with Au NPs was developed. The photopotential peaks were obtained on the basis of PICS. The response peak redshifted linearly as the refractive index of the sample solution increased. Since light need not pass through a sample solution, the sensor can be applied to a colored and turbid sample liquid. Work is currently underway to improve the sensor performance by using anisotropic NPs. References (1) T. Tatsuma, Y. Katagi, S. Watanabe, K. Akiyoshi, T. Kawawaki, H. Nishi, and E. Kazuma, Chem. Commun., 51, 6100 (2015). (2) Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005). Figure Caption. (a) Mechanism of the potentiometric LSPR sensor. (b) Typical photopotential action spectra of a Au/TiO2 film. Figure 1

Metal nanoparticles (MNPs) have potential uses in biochemical sensing applications owing to their localised surface plasmon resonance (LSPR) characteristics. In practice, selective biochemical sensors based on LSPR are constructed by MNPs and selective receptors for specific biochemical molecules. This Letter studies the effects of receptor thickness on LSPR sensing properties experimentally and theoretically. Experimental results demonstrate that an increase in polymethyl methacrylate (PMMA) receptor thickness induces a red shift of LSPR spectrum. For a 42 nm PMMA receptor, the maximum extinction efficiency is more sensitive to the test chloroform vapour than the peak wavelength. This receptor thickness effect is confirmed by electrodynamics calculations: a relatively thin receptor layer will create high peak wavelength sensitivity and low extinction efficiency sensitivity. However, it is the opposite for a relatively thick receptor layer. The turning point between thick and thin is approximately 20 nm in this Letter. These insights can be used as guidelines in fabricating high-sensitive LSPR-based biochemical sensors.

Quantum state engineering, the cornerstone of quantum photonic technologies, mainly relies on spontaneous parametric downconversion and four-wave mixing, where one or two pump photons spontaneously decay into a photon pair. Both of these nonlinear effects require momentum conservation for the participating photons, which strongly limits the versatility of the resulting quantum states. Nonlinear metasurfaces have subwavelength thickness and allow the relaxation of this constraint; when combined with resonances, they greatly expand the possibilities of quantum state engineering. Here, we generated entangled photons via spontaneous parametric downconversion in semiconductor metasurfaces with high-quality factor, quasi-bound state in the continuum resonances. By enhancing the quantum vacuum field, our metasurfaces boost the emission of nondegenerate entangled photons within multiple narrow resonance bands and over a wide spectral range. A single resonance or several resonances in the same sample, pumped at multiple wavelengths, can generate multifrequency quantum states, including cluster states. These features reveal metasurfaces as versatile sources of complex states for quantum information.

To improve the color-conversion efficiency based on a quantum-well (QW) light-emitting diode (LED), a more energy-saving strategy is needed to increase the energy transfer efficiency from the electrical input power of the LED into the emission of over-coated color-converter, not just from LED emission into converted light. In this regard, the efficiency of energy transfer of any mechanism from LED QW into the color-converter is an important issue. By overlaying blue-emitting QW structures and GaN templates with both deposited metal nanoparticles (DMNPs) and color-converting quantum dot (QD) linked synthesized metal nanoparticles (SMNPs) of different localized surface plasmon (LSP) resonance wavelengths for producing multiple surface plasmon (SP) coupling mechanisms with the QW and QD, we study the enhancement variations of their internal quantum efficiencies and photoluminescence decay times. By comparing the QD emission efficiencies between the samples with and without QW, one can observe the advantageous effect of QW coupling with LSP resonances on QD emission efficiency. Also, with the LSP resonance wavelengths of both DMNPs and SMNPs close to the QW emission wavelength for producing strong SP coupling with the QW and hence QD absorption, a higher QD emission or color-conversion efficiency can be obtained.

In this work, we introduce a new perspective on the development of Localized Surface Plasmon Resonance (LSPR) optical biosensors. Computational simulations, focused on the assessment of the LSPR spectrum and spatial distribution of the electromagnetic field enhancement near a metallic nanoparticle, elucidated the behavior of crucial parameters, as figure of merit, bulk and molecular sensitivity, which governs a LSPR sensor performance. Gold and silver nanospheres were explored as starting point to assess plasmonic optical characteristics of the nanostructured sensor platform. Here, for the first time in the literature, Campbell’s model was evaluated exploiting a NP size-dependence approach. The theoretical analyses indicate a nonlinear behavior of the bulk and molecular sensitivity as function of the NP size. Substantial LSPR peak shifts due to the adsorption of molecules layer on a NP surface were observed for nanoparticles with ~5 nm and ~40 nm radius. Moreover, on molecular sensing, LSPR peak shift is also determined by the thickness of adsorbed molecular shell layers. We observed that for 40 nm radius gold and silver nanospheres, significant LSPR peak shift could be induced by small (few nm) thickness change of the adsorbate shell layer. Moreover, this work provides insights on the LSPR behavior due to adsorption of molecular layer on a NP surface, establishing a new paradigm on engineering LSPR biosensor. Furthermore, the proposed approach can be extended to engineer an efficiently use of different nanostructures on molecular sensing.

Surface plasmon resonance optical sensors involving nanomaterials as reliable analytical tools: A critical view about performance and applications.

Nonlinear crystals are typically used when interaction of different frequencies of light is requested. In classical optics these nonlinear phenomena are used for second-harmonic generation, sum-frequency generation, optical parametric amplification and many other effects. In quantum optics, dealing with optical interaction on the level of individual photons, the most prominent process is spontaneous parametric down-conversion (SPDC),1 where the crystal is pumped by intensive laser light and the crystal can mediate the splitting of a pump photon to a photon pair. The two generated photons are typically called signal and idler. Influence of magnetic field on these nonlinear processes was not thoroughly tested yet. This topic deserves intensive study both from theoretical and experimental point of view, because the magnetic field can decrease the symmetry of the nonlinear crystal and so it may allow to use new types of phase-matching conditions. We started to test the SPDC process in BBO crystals. Nonlinear magneto-optic tensor of this material is not known and we can hardly predict it. According to our first theoretical derivations the efficiency of the nonlinear processes has to oscillate when rotating the magnetic-field orientation.

Entangled photon pairs are a crucial resource in quantum information processing. However, common sources of entangled photon pairs, such as generation by spontaneous parametric down-conversion, impose a Poisson distribution on the pairs, limiting their applicability to quantum information processing. Therefore, generating entangled photon pairs exhibiting a sub-Poisson distribution (i.e., entangled photon bundle) would be crucial/an important advancement in the current research context. Recently, it has been proposed that antibunched photon pairs can be generated using pulsed excitation of quantum dots based on photon blockade. In this work we propose a scheme to generate entangled photon pairs with strong antibunching properties via continuous driving. Under two-photon resonant continuous pumping, a quantum dot is first excited to a biexciton state and then emits a pair of entangled photons into a two-mode cavity. Owing to the strong coupling between the quantum dot and optical modes of the cavity, at most one pair of left-/right-polarized photons exists at a time in the cavity. Specifically, the antibunching effect, average photon number, and entanglement of the left- or right-polarized photons can be resonantly enhanced when the coupling strengths between the exciton (biexciton) transition and cavity modes are equal. Moreover, our results show that the binding energy of the biexciton substantially affects the antibunching effect, average photon number, and entanglement of the left-/right-polarized photons. With suitable parameters, we can realize photon pairs with strong antibunching, a large average photon number, and deep entanglement. Hence, the studied quantum dot-cavity system is a promising source of antibunched entangled photon pairs.

Deterministic sources of high-fidelity entangled qubit pairs encoded in the dual-rail photonic basis, i.e., presence of a single photon in one of two orthogonal modes, are a key enabling technology of many applications of quantum information processing, including high-rate, high-fidelity quantum communications over long distances. The most popular and mature sources of such photonic entanglement, e.g., those that leverage spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing, generate an entangled (so-called continuous-variable) quantum state that contains contributions from high-order photon terms that lie outside the span of the dual-rail basis, which is detrimental to most applications. One often uses low pump power to mitigate the effects of those high-order terms. However, that reduces the pair generation rate, and the source becomes inherently probabilistic. We investigate a cascaded source that performs a linear-optical entanglement swap between two SPDC sources, to generate a heralded photonic entangled state that has a higher fidelity (to the ideal Bell state) compared to a free-running SPDC source. Furthermore, with the Bell swap providing a heralding trigger, we show how to build a multiplexed source, which despite reasonable switching losses and detector loss and noise, yields a fidelity versus success probability trade-off of a high-efficiency source of high-fidelity dual-rail photonic entanglement. We find, however, that there is a threshold of $1.5$ dB of loss per switch, beyond which multiplexing hurts the fidelity versus success probability trade-off.

We propose a method for generation of single-cycle squeezed light by parametric down-conversion in a chirped quasi-phase-matched nonlinear crystal. We find an exact quantum solution for this process valid for an arbitrary parametric gain (under an assumption of nondepleted pump), and discover an ultrabroadband squeezing in the down-converted light with a flat squeezing spectrum comprising the full optical octave. We describe a scheme for observation of this kind of squeezing using second-harmonic generation as an ultrafast correlator.