Double blind ultrafast pulse characterization by mixed frequency generation in a gold antenna

Ultrafast pulse characterization requires the analysis of correlation functions generated by frequency mixing of optical pulses in a nonlinear medium. In this work, we use a gold optical nanoantenna to generate simultaneously Four Wave Mixing and Sum Frequency Generation across the tuning range of a Ti: Sapphire and Optical Parametric Oscillator (OPO) system. Since metal nanoparticles create remarkably strong nonlinear responses for their size without the need for phase matching, this allows us to simultaneously characterize the unknown OPO pulse and its pump pulse using a single spectrogram. The nonlinear mixing is efficient enough to retrieve pulses with energies in the picojoule range

Optically-Triggered Nanoscale Memory Effect in a Hybrid Plasmonic-Phase Changing Nanostructure

Nanoscale devices, such as all-optical modulators and electro-optical transducers, can be implemented in heterostructures that integrate plasmonic nanostructures with functional active materials. Here we demonstrate all-optical control of a nanoscale memory effect in such a heterostructure by coupling the localized surface plasmon resonance (LSPR) of gold nanodisk arrays to a phase-changing material (PCM), vanadium dioxide (VO<inf>2</inf>). By latching the VO<inf>2</inf> in a distinct correlated metallic state during the insulator-to-metal transition (IMT), while concurrently exciting the hybrid nanostructure with one or more ultraviolet optical pulses, the entire phase space of this correlated state can be accessed optically to modulate the plasmon response. We find that the LSPR modulation depends strongly but linearly on the initial latched state, suggesting that the memory effect encoded in the plasmon resonance wavelength is linked to the strongly correlated electron states of the VO<inf>2</inf>. The continuous, linear variation of the electronic and optical properties of these model heterostructures opens the way to multiple design strategies for hybrid devices with novel optoelectronic functionalities, which can be controlled by an applied electric or optical field, strain, injected charge, or temperature.Department of Applied Physic

Spectral interferometric microscopy reveals absorption by individual optical nanoantennas from extinction phase

Optical antennas transform light from freely propagating waves into highly localized excitations that interact strongly with matter. Unlike their radio frequency counterparts, optical antennas are nanoscopic and high frequency, making amplitude and phase measurements challenging and leaving some information hidden. Here we report a novel spectral interferometric microscopy technique to expose the amplitude and phase response of individual optical antennas across an octave of the visible to near-infrared spectrum. Although it is a far-field technique, we show that knowledge of the extinction phase allows quantitative estimation of nanoantenna absorption, which is a near-field quantity. To verify our method we characterize gold ring-disk dimers exhibiting Fano interference. Our results reveal that Fano interference only cancels a bright mode’s scattering, leaving residual extinction dominated by absorption. Spectral interference microscopy has the potential for real-time and single-shot phase and amplitude investigations of isolated quantum and classical antennas with applications across the physical and life sciences

Non-plasmonic nanoantennas for surface enhanced spectroscopies with ultra-low heat conversion

Nanoplasmonics has recently revolutionized our ability to control light on the nanoscale. Using metallic nanostructures with tailored shapes, it is possible to efficiently focus light into nanoscale field 'hot spots'. High field enhancement factors have been achieved in such optical nanoantennas, enabling transformative science in the areas of single molecule interactions, highly enhanced nonlinearities and nanoscale waveguiding. Unfortunately, these large enhancements come at the price of high optical losses due to absorption in the metal, severely limiting real-world applications. Via the realization of a novel nanophotonic platform based on dielectric nanostructures to form efficient nanoantennas with ultra-low light-into-heat conversion, here we demonstrate an approach that overcomes these limitations. We show that dimer-like silicon-based single nanoantennas produce both high surface enhanced fluorescence and surface enhanced Raman scattering, while at the same time generating a negligible temperature increase in their hot spots and surrounding environments

Room temperature plasmon laser by total internal reflection

Plasmon lasers create and sustain intense and coherent optical fields below light's diffraction limit with the unique ability to drastically enhance light-matter interactions bringing fundamentally new capabilities to bio-sensing, data storage, photolithography and optical communications. However, these important applications require room temperature operation, which remains a major hurdle. Here, we report a room temperature semiconductor plasmon laser with both strong cavity feedback and optical confinement to 1/20th of the wavelength. The strong feedback arises from total internal reflection of surface plasmons, while the confinement enhances the spontaneous emission rate by up to 20 times.Comment: 8 Page, 2 Figure

IR hot carrier based photodetection in titanium nitride oxide thin film-Si junctions

Hot carrier based methods constitute a valuable approach for efficient and silicon compatible sub-bandgap photodetection. Although, hot electron excitation and transfer have been studied extensively on traditional materials such as Au and Ti, reports on alternative materials such as titanium nitride (TiN) are rare. Here, we perform hot hole photodetection measurements on a p-Si/metal thin film junction using Ti, Au and TiN. This material is of interest as it constitutes a refractory alternative to Au which is an important property for plasmonic applications where high field intensities can occur. In contrast to Au, a TiN/Si junction does not suffer from metal diffusion into the Si, which eases the integration with current Si-fabrication techniques. This work shows that a backside illuminated p-Si/TiN system can be used for efficient hot hole extraction in the IR, allowing for a responsivity of 1 mA/W at an excitation wavelength of 1250 nm and at zero bias. Via a comparison between TiN and other commonly used materials such as Au, the origin of this comparably high photoresponse can be traced back to be directly linked to a thin TiO2-x interfacial layer allowing for a distinct hot-hole transfer mechanism. Moreover, the fabrication of TiN nanodisk arrays is demonstrated which bears great promise to further boost the device efficiency

Developmental motor problems and health-related quality of life in 5-year-old children born extremely preterm: A European cohort study

Aim To measure the association between cerebral palsy (CP) and non-CP-related movement difficulties and health-related quality of life (HRQoL) among 5-year-old children born extremely preterm (<28 weeks gestational age). Method We included 5-year-old children from a multi-country, population-based cohort of children born extremely preterm in 2011 to 2012 in 11 European countries (n = 1021). Children without CP were classified using the Movement Assessment Battery for Children, Second Edition as having significant movement difficulties (<= 5th centile of standardized norms) or being at risk of movement difficulties (6th-15th centile). Parents reported on a clinical CP diagnosis and HRQoL using the Pediatric Quality of Life Inventory. Associations were assessed using linear and quantile regressions. Results Compared to children without movement difficulties, children at risk of movement difficulties, with significant movement difficulties, and CP had lower adjusted HRQoL total scores (beta [95% confidence interval] = -5.0 [-7.7 to -2.3], -9.1 [-12.0 to -6.1], and - 26.1 [-31.0 to -21.2]). Quantile regression analyses showed similar decreases in HRQoL for all children with CP, whereas for children with non-CP-related movement difficulties, reductions in HRQoL were more pronounced at lower centiles. Interpretation CP and non-CP-related movement difficulties were associated with lower HRQoL, even for children with less severe difficulties. Heterogeneous associations for non-CP-related movement difficulties raise questions for research about mitigating and protective factors.Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Grant/Award Number: SFRH/BPD/117597/2016; Horizon 2020 Framework Programme, Grant/Award Number: No 633724 and No 733280; Seventh Framework Programme, Grant/Award Number: No 25988

Optimizing hot electron harvesting at planar metal–semiconductor interfaces with titanium oxynitride thin films

Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO2 and titanium oxynitride (TiON)/TiO2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO2, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO0.5N0.5) exhibits the highest carrier extraction efficiency (NFC ≈ 2.8 × 1019 m-3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (NHE ≈ 1.6 × 1018 m-3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride

Comparison of the ultrafast hot electron dynamics of titanium nitride and gold for plasmonic applications

With similar optical properties to gold and high thermal stability, titanium nitride continues to prove itself as a promising plasmonic material for high-temperature applications in the visible and near-infrared. In this work, we use transient pump probe differential reflection measurements to compare the electron energy decay channels in titanium nitride and gold thin films. Using an extended two temperature model to incorporate the photoexcited electrons, it is possible to separate the electron-electron and electron-phonon scattering contributions immediately following the arrival of the pump pulse. This model allows for incredibly accurate determination of the internal electronic properties using only optical measurements. As the electronic properties are key in hot electron applications, we show that titanium nitide has substantially longer electron thermalization and electron-phonon scattering times. With this, we were also able to resolve electron thermal conduction in the film using purely optical measurements