Mid-infrared-perturbed molecular vibrational signatures in plasmonic nanocavities.

Recent developments in surface-enhanced Raman scattering (SERS) enable observation of single-bond vibrations in real time at room temperature. By contrast, mid-infrared (MIR) vibrational spectroscopy is limited to inefficient slow detection. Here we develop a new method for MIR sensing using SERS. This method utilizes nanoparticle-on-foil (NPoF) nanocavities supporting both visible and MIR plasmonic hotspots in the same nanogap formed by a monolayer of molecules. Molecular SERS signals from individual NPoF nanocavities are modulated in the presence of MIR photons. The strength of this modulation depends on the MIR wavelength, and is maximized at the 6-12 μm absorption bands of SiO2 or polystyrene placed under the foil. Using a single-photon lock-in detection scheme we time-resolve the rise and decay of the signal in a few 100 ns. Our observations reveal that the phonon resonances of SiO2 can trap intense MIR surface plasmons within the Reststrahlen band, tuning the visible-wavelength localized plasmons by reversibly perturbing the localized few-nm-thick water shell trapped in the nanostructure crevices. This suggests new ways to couple nanoscale bond vibrations for optomechanics, with potential to push detection limits down to single-photon and single-molecule regimes.We acknowledge support from European Research Council (ERC) under Horizon 2020 research and innovation programme THOR (Grant Agreement No. 829067) and POSEIDON (Grant Agreement No. 861950). We acknowledge funding from the EPSRC (Cambridge NanoDTC EP/L015978/1, EP/L027151/1, EP/S022953/1, EP/P029426/1, and EP/R020965/1). R.C.acknowledges support from Trinity College, University of Cambridge

Fibre-optic metadevice for all-optical signal modulation based on coherent absorption

Recently, coherent control of the optical response of thin films of matter in standing waves has attracted considerable attention, ranging from applications in excitation-selective spectroscopy and nonlinear optics to demonstrations of all-optical image processing. Here we show that integration of metamaterial and optical fibre technologies allows the use of coherently controlled absorption in a fully fiberized and packaged switching metadevice. With this metadevice, that controls light with light in a nanoscale plasmonic metamaterial film on an optical fibre tip, we provide proof-of-principle demonstrations of logical functions XOR, NOT and AND that are performed within a coherent fully fiberized network at wavelengths between 1530 nm and 1565 nm. The metadevice performance has been tested with optical signals equivalent to a bitrate of up to 40 Gbit/s and sub-milliwatt power levels. Since coherent absorption can operate at the single photon level and also with 100 THz bandwidth, we argue that the demonstrated all-optical switch concept has potential applications in coherent and quantum information networks.Comment: 9 pages, 6 figure

Merging metamaterial and fiber technologies

We report on integration of plasmonic and all-dielectric metamaterials into active photonic devices on the fiber platform. These include all-optical and electro-optical phase change and nano-opto-mechanical switching devices, dispersion control solution and coherent control metadevices

Controlling Optically Driven Atomic Migration Using Crystal-Facet Control in Plasmonic Nanocavities.

Plasmonic nanoconstructs are widely exploited to confine light for applications ranging from quantum emitters to medical imaging and biosensing. However, accessing extreme near-field confinement using the surfaces of metallic nanoparticles often induces permanent structural changes from light, even at low intensities. Here, we report a robust and simple technique to exploit crystal facets and their atomic boundaries to prevent the hopping of atoms along and between facet planes. Avoiding X-ray or electron microscopy techniques that perturb these atomic restructurings, we use elastic and inelastic light scattering to resolve the influence of crystal habit. A clear increase in stability is found for {100} facets with steep inter-facet angles, compared to multiple atomic steps and shallow facet curvature on spherical nanoparticles. Avoiding atomic hopping allows Raman scattering on molecules with low Raman cross-section while circumventing effects of charging and adatom binding, even over long measurement times. These nanoconstructs allow the optical probing of dynamic reconstruction in nanoscale surface science, photocatalysis, and molecular electronics.ER

Split-cube-resonator-based metamaterials for polarization-selective asymmetric perfect absorption

Abstract: A split-cube-resonator-based metamaterial structure that can act as a polarization- and direction-selective perfect absorber for the infrared region is theoretically and experimentally demonstrated. The structure, fabricated by direct laser writing and electroless silver plating, is comprised of four layers of conductively-coupled split-cube magnetic resonators, appropriately rotated to each other to bestow the desired electromagnetic properties. We show narrowband polarization-selective perfect absorption when the structure is illuminated from one side; the situation is reversed when illuminating from the other side, with the orthogonal linear polarization being absorbed. The absorption peak can be tuned in a wide frequency range by a sparser or denser arrangement of the split cube resonators, allowing to cover the entire atmospheric transparency window. The proposed metamaterial structure can find applications in polarization-selective thermal emission at the IR atmospheric transparency window for radiative cooling, in cost-effective infrared sensing devices, and in narrowband filters and linear polarizers in reflection mode

Fibre-coupled photonic metadevices

We report on metadevices realised by integration of functional metamaterials with single-mode telecoms fibres. These include plasmonic and all-dielectric nonlinear, nano-opto-mechanical and phase-change switching, dispersion manipulation and coherent absorber metadevices

Coherent all-optical signal processing using fibre-optic metadevices

This Thesis merges the physics of metamaterials with optical fibre technology in order to demonstrate low-power, high-bandwidth signal processing applications. Control over optical absorption using linear coherent interactions of light beams with metasurfaces of deeply subwavelength thickness offers a range of novel opportunities. Here I report on:♦ The first demonstration of a fibre-optic metadevice for coherent all-optical signal processing. The fibre metamaterial has been integrated and packaged resulting in a device that is compatible with standard fibre-optics components.♦ All-optical signal switching, effective nonlinearity and logical functions XOR, NOT and AND performed within a coherent fibre network at wavelengths between 1530 and 1565 nm. The metadevice has been tested at up to 40 Gbit/s with energy consumption as low as 2.5 fJ/bit.♦ Dark pulse generation, selective transmission/absorption of 1 ps pulses and all-optical pulse shaping in the telecommunications C-band with 1 THz bandwidth in-fibre.♦ The first demonstration of a fibre-optic plasmonic/metamaterial device for data security applications. I introduced the concept of coherent cryptography, an optical layer secure communication protocol that does not rely on nonlinear optical processes but instead uses energy redistribution of coherent optical waves interacting on a metamaterial absorber. I demonstrated different types of encryption modes and reported a scheme providing perfect secrecy.♦ Nonlinear control of coherent absorption in a nonlinear fibre network containing a metamaterial absorber. I exploited power-dependent phase retardation arising from the Kerr effect for all-optical noise suppression, power-limiting, pulse restoration, pulse splitting and signal transfer between carrier wavelengths.In addition, I have developed and fabricated fibre metadevices, which have enabled:♦ The first demonstration of coherent perfect absorption and transmission for a single photons in a stabilized quantum fibre network by collaborators.To conclude, this Thesis investigates all-optical solutions provided by plasmonic metamaterials for coherent signal processing within fibre networks. The above proof-of-principle demonstrations show the appropriateness of such metasurfaces for fibre integration and illustrate application opportunities ranging from all-optical switching and pulse shaping to optical encoding and stabilization of fibre-optic classical and quantum information networks

Multi-wavelength lock-in spectroscopy for extracting perturbed spectral responses: molecular signatures in nanocavities.

Detecting small changes in spectral fingerprints at multiple wavelength bands simultaneously is challenging for many spectroscopic techniques. Because power variations, drift, and thermal fluctuations can affect such measurements on different timescales, high speed lock-in detection is the preferred method, however this is typically a single channel (wavelength) technique. Here, a way to achieve multichannel (multi-wavelength) lock-in vibrational spectroscopy is reported, using acousto-optic modulators to convert nanosecond periodic temporal perturbations into spatially distinct spectra. This simultaneously resolves perturbed and reference spectra, by projecting them onto different locations of the spectrometer image. As an example, we apply this multichannel time-resolved methodology to detect molecular frequency upconversion in plasmonic nanocavities from the perturbed Raman scattering at different wavelengths. Our phase-sensitive detection scheme can be applied to any spectroscopy throughout the visible and near-infrared wavelength ranges. Extracting perturbed spectra for measurements on nanosecond timescales allows for capturing many processes, such as semiconductor optoelectronics, high-speed spectro-electrochemistry, catalysis, redox chemistry, molecular electronics, or atomic diffusion across materials

Data Supporting "Mid-infrared-perturbed Molecular Vibrational Signatures in Plasmonic Nanocavities"

This data set contains the experiment and simulation data for the manuscript. This includes SERS scattering spectra and Finite Difference Time Domain simulations. All files are given in .txt