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This Thesis reports on the study of atomic scale dynamics of thermal and driven motion in nanomechanical and nano-optomechanical photonic metamaterials system including their atomic scale movement visualization and control. I have developed a sub-atomic motion visualization technique combining picometric displacement sensitivity with the nanometric spatial resolution of a conventional scanning electron microscope, and demonstrated its application in characterization of thermomechanical (Brownian) motion in nanomechanical structures, nanomechanical photonic metamaterials, NEMS/MEMS devices and biological structures. Using this technique, I have reported on the first observation of short-timescale ballistic motion in the flexural mode of a nano-membrane cantilever, driven by thermal fluctuations of flexural phonons. Within intervals < 10 µs, the membrane moves ballistically at a constant velocity of ~300 µm/s, on average. Access to ballistic regime provides the first experimental verification of the equipartition theorem and Maxwell-Boltzmann statistics for flexural modes. For the first time I have optically resolved the average position of a nanowire with an absolute error of ~30 pm using light at a wavelength of λ= 488 nm, thus providing the first example of sub-Brownian metrology with λ/10,000 resolution. To localize the nanowire, I employed a deep learning analysis of the scattering of topologically structured light, which is highly sensitive to the nanowire's position. For the first-time, I have demonstrated: a) optical parametric control of the spectrum of thermomechanical motion on an array of nano-opto-mechanical resonators; b) phononic frequency comb generation by the array; c) thermal energy exchange between two coupled oscillators within an optically driven array. Collectively, these works advance the visualization and control of photonic nanostructures at the picometre scale, thus opening up the exciting field of picophotonics.

Picophotonics is the emerging science of light-matter interactions at the subnanometre scale. In nanomechanical metamaterials (NMs), picometric movements driven by thermal forces can be amplified by optical forces, opening up a novel way to explore their physics and applications. In this work I have: • Designed and constructed a unique apparatus for investigation of thermal fluctuations and directional asymmetry of optical properties in NMs. This experimental setup is part-fiberized for stability, operating at telecommunications C-band wavelengths. • For the first time, observed thermal fluctuations in the optical properties of metamaterials. High-frequency time-domain fluctuations of the optical properties of NMs are directly linked to picometre thermal motion of their components and can give information on fundamental mechanical frequencies and damping of mechanical modes. At room temperature the magnitude of metamaterial transmission and reflection fluctuations is of order 0.1% but may exceed 1% at optical resonances. • Demonstrated, for the first time, that the natural frequencies and thermal fluctuation amplitudes of NMs can be optically controlled at µW/µm² intensities. The few - MHz natural frequencies of beams shift up to 3.6% and few tens of pm displacement amplitudes of thermal fluctuations vary up to 4.3% with light intensity of 0.8µW/µm², providing active control of frequency response and may serve as a basis for bolometric, mass and stress sensing. • For the first time, reported asymmetric transmission in a nanomechanical metamaterial driven by optical forces. I have experimentally demonstrated in NMs that resonant excitation of optical and mechanical sub-systems can lead to profound light-induced transmission asymmetry reaching 16% at µW power levels, making it suitable for a range of laser technology and fibre telecom applications. • Shown, for the first time, that a free-standing, homogenous dielectric thin film can exhibit an optical magnetic response, i.e. without metamaterial nanostructuring. Indeed, such a response is an essential feature of homogeneous dielectric films at Fabry-Perot resonances, which are formed by the interference of electromagnetic multipoles, including the magnetic dipole.

This thesis reports on my research efforts towards all-dielectric metamaterials with reconfigurable functionalities: • I have reported the first optomechanical nonlinear dielectric metamaterial. I have shown that such metamaterials provide extremely large optomechanical nonlinearities at near infrared, operating at intensities of only a few μW per unit cell and modulation frequencies as high as 152 MHz, thereby offering a path to fast, compact, and energy efficient all-optical metadevices. • I have experimentally demonstrated the first all-dielectric electro-optical nanomechanical modulator based on all-dielectric nanomembrane metamaterial. Furthermore, I have shown the dynamical control of optical properties of this device, with modulation frequency up to 7 MHz. I have also establish an encapsulation technique where any nano-membrane can be embedded within a fiber setup with electrical feedthroughs and pressure control. • I have studied for first time the optical properties of Diamond nano-membrane metamaterials. Diamond membranes after nanostructuring with Focus Ion Beam, present broadband, polarization-independent absorption that can be used as efficient coherent absorbers for optical pulses as short as 6 fs. This novel class of metamaterials have been used for coherent modulation with modulation contrast up to 40% at optical fluences of few nJ/cm2 across the visible spectrum. • I have reported the first optically-switchable, all-chalcogenide phase-change metamaterial. Germanium antimony telluride alloys (GST) after nanostructuring subwavelength-thickness films of GST present high-quality resonances that are spectrally shifted by laser-induced structural transitions, providing reflectivity and transmission switching contrast ratios of up to 5:1 (7 dB) at near-infrared wavelengths selected by design, or strong colour contrast in visible due to its plasmonic nature. • This work has introduced dielectric nano-membrane metamaterials, as a platform to provide optically switchable, nonlinear, reconfigurable responses. Due to nanomechanical actuation based on optical/electromagnetic forces, coherent modulation based on the diamond absorbers and phase change media of Chalcogenide glasses.

This thesis reports on my research efforts towards controlling light with photonic metamaterials for desired functionalities: I have demonstrated a new family of continuously metallic metamaterials-‘intaglio’and ‘bas-relief’ metamaterials. They are formed of indented or raised sub-wavelength patterns with depth/height of the order 100 nm and offer a robust and fexible paradigm for engineering the spectral response of metals in the vis-NIR domains. Controlling the colour of metals by intaglio/bas-relief metamaterials has been realized. I have also demonstrated the concept of ‘dielectric loaded’ metamaterials where nanostructured dielectrics on unstructured metal surfaces work as optical frequency selective surfaces. I have demonstrated for the first time controlling light with light without nonlinearity using a plasmonic metamaterial. I have experimentally shown that the interference of two coherent beams can eliminate the plasmonic Joule losses of light energy in the metamaterial with thickness less than one tenth of the wavelength of light or, in contrast, can lead to almost total absorbtion of light. The phenomenon provides functionality that can be implemented freely across a broad visible to infrared range by varying the structural design. I have demonstrated for the first time that a strong light-driven force can be generated when a plasmonic metamaterial is illuminated in close proximity to a dielectric or metal surface. This near-field force can exceed radiation pressure to provide an optically controlled adhesion mechanism mimicking the gecko toe. I have first demonstrated resonant optical forces which are tens of times stronger than radiation pressure within planar dielectric metamaterials and introduced the concept of optomechanical metamaterials. An optomechanical metamaterial consisting of an array of dielectric meta-molecules supported on free-standing elastic beams has been designed. It presents a giant nonlinear optical response driven by resonant optomechanical forces and exhibits optical bistability and nonlinear asymmetric transmission at intensity levels of only a few hundred μW/μm2. Furthermore, I have experimentally demonstrated optical magnetic resonances in all-dielectric metamaterials. I have demonstrated for the first time a non-volatile bi-directional all-optical switching in a phase-change metamaterial. By functionalising a photonic metamaterial with the phase-change chalcogenide glass, phase transitions across a 2000 μm2 area are initiated uniformly by single laser pulse. Reversible switching both in the near- and mid-infrared spectral ranges with a shift of optical resonance position up to 500 nm has been achieved at optical excitation levels of 0.25 mW/μm2, leading to a reflection contrast ratio exceeding 4:1 and transmission contrast around 3.5:1.

This thesis reports on the development of new types of nanoscale optical light sources driven by free-electrons and on the investigation of the underpinning physical phenomena. It is focused on three types of nanoscale light sources with increasing degree of complexity: a nanoscale antenna, an undulator-based tuneable nanoscale light source and a metamaterial-based spatially coherent light source. I have demonstrated for the first time that a nanoscale Hertzian antenna can be driven by free-electrons. The studied nano-antennas consisting of pair of gold nano-rods spaced by a gap resemble conventional radio dipole antennas. Nanoantennas, driven by electron beam produce emission in the visible part of the spectrum. I show that these nanoantennas are most efficiently fed in the region of closest proximity between their two elements, in particular, where the light density of states reaches a maximum. I have developed for the first time a tuneable nanoscale light source driven by free-electrons, the light-well. Alike a free-electron laser that exploits magnet-based undulators, the light-well is based on a nanoscale undulator, a cylindrical channel through alternating metal-dielectric nano-layers. A fast electron propagating through the channel emit light at the wavelength linked to the electron energy and the period of the undulator, thus allowing for a continuous tuning of the output radiation. It has been achieved a 200 nm tunability range in the vis-NIR range at wavelengths between 750 and 950 nm with light at the level of 200W/cm2. Furthermore, I have performed a comprehensive numerical analysis of the light-well by modeling a free electron propagating in the undulator which has lead to a deeper understanding of its physical mechanisms. I demonstrate for the first time an electron-beam driven metamaterial light source that converts the kinetic energy of tree-electrons into spatially coherent optical radiation. It is based on a fundamentally new radiation phenomenon: the injection of free electrons into the metamaterial leads to a directed light emission that comes from synchronized plasmonic oscillations of the ensemble of metamolecules, the individual building blocks of the nanostructure. The effect results from the synchronizing interactions between the metamolecules leading to the spectrum narrowing with the increasing number of metamolecules involved. Depending on the type of metamaterial used I observed emission of narrow-divergent radiation in the visible range from 640 to 760 nm. These results - in principle - demonstrate an alternative to the laser, a threshold-free way of generating spatially coherent and spectrally narrow electromagnetic radiation.