Photomechanics, i.e., the conversion of light into thermal and mechanical work is of significant importance for energy conversion/reconfigurable technologies. Advantages of such photo-thermal mechanisms for transducers include remote energy transfer, remote controllability, control of actuation using number of photons (intensity) and photon energies (wavelength), fast actuation (milliseconds), low signal to noise ratio, high stored elastic strain energy densities with hyperelastic elastomers and scalability at different length scales using batch fabrication and high-volume semiconductor manufacturing. However, only a few materials exist that can convert light into mechanical work. Azobenzene liquid crystal elastomers were one of the first materials to exhibit the photomechanical effect. However, their application required two different light sources for reversible thermal switching (420 nm and 365 nm) between an extended trans and a shorter cis configuration. In this talk, we will cover how light is used with new materials to create the mechanical effect. New nanomaterials, when mixed with polymeric materials, show the unusual photomechanical effect that can be practically harnessed for real-world application. Straining new 2D nanomaterials such as graphene, MoS2 and others creates a new effect called the coupled straintronic photo-thermic effect enables large light absorption and also increase in mechanical effect. The talk will go through an overview of this new and upcoming area of research based on light-matter interaction in 1D and 2D nanomaterial composites
Atomically-thin two-dimensional (2D) materials have attracted remarkable interest in a wide range of research and applications. However, one of the main challenges is how to visualise the extremely thin films and accurately identify its layer thickness. Due to the ultimately thin thickness and the low absorption of light, many 2D films, such as graphene, graphene oxide and hexagonal boron nitride (hBN) are nearly completely transparent on most surfaces. They are only visible when deposited on specific contrast-enhancing substrates. However, there is no universal substrates which can be used to visualise all 2D materials. A substrate often can only provide enhanced visibility for a specific category of 2D materials. For instance, the oxidised Si substrates can considerably enhance the optical contrast of graphene, but produce negligible enhancing effect on hBN. It is therefore desirable to develop a general theoretical guidance on how to design contrast-enhancing substrate for any given 2D materials.
Here we report a universal theoretical model which can be employed to design high-contrast substrates for any 2D materials. For a given thin film of known optical properties, the optical contrast is completely defined by the complex reflectivity of the underlying substrate. By engineering the optical properties of the underlying substrate, we fabricated a range of structures, significantly enhancing the contrast of graphene, graphene oxides and hBN. Monolayers of these transparent 2D films are readily visible (>10% contrast) on a range of substrates with metallic or dielectric materials as top surface layers. The measured optical contrasts excellently match theoretical calculations.
Under extreme conditions, when the coupling between photon and exciton is sufficiently strong, a hybridised quantum state called polariton is formed. Polaritons exhibit intriguing features, such as Bose-Einstein condensation and Rabi splitting, and have applications in many areas, including molecular sensors, light harvesting and quantum optical devices.
Previously polaritons are often produced in microcavities at low temperatures, with a cavity volume at the order of m3. Plasmonic junctions provide extreme confinement and enhancement of optical fields within ~nm3 cavity, about 8-9 orders smaller than that of microcavity, thus producing extremely strong Purcell effect, which renders the observation of strong-coupling between plasmon and exciton at room temperature, so called plexciton.
Here we report the observation of strong coupling between localised surface plasmon (LSP) and the excitons of fluorescent graphene quantum dots. We adopt a nanoparticle-on-mirror (NPoM) plasmonic structure, comprised of a Au nanoparticle on top of a reflective Au substrate (the 'mirror'). Extremely strong field enhancement is produced within the nanometer-scale junction. The Au nanoparticle is encapsulated with a thin layer of graphene shell. The measured scattering spectra of Au nanoparticles show distinct splitting double peaks, a characteristic feature of strong coupling. In addition, we demonstrate that the strong coupling is configurable. The splitting can be tuned with a low-power laser irradiation, exhibiting typical anti-crossing behaviour as a result of tuned LSP resonance and the oscillator strength of nano graphene. Our results demonstrate a new avenue for investigating strong-coupling at room temperature and provide opportunities for developing tuneable quantum optical devices.
We investigate the mechanisms for fluorescence enhancement and energy transfer near a gold tip in apertureless
scanning near-field optical microscopy (ASNOM) and provide a demonstration of sub-diffraction tip-enhanced
fluorescence imaging. We have imaged the fluorescence from a single quantum dot cluster using ASNOM and find that
when a sharp gold tip is brought within a few nanometres from the sample surface, the resulting enhancement in
quantum dot fluorescence in the vicinity of the tip leads to a resolution of about 60 nm. We determine this enhancement
of the fluorescence to be about four-fold in magnitude, which is consistent with the value calculated with a simple
quasistatic model. Using this model we show that the observed enhancement of fluorescence results from a competition
between enhancement and quenching, dependent on a range of experimental parameters. We also demonstrate that
optical signals measured in ASNOM under ambient conditions are found to be affected significantly by the thin water
layer absorbed on the surface under investigation.