The interaction of ultrashort pulses with matter plays an important role in many aspects of modern optical science since key processes in biology, chemistry and physics all take place on femtosecond timescales. At the same time our rapidly increased ability to fabricate complex nanostructures with typical dimensions smaller than the wavelength of light has greatly enhanced our control over light as materials structured on the nanoscale can show optical properties very different to those they exhibit on a macroscale, enabling the engineering of unique functionality and applications.
Recent progress in nanophotonics means that the control of light on the nano-scale by way of photonic crystals, plasmonics and left-handed materials is no longer just an exciting theoretical approach, but a practical possibility. Understanding the interaction of ultrashort pulses with these nanostructured materials is particularly interesting as ultrashort pulses excite nanostructures over a wide range of different frequencies so that the resulting superposition of these field distributions, with different frequencies, determines the actual local field evolution. Hence, controlling the spectral phase of the illuminating laser pulses offers direct control over the space- and time-evolution of the local field on femtosecond timescales.
In this project we aim to combine two powerful techniques, near-field scanning optical microscopy and spectral interferometry, to measure full time-dependent optical fields in and around nanostructures for ultraweak femtosecond pulses with nanometer spatial and femtosecond temporal resolution. Because the technique is linear it can be used for extremely weak pulses that on average contain less than a fraction of a photon per pulse. This extreme sensitivity opens up many fascinating research avenues in coherent control, nano-optics, quantum information and other research fields that investigate the interaction between light and matter with high spatial and temporal resolution.
In collaboration with Ashley Toye and George Banting (Department of Biochemistry)
The ability to visualize, track and quantify molecules and events directly inside living cells with high spatial and temporal resolution is essential for understanding biological systems. Over the past decades the development of new fluorescent proteins has started a revolution by allowing complex biochemical processes to be correlated with the functioning of proteins in living cells. Since most (bio-)molecules are non-fluorescent, they can only be followed by linking to efficient fluorescent markers, in many cases influencing the behaviour of the molecule of interest. Moreover, a major drawback of fluorescent markers is that after a finite number of photocycles they will convert to a non-fluorescent state (photobleach) thereby limiting the available observation time.
An attractive alternative to fluorescent markers is given by metallic nano-particles, which have been used extensively as a biomolecular label in optical and electron microscopy. In particular, gold nanoparticles have the advantage that they are relatively inert, do not photobleach and can be easily linked to (bio-)molecules via well-established procedures. The challenge undertaken in this proposal is to reliably detect and follow nanoparticles smaller than 5 nm in three dimensions at ambient conditions directly inside a living cell with optical methods. This ability would allow to follow a large range of processes occurring inside and outside living cells without having to attach bulky fluorescent labels influencing the behaviour of the molecule of interest.
We aim to compare an existing method relying on photo-induced changes of the local refractive index of the environment of the particle to a method relying on the intrinsic scattering properties of the nano-particle. Combining these methods for detecting small metal nanoparticles in combination with the three-dimensional imaging ability of a confocal microscopy would result in a powerful detection system to monitor biochemical process inside living cells under biological relevant conditions on fast time-scales.
As part of this project we recently demonstrated a new method, Interferometric Cross-Polarization Microscopy, that detects individual nanoparticles by their modification of the polarization state of the light detected in a confocal arrangement. This approach enabled the full retrieval of the amplitude and phase response of individual gold nanoparticles down to 5 nm in diameter at wavelengths far from the plasmon resonance with a signal-to-noise ratio of ~7 as show in Fig 1 and 2. This was realized at the very low excitation intensities (~1µW) needed for single molecule fluorescence and bioimaging experiments.
The attraction of this approach lies in the fact that it in principle can be used for the shot-noise limited detection of a whole range of different particle types, ranging from single molecules to nanometer sized gold particles. As a result new experimental avenues are opened up for co-localization experiments as well as for exploration of interaction between fluorescent emitters and (metal) nanoparticles.