- Measuring the spatio-temporal field of ultrashort pulses near nanostructures
- Ultrasensitive detection of nanometer sized metal particles
Measuring the spatio-temporal field of ultrashort pulses near nanostructures
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.
- “Ultrafast evolution of photonic eigenstates in k-space”, Engelen Rob J. P.; Sugimoto Yoshimasa; Gersen Henkjan; Ikeda Naoki, Asakawa, Kiyoshi, Kuipers, L. (Kobus)) Nature Physics, 3, 401-405 DOI: 10.1038/nphys576 (2007)
- “Tracking femtosecond laser pulses in space and time”, M.L.M. Balistreri, H. Gersen et al, Science, 294, 1080 (2001)
- “Real-space observation of ultraslow light in photonic crystal waveguides” H Gersen et al, Phys. Rev. Lett, 94, 073903 (2005)
- “Direct observation of Bloch harmonics and negative phase velocity in photonic crystal waveguides”, H Gersen et al., Phys. Rev. Lett, 94, 123901 (2005)
- “Tracking ultrashort pulses through dispersive media: Experiment and theory”, Gersen H; Korterik JP; van Hulst NF; et al. Phys. Rev. E, 68, 026604 (2003)
- “Propagation of a femtosecond pulse in a microresonator visualized in time” Gersen H; Klunder DJW; Korterik JP; et al. OPTICS LETTERS, 29, 1291-1293 DOI: 10.1364/OL.29.001291 (2004)
Ultrasensitive detection of nanometer sized metal particles
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.
- For a recent review see: P. Zijlstra and M. Orrit, “Single metal nanoparticles: optical detection, spectroscopy and applications”, Rep. Prog. Phys. 74 106401 (2011).
- Xin Hong, Erik M.P.H. van Dijk, Simon R. Hall, Jörg B. Götte, Niek F. van Hulst and Henkjan Gersen, “Background-Free Detection of Single 5 nm Nanoparticles through Interferometric Cross-Polarization Microscopy”, Nano Lett., 2011, 11 (2), pp 541–547.
Measuring the broadband spectral response of individual nanostructures
Probing the optical properties of single nanostructures is becoming increasingly important in the context of (bio-)nanotechnology. As the optical properties of nanostructures vary with wavelength, material, local surrounding, geometry and size a full characterization therefore requires measuring individual nanostructures over a broad spectral range. This is however extremely challenging due to their small absorption and scattering cross section.
For that reason most approaches to the detect small nanoparticles rely on amplifying the weak optical signal by interference with a much stronger optical reference. Often these interferometric approaches only use a single, or a few, discrete wavelengths giving only limited optical information on the nano-structure of interest. The use of a broadband supercontinuum light source when combined with interferometric detection, could potentially enable measuring the optical features and properties of individual nanostructure over the full visible - NIR wavelength range (here: 450nm - 2μm).
Unfortunately the major drawback of using these supercontinuum sources for interferometry is however that they are intrinsically noisy - typical rms-noise of 1-5%. As interferometric detection relies on multiplying the signal by a reference from the same source, this noise will quickly start to dominate over any signal from the nano-structures. Currently we are investigate whether balanced detection of both outputs of an interferometer would enable shotnoise limited detection of the optical signal from individual nanostructures over a broad spectral range as a first step towards a full spectral characterization of individual and coupled nanostructures.
Synthesis and detection of Individual Sub-Nanometre Clusters
Working on this: Jamie Shenston, Dr Simon Hall, Dr Henkjan Gersen
Noble metals in bulk form are noted for their appearance and their resistance to oxidation and corrosion. However it is well known that upon reducing these materials to the nanoscale they start to exhibit very different catalytic and optical properties. Strikingly at around the Fermi wavelength of an electron (~0.5nm) the band structure associated with bulk materials breaks up into discrete quantum states. In this smallest size regime (< 2nm) they are referred to as clusters and no longer behave as a metal but instead become molecular species with strong fluorescence.
The exceptional high brightness and long lifetime of these noble metal clusters combined with their small size, water solubility and low toxicity indicates that these tiny clusters with a quantum yield of up to 70% could be extremely attractive for biophysical studies where molecules are tagged and tracked over time. This is enhanced by the fact that these clusters can show fluorescence at red wavelengths far away from the green autofluorescence background in cells with a higher quantum yield than organic dyes in this wavelength regime.
We currently work on the synthesis and characterisation of a range of Gold and Silver clusters utilising various protecting ligands that display the characteristic fluorescence associated with particles of this size. An example of our current results is shown below.
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- Zheng, J.; Nicovich, P. R.; Dickson, R. M. Annual review of physical chemistry. 2007, 58, 409-31.
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Working in this area
The following people are involved in this research: