Super-resolution imaging using nano-bells

Abstract

In this paper we demonstrate a new scheme for optical super-resolution, inspired, in-part, by PALM and STORM. In this scheme each object in the field of view is tagged with a signal that allows them to be detected separately. By doing this we can identify and locate each object separately with significantly higher resolution than the diffraction limit. We demonstrate this by imaging nanoparticles significantly smaller than the optical resolution limit. In this case the “tag” we have used is the frequency of vibration of nanoscale “bells” made of metallic nanoparticles whose acoustic vibrational frequency is in the multi-GHz range. Since the vibration of the particles can be easily excited and detected and the frequency is directly related to the particle size, we can separate the signals from many particles of sufficiently different sizes even though they are smaller than, and separated by less than, the optical resolution limit. Using this scheme we have been able to localise the nanoparticle position with a precision of ~3 nm. This has many potential advantages - such nanoparticles are easily inserted into cells and well tolerated, the particles do not bleach and can be produced easily with very dispersed sizes. We estimate that 50 or more different particles (or frequency channels) can be accessed in each optical point spread function using the vibrational frequencies of gold nanospheres. However, many more channels may be accessed using more complex structures (such as nanorods) and detection techniques (for instance using polarization or wavelength selective detection) opening up this technique as a generalized method of achieving super-optical resolution imaging.

3D phonon microscopy with sub-micron axial-resolution

“…Using higher NA objective lenses could improve this further but this is still limited by diffraction. To move beyond the optical diffraction limit would require, for instance, moving away from planar transducers to implement spherical nano-particles 30,31 .…”

Characterising the size and shape of metallic nano-structures by their acoustic vibrations

“…With the characterisation of the size and shape of the nano-structures with our technique, each individual structure can be super-localised providing a new way of super-resolution imaging. 16 The two current limitations are the heat rise produced by the nanostructures due to the laser excitation and the UV photons used to excite the nano-structures. These two effects could damage the biological specimens as live cells are very sensitive to temperature changes as well as photo-damage.…”

“…This provides a way to achieve suboptical resolution imaging using the characteristic vibrations of each metallic nano-structure. 16 The detection of the vibrational modes of metallic nanostructures is typically performed in a pump-probe configuration. 17,18 Following the excitation by a femtosecond pump pulse, the conduction band of the metal is excited into a nonthermal electronic distribution which relaxes into thermalised electronic distribution via electron-electron interaction on the hundred femtosecond time scale.…”

Transducer-matched multipulse excitation for signal-to-noise ratio improvement in diode laser-based photoacoustic systems

“…2 and Table 2, PRAT). Next, once the pulse repetition rate approaches the absorber resonance, t R ∼ t A , its resonant excitation is possible along a range of sizes [12][13][14][15] (Fig. 2 and Table 2, PR ∼ AT).…”