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Digital holographic microscopy

Digital holography setup, and view of one of the systems

Digital holography is a powerful tool for the complete measurement (amplitude and phase) of propagative optical fields, allowing their 3D reconstruction. We have developed holographic microscopes which take advantage of the holographic amplification (multiplication by the reference wave) to record the photons scattered by single metallic nano-objects, from single nanoparticles to more complex plasmonic systems. In addition, the use of a heterodyne modulation of both the object and reference beams enhances signal to noise ratios and allows the study of modulated phenomena.

Currently, this systems is applied to various studies:

  • 3D reconstruction of the field scattered by metallic nanoantennas.
Electronic microscopy image of a plasmonic nanoantenna (gold on glass, electron lithography, LPN Marcoussis), and 3D reconstructions of the scattered optical field
  • Themal imaging in plasmonic nanoantennas.

In ohmic metals, the plasmonic oscillation of charges induces heating, which can be either maximized (for e.g. photo-thermo-voltaic conversion) or minimized (e.g. to avoid the destruction of the antenna, or of analytes in diagnosis applications). With a modulated heating laser and a heterodyne beating tuned to the excitation frequency, localized heating can be measured or imaged in plasmonic nanoantennas using holography.

Thermal image of a single nanoantenna heated by a 532 nm excitation laser
  • 3D superlocalization of Brownian nanoparticles.

Although holography is a diffraction-limited far field technique, it can be used to localize in 3D nanoparticles down to 10 nm in diameter. Each subwavelength particle yields a diffraction-limited spot, but its center can be determined with an accuracy of 5 nm in the plane of the sample and 15 nm in the perpendicular direction. Tens of particle can be tracked simultaneously in 3D. In the example below, this has been applied to the localization of silver nanoparticles during electrochemical reactions in microfluidic chambers.

3D tracking of a silver nanoparticle in water with subwavelength accuracy.
  • Sub-diffraction imaging with stochastic Brownian nanoprobes.

Once they have been localized, the light scattered by each particle can also be measured, giving a local information on the optical field. Over sufficient times, several particles in Brownian motion will eventually reach all regions of the volume under study, giving an optical measurement limited in resolution by the size of the particle (typ. 50 nm), or by its localization accuracy (5-15 nm). We are currently applying this principle to sub-diffraction optical imaging in liquids.

Reconstruction of the propagation of a laser beam in water, using 1300 nanoparticle localization events.