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We demonstrated the direct and noninvasive imaging of functional neurons by Ionic Contrast Terahertz (ICT)
near-field microscopy. This technique provides quantitative measurements of ionic concentrations in both the
intracellular and extracellular compartments and opens the way to direct noninvasive imaging of neurons during
electrical, toxin, or thermal stresses. Furthermore, neuronal activity results from both a precise control
of transient variations in ionic conductance and a much less studied water exchange between the extracellular
matrix and the intraaxonal compartment. The developed ICT technique associated with a full three-dimensional
simulation of the axon-aperture near-field system allows a precise measurement of the axon geometry and therefore
the direct visualization of neuron swelling induced by temperature change or neurotoxin poisoning. We
also developed Terahertz Attenuated Total Reflection (ATR) devices perfectly suited for studying cell layers.
Inserted in a terahertz time-domain system, and using a high resistivity low loss silicon prism to couple the
terahertz wave into the sample, the detection scheme is based on the relative differential spectral phase of two
orthogonal polarizations. Biological sample imaging as well as subwavelength (λ/16) longitudinal resolution are
demonstrated.
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