4 - Thermal Microscopy Techniques

Summary

This chapter introduces the thermal microscopy techniques that have been developed and used to probe local temperature in plasmonic structures under illumination. Reliably imaging a temperature distribution with a sub-micrometric resolution is not trivial, which certainly explains why the first temperature measurement in plasmonics dates only from 2009.

To date, ten families of techniques have been developed for this purpose. Interestingly, most of them rely on far-field optical microscopy techniques, which explains why part of the optics community is now tackling problems of thermodynamics. Half of the techniques are based on fluorescence measurements of molecular probes and are gathered in the first section. Then, the other techniques are assigned to specific sections, namely nanodiamond spectroscopy, wavefront sensing, Raman scattering spectroscopy, X-ray absorption spectroscopy, and scanning thermal microscopy. Each section of this chapter begins with a rather detailed explanation of the underlying physics in play. For this reason, the interest of this chapter is also to enter the physics of concepts such as fluorescence emission, surface-enhanced Raman scattering, X-ray absorption or nanodiamond's photophysics.

Introduction

Probing or mapping temperature on the submicrometric scale is not an easy task, even in the twenty-first century. Standard IR thermal radiation measurements, which are usually done on the macro scale, no longer apply at such small dimensions since the involved wavelength of the radiations (more than 10 μm) would lead to a very poor spatial resolution [39]. Moreover, most optical components are not transparent in this wavelength range (lenses, glass coverslips, etc.). Unlike light (which is endowed with a propagative nature), heat just diffuses. This makes any temperature distribution arising from a nanosource of heat confined at its vicinity, and not propagating to the far field. For these reasons, first attempts to probe a temperature field at small scales were based on the use of local probes consisting of a small composite tip acting as a nanoscale thermocouple or bolometer. This is the socalled SThM (scanning thermal microscopy) technique, invented in 1986 [80]. It enables a spatial resolution of around 50 nm. This technique would therefore have been ideal for temperature measurements in nanoplasmonics … if only it were not so invasive

Magnetic resonance imaging (MRI) was one of the first thermal imaging techniques used in thermoplasmonics. The groups of West and Halas used MRI measurement to retrieve the temperature in living animals in the context of photothermal therapies [33].