Investigation of Short-Time Heat Transfer Effects by an Optical Pump–Probe Method

Abstract

The study of heat transfer properties on micro- and nanoscales generally requires one to work on time scales ranging between a few picoseconds and a few nanoseconds, whether one is concerned with diffusion over short length scales or heat exchanges involving small volumes of matter. Optical methods are particularly well-suited to the study of small systems for various reasons:

• Non-contact measurements are possible.

• The optical penetration length can be extremely short, e.g., 10–30 nm in metals.

• Measurements can be made with a broad temporal dynamic range, whether one is working in the frequency domain, e.g., photothermal measurements with modulated optical sources, or in the time domain.

In this context, the development of reliable and easy-to-use femtosecond laser sources has considerably enhanced the potential of optical methods. In the present Chapter, we describe how femtosecond lasers are used, and in particular, the pump–probe method for studying heat transfer in micro- and nanoscopic systems. In this technique, an ultrashort laser pulse is split into two parts: one (the pump), very intense, excites the medium under investigation, whilst the other (the probe), weaker and slightly delayed with respect to the first by being made to follow an optical path of variable length, is used to detect the physical effects induced by the first. By controlling the path of the probe beam to within 1 μm, a time resolution well below the picosecond is possible, more than adequate to study heat transfer phenomena.

Although thermal and elastic effects may be quite well decoupled on long time scales, this is certainly not the case for the time scales considered here (picosecond to nanosecond). We therefore begin by describing acoustic effects caused by absorption of a femtosecond pulse in a simple geometry, namely, an opaque film, neglecting heat diffusion (Sect. 1.1). Having understood the way acoustic effects manifest themselves in the response of a system heated by a first pump pulse, we shall examine three simple situations (Sect. 1.2):

• heat diffusion in an opaque film,

• heat diffusion in a substrate in the presence of an interfacial thermal resistance,

• cooling of a nanoparticle in solution or embedded in a matrix.

We then tackle the mechanisms relevant to optical detection of heat or acoustic gradients, and discuss the difficulties involved in determining the heat diffusion (Sect. 2). Finally, in Sect. 3, we describe the main features of a pump–probe experiment, including several detection devices (reflectometry and interferometry). We also discuss effects induced by the repeat rate of pulsed lasers.