A simple cavity-enhanced laser-based heater for reflective samples.

Abstract

Surface science instruments require excellent vacuum to ensure surface cleanliness; they also require control of sample temperature, both to clean the surface of contaminants and to control reaction rates at the surface, for example, for molecular beam epitaxy and studies of heterogeneous catalysis. Standard approaches to sample heating within high vacuum chambers involve passing current through filaments of refractory metals, which then heat the sample by convective, radiative, or electron bombardment induced heat transfer. Such hot filament methods lead to outgassing of molecules from neighboring materials that are inadvertently heated; they also produce electrons and ions that may interfere with other aspects of the surface science experiment. Hot filaments may even disintegrate when used in the presence of gases introduced to induce surface reactions on the sample. Optical heating using lasers can deliver energy directly to the sample, ensuring that only the sample is heated and surroundings within the vacuum chamber are not, while simultaneously eliminating the need for hot filaments. Despite this advantage, optical heating is not commonly employed-such methods are considered complex, expensive, and unreliable. More fundamentally, surface scientists are often interested in metallic samples, whose reflectivity may limit the efficiency of laser heating. In this paper, we describe a simple and inexpensive sample heater based on a commercial diode laser, whose heating efficiency is enhanced by a concave aluminum mirror placed behind the sample. The geometry of the reflector and sample ensures that a stable optical cavity is produced. Using only 26 W of laser power directed to the sample with a fiber optic, a 1-cm diameter × 2-mm thick Pt sample could be heated to 1400 K within 1 min. Excellent programmable temperature control and long-term temperature stability are also demonstrated. Sample heating to 900 °C was performed with negligible increase in chamber pressure. The entire setup comprises components costing less than typical electron bombardment heaters.