Mirage effect from thermally modulated transparent carbon nanotube sheets

Abstract

The single-beam mirage effect, also known as photothermal deflection, is studied using afree-standing, highly aligned carbon nanotube aerogel sheet as the heat source. The extremelylow thermal capacitance and high heat transfer ability of these transparent forest-drawncarbon nanotube sheets enables high frequency modulation of sheet temperature over anenormous temperature range, thereby providing a sharp, rapidly changing gradient ofrefractive index in the surrounding liquid or gas. The advantages of temperature modulationusing carbon nanotube sheets are multiple: in inert gases the temperature can reach > 2500 K; the obtained frequency range for photothermal modulation is ∼ 100 kHz in gases and over 100 Hz in high refractive index liquids; and the heat source istransparent for optical and acoustical waves. Unlike for conventional heat sources forphotothermal deflection, the intensity and phase of the thermally modulated beamcomponent linearly depends upon the beam-to-sheet separation over a wide range ofdistances. This aspect enables convenient measurements of accurate values for thermaldiffusivity and the temperature dependence of refractive index for both liquids and gases.The remarkable performance of nanotube sheets suggests possible applications asphoto-deflectors and for switchable invisibility cloaks, and provides useful insights into theiruse as thermoacoustic projectors and sonar. Visibility cloaking is demonstrated in a liquid.