The single-beam mirage effect, also known as photothermal deflection, is studied using a free-standing, highly aligned carbon nanotube aerogel sheet as the heat source. The extremely low thermal capacitance and high heat transfer ability of these transparent forest-drawn carbon nanotube sheets enables high frequency modulation of sheet temperature over an enormous temperature range, thereby providing a sharp, rapidly changing gradient of refractive index in the surrounding liquid or gas. The advantages of temperature modulation using carbon nanotube sheets are multiple: in inert gases the temperature can reach over 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 is transparent for optical and acoustical waves. Unlike for conventional heat sources for photothermal deflection, the intensity and phase of the thermally modulated beam component linearly depends upon the beam-to-sheet separation over a wide range of distances. This aspect enables convenient measurements of accurate values for thermal diffusivity and the temperature dependence of refractive index for both liquids and gases. The remarkable performance of nanotube sheets suggests possible applications as photo-deflectors and for switchable invisibility cloaks, and provides useful insights into their use as thermoacoustic projectors and sonar. Visibility cloaking is demonstrated in a liquid.
(a) SEM image of a self-supported, aligned MWNT sheet that was drawn from an MWNT forest. (b) Photograph of the 2.5× 5 cm2 MWNT sheet of (a), which is attached under tension between two copper electrodes. The box shows the approximate region of the sheet where the SEM image of (a) was obtained. (c) Photograph showing incandescence, by radiation of polarized light, for this MWNT sheet when it is resistively heated at ∼1650 K.
The optical and acoustical transparency of free-standing carbon nanotube films makes possible their use for large-area cloaking systems. Application of a nanotube sheet as
concealment cloak is demonstrated in water. These nanotube sheets are of considerable interest for loudspeakers and sonar projectors, and it is believed that this investigation of photothermal deflection will help optimize performance for these applications.