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) Schematic diagram of large-area cloaking system. (b), (c) Underwater mirage effect created by 5 × 12 cm2 MWNT sheet, (P = 6.6 W).
The temperature-dependent onset for modulated blackbody radiation at ∼1000–2000 Hz is consistent with the observed thermal inertia of large bundles which provide the main current pathways. Decreasing bundle size from the 100–150 nm diameter bundles existing in both aligned MWNT and randomly deposited SWNT sheets, while maintaining the will increase rate performance in liquids, gases and vacuum. Evidence for this is in the increased onset frequency for MWNT sheets grown from a decreased height forest, which display less bundling.
While such improvements are possible, the high temperature modulation rates of the present carbon nanotube aerogel sheets already enable fast, reliable measurements of thermal diffusivity and the temperature dependence of the refractive index for liquids and gases. These refractive measurements are facilitated by the long separation range where the intensity of the modulated laser beam and the phase
of this beam (relative to the ac electrical input) linearly depend upon beam-to-sheet separation.
The high modulation rate achieved for incandescent radiation in vacuum (and the higher modulation rate in inert gas) suggests the use of the carbon nanotube aerogel sheets as the source of polarized light for active night vision systems that use phase-sensitive detection. The usually used filament arrays for these systems provide slow frequency response and light that is largely unpolarized. The high deflection angles and modulation rate in inert gases and non-wetting liquids enable use of these sheets for optical deflectors. The MWNT sheet-based light beam deflection devices possess a high degree of versatility, have a reasonably simple structure and are easily fabricated. Benefits over acousto-optic deflectors are the absence of an undeflected beam and elimination of the need for above 10 MHz acoustic waves to obtain high angle beam deflection. Their principal attraction, however, is for large, lightweight systems requiring wide-band aperiodic, rapid-access modes of operation. Low cost is a benefit over crystal-based deflectors based on Kerr and Pockels effects.
Further development work is required before systems of this type can achieve their full potential. Specifically, effort should be directed towards high pressure xenon encapsulated cells.
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 a mirage based 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.