Highly Efficient Photoacoustic Conversion by Facilitated Heat Transfer in Ultrathin Metal Film Sandwiched by Polymer Layers

Abstract

of biomaterials (tissue damage threshold, 20 mJ cm −2 [6] ). Therefore, there is a strong need to find a system that can increase the PA conversion efficiency.PA conversion, particularly via the thermoelastic effect, depends on material properties such as thermal expansion coefficient (β) and light absorption coefficient (α) (i.e., Photoacoustic pressure amplitude, P = Γ(β)αF where the Grüneisen parameter, Γ is a function of β, and F is the laser fluence). [2] Efficient PA materials should have both high thermal expansion and good light absorption. Among many materials, metal films (typically, hundreds of nanometers thick) have been widely used due to easy fabrication and decent light absorption (e.g., >50% light absorption for Cr 100 nm at 532 nm wavelength). However, the PA conversion of metals is considered to be inefficient because of their low thermal expansion. Alternatively, researchers have developed composite materials with both high thermal expansion and good light absorption, [7][8][9][10][11] which consist of lightabsorbing fillers embedded in transparent mediums with high thermal expansion. Specifically, an early attempt to make such composites was to use carbon black mixed with polydimethylsiloxane (PDMS), which showed an order of magnitude high PA amplitude compared to a metal film. [7] Recently, besides carbon black, other carbon materials including CNTs, [8,9] rGO, [10] candle soot nanoparticles, [11] and nanofiber [12] have been utilized as light-absorbing fillers, showing improved PA conversion efficiency. In addition to the carbon-based broadband absorbers, a metal-based PDMS composite consisting of gold nanoparticles was developed, showing high PA conversion efficiency compared to aluminum thin film. [13] Beyond imaging and sensing applications, such highly efficient PA materials have allowed new applications, e.g., PA cavitation therapy capable of selectively removing biomaterials, cells or tissues. [14][15][16][17] Although the composite materials are proven to be promising, they have several disadvantages. For example, the composites require arduous preparation processes (e.g., high temperature CVD growth of CNT forest [8] ). Also, the carbon fillers are easily agglomerated while mixing with polymers, thereby requiring surface functionalization of the fillers. [9] Moreover, for solutionprocessed composites, it is difficult to control the thickness of the films, especially if the composites contain a high concentration of fillers and thus become highly viscous. [8,18] Even though main contribution to efficient PA conversion is believed to be Photoacoustic (PA) conversion of metal film absorbers is known to be inefficient because of their low thermal expansion and high optical reflection, as compared to polymeric materials containing light absorbing fillers. Here, highly efficient PA conversion is demonstrated in metal films. By using a metal film absorber sandwiched by transparent polymer layers, PA conversion is significantly enhanced, which is even comparable to in the highest reported in...

Efficient Photoacoustic Conversion in Optical Nanomaterials and Composites

“…To demonstrate an efficient metal‐based photoacoustic transmitter, we used a thin metal film sandwiched by polymer layers, as illustrated in Figure b . At first, we tried to maximize the heat transfer from the metal film to the surrounding polymers, because the photoacoustic conversion in the metal film is not efficient and the thermal energy stored in the metal film is just wasted.…”

“…The metal film was too thin to effectively absorb light for photoacoustic generation. This issue was addressed by using a resonance cavity, which was realized by adding an additional metal layer as an optical reflector, showing 90% of light absorption in the metal film . The photoacoustic conversion of the metal film composite was comparable to carbon‐based polymer composites (e.g., CNT‐PDMS composites).…”

“…By plugging into Equation , . For polymer composites, the photoacoustic amplitude is mainly determined by the thermal energy delivered to the constituent polymers (i.e., ).…”

A Fiber Optic Ultrasonic Sensing System for High Temperature Monitoring Using Optically Generated Ultrasonic Waves

“…However, the photoacoustic conversion efficiency of metals is low since their low thermal expansion. Researchers have developed composite materials with both high thermal expansion and light absorption, such as gold-nanocomposites [16,17,18], carbon black combined with PDMS (black PDMS) [19,20], CNTs [21], candle soot nanoparticles [22], and polymer-thin metal-polymer [23]. In this paper, black PDMS material was used as a photoacoustic material.…”

Notice of Removal: Acoustical characterisation of carbon nanotube-loaded polydimethylsiloxane used for optical ultrasound generation

“…Recent research efforts have focussed on the use of polydimethylsiloxane (PDMS) as a host material due to its high thermal expansion coefficient and its ease of fabrication and handling [3]. A variety of optical absorbers have been used, ranging from metals [11], [12] to carbon-based materials [3], [13]- [16].…”