Carbon nanotube composite optoacoustic transmitters for strong and high frequency ultrasound generation

Abstract

We demonstrate carbon nanotube ͑CNT͒ composite-based optoacoustic transmitters that generate strong and high frequency ultrasound. The composite consists of CNTs grown on a substrate, which are embedded in elastomeric polymer used as an acoustic transfer medium. Under pulsed laser excitation, the composite generates very strong optoacoustic pressure: 18 times stronger than a Cr film reference and five times stronger than a gold nanoparticle composite with the same polymer. This enhancement persists over a broadband frequency range of up to 120 MHz and is confirmed by calculation. We suggest the CNT-polymer composites as highly efficient optoacoustic transmitters for high resolution ultrasound imaging. © 2010 American Institute of Physics. ͓doi:10.1063/1.3522833͔Laser-induced ultrasound generation is an effective way to make high frequency ultrasound transmitters by exploiting the high frequency spectra of laser pulses to achieve broad acoustic bandwidths. Typically, such transmitters are made of light-absorbing thin films containing specific structures designed to have high optical absorption that are capable of efficient optoacoustic conversion, for example, thin metal, 1 dye-doped polymer composites, 2,3 and two-dimensional ͑2D͒ gold nanoparticle ͑AuNP͒ arrays. 4 They are often integrated with optical interferometric detectors ͑e.g., FabryPérot etalon͒ 5 to make all-optical ultrasound transducers working over broadband and high frequency in 2D array platforms. In these arrays, the size of each element was determined by the spot size of the focused laser beam that is of the order of several microns.Thin metallic coatings on solid substrates are suitable as a common reference material for qualifying the performance of optoacoustic transmitters.1,2 While these thin films ͑Ͻ1 m͒ can be used as high frequency ultrasound sources their optoacoustic conversion efficiency is poor mainly because of the low thermal expansion. Also, a significant percentage of light is reflected back from the film surface.As large thermal expansion is desirable for strong pressure generation, an elastomeric polymer, polydimethylsiloxane ͑PDMS͒, has been used as an acoustic transfer medium to interface with light-absorbers.2-5 A composite film of PDMS with carbon black as a light-absorber has shown nearly 20 dB improvement in optoacoustic signal strength as compared to a reference Cr film alone.2 However, high frequency response was severely limited due to the composite film thickness ͑ϳ25 m͒ due to the acoustic attenuation. This is a serious issue because high frequency performance is vital for optoacoustic transmitters. Moreover, it is challenging to obtain uniform mixing and dispersion of carbon black particles in the PDMS matrix. Agglomeration of carbon black can cause uneven light absorption within the same film. Significant progress has been recently made using a planar array of AuNPs with an overlying PDMS layer of several microns. 4 High frequency output was improved by ϳ5 dB over 70-100 MHz as compared with those carbon black-PDMS...

Laser-generated ultrasound with optical fibres using functionalised carbon nanotube composite coatings

“…Several absorbers have been considered, including gold nanostructures, 5 carbon black, 6 graphite, 7,8 graphene, 9 and carbon nanotubes (CNTs). 10 The use of polydimethylsiloxane (PDMS) as an elastic component with a large thermal expansion coefficient was highlighted in several studies. 5,6,10 To achieve micron-scale thicknesses for high-frequency ultrasound generation, spin coating was used to apply thin PDMS layers onto CNTs that were grown onto a substrate.…”

“…Previous studies highlighted the need for thin coatings due to attenuation of high frequencies within the composite. 10 Thinner coatings might be achieved with a decrease in the withdrawal speed during dip coating 12 or with an increase in the xylene concentration to reduce the viscosity. However, a decrease in the coating thickness is likely to decrease the optical absorption and the corresponding ultrasound pressure.…”

“…5,6,10 To achieve micron-scale thicknesses for high-frequency ultrasound generation, spin coating was used to apply thin PDMS layers onto CNTs that were grown onto a substrate. 10 While spin coating is well suited for depositing PDMS across large areas, it is ill-suited to coating the distal ends of optical fibres.…”

Review of Laser-Generated Ultrasound Transmitters and Their Applications to All-Optical Ultrasound Transducers and Imaging

“…The amplitude of ultrasound produced by CB-PDMS is slightly less than that by AuNPs-PDMS [43]. (ii) CNT nanocomposites are demonstrated as highly-efficient and high-frequency LGUS transmitters [30,[53][54][55]. The CNT nanocomposite shows five times enhancement compared with the above-mentioned 2D AuNPs nanocomposite with the same polymer PDMS [43] and a broad bandwidth up to 120 MHz.…”

A new fiber-optic non-contact compact laser-ultrasound scanner for fast non-destructive testing and evaluation of aircraft composites

“…10 When the laser pulse duration is varied from $100 ps to $100 ns, generated pressure amplitudes can achieve tens of or even one hundred of MPa, with relatively low (tens of mJ) laser pulse energies. 11,12 A unique feature of laser generated ultrasound (US), or optoacoustic (OA) signals, is their unipolar or bipolar 10 impulse response, which cannot be efficiently duplicated with piezoelectric excitation. Ultimately, the temporal profile of optically generated acoustic signals mimics the laser pulse envelope, i.e., it can be perfectly Gaussian.…”