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Abstract
We demonstrate a photoacoustic sensor capable of measuring high-energy nanosecond optical pulses in terms of temporal width and energy fluence per pulse. This was achieved by using a hybrid combination of a carbon nanotube-polydimethylsiloxane (CNT-PDMS)-based photoacoustic transmitter (i.e., light-to-sound converter) and a piezoelectric receiver (i.e., sound detector). In this photoacoustic energy sensor (PES), input pulsed optical energy is heavily absorbed by the CNT-PDMS composite film and then efficiently converted into an ultrasonic output. The output ultrasonic pulse is then measured and analyzed to retrieve the input optical characteristics. We quantitatively compared the PES performance with that of a commercial thermal energy meter. Due to the efficient energy transduction and sensing mechanism of the hybrid structure, the minimum-measurable pulsed optical energy was significantly lowered, ~157 nJ/cm2, corresponding to 1/760 of the reference pyroelectric detector. Moreover, despite the limited acoustic frequency bandwidth of the piezoelectric receiver, laser pulse widths over a range of 6–130 ns could be measured with a linear relationship to the ultrasound pulse width of 22–153 ns. As CNT has a wide electromagnetic absorption spectrum, the proposed pulsed sensor system can be extensively applied to high-energy pulse measurement over visible through terahertz spectral ranges.
Highlights
Nanocomposite thin films, consisting of carbon nanotube (CNT) and polydimethylsiloxane (PDMS), have been widely used for highly efficient photoacoustic energy conversion [1,2,3].Under pulsed laser irradiation with a short temporal width, these thin-film transmitters can produce pressure pulses with a broad frequency bandwidth over tens of MHz and a high peak amplitude on the order of MPa
CNT-PDMS transmitters have been utilized for all-optical ultrasound transducers [5,6], fiber-optic ultrasound generators [7], energy conversion lenses for laser-generated focused ultrasound (LGFU) [8,9] and transduction layers to generate micro-bubbles and shear forces for safe harvesting of cultured cells [10]
The optical absorption, the incident optical fluence, the sound speed, the laser pulse width and the where P, Γ, A, F, c, τ and α are the pressure amplitude, the dimensionless Grüneisen coefficient, the depth of optical absorption, respectively. This shows the linear dependence between the incident optical absorption, the incident optical fluence, the sound speed, the laser pulse width and the depth laser energy fluence and the output pressure of the transmitter
Summary
Under pulsed laser irradiation with a short temporal width, these thin-film transmitters can produce pressure pulses with a broad frequency bandwidth over tens of MHz and a high peak amplitude on the order of MPa. The nanocomposite structure exhibits not just efficient energy conversion and strong robustness allowing high-energy optical absorption (>100 mJ/cm2 ) without laser-induced ablation [4]. The nanocomposite structure exhibits not just efficient energy conversion and strong robustness allowing high-energy optical absorption (>100 mJ/cm2 ) without laser-induced ablation [4] Due to these advantages, CNT-PDMS transmitters have been utilized for all-optical ultrasound transducers [5,6], fiber-optic ultrasound generators [7], energy conversion lenses for laser-generated focused ultrasound (LGFU) [8,9] and transduction layers to generate micro-bubbles and shear forces for safe harvesting of cultured cells [10].
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