Efficient Photoacoustic Conversion in Optical Nanomaterials and Composites

Abstract

approaches. While addressing the limitations of the electrical counterparts (e.g., massive amounts of radio frequency (RF) cabling and electromagnetic (EM) interference), a photoacoustic approach for gene rat ing high-amplitude ultrasound has created new opportunities.Active research on efficient photo acoustic materials has started with the development of photoacoustic contrast mediums (e.g., exogenous contrast particles) for contrast enhancement. [3][4][5] Recent advances in nanofabrication technologies have allowed to develop more efficient photoacoustic contrast mediums, [3] such as metal nanoparticles having enhanced optical absorption due to optical resonance. Later, by coating such nanoparticles on a flat substrate, layered structures were developed for generating high-amplitude ultrasound. Among many nanoparticles, gold nanoparticles were widely used owing to strong resonance. [6] In fact, these initial efforts to increase photoacoustic conversion were focused on increasing light absorption, while the critical role of thermal expansion for photoacoustic generation were often overlooked.To enhance thermal expansion for photoacoustic generation, researchers have started to fabricate composite materials, composed of light-absorbing materials mixed with polymers with high thermal expansion coefficients. These polymer-based composites significantly enhance photoacoustic conversion efficiency. Among many polymers, polydimethylsiloxane (PDMS) is exclusively used because of its high thermal expansion, optical transparency, and acoustic impedance comparable to that of water. [7][8][9][10][11] Photoacoustic pulses generated with PDMSbased composites have the characteristics of high amplitude and high frequency, which have enabled interesting applications such as cavitation-based therapy [12] and imaging. [13] For photoacoustic composites, there are a myriad of light-absorbing nanomaterials such as metal absorbers [6] and carbon nanomaterials (e.g., carbon black (CB), [14] carbon nanotube (CNT), [7,10] carbon nanofiber (CNF), [9] candle soot (CS), [8] and reduced graphene oxide (rGO) [15] ). These light-absorbing materials of nanometer sizes have an advantage over microscale absorbing materials [7] in facilitating heat transfer from the light absorbers into the surrounding mediums. Such facilitated heat transfer is another key factor that contributes to photoacoustic energy conversion.In this Review, we will start with general considerations of photoacoustic generation in nanomaterial composites. Various photoacoustic composites enabling interesting applications will be introduced. In addition, recent efforts will be reviewed from an application perspective. Also, we will discuss outlook and future directions.Photoacoustic pulses generated by pulsed laser irradiation have the characteristics of high frequency and wide bandwidth, which are desirable for imaging and sensing. Efficient photoacoustic composites have been developed for fabricating photoacoustic transmitters capable of generating highamplitude ultrasound....

Optoacoustic brain stimulation at submillimeter spatial precision

“…More specifically, optoacoustic tomography has allowed imaging of brain structures, as well as function in a non-invasive manner [23][24][25][26] . Beyond imaging, recent advances in developing optoacoustic materials have enabled highly efficient optoacoustic conversion 27 . Pulsed light excitation of these optoacoustic materials generates ultrasound waves at high amplitude, which allowed for all-optical ultrasound imaging 28,29 , tissue cavitation 30,31 , and precision surgical guidance of lumpectomy 32 .…”

Photoacoustic Energy Sensor for Nanosecond Optical Pulse Measurement

“…For CNT-PDMS composite transmitters, a photoacoustic output pressure can be represented approximately by using the following equation [ 2 ]: where P , Γ , A , F , c , τ and α are the pressure amplitude, the dimensionless Grüneisen coefficient, the optical absorption, the incident optical fluence, the sound speed, the laser pulse width and the depth of optical absorption, respectively. This shows the linear dependence between the incident laser energy fluence and the output pressure of the transmitter.…”

“…Then, most of the thermal energy optically deposited within the CNT can be transferred rapidly to the surrounding PDMS (>90% during optical pulse width). As the PDMS with a high coefficient of thermal expansion (0.92 × 10 −3 K −1 ) is configured as the surrounding matrix, the composite transmitter enables thermoacoustic generation of high-amplitude pulsed ultrasound [ 2 , 4 ].…”

“…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.…”

“…Moreover, the latter has enabled development of photoacoustic transmitters, for example, using CNT-PDMS composite films [ 1 , 3 ]. In the CNT-PDMS composite structure, PDMS receives the thermal energy from the nanoscale heater and then generates acoustic pressure via volume expansion [ 1 , 2 , 3 , 4 , 8 ]. In this photoacoustic process, the incoming laser beam characteristics are fully reflected into the acoustic output in terms of temporal pulse width, peak magnitude and waveform [ 1 ].…”