Photosensitization of nonlinear scattering and photoacoustic emission from single-walled carbon nanotubes

Abstract

Enhancement of laser-induced nonlinear scattering has been observed from an aqueous suspension of single-walled carbon nanotubes (SWCNTs) doped with near-infrared dye chromophores. The underlying mechanism involves cavitation triggered by optical heating and bubble pulsation, which are further enhanced by the dye adsorbed on SWCNT surfaces, resulting in the scattering enhancement and a modification of the accompanying photoacoustic waveform. These photosensitizing effects suggest potential feasibility of dye-doped nanotubes as optically switchable nanoemitters of ultrasound for biomedical applications such as ultrasonic therapy and imaging.

Photoacoustic Sensitization and Laser-Induced Cavitation in Polymer Solutions by Carbon Nanotubes

“…A knife-edge technique was used to block half of the laser beam, making the illuminated volume a semicylindrical (chopped Gaussian) form, which prevented direct illumination at the sensor and also enhanced the photoacoustic signal by shaping the acoustic wavefront pseudoplanar. 7 Signals were fed into a high-speed amplifier (Hamamatsu C6438) and monitored with an oscilloscope (Tektronix TDS-3032). The observed signal waveform depended on the circuit impedance due to the capacitive nature of the sensor, so that the true transient pressure changes at the film surface were recovered by appropriate Fourier filtering.…”

“…Recently, we have observed laser-induced nonlinear optical scattering and photoacoustic wave generation in a suspension of single-walled carbon nanotubes (SWCNTs). 7 The strong absorption of photons at the surface of the SWCNT aggregates causes cavitation and even microplasma formation: the former is an involved process including solvent evaporation and extraction of dissolved gas into embryonic bubble seeds followed by acoustic (i.e., pressure-driven) or thermal expansion; the latter results from the exceedingly intensive optical heating that is microscopically localized at the skin depth of SWCNTs. The size distribution of the emerging bubbles often falls in the range comparable to the optical wavelengths, so that the origin of the nonlinear optical scatteringswhich is transient in nature and has a strongly nonlinear, threshold-like dependence on the incident laser intensitysis attributed to the onset of cavitation.…”

“…For example, photoacoustic spectroscopy has widely been used to obtain absorption spectra of powders and suspensions . Besides, highly efficient photon absorbers such as dyes and nanoparticles can be incorporated into samples to enhance photoacoustic signal generation . The resultant sound wave is often temporarily and spatially localized and extremely intense, causing optical and acoustic nonlinearity as well as cavity formation.…”

“…Use of NIR lasers is also beneficial in biomedical applications, where deep penetration into tissues is desirable. Recently, we have observed laser-induced nonlinear optical scattering and photoacoustic wave generation in a suspension of single-walled carbon nanotubes (SWCNTs) . The strong absorption of photons at the surface of the SWCNT aggregates causes cavitation and even microplasma formation: the former is an involved process including solvent evaporation and extraction of dissolved gas into embryonic bubble seeds followed by acoustic (i.e., pressure-driven) or thermal expansion; the latter results from the exceedingly intensive optical heating that is microscopically localized at the skin depth of SWCNTs.…”

“…6 Besides, highly efficient photon absorbers such as dyes and nanoparticles can be incorporated into samples to enhance photoacoustic signal generation. 7 The resultant sound wave is often temporarily and spatially localized and extremely intense, causing optical and acoustic nonlinearity as well as cavity formation. Laser-induced nonlinearity has attracted much attention in relation to ultrafast optical switching, optical limiting, and telecommunications.…”