Hyoung Won Baac

Carbon-Nanotube Optoacoustic Lens for Focused Ultrasound Generation and High-Precision Targeted Therapy

We demonstrate a new optical approach to generate high-frequency (>15 MHz) and high-amplitude focused ultrasound, which can be used for non-invasive ultrasound therapy. A nano-composite film of carbon nanotubes (CNTs) and elastomeric polymer is formed on concave lenses, and used as an efficient optoacoustic source due to the high optical absorption of the CNTs and rapid heat transfer to the polymer upon excitation by pulsed laser irradiation. The CNT-coated lenses can generate unprecedented optoacoustic pressures of >50 MPa in peak positive on a tight focal spot of 75 μm in lateral and 400 μm in axial widths. This pressure amplitude is remarkably high in this frequency regime, producing pronounced shock effects and non-thermal pulsed cavitation at the focal zone. We demonstrate that the optoacoustic lens can be used for micro-scale ultrasonic fragmentation of solid materials and a single-cell surgery in terms of removing the cells from substrates and neighboring cells.

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

We demonstrate carbon nanotube (CNT) 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.

Low density carbon nanotube forest as an index-matched and near perfect absorption coating

We demonstrate broadband, near perfect absorption with a conformal coating of a multi-walled carbon nanotube (CNT) forest on an arbitrarily shaped surface. The complex refractive index of such a CNT forest is retrieved from the measured transmission and reflection spectra using Kramers-Kronig constrained variational analysis, which gives a typical value of neff = 1.04 + 0.01i at visible wavelengths. Therefore, when used as a conformal coating on an object, a thick layer of the CNT forest can provide an excellent impedance match to air and near perfect absorption, preventing any detectable light reflection and scattering from the object.

A 16-site neural probe integrated with a waveguide for optical stimulation

In this paper, we report a neural probe which can selectively stimulate target neurons optically from an integrated optical waveguide and also monitor extracellular neural signals in electrical recording sites. The waveguide is composed of SU-8 core and oxide cladding layer to guide a light from optical source. A U-groove has been formed at the end of the waveguide for easy alignment with an optical fiber. The coupling loss between the optical fiber and waveguide has been measured below −3.7 dB with a waveguide loss of −0.22 dB/mm. We have successfully transmitted a light of 470nm in wavelength through the integrated polymer waveguide on the neural probe.

Photoacoustic correlation spectroscopy and its application to low-speed flow measurement.

A photoacoustic correlation technique, inspired by its optical counterpart-the fluorescence correlation spectroscopy (FCS)-was tested for the first time, to our knowledge, to demonstrate the feasibility of low-speed flow measurement based on photoacoustic signal detection. A pulsed laser was used to probe the flow of light-absorbing beads. A photoacoustic correlation system of 0.8 s temporal resolution was built and flow speeds ranging from 249 to 14.9 microm/s with corresponding flow times from 4.42 to 74.1 s were measured. The experiment serves as a proof of concept for photoacoustic correlation spectroscopy, which may have many potential applications similar to the FCS.

Rapid anisotropic photoconductive response of ZnO-coated aligned carbon nanotube sheets.

We investigate the rapid and anisotropic UV-induced photoconductive response of hybrid thin films comprising zinc oxide (ZnO) nanowires (NWs) directly grown on horizontally aligned (HA-) carbon nanotube (CNT) sheets. The films exhibit anisotropic photoconductivity; along the CNTs, conductivity is dominated by the CNTs and the photoconductive gain is lower, whereas perpendicular to the CNTs the photoconductive gain is higher because transport is influenced by ZnO nanoclusters bridging CNT-CNT contacts. Because of the distributed electrical contact provided by the large number of ZnO NWs on top of the HACNT film, this hybrid nanoarchitecture has a significantly greater photocurrent than reported for single ZnO NW-based devices at comparable UV illumination intensity. Moreover, the hybrid architecture where a thin basal film of ZnO ohmically contacts metallic CNTs enables rapid transport of photogenerated electrons from ZnO to CNTs, resulting in sub-second photoresponse upon pulsed illumination. The built-in potential generated across ZnO-CNT heterojunctions competes with the externally applied bias to control the photocurrent amplitude and direction. By tuning the anisotropic conductivity of the CNT network and the morphology of the ZnO or potentially other nanostructured coatings, this material architecture may be engineered in the future to realize high-performance optical and chemical sensors.

Micro-ultrasonic cleaving of cell clusters by laser-generated focused ultrasound and its mechanisms

Laser-generated focused ultrasound (LGFU) is a unique modality that can produce single-pulsed cavitation and strong local disturbances on a tight focal spot (<100 μm). We utilize LGFU as a non-contact, non-thermal, high-precision tool to fractionate and cleave cell clusters cultured on glass substrates. Fractionation processes are investigated in detail, which confirms distinct cell behaviors in the focal center and the periphery of LGFU spot. For better understanding of local disturbances under LGFU, we use a high-speed laser-flash shadowgraphy technique and then fully visualize instantaneous microscopic processes from the ultrasound wave focusing to the micro-bubble collapse. Based on these visual evidences, we discuss possible mechanisms responsible for the focal and peripheral disruptions, such as a liquid jet-induced wall shear stress and shock emissions due to bubble collapse. The ultrasonic micro-fractionation is readily available for in vitro cell patterning and harvesting. Moreover, it is significant as a preliminary step towards high-precision surgery applications in future.

Localized micro-scale disruption of cells using laser-generated focused ultrasound

We utilize laser-generated focused ultrasound (LGFU) to create targeted mechanical disturbance on a few cells. The LGFU is transmitted through an optoacoustic lens that converts laser pulses into focused ultrasound. The tight focusing (<100 µm) and high peak pressure of the LGFU produces cavitational disturbances at a localized spot with micro-jetting and secondary shock-waves arising from micro-bubble collapse. We demonstrate that LGFU can be used as a non-contact, non-ionizing, high-precision tool to selectively detach a single cell from its culture substrate. Furthermore, we explore the possibility of biomolecule delivery in a small population of cells targeted by LGFU at pressure amplitudes below and above the cavitation threshold. We experimentally confirm that cavitational disruption is required for delivery of propidium iodide, a membrane-impermeable nucleic acid-binding dye, into cells.

Broad-band high-efficiency optoacoustic generation using a novel photonic crystal-metallic structure

Various optical structures have been investigated for high-frequency optoacoustic generation via thermoelastic effect, including metal films, mixture of polydimethylsiloxane (PDMS) and carbon black, two-dimensional (2-D) gold nanostructure with PDMS film, etc. However, they suffer from either low light absorption efficiency which affects the amplitude of generated ultrasound, or thick films that attenuate the amplitude and restrict its spectra bandwidth. Here we propose a novel one-dimensional photonic crystal-metallic (PCM) structure, which can be designed to absorb 100% optical energy of specific wavelengths in a total-internal-reflection geometry. The unique configuration enables us to choose suitable polymer films on top of the metallic structure, which can act as an ideal ultrasound transmitter to generate broad-band ultrasound with high conversion efficiency. Experimental results show that the PCM structure generated several times stronger ultrasound pressure than our previously demonstrated 2-D gold nanostructures [Appl. Phys. Lett. 89, 093901 (2006)]. Moreover, the generated ultrasound exhibited almost the same frequency spectrum as the input laser pulse (duration width 6 ns). This shows that the PCM structure has great potential to generate broad-band ultrasound signal. It is also important to mention that the simple PCM structure with the polymer film forms a Fabry-Pérot resonator and can play a role of an ultrasound receiver, which provides a convenient method to construct a broad-band and all-optical ultrasound transducer.

Dual-frequency focused ultrasound using optoacoustic and piezoelectric transmitters for single-pulsed free-field cavitation in water

Pulsed ultrasonic cavitation is a promising modality for non-contact targeted therapy, enabling mechanical ablation of the tissue. We demonstrate a spatio-temporal superposition approach of two ultrasound pulses (high and low frequencies) producing a tight cavitation zone of 100 μm in water, which is an-order-of-magnitudes smaller than those obtained by the existing high-amplitude transducers. Particularly, laser-generated focused ultrasound (LGFU) was employed for the high-frequency operation (15 MHz). As demonstrated, LGFU plays a primary role to define the cavitation zone. The generation rate of cavitation bubbles could be dramatically increased up to 4.1% (cf. 0.06% without the superposition) with moderated threshold requirement.