Optical Generation and Detection of High-Frequency Focused Ultrasound and Associated Nonlinear Effects.

In this thesis, optical generation and detection of high-frequency ultrasound are presented. On the generation side, high-efficiency optical transmitters have been devised and developed which can generate high-frequency and high-amplitude pressure. Conventional optoacoustic transmitters have suffered from poor optoacoustic energy conversion efficiency (10-7~10-8). Therefore, pressure amplitudes were usually weak for long-range imaging (several cm) and too weak to induce any therapeutic effects. Here, far beyond such traditional regime, therapeutic pressure amplitudes of tens of MPa were achieved optoacoustically. First, high-efficiency optoacoustic sources were developed in planar geometries by using carbon nanotube-polymer composites. The planar transmitters could generate 18-fold stronger pressure than thin metallic films used as references, together with providing broadband and high-frequency spectra over 120 MHz. Then, the thin-film transmitters were formed on concave substrates to generate and simultaneously focus the ultrasound. Unprecedented optoacoustic pressure was achieved at lens focus: >50 MPa in positive and >20 MPa in negative peaks. These amplitudes were sufficient to induce strong shock waves and acoustic cavitation. Due to the high-frequency operation, such therapeutic pressure and the induced effects were tightly localized onto focal widths of 75 um in lateral and 400 um in axial directions, which are an order of magnitude smaller than those of traditional piezoelectric transducers. The shock waves and the cavitation effects were investigated in various ways. High focal gains and short distances for shock formation were suggested as main features. The optoacoustic approach is expected to open numerous opportunities for a broad range of biomedical applications demanding high-accuracy treatment with minimal damage volumes around focal zones. For optical detection of ultrasound, optical microring resonators have been used due to their broadband frequency responses (~100 MHz) and high sensitivity. However, their spatial responses due to the particular ring shape have not been investigated especially for high-frequency ranges. Here, the microring responses were characterized in this regime. As a final subject, the microrings were used to detect focused ultrasound and realize novel optoacoustic 4f imaging systems which have capabilities of fast 3-D imaging without requiring mathematical reconstruction steps. High-resolution performances were demonstrated by resolving polymer microspheres of 100-um diameter.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91573/1/hwbaac_1.pd

Efficient Photoacoustic Conversion in Optical Nanomaterials and Composites

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 high‐amplitude ultrasound. Here, recent advances in photoacoustic transmitters are reviewed from an application perspective, starting with the fundamental aspects of photoacoustic generation. The topics discussed include various composite materials for photoacoustic generation, and their applications such as high‐amplitude therapy, imaging and sensing, and photoacoustic waveform control.Photoacoustic transmitters using pulsed laser irradiation onto optical nanomaterials have been developed for generating strong photoacoustic pulses, enabling interesting applications. Recent advances in photoacoustic transmitters are reviewed from an application perspective, starting with the fundamental aspects of photoacoustic generation.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/147165/1/adom201800491_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147165/2/adom201800491.pd

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

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98687/1/ApplPhysLett_99_211103.pd

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

Simulation Study: The Impact of Structural Variations on the Characteristics of a Buried-Channel-Array Transistor (BCAT) in DRAM

As the physical dimensions of cell transistors in dynamic random-access memory (DRAM) have been aggressively scaled down, buried-channel-array transistors (BCATs) have been adopted in industry to suppress short channel effects and to achieve a better performance. In very aggressively scaled-down BCATs, the impact of structural variations on the electrical characteristics can be more significant than expected. Using a technology computer-aided design (TCAD) tool, the structural variations in BCAT (e.g., the aspect ratio of the BCAT recess-to-gate length, BCAT depth, junction depth, fin width, and fin fillet radius) were simulated to enable a quantitative understanding of its impact on the device characteristics, such as the input/output characteristics, threshold voltage, subthreshold swing, on-/off-current ratio, and drain-induced barrier lowering. This work paves the road for the design of a variation-robust BCAT

Comparative Study of Novel u-Shaped SOI FinFET Against Multiple-Fin Bulk/SOI FinFET

Superior scalability and better gate-to-channel capacitive coupling can be achieved with adopting gate-all-around (GAA) device architecture. However, compared against FinFET device structure, the GAA device is not very cost-effective. In addition, its yield for mass production is not as high as expected. In order to explore the device design option for extending the current FinFET device out, we have come up with a new idea, i.e., novel device structure to increase the effective channel width without affecting the contacted poly pitch (CPP). A novel u-FinFET structure is a type of FinFET that has a u-shaped channel. By using 3-D TCAD simulation, it turned out that the u-FinFET (vs. conventional bulk/SOI FinFETs) shows a 27.3% higher drain current at $V_{\mathrm {GS}}$ = 0.7 V because of its 28.7% wider effective channel width. Especially, the u-FinFET can be adopted for high performance applications because it can implement a wider channel width for a given layout area. Moreover, taking advantages of using the current processes and materials for bulk/SOI FinFET, the u-FinFET can be fabricated (which must be very cost-effective). Hence, with taking advantages of using the u-FinFET structure, the current FinFET platform would last more

Photoacoustic Energy Sensor for Nanosecond Optical Pulse Measurement

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

Side-Polished Fiber-Optic Line Sensor for High-Frequency Broadband Ultrasound Detection

We demonstrate a side-polished fiber-optic ultrasound sensor (SPFS) with a broad frequency bandwidth (dc–46 MHz at 6-dB reduction) and a wide amplitude detection range from several kPa to 4.8 MPa. It also exhibits a high acoustic sensitivity of 426 mV/MPa with a signal-to-noise ratio of 35 dB and a noise-equivalent pressure of 6.6 kPa (over 1–50 MHz bandwidth) measured at 7-MHz frequency. The SPFS does not require multi-layer-coated structures that are used in other high-sensitivity optical detectors. Without any coating, this uses a microscale-roughened structure for evanescent-field interaction with an external medium acoustically modulated. Such unique structure allows significantly high sensitivity despite having a small detection area of only 0.016 mm2 as a narrow line sensor with a width of 8 μm. The SPFS performance is characterized in terms of acoustic frequency, amplitude responses, and sensitivities that are compared with those of a 1-mm diameter piezoelectric hydrophone used as a reference

Graphene- and Carbon-Nanotube-Based Transparent Electrodes for Semitransparent Solar Cells

A substantial amount of attention has been paid to the development of transparent electrodes based on graphene and carbon nanotubes (CNTs), owing to their exceptional characteristics, such as mechanical and chemical stability, high carrier mobility, high optical transmittance, and high conductivity. This review highlights the latest works on semitransparent solar cells (SSCs) that exploit graphene- and CNT-based electrodes. Their prominent optoelectronic properties and various fabrication methods, which rely on laminated graphene/CNT, doped graphene/CNT, a hybrid graphene/metal grid, and a solution-processed graphene mesh, with applications in SSCs are described in detail. The current difficulties and prospects for future research are also discussed