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  4. Femtosecond-Laser-Based Nano-Acoustic Systems: Principles and Applications
 
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Femtosecond-Laser-Based Nano-Acoustic Systems: Principles and Applications

Date Issued
2006
Date
2006
Author(s)
Lin, Kung-Hsuan
DOI
en-US
URI
http://ntur.lib.ntu.edu.tw//handle/246246/50771
Abstract
Piezoelectric semiconductor nanostructures can be treated as optical piezoelectric transducers to generate and detect acoustic nanowaves through the piezoelectric effects by femtoseocond pump-probe techniques. With the optical piezoelectric transducer, several physical issues and applications in nano-ultrasonics can be investigated. In this thesis, the theories of generation and detection of acoustic nanowaves in the optical piezoelectric transducers have been presented, and verified by experiments. With the theories and optical techniques, the design principles and characterization methods of the optical piezoelectric transducers have been illustrated from the viewpoint of acoustic engineering. We have discussed several topics such as acoustic sensors, narrowband and broadband acoustic wave generator, and frequency-tunable acoustic transducers. Based on the nano-acoustic system, several novel applications have been demonstrated in this thesis. Firstly, we have found that the energy distributions of electrons and holes can follow the band modulation due to the acoustic nanowaves in THz regime. These investigations imply the feasibility to utilize acoustic nanowaves for controlling electronic devices. Secondly, we have demonstrated that acoustic nanowaves can be utilized for ultrasonic imaging. The accuracy of one-dimensional ultrasonic scan measurement is less than one nanometer. For resolving a two-dimensional subsurface structure, the axial resolution is less than 20 nm while the transverse resolution can be less than 100 nm if the images are post-processed. We have demonstrated that the novel nano-ultrasonic technology has the advantages of non-destructive measurement, three-dimensional imaging capability, and nanometer resolutions, of which another state-of-the-art imaging technology does not possess all. Thirdly, to directly obtain a three-dimensional ultrasonic image with nanometer resolutions, the technique of generating acoustic nanospots has been demonstrated. With beam shaped optical pulses for exciting saturation carriers, the acoustic spot size can be smaller than the optical spot size, which is on the micron scale due to the diffraction limit. On the other hand, we have also used the nano-acoustic system to characterize nanowave devices. By using a single-quantum-well structure optical piezoelectric transducer, the waveforms of sub-THz or THz acoustic nanowaves can be directly measured in the time domain. We have used this method to experimentally obtain the reflection transfer function of a phononic bandgap nano-crystal, which can be served as a mirror for acoustic nanowaves. Moreover, we have investigated the energy propagation of bandgap waves in the phononic bandgap nano-crystal. Besides the piezoelectric semiconductor heterostructures, we have also demonstrated that acoustic nano-pulses can be generated in the depletion region of a p-n junction through the piezoelectric effects. The shape and width of the acoustic nano-pulses are determined by the depletion region, which can be controlled by an external bias. This interesting demonstration has shown the possibility to integrate electronic technologies for controlling acoustic nanopulses. In this study, the high optoacoustic conversion efficiency of the piezoelectric effects has also been experimentally confirmed in piezoelectric semiconductors.
Subjects
音波
奈米
半導體
超快
飛秒
雷射
奈米超音波
acoustic wave
nano
semiconductor
ultrafast
femtosecond
laser
nano-ultrasonics
Type
thesis
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