Studies on Electrically Manipulated Coherent Acoustic Phonons
Date Issued
2009
Date
2009
Author(s)
Wang, Pei-Hsun
Abstract
Coherent acoustic phonons (CAPs) in GaN-based materials have been widely discussed in recent years. Due to the large strain-induced piezoelectric fields in InGaN/GaN multiple-quantum-wells (MQWs), we can generate CAPs in MQWs using femtosecond laser techniques. The photo-induced carriers generated by femtosecond pulse excitation spatially separate and screen the piezoelectric field in the well region. This screening effect induces the lattice vibration and produces the longitudinal CAPs. The oscillation frequency in the terahertz (THz) region is decided by the periods of MQWs.hen the acoustic waves propagate through piezoelectric materials, the strain affects the piezoelectric fields and thus modulates the optical absorption. The longitudinal CAPs induce a large absorption variation while the optical probe was near the energy gap of the materials. Therefore, the MQWs can be treated as an optical piezoelectric transducer (OPT) to detect the THz CAPs.For the optically detected oscillations of CAPs in MQWs, the sensitivity for CAP detection strongly depended on the piezoelectric field and the quasi-Fermi levels. As a result, if we change the piezoelectric field by external voltages, the detected oscillations of CAPs in MQWs can be altered by using electrical control while optical excitation condition remains the same.n this thesis, the author investigates the electrical control of the optical sensitivity for CAP detection by applying reverse voltages across InGaN/GaN MQWs light emitting diodes (LEDs). The author took 10-period MQWs in GaN-based p-i-n structure as the CAP source and detector simultaneously. The frequency of the CAPs generated in the MQWs was 0.54THz with well/barrier width 3/12nm. Through the piezoelectric effect, the CAPs modified the energy of subbands and thus modulated the transmission changes. By electrically controlling the quasi-Fermi levels, we can observe a large decrease of the strain-induced transmission modulation in the MQWs. As applying reverse bias voltages on the LEDs, the sensitivity for CAP detection drastically decreased and the CAP detection was possibly turned off.
Subjects
Coherent Acoustic Phonons
Femtosecond Laser
Piezoelectric Materials
Light Emitting Diodes
Nano-Ultrasonics
Type
thesis
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