Cheng-Ta Yu

Two-dimensional nanoultrasonic imaging by using acoustic nanowaves

Two-dimensional ultrasonic imaging is demonstrated by using acoustic nanowaves. With a 14nm acoustic wavelength, both axial and transverse resolutions of a few tens of nanometers are thus achieved. This ultrasonic-based nondestructive technique not only images but also reconstructs the subsurface nanostructures including the depth positions of the buried interfaces. By demonstrating two-dimensional nanoultrasonic scans in depth and transverse (or z-x) axes, we show that acoustic nanowaves can be a promising tool for future subsurface three-dimensional noninvasive imaging with nanometer resolutions.

Optical piezoelectric transducer for nano-ultrasonics

Piezoelectric semiconductor strained layers can be treated as piezoelectric transducers to generate nanometer-wavelength and THz-frequency acoustic waves. The mechanism of nano-acoustic wave (NAW) generation in strained piezoelectric layers, induced by femtosecond optical pulses, can be modeled by a macroscopic elastic continuum theory. The optical absorption change of the strained layers modulated by NAW through quantum-confined Franz-Keldysh (QCFK) effects allows optical detection of the propagating NAW. Based on these piezoelectric-based optical principles, we have designed an optical piezoelectric transducer (OPT) to generate NAW. The optically generated NAW is then applied to one-dimensional (1-D) ultrasonic scan for thickness measurement, which is the first step toward multidimensional nano-ultrasonic imaging. By launching a NAW pulse and resolving the returned acoustic echo signal with femtosecond optical pulses, the thickness of the studied layer can be measured with <1 nm resolution. This nano-structured OPT technique will provide the key toward the realization of nano-ultrasonics, which is analogous to the typical ultrasonic techniques but in a nanometer scale.

Generation of picosecond acoustic pulses using a p‐n junction with piezoelectric effects

We demonstrate the generation of picosecond acoustic pulses using a piezoelectric-semiconductor-based p‐n junction structure. This p‐n junction picosecond ultrasonic experiment confirms that the piezoelectric effect dominates the thermal expansion and deformation-potential coupling in the generation of picosecond acoustic pulses. The characteristics of the p‐n initiated acoustic pulses are determined by the width and the field strength inside the depletion region. Our study indicates the future possibility to electrically control the acoustic pulse characteristics if we could apply an external bias to modulate the depletion region width.

Generation of frequency-tunable nanoacoustic waves by optical coherent control

We have developed a system to generate arbitrary wave-form nanoacoustic waves (NAWs) with a piezoelectric InGaN∕GaN single-quantum well. Based on an optical coherent control technique, acoustic frequency tunability in the subterahertz range is realized within only one fixed sample. The acoustic generation mechanisms, especially the in-well piezoelectric field Coulomb screening which tends to be saturated at high carrier concentrations, are discussed with optical power dependency. With the generated NAWs propagating in the c axis of a GaN thin film, the lifetime of the 500 GHz longitudinal-acoustic phonon pulses in GaN is measured to be longer than 420 ps, corresponding to a GaN depth more than 3.3μm.

2GHz repetition-rate femtosecond blue sources by second harmonic generation in a resonantly enhanced cavity

We report a 2GHz repetition-rate, all-solid-state femtosecond blue source. Pumped by a 740mW femtosecond Ti:sapphire laser with the same repetition rate, 150mW femtosecond pulses at 409nm can be efficiently generated from the external resonant cavity with a lithium triborate crystal.

Generation of frequency tunable nano-acoustic waves by optical coherent control

We have demonstrated an optical coherent control technique to generate nano-acoustic waves in InGaN single-quantum-well with tunable acoustic frequency. The acoustic phonon lifetime of the generated nano-acoustic wave was also measured.

1D nano-ultrasonic scan with 1-nanometer spatial resolution

Piezoelectric quantum wells can be treated as opto-acoustic transducers to generate and detect acoustic waves with nanometer wavelengths. This opto-acoustic transducer was utilized for 1D ultrasonic scan with 1 nanometer demonstrated resolution.

Generation, detection, and propagation of nano-acoustic waves in piezoelectric semiconductors (Invited Paper)

Piezoelectric semiconductor with heterostructure can be treated as a piezoelectric transducer for the generation of acoustic waves with wavelength less than 10 nm (nano-acoustic waves) by optical technique. This optical piezoelectric transducer has also been utilized for the detection of the nano-acoustic waves (NAW). In this paper, we discuss the generation, detection, and propagation of nano-acoustic waves in piezoelectric semiconductors. We demonstrate that the acoustic frequency of the NAW can be tuned by an optical control technique. Besides, we have also developed an acoustic sensor with THz bandwidth which can be used to study NAW propagation control devices such as nano-phononic bandgap crystal. We demonstrated that the roughness of an interface can be evaluated by the NAW with a resolution less than 1 nm through the acoustic phasefront distortion effect. With the optical piezoelectric transducer, nano-ultrasonics, which is analogous to typical ultrasonics but on the nanometer scale, has been successfully developed.