Zenghu Chang
Professor
Address:	330 Cardwell Hall
Phone:	(785) 532-1621
E-mail:	chang@phys.ksu.edu
Group Webpage
Ph.D. Xi'an Institute of Optics & Precision Mechanics, 1988
M.S. Xi'an Institute of Optics & Precision Mechanics, 1985
B.E. Jiaotong University, 1982

Research Area

Atomic, Molecular & Optical Physics

According to the well-accepted Maxwell’s theory, light is an electromagnetic wave. However, since the oscillation period of the wave is very fast (on the order of one femtosecond), it was impossible to observe the oscillation in the time domain. In order to see something that changes on the femtosecond scale, one needs a detector or a probe that is even faster. Our group is working on the generation of attosecond soft x-ray pulses that will serve as such an ultrafast probe. The attosecond pulses can be used for other research, such as measuring vibration of molecules or even motion of electrons in atoms.

The attosecond pulses are generated by exciting argon atoms with very short intense laser pulses. When the duration of laser pulses is comparable to an optical cycle, their carrier envelope phase becomes a very important parameter that affect the quality of the attosecond pulses. The carrier envelope(CE) phase is the relative position of electric field oscillation with respect to the peak of the laser pulse envelope. We have worked on stabilizing the carrier envelope phase of the Kansas Light Source laser. The technology we used is developed by two of the 2005 Nobel Prize winners in physics. We stabilized the carrier envelope phase of the laser oscillator by stabilizing the carrier envelope offset frequency of the oscillator. Then we measured slow drift of the carrier envelope phase in the chirped pulse amplifier, which will be corrected. Significant progress has been made. For many other experiments that involve the coincident measurement of electrons and ions, the signal count rate could be so low that several hours of signal accumulation is necessary even when kilohertz lasers are used. In this case, one needs to know the long term carrier envelope phase characteristics in order to either correct or measure the phase drift. We locked the CE phase of a femtosecond oscillator and studied the carrier envelope phase of the amplified pulses over three hours. This is the first time that such a measurement has been done.

We can also take advantage of the fact that attosecond pulse spectrum is sensitive to the carrier envelope phase. If the XUV spectrum can be measured and analyzed for every laser shot, then it is possible to determine the shot to shot variation of the CE phase. We have improved the intensity of the attosecond spectra and have obtained single shot attosecond spectra.

Research Support

• Department of Energy
• National Science Foundation
• Department of Defense

Recent Selected Publications

• “Precision control of carrier-envelope phase in grating based chirped pulse amplifiers,” Chengquan Li E. Moon, Hiroki Mashiko, C. Nakamura, P. Ranitovic, C. L. Cocke, and Zenghu Chang, G. G. Paulus, Optics Express 14, 11468-11476 (2006).
• “Reduction of fast carrier-envelope phase jitter in femtosecond laser amplifiers,” E. Moon, Chengquan Li, Zuoliang Duan, J. Tackett, K. L. Corwin, B. R. Washburn, and Zenghu Chang, Optics Express 14(21), pp. 9758-9763 (2006).
• “Carrier-envelope phase shift caused by variation of grating separation,” Chengquan Li, Eric Moon, and Zenghu Chang, Optics Letters 31, pp. 3113-3115 (2006).
• “Carrier envelope phase shift caused by grating-based stretchers and compressors,” Zenghu Chang, Applied Optics 45, 8350(2006).
• “Pulse duration measurements of grazing incidence pumped table-top Ni-like Ag and Cd transient soft x-ray lasers,” M. A. Larotonda, Y. Wang, M. Berrill, B. M. Luther and J. J. Rocca, Mahendra Man Shakya, S. Gilbertson and Zenghu Chang, Optics Letters 31, 3043-3045 (2006).
• “Probing the intrinsic conductivity of multiwalled carbon nanotubes,” M. Zamkov, A. S. Alnaser, N. Woody, B. Shan, Z. Chang, and P. Richard , Applied Physics Letters 89, 093111 (2006).
• “High peak-power kilohertz laser systems employing singe-stage multi-pass amplification”, Bing Shan, Chun Wang and Zenghu Chang, U. S. Patent No. 7,050,474, issued on May 23, 2006.