CWRU Physics Faculty

Jie Shan
Associate Professor
Diploma, Moscow State University, Russia (1996)
Ph.D., Columbia University (2001)
Experimental Condensed Matter Physics, Ultrafast Optics, Terahertz Time-domain Spectroscopy
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Since the first demonstration of the generation of pulses of coherent optical radiation from a mode-locked ruby laser in 1965, the generation of ultrashort light pulses has advanced dramatically.  Recently light pulses as short as two optical cycles (~ 5 femtoseconds)  have been produced from state-of-the-art Ti:sapphire mode-locked lasers.  These pulses have had a significant impact on many areas of spectroscopy.  One of these is the time-domain spectroscopy of the terahertz (THz) or far-infrared spectral region (1 THz = 300 µm = 33.3 cm-1 = 4.1 meV = 47.6 K). This spectral region corresponds to many fundamental excitations in solids and molecules, including phonons, low-frequency vibrational modes, rotations, and certain collective electronic excitations.  Despite the importance of this part of the spectrum, it has remained difficult to access because of the lack of convenient coherent sources and sensitive detectors.  Through the use of a photoconductor or, by means of the non-resonant nonlinear response of suitable materials, ultrafast laser pulses permit one to produce and detect far-infrared radiation with controlled electric-field waveforms on the femtosecond time scale.  This may be considered as the extension of electronics, which provides signal generators and oscilloscopes in the rf regime, to the far-infrared.  The method may be adapted to electromagnetic pulses propagating on transmission lines or through free space.  In either case, the approach provides direct detection of the temporal evolution of the electric-field waveform and permits one to perform THz time-domain spectroscopy.

As a coherent technique, THz time-domain spectroscopy has high sensitivity.  It also provides a broad spectral range, corresponding to the high temporal resolution of current ultrafast laser pulses.  Since this scheme yields information on the electric field itself, it permits us to observe changes in both its amplitude and phase during propagation.  We may consequently extract both the real and imaginary part of the dielectric function of any material placed in the path of the beam.  Another very significant aspect of the technique, derived from its use of pulsed radiation, is its suitability for probing rapidly evolving systems.

Currently we are focusing primarily on applications of ultrafast spectroscopy, in particular, THz time-domain spectroscopy to the study of various condensed-phase systems including conventional materials and strongly correlated systems.  Although both equilibrium and nonequilibrium properties are investigated, we emphasize pump/probe techniques to examine the dynamical properties of materials, such as carrier and spin dynamics and energy relaxation.  At present, we are particularly interested in probing the effects of reduced dimensionality associated with interfaces and nanostructures.