Zhelin Zhang

Growth and depth dependence of visible luminescence in wurtzite InN epilayers

We present detailed investigation of growth and depth dependence of visible (∼1.9eV) photoluminescence (PL) in wurtzite InN epilayers grown by magnetron sputtering. For normal surface incidence, PL peak was found to redshift with increasing growth temperatures. Cross-sectional PL measurements were able to separate contributions from the InN epilayers and sapphire substrates, which not only demonstrated the visible luminescence in InN but also revealed the blueshift of the PL peak with laser spot focusing from epilayer surface toward the interface. The results have been well explained by the growth mechanism and residual strain along growth direction of InN epilayers.

Determination of the third- and fifth-order nonlinear refractive indices in InN thin films

We have combined the reflection and transmission Z-scan (RZ- and TZ-scans) techniques under femtosecond laser at 800nm to extract both the third- and fifth-order nonlinear refractive indices (n2 and n4) in InN thin films. The observation of the nonlinear refractive index saturation in the intensity-dependent RZ-scan measurement indicates the existence of the fifth-order effect. By the aid of the TZ-scan, the fifth-order nonlinear effect has been enhanced by enlarging the cascaded contribution from the increased laser interaction length, where large n2 of −2.5×10−11cm2∕W and n4 of 2.1×10−19cm4∕W2 have been determined.We have combined the reflection and transmission Z-scan (RZ- and TZ-scans) techniques under femtosecond laser at 800nm to extract both the third- and fifth-order nonlinear refractive indices (n2 and n4) in InN thin films. The observation of the nonlinear refractive index saturation in the intensity-dependent RZ-scan measurement indicates the existence of the fifth-order effect. By the aid of the TZ-scan, the fifth-order nonlinear effect has been enhanced by enlarging the cascaded contribution from the increased laser interaction length, where large n2 of −2.5×10−11cm2∕W and n4 of 2.1×10−19cm4∕W2 have been determined.

Dual-frequency terahertz emission from splitting filaments induced by lens tilting in air

Dual-frequency terahertz radiation from air-plasma filaments produced with two-color lasers in air has been demonstrated experimentally. When a focusing lens is tilted for a few degrees, it is shown that the laser filament evolves from a single one to two sub-filaments. Two independent terahertz sources emitted from the sub-filaments with different frequencies and polarizations are identified, where the frequency of terahertz waves from the trailing sub-filament is higher than that from the leading sub-filament.

Plasma optical shutter in ultraintense laser-foil interaction

We report on a plasma optical shutter to reduce the intensity level of a nanosecond-duration pedestal of amplified spontaneous emission (ASE) using an ultrathin foil. The foil is ionized by the ASE prepulse and forms an expanding underdense preplasma, which enables the main laser pulse transmission, leading to an enhancement in temporal contrast. When such a plasma shutter is placed in front of a main target of interest, the preplasma profiles observed are similar to that produced from a single-layer reference target irradiated by a high-contrast laser, and can be finely tuned by varying the shutter thickness. Proton beams with significantly reduced divergence and higher flux density were measured experimentally using the double-foil design. The reduction in beam divergence is a characteristic signature of higher contrast laser production as a combined consequence of less target deformation and flatter sheath-acceleration field, as supported by the two-dimensional (2D) hydrodynamic and particle-in-cell si...

Spectral interference of terahertz pulses from two laser filaments in air

Spectral interference is experimentally demonstrated by two terahertz pulses emitting from filaments induced by two successive femtosecond laser pulses in air. Here, a leading pulse is set to be weaker than a trailing pulse and their temporal separation is larger than the pulse duration of the terahertz pulses. When the leading pulse is stronger than the trailing pulse, the frequency modulation within the whole terahertz envelope is greatly deteriorated due to nonlinear effects applying on the trailing pulse through the plasmas generated by the leading pulses. Such unique terahertz spectrum may find applications in terahertz spectroscopy.