As the typical example of the third wide band gap semiconductors, GaN, AlN and their alloys have been widely used in optoelectronic devices such as blue, green LED, LD, and high speed electronic devices such as HEMT. However, few studies of the nonlinear optical effects of the materials have been reported so far, especially the second-order nonlinear optical susceptibility associated with the intersubband transitions in quantum well. In the early 1990s, the idea of enhancing the second-order nonlinear optical susceptibility by utilizing the intersubband transitions in quantum wells has been proposed, and first demonstrated in GaAs materials. But until recently, the second harmonic generation based on the idea has been demonstrated in GaN materials, however, the measured second-order nonlinear optical susceptibility is far smaller than the predicted value. There are two probable reasons; one is the error between the actual structure parameters and the designed, the other is the quantum well structure designed in this study cannot fully utilizing the enhancement effect of the intersubband transitions, thus makes the second-order nonlinear optical susceptibility small. As we know, a high second-order nonlinear optical susceptibility is critical to second harmonic generation.
To solve the above mentioned problems, the advanced semiconductor team led by Professor Chen Changqing from the Wuhan National Laboratory for Optoelectronics proposed a new quantum well structure-----GaN/AlGaN step quantum well structure. The conventional single quantum well structure cannot achieve large second-order nonlinear optical susceptibility because the double-resonance enhancement effect cannot be realized. But situation is different in step quantum well structure, the double-resonance condition can be readily realized by changing the width of the well and the step well or the Al mole composition of the barrier and step well, thus the second-order nonlinear optical susceptibility can be enhanced by one order of magnitude greater than that in single quantum well structure.The results indicate that nonlinear optical elements based on GaN/AlGaN step quantum wells are very promising for SHG in a wide range of wavelengths from telecommunication to mid-infrared, especially effective in longer wavelength.
This work was supported by the National Basic Research Program of China (Grant No. 2012CB619302, 2010CB923204), the National Natural Science Foundation of China (Grant No. 60976042, 51002058, 10990102), the Science and Technology Bureau of Wuhan City (Grant No. 2014010101010006).