Study On Optical Gain Characteristics Of Semiconductor Nano-structures

To study the optical gain characteristics of nano-structures, gain characteristics of photonic crystal (PhC) waveguides were analyzed. The multiple-quantum-well (MQW) polarization-insensitive superluminescent diodes (SLD) was designed theoretically and demonstrated experimentally. Also a structure of quantum-well (QW) with asymmetric barrier was proposed, which can increase the differential gain of the active layer dramatically through the disappearance of confined light-hole states in the quantum well.Based on the plane wave method, the energy band structures and the density of states (DOS) of the 1.55μm InGaAsP PhC waveguides were simulated, and the idea that the decrease of the group velocity of the defect mode near the band edge would increase the optical gain was demonstrated. It was proposed that the natural broadening of the active materials would also affect the gain; moreover, the DOS in the PhC waveguides would change the spontaneous emission rates, and then affect the naturally broadening and the gain. The gain at the band edge where the group velocity turned to be zero was simulated based on these analyses. The calculated gain spectra can explain the experimental results of the gain in PhC waveguide on some published papers.The 1.3μm InGaAsP polarization-insensitive superluminescent diodes with weak tensile strained MQW and complex strained MQW were demonstrated theoretically based on the simulation of energy band structures and gain spectra at different injected carrier density. The complex strained MQW got good polarization-insensitive spectra of TE and TM modes over a wide bandwidth of 220nm. The output optical spectra and the I-P line were also simulated based on the amplified spontaneous emission model and rate equations in the polarization-insensitive SLD. Then the complex strained MQW SLD was fabricatedand its testing results demonstrated the effectivity of the design.Dramatic improvements in differential gain for 1.3μm AlGaInAs QW with asymmetric barrier under critical strains were demonstrated and analyzed theoretically. It is found that this phenomenon is caused by the interaction of the heavy- and light-holes in the strained QW. It is the disappearance of the confined light-hole states in the QW with asymmetric barrier under the critical strain that improves the gain characteristics of the QW.