Semiconductor nanowires have been the subject of intense study in recent years. In particular, their ability to confine electrons/holes and photons makes them attractive as potential building blocks of nanoscale electronics and optoelectronic devices.1-3 For UV/blue light emitting devices, the optical properties of gallium nitride (GaN) nanowires are suitable due to their wide band gap (3.4 eV) and the absence of threading dislocation in the lateral dimensions.4-7 Therefore, the optical properties of GaN nanostructures have been widely investigated, which include stimulated emission processes such as lasing and amplified spontaneous emission (ASE). These can have a positive impact on the optoelectronic devices based on GaN nanowires. In this Note, we examine the stimulated emission process of GaN nanorods using optical excitation of femtosecond laser pulses.The shapes and lengths of the GaN nanostructures were characterized by scanning electron microscopy (SEM), whose result is presented in Figure 1(a). The diameters of nanorods were in the range of 150-400 nm and the lengths were 500-1000 nm. The as-grown nanorods were excited with a He-Cd laser (325 nm, Kimmon) to obtain a normal photoluminescence spectrum at room temperature ( Figure 1(b)). In the UV region an emission peak was observed at 368 nm (3.4 eV). As the exciton binding energy in GaN is in the range of 20-25 meV, 4,8 emission of the exciton state was not expected due to the thermal energy at room temperature (26 meV). Therefore, the UV emission is attributed to band edge emission (BGE), which indicates the band gap energy.
4-8To measure the nonlinear optical response, GaN nanorods were excited by the second harmonics (355 nm) of a cavitydumped oscillator (Mira/PulseSwitch, Coherent, 1 MHz, 710 nm, 150 fs) using a UV microscope objective.9,10 The emissions were collected by the same objective, resolved spectrally by a monochromator, and detected by a photomultiplier. The shape of BGE was not changed when a low excitation intensity was used (< 25 µJ/cm 2 ), as shown in Figure 2(a). The intensity of BGE increased almost linearly in this low excitation intensity regime, which is presented in Figure 2(b). On the other hand, a superlinear increase in the BGE intensity was observed at an excitation intensity of 30 µJ/cm 2 . In addition, the bandwidth of BGE decreased with increasing excitation intensity above a threshold of 30 µJ/ cm 2 . For example, the full-width at half maximum (FWHM) of the band was found to be 17 nm at an excitation intensity of 10 µJ/cm 2 , which decreased to 15 nm at an excitation intensity of 50 µJ/cm 2 , as seen in the inset of Figure 2(a). The superlinear increase in the BGE intensity and the narrowing of the bandwidth can be attributed to the waveguiding effect.
