Optical pumping of a-plane ZnO/ ZnMgO MQWs

At high excitation intensity (~55.7MWcm^-2), many sharp peaks emerge on the spectra (Fig5.5). The FWHM of each individual peaks was about 0.37nm and the mode spacing was about 0.65nm. In order to figure out this phenomenon, from low to high pumping density was used to observe the PL spectra.

Fig 5.6 clearly displayed various spectra under different pumping intensity. At 3.9MWcm^-2 pumping density, the PL spectrum was as similar as the previous measurement result. We named this emission mechanism as spontaneous emission and the peak energy was around 3.313eV. At higher (~31.3MWcm^-2) pumping density, another peak emerges at the lower shoulder of the spontaneous emission which has peak energy around 3.22eV. This behavior was mentioned above as ZnO P-band emission which was induced by exciton-exciton scattering. As the excitation density further increasing, the EHP band emission appeared at around 3.139eV. The peak energy of EHP band is lower than P band emission. This is due to the excitons density exceeded the Mott density, the excitons can not exist as individual quasiparticles.

Then, electron–hole plasma was formed under this condition. From ref [47], we get

the relation between spontaneous emission and P band emission.

We can use Eq5.2 to obtain the exciton binding energy which is about 66meV. This result is close to the previous fitting result. There are many individual peaks in the EHP band emission. We presumed the random lasing behavior of a-plane ZnO/ZnMgO MQWs was observed. Random laser represented a laser amplifier due to random scattering mechanism, as opposed to the reflective feedback by the mirrors, so we also called the random laser as a mirrorless laser. In the late 1990s, random lasers with coherent feedback were realized with disordered semiconductor and organic materials [48]. The interference of waves produces the amplitude feedback. In the active medium of a random laser, light is scattered by some disorder and a random trip is achieved before leaving the active medium. A photon may generate the stimulated emission of a second photon as it travels through a gain medium. There are characteristics length scales. We use it to describe the action of photon in the medium.

The gain length is the distance a photon travels before generating a second photon.

The average length is that a photon travels in the gain medium. With the number of scattering centers increase, the average path length of photons increases. When it is equal to the gain length, each photon will induce another photon before leaving the medium. This situation has been called lasing without coherent feedback. When

scattering gets stronger, after multiple scattering light may return to the scattering center from which it was scattered before. A closed-loop path for light is formed.

When the gain along the loop reaches the loss, lasing oscillation occurs in this closed loop which forms a cavity. Their phase relationship determines the lasing frequencies.

The feedback is provided by the disorder-induced scattering. In ZnO system, this behavior was extensively observed. For example, Z. K. Tang et al [49] had discovered ultraviolet laser emission from self-assembled Zno microcrystallite thin films at room-temperature. H. GaO et al [48] had observed ultraviolet lasing formed by scattering in semiconductor polycrystalline films. S. F. Yu et al [50] also discovered the random laser action in ZnO nanorod arrays in ZnO epilayers.

Fig 5.7 shows the optical pumping result. At low excitation density, single-broad emission spectra with a FWHM of around 20nm were observed. When excitation intensity increased; many individual peaks appeared on the spectra. The FWHM of each individual peaks was about 0.37nm. The mode spacing roughly equated to 0.65nm. A further increase in pump intensity increases the number of lasing modes as the increase in optical gain excites more cavity modes with higher losses. The central position of lasing peak energy was gradual red-shift, when excitation intensity increased. This is so called bandgap renormalization (BGR) phenomenon. This phenomenon is a characteristic of EHP lasing behavior. We further fitted the lasing

threshold. The lasing threshold was about 47.33MWcm^-2. Fig 5.9 shows the evolution of the emission spectra when the excitation area was varied at a fixed pump intensity.

When the excitation area was increased, more lasing peaks emerged in the emission spectra. This is because in a large excitation area, more closed-loop paths for light can be formed. As a result, random laser action could occur in more cavities formed by recurrent scattering. On the other hand, when the excitation area was reduced to below a critical size, laser oscillation stopped. This is because if the closed-loop paths are too short, the amplification along the loop was not high enough achieve lasing. A major advantage of random lasers over regular lasers is that their production is cheap and the required technology relatively simple. The high-precision methods needed to create ultraprecise microcavities, used in for, example, diode lasers, are not required here. In addition, the materials can be produced on a large scale and have high emission efficiency. The main challenge for the development of future applications is that of electrical excitation of a random-laser material, which is crucial for display and lighting technology. Electrical conductance is going to be an important issue here, owing to the disordered and often porous character of the materials under study.

5.3 Conclusion

In this chapter, we demonstrated the random lasing behavior of a-plane ZnO/ZnMgO MQWs. At high excitation intensity (~55.7MWcm^-2), many sharp peaks

emerge on the spectra (Fig5.5). The FWHM of each individual peaks was about 0.37nm and the mode spacing was about 0.65nm. From Eq 5.2 we had calculated the exciton binding energy is around 66meV which is closed to the activation energy. We attributed this lasing behavior from the forming of electron-hole plasma and the random lasing was achieved under the high excitation density. The threshold was about 47.33MWcm^-2. For constant pump intensity of 113 MWcm^-2, larger excitation area produces EHP lasing behavior easier. This is also a characteristic behavior in random lasing system.

Fig5.1 Micro-PL system

Fig 5.2 A simple graph and the processes that occur with increasing excitation intensity.

Fig 5.3 A diagram of exciton-exciton scattering.

Fig 5.4 A function of the electron–hole pair density nP the renormalization of the gap

Fig5.5 The PL spectra at high excitation intensity.

Fig5.6 Various spectra under different pumping intensity.

FWHM~0.37nm

30 35 40 45 50 55 60 65

Emission int e n s it y ( a rb .u nit s )

Power density(MW/cm2) Threshold~47.33MW/cm

Fig5.7 Optical pumping measurement results.

Fig5.8 The fitting result of lasing threshold.

3.000 3.05 3.10 3.15 3.20 3.25

Fig5.9 The evolution of the emission spectra when the excitation area was varied at a fixed pump intensity.

c

2.95 3.00 3.05 3.10 3.15 3.20 3.25

Intensi ty (ar b .units )

Photon energy(eV)

Pump intensity~113MW/cm² a

b d

Excitation Area a~5.0×10^-5cm² b~4.4×10^-5cm² c~3.8×10^-5cm² d~2.8×10^-5cm²