Effects of excitonic diffusion on stimulated emission in nanocrystalline ZnO

Abstract

We present optically pumped emission data for ZnO, showing that high excitation effects and stimulated emission/lasing are observed in nanocrystalline ZnO thin films at room temperature, although such effects are not seen in bulk material of better optical quality. A simple model of exciton density profiles is developed which explains our results and those of other authors. Inhibition of exciton diffusion in nanocrystalline samples compared to bulk significantly increases exciton densities in the former, leading-via the nonlinear dependence of emission in the exciton bands on the pump intensity-to large increases in emission and to stimulated emission. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2174107͔ ZnO has recently emerged as a strong candidate for ultraviolet photonic devices, with attractive properties, including a large exciton binding energy of 60 meV enabling efficient excitonic emission at room temperature. Many groups have studied stimulated emission and lasing properties of bulk ZnO, under electron beam 1 and optical pumping, 2 but evidence for stimulated emission and lasing has only been seen at rather low temperatures. Recent studies of optically pumped nanocrystalline samples reveal stimulated emission and lasing even at room temperature, in contrast to the bulk studies, even in samples of similar or inferior quality compared to bulk crystals. [3][4][5][6] We have studied optically pumped emission at room temperature from bulk single-crystal material ͑Eagle-Picher Corporation͒ and samples grown by pulsed laser deposition ͑PLD͒ on c sapphire. Details of the sample growth are given elsewhere. 7 Some films were annealed in O 2 ͑0.3 mbar͒ between 400°C and 600°C in the growth chamber after deposition leading to a consistent increase in grain size ͑30 → 80 nm͒ with annealing temperature. 7 All films show evidence of electric-field damping of the free exciton ͑FE͒ and film thicknesses are in the range of 200-250 nm. 7,8 The samples were studied using continuous-wave photoluminescence ͑PL͒ and high excitation pumping with a frequencytripled Nd:YAG laser ͑355 nm 10 Hz 6 ns pulse width͒ focused on the sample to a diameter of ϳ1.2 mm ͑determined by the knife-edge method͒ enabling excitation intensities in the range of 500-7000 kW/ cm 2 . Full details are described in Refs. 7 and 9. Figure 1͑a͒ shows emission at 300 K from a PLD sample ͑annealed at 500°C͒ as a function of increasing Nd:YAG excitation power. The data from this sample are representative of all the PLD samples. With increasing excitation, one observes the growth first of the FE band at 3.33 eV and then the P band at 3.23 eV, corresponding to the well-known exciton-exciton collision process, ϳ100 meV below the FE. 2 The emission bands are significantly broadened due to the damping effects mentioned earlier. The plot of integrated emission intensity versus excitation power shown in Fig. 1͑b͒ exhibits a distinct threshold at a pump power slightly above 2000 kW/ cm 2 , which is not explained by a superlinear power-law dependence of emis...

Low-threshold and quasi-single-mode random laser within a submicrometer-sized ZnO spherical particle film

“…Note that because the excitation intensity of 0.5 Â I th was enough higher than the threshold at the defect (about 6 Â I d_th ), broad emission spectrum already redshifted compared with that at the defect owing to the increase in the exciton density or formation of electron-hole plasma with high excitation intensity. 27 On the other hand, when we measured at the point defect (arrow 1 in Fig. 1(d)), we found that the behaviors were clearly different from the results at the defect-free sites (conventional random lasers), where a single sharp peak was observed at 381 nm (see Fig.…”

Stimulated emission from ZnO thin films with high optical gain and low loss

“…The second suggested explanation is a result of weak exciton confinement due to the relatively large grain size. This is based on work suggesting that grain boundaries confine the excitons, which leads to increased n ex [24] which augments the gain.…”

Multiphoton absorption induced amplified spontaneous emission from biocatalyst-synthesized ZnO nanorods

“…Since the diffusion length of free excitons in ZnO can be as large as ϳ2 m, 15,22,23 ZnO NRs with the longitudinal dimension ͑ϳ1 m͒ substantially less than the diffusion length could provide the enhanced exciton confinement and, consequently, a more efficient optical amplification process as compared to those long ͑l Ͼ 10 m͒ ZnO NWs. It has been reported that the radiative recombination via exciton-related processes ͑exciton-exciton collision, exciton-carrier collision, etc.͒ dominates the MPA-induced ASE and lasing actions in ZnO NWs under low-and intermediate-level excitation.…”