7. Non-linear optical properties and Lasing action in ZnO nanorods
7.7 Multi Photon Absorption Induced Lasing Action in ZnO Nanorods… 126
been utilized by pumping with deep UV-lasers. Multi-photon absorption (MPA) process [248, 249] by pumping with IR lasers for frequency conversion is another potential solution for the generation of UV laser in ZnO. Multi photon absorption in ZnO utilizes non-linear interaction between applied intense optical field and ZnO nanostructures. This leads to simultaneous absorption of two or more photons of sub band gap energy. These virtual states interband transition produces electron-hole pairs in the excited states and subsequently, the band edge emission from ZnO nanostructures takes place via radiative recombination. MPA has been
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127 | P a g e demonstrated as an efficient excitation process to pump carriers in ZnO thin films with green nano-second lasers as well as near IR femtosecond lasers [88, 249-251].
MPA induced lasing action in ZnO nanorods can take place via three mechanisms; two photon absorption (TPA) using second harmonic at 532 nm from ZnO (generated via two photon direct absorption in ZnO, Sec 6.5), direct three photon absorption (3PA) of 1064 nm and sum frequency generation utilizing both 532 and 1064 nm. Out of these, possibility of sum frequency generation along the c-axis of ZnO nanorods in the present setup can be ruled out because phase matching condition will not be satisfied. It may be possible that both TPA and 3PA processes are taking part simultaneously in inducing lasing action in ZnO nanorods [252, 253].
MPA induced lasing action in ZnO NRs have been recorded by exciting with 1064 nm wavelength of 8 ns Q-switched Nd:YAG laser. Initially at low excitation intensities, broad UV-PL emission with excitonic features, situated around 386 nm has been observed (Fig.7.9).
In addition to MPA induced UV emission from ZnO NRs, second harmonic generation at 532 nm and green band emission centered at 540 nm has been also observed. With an increase of the pump intensity to 28 MW/cm2, broad PL spectra switches over to an intense and relatively sharp peak centered around ~ 392 nm, indicating a transition from spontaneous emission to amplified spontaneous emission (ASE). Variation of laser intensity at 392 nm, SHG at 532 nm and green band emission at 540 nm with respect to pump intensity is plotted in Fig. 7.10.
Beyond lasing threshold (marked by Ith in Fig.7.10), intensity of ASE peak increases almost exponentially with pump as shown in Fig.7.10, where as below threshold UV output intensity varies linearly with pump intensity.
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128 | P a g e Figure 7.9 MPA induced PL spectra with second harmonic generation from ZnO NRs excited by 1064 nm Nd:YAG laser with 8 ns pulse width. (Inset: expanded view of MPA induced UV-PL emission from ZnO NRs)
Figure 7.10 Variation of emission intensity of MPA induced UV emission at 392 nm, green band emissions at 540 nm, SHG emission at 532 nm and FWHM of UV emission at 392 nm with excitation intensity.
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129 | P a g e Super linear as well as exponential growth in the output lasing intensity with UV pump was documented in earlier reports on ZnO based lasers [95, 96, 254-256]. Further the growth of the laser radiation (above threshold) for the pulsed system is exponential. Defect related green band emission peak increases linearly with pump intensity while SHG signal intensity at 532 nm showed quadratic dependence on pump intensity. The drastic decrease in FWHM of UV emission at 392 nm as a function of pump intensity as shown in Fig.7.10, further confirms the lasing action in ZnO NRs. At very high pump intensities, nearly equispaced multiple peaks with very narrow line width (~ 0.5-0.8 nm) were observed in the ASE spectra, as shown in the inset of Fig.7.9. This multiple peak structure at high excitation intensities corresponds to longitudinal lasing modes arising because of micro cavity formation in NR arrays with lateral facets acting as weak reflectors [257], as shown schematically in Fig.7.11.
Figure 7.11 Schematic of ZnO NR cavity formed to sustain lasing emission along the c-axis of NRs.
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130 | P a g e ZnO nanarod structures not only support longitudinal modes (as in the case of classical FP cavity) but also support transverse modes due to strong optical confinement. These transverse modes travel through the ZnO NR waveguide via total internal reflection taking place at NR boundary walls. The NR‟s behaves as a wave guide as the refractive index of ZnO (~2.4) is higher than that of air (1.0). The lasing modes propagate in a manner similar to that of a wave guide and hence low reflectivity at the lateral facets is sufficient to provide the optical feedback to support the lasing action. The observed mode spacing (~ 1 nm) between multiple longitudinal modes indicates the presence of coherent scattering of light in the dense ZnO nano rods. The optical amplification in ZnO nanostructures is generally considered to arise from the exciton-exciton scattering [258]. But, in the present case, the red shift of lasing peak at 392 nm from the position of free exciton emission peak at 386 nm is much smaller than that of expected from exciton-exciton scattering (83 meV @ 300K) so it can not be attributed to exciton-exciton scattering . This shift and ASE may be due to exciton-carrier interaction and /or exciton–phonon interaction. At extremely high intensity of pump, near to damage threshold, recombination of electron hole plasma (EHP) may also contribute towards the gain of ASE [259].