The result of the morphological study of the ZnO nanorods is shown in Fig. 1. It shows the FESEM image of the ZnO nanorods. From this figure the average diameter and length of nanorods were found to be 33 nm and 270 nm respectively.
The SHG spectrum for the fs pulses of central wavelength 1000 nm, 1100 nm and 1300 nm are shown in Fig. 3a, Fig. 3b and Fig. 3c respectively. As it can be seen from this figure, the central wavelength of SHG pulses are at wavelengths 500 nm, 550 nm, 650 nm respectively. This figure clearly confirms that a single sample of ZnO nanorods can be used for SHG of fs laser pulses of central wavelength 1000 nm, 1100 nm, 1300 nm.
So in this work we have demonstrated that SHG of fs pulses of other central wavelengths other than 800 nm can also be done using the ZnO nanorods. The more interesting fact here is the SHG of the multiple wavelength ultrafast pulses were done using a single ZnO nanorods sample at only one angle of incident. This become possible because of the fact that SHG in the present case has be done with the ZnO nanorods having length much less than the coherence length (lc). This can be easily verified from the Fig. 4. This figure indicates the plot of lc versus fundamental wavelength. In particular, the Fig. 4 (a) and Fig. 4 (b) represent the variation of lc for broad wavelength and short wavelength range respectively.
The following equation was used to estimate the lc (Boyd 2003)
Where nω= refractive index for fundamental wavelength, n2ω= refractive index for second harmonic wavelength, λ = wavelength of fundamental laser radiation. The nω, n2ω are estimated from the Sellmeier equation (Bond 1965)
From Fig. 4, the lc for the SHG of laser pulses within the wavelength range 800 nm – 4000 nm is found to be ≥ 1200 nm. This indicate if the used ZnO nanorods are having length much less than 1200 nm then they can be easily used for SHG of fs pulses of any possible wavelength in the aforementioned range without going for phase matching process. This is the case in our present study as the length of the used ZnO nanorods are of 270 nm length.
It is also to note here that conventionally, the SHG of ultrafast pulses of various central wavelengths is mostly done by using the angular tuning or temperature method methods in the commercial nonlinear optical (NLO) crystals. One needs sophisticated extra arrangement to realize such tuning. In our case no such tuning is required. So the process reported here is much simpler. There are a few other advantages of our methods. Firstly the angular or temperature tuning supports the SHG of ultrafast pulses in a limited wavelength range. But as the method reported here is based on non-phase matching process, it can have much broader working range.
Finally, it is also to note that the SHG study in this work has been done by taking the outputs of an OPO. As an OPO can generate fs pulses of any wavelength within its working range so the SHG of them can be easily done by a single ZnO nanorods sample at only one angle of incident. It is to mention here that the SHG of ZnO can be easily used for realization of ultrafast laser pulse diagnostics system. This has already been demonstrated for the fs laser pulses of 800 nm (Panda and Das 2017). As a single ZnO nanorods sample can be used for SHG in a single angle of incident, a broad wavelength range ultrafast pulse diagnostics system can be easily realized using this concept. This will be particularly very useful to characterize the fs pulses generating from the OPO.