Second-order Susceptibility Measurement Of CBN By Relative Method

Cubic boron nitride (cBN) is a kind of artificial synthetic crystal with band-gap of about 6.3 eV, which has zinc-blende structure and 4 3m. Within the range of all visible light, most infrared and ultraviolet spectra, cBN crystal is transparent. cBN crystal could be a good candidate for a nonlinear optical material used inshort-wavelength region. It can be used for SHG crystal. It will have wide applications in this field.The second-order nonlinear optical characteristics of cBN crystal have not been investigated by now. We often use two sorts of methods called the absolute and the relative measurement techniques to determine the second-order susceptibilities of crystals. We introduced several conventional methods in both the absolute and the relative measurement techniques. We introduced the phase-matched SHG method and the parametric fluorescence method which belong to the absolute measurement techniques, on the other hand, the wedge technique and rotational Maker-fringe technique were referred as the relative measurement techniques. By studying these theories, we find that these theories are not suitable for our experiment because of such reasons as followed: Firstly, the absolute measurement techniques which contains the phase-matched SHG method and the parametric fluorescence method cannot be applied for optically isotropic materials such as zinc-blende structured crystals because that the condition of phase-matched can not be achieved by this type of structure. Unfortunately, Cubic boron nitride (cBN) has zinc-blende structure and (4|-)3m so that we cannot use the absolutemeasurement techniques to determine the second-order susceptibility of cBN. Secondly, the relative measurement techniques can be applied for this experiment in theory., however, because the synthesis of cBN crystal is extremely difficult, and synthetic monocrystal is very small, whose maximal size is about 3 mm only, then it is difficult to fit the conditions that must be achieved in the relative measurement techniques: ( 1 ) The wedge samples used were typically 100μm thick and had nearly parallel surfaces with an apex angle of 0.1–0.20 in the wedge technique, but in fact, the synthesis of cBN crystal is too small to be fit for this condition. ( 2 ) Since the synthesis of cBN is very small, then we can notice that the rotational angle will be limited awfully when we want to determine the second-order susceptibility of cBN. Therefore, we can draw a conclusion that the relative measurement techniques are difficult to carry out in our experiment.According to those methods, we find another method called comparing method that can be carried out easily in our experiment. We use Q-switching Nd:YAG laser, with the wavelength of 1064 nm, the peak power more than 700 W, the repeated frequency of 2 kHz and the pulse wide less than 20 ns. The second-order harmonic generation of both the cBN crystal and the GaP crystal was observed, then measured the relative SH intensity of both the cBN crystal and the GaP crystal. According to the theory of comparing method, we can see the the relation of second-order susceptibilities between the cBN crystal and the GaP crystal. Then it is easy to determine the second-order susceptibility of cBN only by measuring the SHG's beam intensity ratio between the cBN and the GaP crystals. It is more availably n another methods to carry out.By using the comparing method, we determined the second-order susceptibility of cBN, the value is about 83.2pm/v under the wavelength of 1064nm. Then we make such error analysis as followed:Firstly, the reference material GaP is not very fit for this experiment. The GaP crystal's band-gap is about 2.26ev, then we can use the relationship about the band-gap and the absorption edge to determine Gap's absorption edge (λ=1.24/E_g, the unit of wavelength isμm; the unit of band-gap is ev), it is about 0.549μm. But, we use Q-switching Nd:YAG laser, with the wavelength of 1064nm in this experiment, then the wavelength of SHG is 532nm(0.532μm). Obviously, it must be absorbed by the reference material GaP(0.532μm<0.549μm). It will lead that the value of the second-order susceptibility of cBN is more higher than the true value. According to it, we can select another reference material such as ZnSe which has zinc-blende structure and (4|-)3m, and the band-gap is more broader than GaP. Or we can use another laser which wavelength is longer than 1064nm. Then the problem of absorption will be solved at last.Secondly, in this paper, the comparing method depends on that the index of refraction of SHG is approximately equal to the index of refraction of base frequency light. But, it can bring some error with this type of approximation. So we can put forward a preliminary plan for test that can make the error lower than before. According to the conception of Brewster's angle, we can use this property to measure the index of refraction of the cBN crystal and the GaP crystal under both of the base frequency light and the SHG. We use this kind of method to determine the index of refraction of glass, the value is about 1.5 with He-Ne laser that meets necessity to the true. So we can believe that this plan is available to carry out. Moreover, we determined the index of refraction both of the cBN crystal and the GaP crystal with He-Ne laser (n_cBN=1.9, n_GaP=3.16). Then the next time we can use this method to determine the index of refraction of the cBN crystal and the GaP crystal under both of the base frequency light and the SHG then can make the value more exactly.