Since the original concept pioneered by Snitzer, vector optical beams have attracted tremendous attention and academic interest owing to its potential applications in various research realms, such as super-resolution imaging, optical trapping, laser micromachining, and so on. However, the intensity profile or the radius of the traditional vector beams is strongly dependent on the absolute value of TC which makes it difficult to couple into a fiber. Besides, for the fields of vector beams optical manipulation, the radius changing of the traditional vector beams with different TC is also not a stable way to control the particles trajectory. In this study, we propose a method to generate the perfect vector (PV) beams both in fundamental frequency (FF) and second harmonic (SH) waveband at the same time.

Fig. 1. Schematic of the experimental setup. Insets (a) and (b) represent the spatial intensity and polarization distributions of the simulated FF and SH PV beams, respectively.

The experimental setup is illustrated in Fig. 1. By simply rotating the axis direction of the HWP, we can flexibility control the polarization states of PV beams in dual waveband. Figure 2 shows the corresponding experimental results. The first and third rows depict the FF PV beams and the corresponding SH PV beams are displayed in the second and forth rows. Note that the generated SH PV beams are completely different from the FF ones as Type-I (oo-e) nonlinear processes occur. It is clear to see that two crescent moon patterns of FF PV beams located at different positions while the axis direction of the HWP varies as shown in first and third rows in Fig. 2. The direction of the crescent moon patterns rotates along the same orientation of GT prism. At the same time, the vectorial properties of the generated SH PV beams are also changed. Four crescent moon patterns appear and rotate while the angle of the GT prism changes.

Fig. 2. The first and third rows are, respectively, the experimental results of FF PV beams with different initial phase. The second and forth rows are, respectively, the SH PV beam patterns corresponding to the FF ones. Arrows in the figures show the polarization angles of GT prism with respect to the positive horizontal direction.

Fig. 3. Radiuses of the generated PV beams in FF (left) and SH (right) wavebands with different TC.

Besides, we measure the radius of the generated PV beams at the focus of the lenses L1, L2 and the results are displayed in Fig. 3 It can be observed that the radius of the FF PV beams and SH PV beams are almost equal whatever the TC changes. In fact, the radius of the generated PV beam can be expressed as  , where   is the focus length of the L1 or L2,   and   refer to the refractive index and base angle of the axicon, respectively. Obviously, the radius of generated PV beams can be undoubtedly affected by the parameters of the lens and axicon. The experimental results show that the radius of the PV beams almost keep consistent with the simulated ones and are independent of the TC. In addition, we can choose proper parameters of lens or axicon to fulfill the demand for the different radius. Our studies unambiguously confirm the generation of FF and SH PV beams, which may further generation of PV beams in ultraviolet regimes. Actually, this technique has potential use in other nonlinear processes such as sum-frequency generation, difference-frequency generation, and so on.

This research was published in “Hui Li, Haigang Liu, and Xianfeng Chen, Dual waveband generator of perfect vector beams, Photonics Research, 7(11) 1340 (2019).”