Excitation of second harmonics in a wide spectrum

Since the discovery of second-order nonlinear optical effects in the 1960s, has aroused wide interest of researchers, so far, based on the second harmonic, and frequency effects, has produced from the extreme ultraviolet to the far infrared band of lasers, greatly promoted the development of laser, optical information processing, high-resolution microscopic imaging and other fields. According to nonlinear optics and polarization theory, the even-order nonlinear optical effect is closely related to crystal symmetry, and the nonlinear coefficient is not zero only in non-central inversion symmetric media. As the most basic second-order nonlinear effect, the second harmonics greatly hinder their generation and effective use in quartz fiber because of the amorphous form and the symmetry of center inversion. At present, polarization methods (optical polarization, thermal polarization, electric field polarization) can artificially destroy the symmetry of material center inversion of optical fiber, and effectively improve the second-order nonlinearity of optical fiber. However, this method requires complex and demanding preparation technology, and can only meet the quasi-phase matching conditions at discrete wavelengths. The optical fiber resonant ring based on the echo wall mode limits the wide spectrum excitation of second harmonics. By breaking the symmetry of the surface structure of the fiber, the surface second harmonics in the special structure fiber are enhanced to a certain extent, but still depend on the femtosecond pump pulse with very high peak power. Therefore, the generation of second-order nonlinear optical effects in all-fiber structures and the improvement of conversion efficiency, especially the generation of wide-spectrum second harmonics in low-power, continuous optical pumping, are the basic problems that need to be solved in the field of nonlinear fiber optics and devices, and have important scientific significance and wide application value.

A research team in China has proposed a layered gallium selenide crystal phase integration scheme with micro-nano fiber. By taking advantage of the high second-order nonlinearity and long-range ordering of gallium selenide crystals, a wide-spectrum second-harmonic excitation and multi-frequency conversion process are realized, providing a new solution for the enhancement of multi-parametric processes in fiber and the preparation of broadband second-harmonic light sources. The efficient excitation of the second harmonic and sum frequency effect in the scheme mainly depends on the following three key conditions: the long light-matter interaction distance between gallium selenide and micro-nano fiber, the high second-order nonlinearity and long-range order of the layered gallium selenide crystal, and the phase matching conditions of the fundamental frequency and frequency doubling mode are satisfied.

In the experiment, the micro-nano fiber prepared by the flame scanning tapering system has a uniform cone region in the order of millimeter, which provides a long nonlinear action length for the pump light and the second harmonic wave. The second-order nonlinear polarizability of the integrated gallium selenide crystal exceeds 170 pm/V, which is much higher than the intrinsic nonlinear polarizability of the optical fiber. Moreover, the long-range ordered structure of the gallium selenide crystal ensures the continuous phase interference of the second harmonics, giving full play to the advantage of the large nonlinear action length in the micro-nano fiber. More importantly, the phase matching between the pumping optical base mode (HE11) and the second harmonic high order mode (EH11, HE31) is realized by controlling the cone diameter and then regulating the waveguide dispersion during the preparation of micro-nano fiber.

The above conditions lay the foundation for the efficient and wide-band excitation of second harmonics in micro-nano fiber. The experiment shows that the output of second harmonics at the nanowatt level can be achieved under the 1550 nm picosecond pulse laser pump, and the second harmonics can also be excited efficiently under the continuous laser pump of the same wavelength, and the threshold power is as low as several hundred microwatts (Figure 1). Further, when the pump light is extended to three different wavelengths of continuous laser (1270/1550/1590 nm), three second harmonics (2w1, 2w2, 2w3) and three sum frequency signals (w1+w2, w1+w3, w2+w3) are observed at each of the six frequency conversion wavelengths. By replacing the pump light with an ultra-radiant light-emitting diode (SLED) light source with a bandwidth of 79.3 nm, a wide-spectrum second harmonic with a bandwidth of 28.3 nm is generated (Figure 2). In addition, if chemical vapor deposition technology can be used to replace the dry transfer technology in this study, and fewer layers of gallium selenide crystals can be grown on the surface of micro-nano fiber over long distances, the second harmonic conversion efficiency is expected to be further improved.