Fig. 1
Field-induced second-order nonlinear coefficient (d11) in a CdS nanobelt. a Schematic of a CdS nanobelt device with the source (S) and drain (D) electrodes. The long-axis of the nanobelt (x-axis) is determined to be CdS a-axis, which is perpendicular to the z-axis (or CdS c-axis). The fundamental wave at the frequency of ω is normally incident upon the belt and excites the second-harmonic wave at 2ω which is back-scattered. b Schematic of the response of the electron wave function to a strong d.c. applied field. An atom with a positively charged nucleus (red dot) is surrounded by electron cloud (blue dots). If there is no external electric field, the electron wave function is symmetric along the x-axis, which can be distorted by an external electric field, thus turning on certain field-induced second-order nonlinear coefficients. c Voltage-dependence of d11-SHG signal. Inset: device image when the fundamental wave (FW) is incident upon the CdS nanobelt near the source electrode (S). The S-electrode is grounded (0 V) and the drain electrode (D) is biased. When applying a positive bias (VDS > 0), the electric current is along the x-axis (from D-electrode to S-electrode), with the reverse bias on the S-contact (VDS > 0). d Normalized conversion efficiency (\(\eta _{2\omega } = P_{2\omega }/P_\omega ^2\)) of second-harmonic generation (SHG) resulting from different nonlinear coefficients (d11 and d33) at different applied voltages (VDS), 0 and 60 V. In order to detect the d11-SHG signal, we focused the FW polarized along the x-axis (Iω,x) and detected the SHG component polarized along the x-axis (I2ω,x) by adjusting the polarizer in front of the detector. Similarly for d33-SHG, we employ Iω,z to detect I2ω,z
