E-Mail IMAGE: The SH light produced in the MoS2 double layer has the same phase as the MoS2 layer so the overlaid secondary harmonic wave has linear polarization. However, in a heterobilayer... view more Credit: POSTECH Since the invention of world's first laser - the ruby laser - in 1960, the human desire to control light has spread to various industries, including telecommunications, medicine, GPS, optical sensors and optical computers. Recently, a POSTECH research team has taken a step closer to its goal of controlling light by identifying nonlinear optical phenomena occurring in heterobilayers composed of two-dimensional materials. A nonlinear optical phenomenon refers to the occurrence of light whose intensity is not doubled when optical input intensity becomes doubled, in which the resulting output has different frequencies from original input. This phenomenon is easily understood if you think of electrons and nuclei as spring-connected oscillators. When the spring is moved at a constant cycle, light is generated by the oscillation of electrons and nuclei. If the spring-pulling force is small, only light with the same frequency as the applied external force is formed, but when a strong force is exerted, light with multiple frequency is produced. Among these, light with twice the input frequency is called second-harmonic generation (SHG). The secondary harmonic wave phenomenon can occur in substances that are not point-symmetric, and it has recently been discovered that efficiency is high in 2D semiconductor crystals such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2).