Credit:
Pohang University of Science & Technology (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 the 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
frequencies is produced. Among these, light with twice the input frequency is
known as 'second-harmonic generation' (SHG) light. 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 2-D semiconductor crystals
such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2).
A research
team led by Professor Sunmin Ryu and Wontaek Kim in the MS/Ph.D integrated
program in the Department of Chemistry at POSTECH noted that the secondary
harmonic wave produced by a heterobilayer material (MoS2/WS2) could not be
explained by the existing model, and confirmed that it was caused by the SHG
interference with different phases. The team anticipated the phase difference
in SHG through the results of polarizing spectroscopy of heterolayers that
showed the elliptically polarized SHG light. The phase difference directly
measured through the secondary harmonic wave interferometer was quantitatively
consistent with the results obtained from polarizing spectroscopy, proving
their hypothesis. In addition, DFT calculations were able to support these
results.
So far SHG
studies of 2-D materials have mostly been limited to their intensity, but this
is the first time that the SHG phase was measured and it was shown that there
is a difference in SHG phase between the two materials. The research showed the
possibility of controlling an SHG's phase.
"The
conventional research was biased toward identifying the orientation of 2-D
crystal samples using SHG intensity and controlling it through external
stimuli," remarked Professor Sunmin Ryu who led the study. He added,
"This study not only broadened our understanding of nonlinear optical
phenomena of 2-D materials, but also opened new possibilities for nonlinear spectroscopic
control methods." He concluded, "The research results are expected to
greatly contribute to the control of nonlinear optical phenomena by using 2-D
materials to produce new photons with twice the frequency of vibration and
controlled phase."