Professor Park Hong-gyu’s Group Develops Technology for Controlling the Position and Polarization of a Single-Photon Emitter
A local deformation was applied to an atomically thin two-dimensional material to generate a new type of single-photon emitter, the position and polarization of which are controlled.
The results were published online in Nano Letters, a renowned international journal in the field of nanotechnology.

▲ Professor Park Hong-Gyu from the Department of Physics, College of Science (left, corresponding author), Ph. D student So Jae-pil (right, first author)

Professor Park Hong-gyu’s group from the Department of Physics, College of Science, conducted a joint research project with Professor Nam Sung-woo from the University of Illinois at Urbana-Champaign (UIUC) and developed a new type of single-photon emitter which can emit a single photon at a time and can have its optical characteristics controlled. The research group successfully controlled the emission positions and polarization direction of single photons and increased the single-photon emission efficiency through coupling with a nano-optical cavity.
*Single-photon emitter: A quantum device that emits a single photon at a time in contrast to classic light sources from which a bunch of photos are emitted together.
*Polarization: A specific state of light in which light advances while vibrating in a specific direction.
* Nano-optical cavity: A nano-sized device that can confine light.  

The single-photon emitter is an essential basic component in quantum cryptography, such as quantum key distribution. In particular, a technology for controlling the properties of photons emitted from a single-photon emitter is necessary to commercialize quantum cryptography. Recently, many studies have been conducted to generate single photons by using the quantum confinement in atomically thin two-dimensional materials based on the naturally- or artificially-occurring defects. However, the emission efficiency is very low due to the randomness of the single-photon emission position and polarization direction.
*Quantum key distribution: The most representative system of quantum cryptography. In contrast to classic communication based on wavelength or amplitude, signals are carried in units of single photons. Eavesdropping based on the collection of some quantums is impossible in this system because single photons are used for communication.
*Two-dimensional materials: Semiconductor materials, like graphene, in which atoms of several nanometers are arranged in a single layer.

The research group realized a new single-photon emitter having controllable emission positions and polarization directions through causing a local deformation by applying a nano-structure with a nanogap to two-dimensional semiconductor materials of tungsten selenide (WSe2). The yield of the prepared single-photon emitter was 83%, and its purity was 0.01 or lower. The optical repetition rate was as high as 15 MHz. In particular, the polarization direction of the emitted single photons was controlled at 100% efficiency: The polarization direction was horizontal when the nanogap was smaller than 90 nm and vertical when the nanogap was 90 nm or larger. In addition, the prepared single-photon emitter was atomically thin and thus was easily coupled with a nano-optical cavity. The coupling of the single-photon emitter and the nano-optical cavity doubled the single-photon emission efficiency.
  
The significance of the results is that a new physical phenomenon was discovered that increased the fundamental understanding of quantum communication. Since the emission positions and polarization directions are controlled, the developed single-photon emitter, when coupled with a nano-optical cavity, may be used as an on-chip photon source in quantum cryptography. Because the technology is a groundbreaking process for future information security, the results are expected to create a huge ripple effect in military or economic aspects in countries with developed internet technology such as Korea. The study was supported by the Korea-US Air Forces International Joint Research Project of the Information Protection Core and Source Technology Development Program and the Quantum Cryptography Communication Integration and Transfer Technology Advancement Program promoted by the Institute for Information & Communications Technology Promotion (IITP). The results of the study were published online in Nano Letters, a renowned international journal in the field of nanotechnology, on January 27.
-Title of Article: “Polarization Control of Deterministic Single-Photon Emitters in Monolayer WSe2”
-Author Details: Professor Park Hong-Gyu (corresponding author, Korea University), Professor Nam Sung-woo (co-corresponding author, UIUC), Ph. D student So Jae-pil (first author, Korea University), Dr. Jeong Kwang-yong (first co-author, Korea University) and Dr. Lee Jung-min (first co-author, Korea University).


[ Figure Description ]

            

(Figure 1) A single-photon emitter made of a two-dimensional material of which the emission position and polarization direction are simultaneously controlled by local strain deformation.
(Left) A schematic of the principle of controlling emission position and polarization direction of the single-photon emitter. (Right) Arrangement of single-photon emitters measured by using a confocal laser scanning microscope. The polarization direction, either horizontal or vertical, is determined by the width of the nanogap.

(Figure 2) The single-photon emitter coupled with a photonic crystal nano-optical cavity.
(A) A schematic of the single-photon emitter coupled with a one-dimensional photonic crystal nano-optical cavity. (B) An SEM image of a single-photon emitter fabricated by using a nanogap coupled with a photonic crystal nano-optical cavity. (C) Time-resolved photoluminescence measurements. The single-photon emission efficiency was doubled by the nano-optical cavity.