A research team led by Professor Cho Yong-hoon of Department of Physics of KAIST announced on the 29th that it has succeeded in developing a technology that increases the single photons purity of a single quantum dot formed at the vertex of the structure of the semiconductor pyramid using focused ion beams.

The technology developed through this research is expected to be used not only in quantum light sources formed by accurately controlling the location such as the vertex of the pyramid but also in various quantum photon devices such as high-density quantum dot-based quantum light source and electricity-driven quantum dot-based quantum light source.

A quantum light source is a light source that releases only one photon without emitting two or more photons at the same time and can be used for quantum communication safe from hacking in that copying single quantum information is impossible under the no-cloning theorem of quantum mechanics. In particular semiconductor-based quantum dots are widely studied as practical quantum light sources as they can be integrated on chips and electrically driven.

However at semiconductor quantum dot-based quantum light sources a background signal generated from the structures around the quantum dot coexists which weakens the property of quantum light causing the possibility for the quantum light to be hacked. Therefore in order to use the semiconductor quantum dot as a practical quantum light source it is important to reduce the background noise and increase the signal-to-noise ratio of quantum light.

Previous studies have used methods such as removing background noise signals with etching or blocking them with metal to improve the signal-to-noise ratio by reducing background noise around quantum light sources. However these methods had the disadvantage of reducing the quantum light signal of the quantum dot or destroying the structure around the quantum dot.

Professor Cho Yong-hoons research team developed a technology that can effectively remove only the background noise signal without weakening the quantum light signal without physically destroying the structure around the quantum dot using a focused ion beam and applied it to quantum dots elaborately formed at the vertex of the semiconductor pyramid structure.

A focused ion beam refers to a deposition using the Gas Injection System (GIS) installed in the process of forming or milling images by detecting electrons and ions after projecting gallium ions accelerated by tens of KeV to a sample. The structure of a focused ion beam device can be largely divided into three parts ion sources parts for beam alignment and focus and parts for sample treatment.

Focused ion beam technology has been widely used in the production of nano structures through etching or of samples for imaging testing in the fields of semiconductor technology and biology. However when it is used to produce a light emitting semiconductor optical component or an optical integrated circuit there was a problem to create a defect structure to the surrounding area much wider than the area where the ion beam was hit greatly weakening the desired light emitting signal. However Professor Cho Yong-hoons research team concentrated on that the precise adjustment of the types and conditions of focused ion beams can selectively let luminescence quenching happens to only the background noise signal to the spatial resolution of nanoscale without destroying the semiconductor structure.

Using this it succeeded in quenching the luminescence of the background noise signal around the quantum dot at the vertex of the semiconductor pyramid structure to nanoscale and the purity of single photons an indicator of how close the resulting light emitting signal is to quantum light was greatly improved.

Professor Cho Yong-hoon who led the research said We have developed a high-resolution technique that can selectively quench unwanted background noise signals using focused ion beams which will be the underlying technology that can be applied to various quantum optical components optical integrated circuits and displays.

The worlds focused ion beam (FIB) market is expected to expand from 800 million dollars in 2019 to 1.1 billion dollars by 2024 with an average annual growth rate of 7.6% from 2019 to 2024.

This research where Dr. Choi Min-ho and Jeon Sung-moon a Ph.D. at KAIST participated as a co-first author was conducted with the support of Samsung Future Technology Fostering Project and Mid-Career Researcher Program of the National Research Foundation of Korea and was officially published on July 27 in ACS Nano a world-renowned journal in the field of nano science. (Thesis title: Nanoscale focus pinspot for high purity quantum emitters via focused-ion-beam induced luminescence quenching)