Researchers Develop Extremely Small Nanocavity Capable of Revolutionizing Quantum Encryption

This stylized rendering of the cross-section of the metallic hybrid nanocavity coupled to a quantum dot shows vertical emission into free space. Credit: Yousif Kelaita, Stanford University

An innovative light-enhancing optical cavity with a height of just 200 nm and a width of just 100 nm has been developed by researchers. Such an innovative nanoscale system symbolizes an initiative toward brighter single-photon sources that can help in propelling quantum-based encryption methods under development.

In quantum encryption methods, perceived to be highly significant for future data encryption methods, individual photons are used as a highly secure way for encoding data. However, a major drawback of these methods is their inability to emit photons at higher rates.

One of the most important figures of merit for single-photon sources is brightness—or collected photons per second—because the brighter it is, the more data you can transmit securely with quantum encryption.

Yousif Kelaita, Stanford University

Kelaita and his collaborators have described their innovative nanocavity in the journal Optical Materials Express published by The Optical Society (OSA). They demonstrated that their nanocavity considerably enhanced the emission brightness of quantum dots, i.e. nanometer-scale semiconductor particles with the ability to release single photons.

The innovative nanocavity was developed by coating the sides of a nanoscale semiconductor pillar - positioned on a substrate - with highly reflective silver. The silver coating causes the light to rebound around in the inner side the nanopillar, altering it into a very small optical cavity. According to the researchers, the same design concept can be employed to construct nanocavities from different materials customized for different single-photon emitters.

Trapping light in a small space

At nanometer levels, the interaction of light with materials occurs in unique ways. One significant example is the Purcell effect that improves the emission efficiency of a quantum dot or any other light emitter enclosed in a small cavity. Systems that demonstrate Purcell enhancement release more photons in a given period of time, enabling quantum encryption systems to operate faster than usual.

Purcell enhancement can be largely achieved using extremely small cavities because transfer of energy between the cavity and the light emitter occurs rapidly. Incorporating a sufficiently high quality factor has also been known to be advantageous, meaning that the cavity’s reflection enables the light get rebounded around for a longer period of time.

We demonstrated a new type of cavity with a volume multiple orders of magnitude lower than the current state of the art in solid-state systems. The system produces strong Purcell enhancement and high light collection efficiency at the same time, which leads to an overall increase in the brightness of the single-photon source.

Yousif Kelaita, Stanford University

Upon testing the new nanocavities, the researchers discovered that compared to quantum dots not positioned inside a cavity, the quantum dots positioned inside the nanocavities released more photons per second.

The open top of the nanocavities enables the emitted light to travel directly into air. Similar nanocavities developed earlier were topped with a metal coating that was imprudent for gathering the emitted photons. The emission profile of the new nanocavities was also observed to be well matched with standard microscope objective lenses, enabling a high proportion of light to enter the lens.

In the earlier nanocavity systems, problematic light loss occurred due to the mismatch between the emission profile and microscope objective lenses.

Making the tiny cavity

The research team applied a modified fabrication method to deal with the problems that occur while coating the nanopillars with metal. The fact that the nanofabrication methods employ a technique through which metal falls straight down onto the device similar to snowfall renders tall and skinny nanostructures to undergo shadowing effects.

If you imagine snow falling on a tree, the snow will cling to itself and pile up on a branch in a way that it forms a larger width, or mound, than the branch itself. This also happens as metal is deposited on top of something like a pillar. As the metal clings to itself, it creates a larger mound than the pillar underneath it, preventing metal from falling underneath the parts that eclipse the pillar. In the end, this shadowing effect creates an air gap in the device.

Yousif Kelaita, Stanford University

In order to overcome this challenge, the scientists simultaneously tilted and rotated the sample to coat all sides of the pillar at the same time. Despite their new approach, to prevent the formation of a connection between the metal on top and the metal coated on the sides of the pillar, the researchers had to be exercise caution with respect to the angle at which the metal was deposited.

This is because if there is a connection formed, then the final stage of ultrasonic removal of the metal cap on top would prove to be challenging or impossible.

Other groups working with metal should be interested in this technique because this shadowing effect occurs even for features that are completely encapsulated in metal,” stated Kelaita.

Even better nanocavities

Currently the research team is working to develop nanocavities of other kinds and with even better characteristics. For instance, they aim to create nanocavities in diamond, which would enable single-photon sources to operate at room temperature, a highly necessary condition for applying quantum encryption to consumer devices.

They are also aiming to integrate the knowledge achieved through their new study with an inverse design algorithm that they have recently created to automatically design photonic devices integrated with silicon chips. Using the algorithm, the engineers designate a desired function, and the instructions for creating a structure performing the designated function are provided by the software.

This new project taught us the merit of aiming for ultra-small volumes. Now we’re trying to leverage that knowledge with experience we have in using computers to design cavities to hopefully create a new type of cavity with even better metrics and figures of merit that push the field ahead even more.

Yousif Kelaita, Stanford University

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