Final Report Summary - PEQUPHOT (Plasmonically-enhanced Quantum Dot Photodetectors)
The detection of optical signals has widespread applications ranging from optical communication, remote sensing, up to digital imaging, resulting in a plethora of photodetector architectures that are each designed specifically for their application. A novel type of nanostructured photodetectors are lead sulfide (PbS) quantum dot photodetectors. These photodetectors have been demonstrated exhibiting a very high sensitivity to light and are promising candidates for short-wave infrared imaging. In general, there is a trend in enhancing the performance of photodetectors by minimizing their electrically active area, with the aim to reduce the noise current and increase their speed. However, the optically active area of these photodetectors needs to stay large enough to capture sufficient photons. One way to maintain a large optically active area of a photodetector, but reducing its electrically active area is employing plasmonic antennas. Plasmonic bow tie antennas, dipole antennas, or bull’s eye structures have been demonstrated to concentrate light at the nanoscale, yet capturing the light over a large area.
This project combined the two novel and challenging fields of research on PbS quantum dot photodetectors and plasmonic antennas to fabricate high sensitivity photodetectors. The project was divided into three main objectives: the design of the plasmonic structure, the fabrication of the plasmonic photodetector, and its characterization. Considering the diversity of the project, the fellow was working in a research team consisting of chemists that performed the quantum dot synthesis, a PhD student who assisted in and developed device fabrication steps supervised by the scientist in charge.
The plasmonic antenna was chosen to be a plasmonic bull’s eye structure that consists of periodically arranged concentric metal grooves that focus the light into its center. Plasmonic bull’s eye structures had been employed to enhance the transmission through a sub-wavelength hole and to direct the light that is transmitted through such a hole. The enhancement in the transmission is explained by light coupling to surface plasmon polaritons when incident on the concentric grooves. These surface plasmon polaritons interfere constructively in the hole in the center of the bull’s eye structure resulting in an enhanced transmission. The possibility to concentrate the light into the center of the bull’s eye structure has inspired their use for nanoscale photodetectors and had been employed to improve the performance of Si and Ge photodetectors. However, in these experiments the photocurrent has only been measured at resonance of the bull’s eye structure and the performance of the bull’s eye photodetector has only been compared to a similar structure either with different or without concentric grooves.
In the course of this project, the fellow has built plasmonic bull’s eye PbS quantum dot photoconductors. First, she characterized the plasmonic bull’s eye structures optically by performing transmission measurements through the structure revealing their resonance and revealing that the resonance can be shifted by varying the distance between the concentric grooves. In a next step, she has measured the photocurrent of the bull’s eye structure spectrally, revealing that the resonance of the bull’s eye structure indeed results into a concentration of light into the PbS quantum dots and increases resonantly the photocurrent. The plasmonic bull’s eye structure has been compared to two references, one reference that had the same design as the bull’s eye structure, but no concentric grooves, and one design where the PbS quantum dot film had the same diameter as the outer ring of the plasmonic bull’s eye structure. The fellow could demonstrate with these experiments that the plasmonic bull’s eye photodetector outperformed both references and so could demonstrate that by reducing the electrically active area of the photodetector but maintaining its optically active area, the performance of photodetectors can be increased.
The fellow’s skills in nanophotonics and optoelectronics were essential for the successful implementation of the project, however, the wide range of tasks from nanofabrication via nanophotonic simulations up to optoelectronic characterization still gave her the possibility to learn and improve on certain tasks. In the course of the project, the fellow could improve and expand her knowledge on nanofabrication by being trained on the focused ion beam and electron beam lithography systems, evaporators and sputtering machines, and reactive ion etching. Further, she got trained in the treatment of quantum dots for devices fabrication. She gained knowledge on nanophotonic simulations based on the finite-difference time domain technique and expand her knowledge on sub-pico Ampere electronic measurements.
This project combined the two novel and challenging fields of research on PbS quantum dot photodetectors and plasmonic antennas to fabricate high sensitivity photodetectors. The project was divided into three main objectives: the design of the plasmonic structure, the fabrication of the plasmonic photodetector, and its characterization. Considering the diversity of the project, the fellow was working in a research team consisting of chemists that performed the quantum dot synthesis, a PhD student who assisted in and developed device fabrication steps supervised by the scientist in charge.
The plasmonic antenna was chosen to be a plasmonic bull’s eye structure that consists of periodically arranged concentric metal grooves that focus the light into its center. Plasmonic bull’s eye structures had been employed to enhance the transmission through a sub-wavelength hole and to direct the light that is transmitted through such a hole. The enhancement in the transmission is explained by light coupling to surface plasmon polaritons when incident on the concentric grooves. These surface plasmon polaritons interfere constructively in the hole in the center of the bull’s eye structure resulting in an enhanced transmission. The possibility to concentrate the light into the center of the bull’s eye structure has inspired their use for nanoscale photodetectors and had been employed to improve the performance of Si and Ge photodetectors. However, in these experiments the photocurrent has only been measured at resonance of the bull’s eye structure and the performance of the bull’s eye photodetector has only been compared to a similar structure either with different or without concentric grooves.
In the course of this project, the fellow has built plasmonic bull’s eye PbS quantum dot photoconductors. First, she characterized the plasmonic bull’s eye structures optically by performing transmission measurements through the structure revealing their resonance and revealing that the resonance can be shifted by varying the distance between the concentric grooves. In a next step, she has measured the photocurrent of the bull’s eye structure spectrally, revealing that the resonance of the bull’s eye structure indeed results into a concentration of light into the PbS quantum dots and increases resonantly the photocurrent. The plasmonic bull’s eye structure has been compared to two references, one reference that had the same design as the bull’s eye structure, but no concentric grooves, and one design where the PbS quantum dot film had the same diameter as the outer ring of the plasmonic bull’s eye structure. The fellow could demonstrate with these experiments that the plasmonic bull’s eye photodetector outperformed both references and so could demonstrate that by reducing the electrically active area of the photodetector but maintaining its optically active area, the performance of photodetectors can be increased.
The fellow’s skills in nanophotonics and optoelectronics were essential for the successful implementation of the project, however, the wide range of tasks from nanofabrication via nanophotonic simulations up to optoelectronic characterization still gave her the possibility to learn and improve on certain tasks. In the course of the project, the fellow could improve and expand her knowledge on nanofabrication by being trained on the focused ion beam and electron beam lithography systems, evaporators and sputtering machines, and reactive ion etching. Further, she got trained in the treatment of quantum dots for devices fabrication. She gained knowledge on nanophotonic simulations based on the finite-difference time domain technique and expand her knowledge on sub-pico Ampere electronic measurements.