A novel approach to design and fabricate thin-film infrared light sources with near-arbitrary spectral output has been developed by engineers from Vanderbilt and Penn State Universities, and could transform molecular sensing technologies.
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The method is driven by heat and employs inverse design, a machine learning methodology, to significantly reduce the time taken to optimize these devices, from several weeks or months using a multi-core computer, to just a few minutes on a consumer desktop.
These inexpensive, efficient, designer infrared light sources could find use in infrared beacons for search and rescue, molecular sensors for monitoring industrial gases, environmental pollutants and toxins, or in free-space communications, an optical communication technology that utilizes light proliferating in free space to transmit data for telecommunications or computer networking wirelessly.
Novel methods
There is a lack of cost-effective narrow-band sources in the mid-to-long wave infrared range. Standard thermal emitters can produce broadband thermal radiation, i.e., thermal radiation that is released over a wide range of wavelengths and over a wide range of angles. However, this restricts their use to simple applications, such as incandescent light bulbs.
In contrast, lasers and light-emitting diodes (LEDs) emit narrow frequency emissions, which make them ideal for numerous applications, but also means they can be inefficient and expensive.
The gap can be filled by wavelength-selective thermal emitters (WS-EMs) which combine the narrow bandwidth of a laser or LED with the simple design of a thermal emitter. However, such thermal emitters with user-defined output spectra require patterned nanostructures and are created with high-cost, low-throughput methods.
Researchers led by Joshua Caldwell, associate professor of mechanical engineering at Vanderbilt, and Jon-Paul Maria, professor of materials science and engineering at Penn State, set out to overcome such challenges while also creating a more effective process.
Their technique uses simple thin-film deposition, an established nano-fabrication technique that applies a very thin film of material, measuring between a few nanometers to around 100 micrometers, onto a substrate or surface to be coated. The technique is aided by key advances in materials and machine learning.
The approach takes advantage of the broad spectral tunability of the semiconductor cadmium oxide (CdO), together with a one-dimensional photonic crystal fabricated with alternating layers of dielectrics, known as a distributed Bragg reflector. These alternating multi-layers cause a so-called “Tamm-polariton,” where the emission wavelength of the device is established by the interactions between these layers.
Such designs were previously limited to a single-designed wavelength output, as designing multiple resonances was much too complex. However, creating multiple resonances at multiple frequencies with user-controlled wavelength, linewidth, and intensity is necessary for matching the absorption spectra of most molecules.
Material Design
Determining the optimal material design was challenging and computationally intense. Since advanced applications require functionality across multiple resonances, the new process was required to dramatically shorten the design time, from several days or months to just minutes.
An inverse design algorithm that calculates an optimized structure within minutes on a consumer-grade desktop was proposed by Mingze He, a PhD student at Vanderbilt, and lead author of the paper published in Nature Materials. His algorithm delivered the ability to match the chosen emission wavelength, linewidth, and amplitude of multiple resonances simultaneously over an arbitrary spectral bandwidth.
Finding a semiconductor material that allowed a large dynamic range of electron densities was also challenging, but the team utilized a doped semiconductor material – cadmium oxide - that allows the intentional design of optical properties.
“This allows the fabrication of advanced mid-infrared light sources at wafer-scale with very low cost and minimal fabrication steps,” said Maria, whose team developed the material at Penn State.
Experimental Results
Experiments were conducted at Penn State, while the devices were categorized by He and J. Ryan Nolen, a graduate of Caldwell’s group. The teams successfully demonstrated the capability of inversely designed infrared light sources with single or multiple emission bands with designable frequencies, line widths and amplitudes.
“The combination of the cadmium oxide material tunability with the fast optimization of aperiodic distributed Bragg reflectors offers the potential to design infrared light sources with user-defined output spectra,” said Caldwell. “While these have immediate potential in chemical sensing, these also exhibit significant promise in a variety of other applications ranging for environmental and remoted sensing, spectroscopy, and infrared signaling and communications.”
The work has enabled the development of a lithography-free, wafer-scale wavelength-selective thermal emitter that is complementary metal-oxide-semiconductor compatible, say the researchers.
The Caldwell group has made the design algorithm open source and is available from Nature Materials website.
References and Further Reading
Vanderbilt School of Engineering (2021) Novel advanced light design and fabrication process could revolutionize sensing technologies, Vanderbilt School of Engineering [Online] Available at: https://engineering.vanderbilt.edu/news/2021/novel-advanced-light-design-and-fabrication-process-could-revolutionize-sensing-technologies/
Mingze, H. et al. (2021) Deterministic inverse design of Tamm plasmon thermal emitters with multi-resonant control, Nature Materials https://www.nature.com/articles/s41563-021-01094-0
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