In our information society, the synthesis, distribution, and processing of radio and microwave signals are ubiquitous in wireless networks, telecommunications, and radars. The current tendency is to use carriers in higher frequency bands, especially with looming bandwidth bottlenecks due to demands for, for example, 5G and the “Internet of Things.” ‘Microwave photonics,’ a combination of microwave engineering and optoelectronics, might offer a solution.

A key building block of microwave photonics is optical frequency combs, which provide hundreds of equidistant and mutually coherent laser lines. They are ultrashort optical pulses emitted with a stable repetition rate that corresponds precisely to the frequency spacing of comb lines. The photodetection of the pulses produces a microwave carrier.

In recent years there has been significant progress on chip-scale frequency combs generated from nonlinear microresonators driven by continuous-wave lasers. These frequency combs rely on the formation of dissipative Kerr solitons, which are ultrashort coherent light pulses circulating inside optical microresonators. Because of this, these frequency combs are commonly called ‘soliton microcombs.’

Generating soliton microcombs requires nonlinear microresonators, and these can be directly built on-chip using CMOS nanofabrication technology. Co-integration with electronic circuitry and integrated lasers paves the way to comb miniaturization, allowing a host of applications in metrology, spectroscopy and communications.

Publishing in Nature Photonics, an EPFL research team led by Tobias J. Kippenberg has now demonstrated integrated soliton microcombs with repetition rates as low as 10 GHz. This was achieved by significantly lowering the optical losses of integrated photonic waveguides based on silicon nitride, a material already used in CMOS micro-electronic circuits, and which has also been used in the last decade to build photonic integrated circuits that guide laser light on-chip.

The scientists were able to manufacture silicon nitride waveguides with the lowest loss in any photonic integrated circuit. Using this technology, the generated coherent soliton pulses have repetition rates in both the microwave K- (~20 GHz, used in 5G) and X-band (~10 GHz, used in radars).

The resulting microwave signals feature phase noise properties on par with or even lower than commercial electronic microwave synthesizers. The demonstration of integrated soliton microcombs at microwave repetition rates bridges the fields of integrated photonics, nonlinear optics and microwave photonics.

The EPFL team achieved a level of optical losses low enough to allow light to propagate nearly 1 meter in a waveguide that is only 1 micrometer in diameter, or about 100 times smaller than a human hair. This loss level is still more than three orders of magnitude higher than the value in optical fibers, but represents the lowest loss in any tightly confining waveguide for integrated nonlinear photonics to date.

Such low loss is the result of a new manufacturing process developed by EPFL scientists—the ‘silicon nitride photonic Damascene process.’ “This process, when carried out using deep-ultraviolet stepper lithography, gives truly spectacular performance in terms of low loss, which is not attainable using conventional nanofabrication techniques,” says Junqiu Liu, the paper’s first author who also leads the fabrication of silicon nitride nanophotonic chips at EPFL’s Center of MicroNanoTechnology (CMi). “These microcombs, and their microwave signals, could be critical elements for building fully integrated low-noise microwave oscillators for future architectures of radars and information networks.”

The EPFL team is already working with collaborators in the US to develop hybrid-integrated soliton microcomb modules that combine chip-scale semiconductor lasers. These highly compact microcombs can impact many applications—e.g. transceivers in datacenters, LiDAR, compact optical atomic clocks, optical coherence tomography, microwave photonics, and spectroscopy.

An ultra-precise microscope that surpasses the limitations of Nobel Prize-winning super-resolution microscopy will let scientists directly measure distances between individual molecules.

UNSW medical researchers have achieved unprecedented resolution capabilities in single-molecule microscopy to detect interactions between individual molecules within intact cells.

The 2014 Nobel Prize in Chemistry was awarded for the development of super-resolution fluorescence microscopy technology that afforded microscopists the first molecular view inside cells, a capability that has provided new molecular perspectives on complex biological systems and processes.

Now the limit of detection of single-molecule microscopes has been smashed again, and the details are published in the current issue of Science Advances.

While individual molecules could be observed and tracked with super-resolution microscopy already, interactions between these molecules occur at a scale at least four times smaller than that resolved by existing single-molecule microscopes.

“The reason why the localisation precision of single-molecule microscopes is around 20-30 nanometres normally is because the microscope actually moves while we’re detecting that signal. This leads to an uncertainty. With the existing super-resolution instruments, we can’t tell whether or not one protein is bound to another protein because the distance between them is shorter than the uncertainty of their positions,” says Scientia Professor Katharina Gaus, research team leader and Head of UNSW Medicine’s EMBL Australia Node in Single Molecule Science.

To circumvent this problem, the team built autonomous feedback loops inside a single-molecule microscope that detects and re-aligns the optical path and stage.

“It doesn’t matter what you do to this microscope, it basically finds its way back with precision under a nanometre. It’s a smart microscope. It does all the things that an operator or a service engineer needs to do, and it does that 12 times per second,” says Professor Gaus.

Measuring the distance between proteins

With the design and methods outlined in the paper, the feedback system designed by the UNSW team is compatible with existing microscopes and affords maximum flexibility for sample preparation.

“It’s a really simple and elegant solution to a major imaging problem. We just built a microscope within a microscope, and all it does is align the main microscope. That the solution we found is simple and practical is a real strength as it would allow easy cloning of the system, and rapid uptake of the new technology,” says Professor Gaus.

To demonstrate the utility of their ultra-precise feedback single-molecule microscope, the researchers used it to perform direct distance measurements between signalling proteins in T cells. A popular hypothesis in cellular immunology is that these immune cells remain in a resting state when the T cell receptor is next to another molecule that acts as a brake.

Their high precision microscope was able to show that these two signalling molecules are in fact further separated from each other in activated T cells, releasing the brake and switching on T cell receptor signalling.

“Conventional microscopy techniques would not be able to accurately measure such a small change as the distance between these signalling molecules in resting T cells and in activated T cells only differed by 4–7 nanometres,” says Professor Gaus.

“This also shows how sensitive these signalling machineries are to spatial segregation. In order to identify regulatory processes like these, we need to perform precise distance measurements, and that is what this microscope enables. These results illustrate the potential of this technology for discoveries that could not be made by any other means.”

Postdoctoral researcher, Dr. Simao Pereira Coelho, together with Ph.D. student Jongho Baek—who has since been awarded his Ph.D. degree—led the design, development, and building of this system. Dr. Baek also received the Dean’s Award for Outstanding Ph.D. Thesis for this work.