Physicists Design Ultrafocused Pulses

Ultrafocus generation using reflections of the electromagnetic waves on the inside of a cylindrical waveguide. (Source: IQOQI)

Microwaves, heat radiation, light and X-radia­tion are examples for electro­magnetic waves. Many appli­cations require to focus the electro­magnetic fields to small spatio­temporal dimensions. Engineers may use different methods to achieve this. Researchers in Oriol Romero-Isart’s group at the Institute of Quantum Optics and Quantum Infor­mation IQOQI and the Insti­tute for Theo­retical Physics at the Uni­versity of Innsbruck together with Ignacio Cirac and Theodor Hänsch at the Max Planck Insti­tute of Quantum Optics, Munich, have reported about a new scheme for generating ultra­focused electro­magnetic fields.

When electric current flows through a coil, it produces electro­magnetic waves that propagate in all directions. When the coil is placed inside a cylinder that reflects the waves perfectly a sur­prising phenomenon happens. “With this setup arbi­trarily focused pulses and quasi-equi­distant pulses could be generated,” says junior scientist Patrick Maurer. “The more waveguide modes are excited, the more focused the electro­magnetic fields become.” The theo­retical physicists analy­tically charac­terized the system to such a degree that, based on the reflections of the electro­magnetic waves on the inside of the cylinder, they were able to design a current pulse that excites a clearly defined number of modes.

“Because of the specific proper­ties of the system, the current pulse needs to be adjusted only slightly in order to change the number of modes or, in other words, to focus the field more strongly. The average frequency of the pulse basi­cally remains the same,” explains Jordi Prat-Camps, Postdoc in Romero-Isart’s research team. The spectrum of the generated field is deter­mined by the radius of the cylinder. For example, focused microwave pulses can be generated by using a cylinder that is several centi­meters thick.

The physi­cists in Innsbruck were able to confirm their analytical calcu­lations with numerical simu­lations. They showed that the fields kept their unique pro­perties for some time after they exited the cylinder through one of the openings. This new concept is of interest for techno­logical appli­cations that require ultra­focused fields to work. For example, in the field of micro­scopy this new scheme could faci­litate the develop­ment of even more precise devices. To implement their scheme, the physicists point out two require­ments: “First, we need to find a material that per­fectly reflects in a broad frequency range,” says Prat-Camps. “In addition, we have to precisely generate the cal­culated current pulse. The better these require­ments are met, the clearer the visi­bility of the desired effect will be.” (Source: IQOQI)

Reference: P. Maurer et al.: Ultrafocused Electromagnetic Field Pulses with a Hollow Cylindrical Waveguide, Phys. Rev. Lett. 119, 043904 (2017); DOI: 10.1103/PhysRevLett.119.043904

Link: Quantum Nanophysics, Optics and Information, Institute of Quantum Optics and Quantum Information IQOQI, Innsbruck, Austria • Theory Division (I. Cirac), Max Planck Institute of Quantum Optics, Munich, Germany

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