Filling the gap of laser sources at the sub-millimeter wavelengths

There is a range of wavelength in the electromagnetic spectrum that is hard to exploit, although it would certainly bring enormous advantages in many fields of application if we were able to build a suitable source. That is known as the THz Gap (The Terahertz Gap), and researchers at Stockholm University seem to have finally found a way around it.

Tunable high intensity THz sources (1 Tera Hertz= 1012 Hertz) are demanded for a large variety of applications ranging from medical imaging, security, industrial and environmental control, telecommunication, as well as for scientific research in condensed matter physics, chemistry, astronomy, biology, etc. Indeed, the majority of important molecular transitions (e.g. water in living cells or carbon dioxide in atmosphere) occur in this frequency range. THz sources would be particularly beneficial for medical and biological studies: radiation at the sub-millimeter wavelength (such as that of THz sources) can penetrate into a wide variety of materials, including body tissues, and therefore could provide an alternative way to X-rays for performing medical diagnostics, as well as security scans, with no secondary effects, since it is a non-ionizing radiation.

The THz gap

At present, microwaves with the wavelength above 1 mm are conveniently generated by semiconducting electronics, while radiation with the wavelength shorter than 100μm is provided by infrared lasers. However, creation of a compact, high power, continuous wave THz lasers in the 0.3-3 THz frequency range remains a difficult technological challenge, often referred to as “the THz gap”.

The work by E.A. Borodianskyi and V.M. Krasnov from the Experimental Condensed Matter Physics group at Fysikum, published recently in Nature Communication, represents a major breakthrough in enhancement of the emission frequency from superconducting oscillators. The authors have demonstrated the first convincing evidence for high-frequency THz emission from small Bi-2212 mesas (layered curate Bi2Sr2CaCu2O8+d)

Superconducting Bi-2212 oscillators

About a decade ago it was demonstrated that large Bi-2212 mesa structures could emit tunable THz radiation. This discovery has boosted research activity towards realization of superconducting sources of coherent THz radiation and gave a hope that this new approach could lead to creation of a novel type of a tunable continuous wave THz laser with a significant emission power. Single crystals of Bi-2212 represent natural stacks of atomic-scale “intrinsic” Josephson junctions. Due to atomic size of intrinsic junctions, a micron-high mesa contains about seven hundred junctions. Mutual phase-locking of so many junctions leads to coherent superradiant enhancement of the emission power, proportional to the square of the number of synchronized junctions (7002=4.9×105). . However, despite the initial success, it has been very difficult to generate frequencies in excess of a THz. The reason is that although large mesas can generate high emission powers, this is accompanied by the proportional increase of the dissipation power, which eventually overheats the device above the superconducting critical temperature at high voltage bias. This prevents reaching high voltages and frequencies. It has long been anticipated that small mesas should have many advantages as THz oscillators for variety of reasons, including small self-heating, which decreases proportionally to the mesa size. Almost all research groups working in this area has tried to make small area Bi-2212 emitters, but failed. The reason of the problem remained a puzzle until now.

The solution: a tunable source

The main key was to use small-but-high mesas (with a small area, but containing many stacked junctions). This way it became possible to achieve tunable THz emission with a good efficacy in an extremely broad and record high frequency range 1-11 THz approaching a theoretical limit for Josephson oscillators set up by the superconducting energy gap. The emission maxima correspond to in-phase cavity modes in the mesa, indicating coherent superradiant nature of the emission. The authors conclude that THz emission requires a threshold number of junctions N>100, suggesting the importance of laser-like cascade amplification of the photon number in the cavity. This also explains the failure of previous attempts, in which mesas with less than 100 junctions were used.

Figure: Spectra of emitted electromagnetic waves by one of the Bi-2212 mesa structures. Vertical lines on top indicate frequencies of cavity modes in the mesa. Inset represents a sketch of the studied sample. It consists of several mesa structures etched from Bi-2212 single crystal with attached metallic electrodes on top.

The motivation of the work is to develop a novel type of compact, continuous-wave, narrow line-width THz lasers, which would also be tunable in a broad frequency range. This is a difficult technological problem, colloquially known as a “Terahertz gap”. Although a significant progress in this direction is achieved by semiconducting quantum cascade lasers, they are generally not tunable. Using superconductors instead of semiconductors, provides an alternative technology for creation of inherently highly tunable continuous-wave, narrow line-width, compact THz sources. – It is remarkable that a single superconducting oscillator is tunable in the full THz frequency range 1-10 THz at the primary oscillation frequency, says Vladimir Krasnov.

Contact information: Vladimir Krasnov (vladimir.krasnov@fysik.su.se)

Read the article: E. A. Borodianskyi and V.M. Krasnov, Nature Communications 8, 1742 (2017)DOI: 10.1038/s41467-017-01888-4

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