Introduction

The field of superconductor-based photonics has seen significant advancements in recent years, pushing the boundaries of what is possible with traditional materials. This area explores the unique optical and electronic properties of superconductors, enabling applications that were once considered science fiction.

Unique Properties of Superconductors

The interesting optical properties of superconductors arise from their unusual electronic structure. Unlike conventional materials, superconductors have a small gap in their electron density of states—a superconducting gap that appears below the critical temperature (Tc). This gap reflects the energy required to break Cooper pairs, and its energy can vary between 1 meV and 80 meV, depending on the type of superconductor.

A superconducting material is transparent to light with energy smaller than this superconducting gap, with a notable exception for high-temperature superconductors, whose gap energy can vary with direction. The diagram below illustrates the difference in dispersion relations between non-superconducting and superconducting states:

Superconducting Dispersion Diagram

Red dashed line represents the non-superconducting dispersion. Blue/Black and Green/Black lines represent the band dispersions in the superconducting state, showing the effect of the superconducting gap.

This property forms the basis for superconducting nanowire photodetectors (SNSPDs), which can detect even single photons, making them invaluable for applications such as ultra-sensitive imaging and quantum computing.

Superconducting Nanowire Photodetectors (SNSPDs)

SNSPDs operate by cooling a superconducting nanowire below its critical temperature and applying a current just below the critical threshold. When photons interact with the nanowire, they can break Cooper pairs locally, inducing a resistive state. This effect is especially useful for detecting THz photons, as demonstrated in the work of Dienst et al. (2011).

One of the advantages of SNSPDs is their ability to detect individual photons with high sensitivity. The nanowire geometry is designed to maximize the detection area, often patterned into a meandering structure to enhance the local effect of the absorbed photons. The sharp transition between superconducting and resistive states allows these detectors to be used in applications requiring ultra-high sensitivity and speed.

Transition-Edge Sensors (TES)

Another application involving superconductors in photonics is the transition-edge sensor (TES). TES devices operate at the edge of superconductivity, where they can transition from a superconducting state to a resistive state with minimal energy input. Incident photons can induce this transition, making TESs useful in non-destructive detection of particles and radiation.

Unusual Applications of High-Temperature Superconductors

The unique electronic structure of high-temperature superconductors has led to several innovative applications in the field of superconducting photonics. For example, the quasi-2D nature of cuprate superconductors allows them to be used as THz emitters. These materials can be modeled as a stack of Josephson junctions, which can be tuned to emit THz radiation through resonant cavity effects.

The Josephson effect, a property of superconductors, involves the tunneling of Cooper pairs across an insulating barrier. When applied to cuprate superconductors, this effect can be exploited to produce narrow-bandwidth THz emitters. As shown in Welp et al. (2013), a stack of Josephson junctions can be tuned to emit THz radiation when the plasma oscillation frequency matches the cavity resonant frequency. This can be achieved by applying a bias voltage or changing the temperature.

An alternative approach involves driving this layered structure with a strong THz field of short duration, which can affect the Josephson coupling between superconducting layers. When the coupling is reduced below a threshold, the superconductor transitions from a zero-resistance state to a finite resistance state, potentially enabling ultrafast nanoelectronics. This was demonstrated in the work of Dienst et al. (2011).

Conclusion

The advancements in superconductor-based photonics continue to push the frontiers of what is possible with traditional materials. From superconducting nanowire photodetectors to transition-edge sensors and THz emitters, the unique properties of superconductors enable a wide range of applications that were once thought impossible. As research in this field advances, we can expect to see even more innovative uses of superconductors in photonic devices.

References

- Dienst, F., et al. (2011). Tuning the Josephson coupling in the insulating state of high-temperature superconductors with terahertz radiation. Nature Photonics.
- Welp, U., et al. (2013). Tunable THz emitters based on Josephson junctions. Nature Photonics.