Polaritons are hybrid quasiparticles that result from the strong coupling of light and matter. They combine the properties of photons (light particles) and phonons (sound particles) or other material excitations, such as electrons or molecules. Polaritons can carry energy and information in novel ways, enabling new possibilities for nanophotonics, sensing, signal processing, energy harvesting, and other technologies.

What are polaritons and why are they important?

Polaritons are not physical particles that can be seen or captured. They are more like ways of describing energy exchange as if they were particles. These can be created in different types of materials and structures, such as metals, dielectrics, semiconductors, superconductors, van der Waals materials, and metamaterials. Depending on the material and the frequency of light, polaritons can have different names, such as surface plasmon polaritons, phonon polaritons, exciton-polaritons, magnon polaritons, and so on.

Polaritons have several advantages over photons or phonons alone. For example:

• Subwavelength confinement: They have a shorter wavelength than light in a vacuum at the same frequency. This means that they can have a higher momentum and local field intensity, as well as subwavelength-scale confinement. This allows polaritons to overcome the diffraction limit and manipulate light at the nanoscale.
• Tunable dispersion: They have a tunable dispersion relation, which means that their speed and direction can be controlled by changing the material parameters or the external stimuli. This enables polaritons to perform functions such as switching, modulation, routing, filtering, and detection of light signals.
• Strong nonlinearity: Polaritons have a strong nonlinearity, which means that their response can be changed by the intensity of light. This allows polaritons to generate new frequencies of light, such as harmonics, or to create entangled states of light, which are useful for quantum information processing.

How are polaritons generated and detected?

To generate polaritons, one needs to match the frequency and momentum of light and matter. However, this is not easy to achieve, because light and matter have different dispersion relations. Therefore, one needs to use a coupling medium or a coupling structure to bridge the gap between them. Some common methods of coupling light and matter are:

To detect polaritons, one needs to convert them back to photons or phonons that can be measured by conventional detectors. This can be done by using the same methods of coupling as mentioned above, or by using other methods, such as:

• Scattering: can scatter light or matter in different directions, depending on their dispersion relation and their interaction with defects or impurities. This can be detected by measuring the scattered intensity or the angular distribution of light or matter.
• Emission: can emit light or heat when they decay or relax to lower energy states. This can be detected by measuring the emitted spectrum or the temperature change of the material.
• Interference: can interfere with each other or with other waves, creating patterns of constructive or destructive interference. This can be detected by measuring the spatial or temporal variation of the intensity or the phase of light or matter.

The latest trends and updates?

Polaritons have been studied for decades, but they have gained renewed interest in recent years, thanks to the advances in nanofabrication, nanospectroscopy, and nanosimulation techniques. Some of the latest trends and updates on polaritons are:

• Ghost polaritons: A new type of infrared polaritons that propagate along the surface of bulk crystals, such as calcite, with complex, out-of-plane momentum. They have been observed for the first time by an international team of researchers using near-field optical microscopy. Ghost polaritons can facilitate a superior control of infrared nano-light for sensing, signal processing, and other applications.
• Polariton cooling: A new way of cooling semiconductors by using polaritons instead of phonons. It has been demonstrated by a team of researchers from Purdue University that using hybrid quasiparticles called “polaritons”. Polariton cooling can overcome the limitations of phonon cooling at the nanoscale and improve the performance and reliability of semiconductor devices.
• Polariton lasing: A new type of lasing that occurs when polaritons reach a threshold density and coherence. It has been achieved by several groups of researchers using different materials and structures, such as organic microcavities, van der Waals heterostructures, perovskite nanowires, and metal halide crystals. Polariton lasing can offer low-threshold, tunable, coherent, and directional light sources for nanophotonics and quantum technologies.

What are some other applications of polaritons?

Polaritons are not only interesting for fundamental research but also for practical applications. Some of the remarkable functionalities exhibited by polaritons include the ability to induce long-range excitation energy transfer, enhance charge conductivity, and inhibit or accelerate chemical reactions. Some of the potential applications of polaritons are:

• Quantum technology: Exciton-polaritons are part-matter, part-light quasiparticles that emerge in semiconductor microcavity systems. They have been observed to undergo Bose-Einstein condensation, where a macroscopic population of polaritons spontaneously emerges in a low-energy polariton state. Owing to the exquisite experimental control in microcavity resonator structure, strong non-linearities, and scalable and versatile platforms, exciton-polaritons offer a feasible alternative to observe unique quantum phenomena that apply to quantum technologies. Polariton condensates are attractive for future quantum technological applications such as quantum simulation, interferometry, information processing, and non-classical state generation, for example.
• Polariton chemistry: They can also interact with molecular vibrations and electronic transitions, creating new hybrid states that can modify the chemical properties of the molecules. This can lead to new ways of controlling molecular dynamics with optical fields, such as manipulating reaction pathways, enhancing or suppressing reaction rates, and creating new reaction products. This emerging field of polariton chemistry has potential applications in catalysis, photochemistry, spectroscopy, and biosensing.

Conclusion

Polaritons are fascinating quasiparticles that open up new possibilities for manipulating light and matter at the nanoscale. They have a wide range of applications in nanophotonics, sensing, signal processing, energy harvesting, and other technologies. They also pose many challenges and opportunities for further research and development. These are the tiny powerhouses transforming semiconductor technology and beyond.