Between electronics and photonics there exists a frequency gap of approximately two octaves, i.e., the frequency range between 100 GHz and 10 THz, across which there are limited capabilities for signal generation, control, guidance, and processing. Researchers have demonstrated e that phonon-polaritons in ionic crystals like LiNbO3 or LiTaO3 may be used to bridge this gap.
Polaritons are quasiparticles that form in semiconductors when an elementary excitation such as an exciton or a phonon interacts sufficiently strongly with light. In particular, exciton–polaritons have attracted tremendous attention for their unique properties, spanning from an ability to undergo ultra-efficient four-wave mixing to superfluidity in the condensed state.
As polaritons are composite particles, their properties are a mixture of the properties of their bare constituents. Because of their photonic part, polaritons propagate rapidly and have long decoherence times. They can be optically excited and observed. On the other hand, because of their excitonic part, polaritons interact with one another and also with the crystal lattice via exciton–phonon interactions. These interactions allow polaritons to scatter from one state to another and furthermore introduce nonlinearity into the system, which is a fundamental ingredient of almost all device proposals.
Polaritonics is an intermediate regime between photonics and sub-microwave electronics. In this regime, signals are carried by an admixture of electromagnetic and lattice vibrational waves known as phonon-polaritons, rather than currents or photons. Since phonon-polaritons propagate with frequencies in the range of hundreds of gigahertz to several terahertz, polaritonics bridges the gap between electronics and photonics. A compelling motivation for polaritonics is the demand for high-speed signal processing and linear and nonlinear terahertz spectroscopy. Polaritonics has distinct advantages over electronics, photonics, and traditional terahertz spectroscopy in that it offers the potential for a fully integrated platform that supports terahertz wave generation, guidance, manipulation, and readout in a single patterned material.
The development of Polaritonics is led by theory, which predicts new effects, designes new structures and proposes new experiments. Fabrication of new generation of opto-electronic devices based on quantum properties of exciton-polaritons would manifest a technology breakthrough in opto-electronics with quantum coherent effects brought into everyday life.
The new quantum light sources and logic gates are important for realization of optical computers and enhancement of capacities of optical communication lines. Compact sources of coherent terahertz radiation have their applications in medicine (skin cancer cure), environment protection, industrial sensing, security.
The University of Southampton has launched a project aimed at the development of several revolutionary new device concepts within the new interdisciplinary research area of Polaritonics, which studies interaction of light with electronic excitations in crystals. This would include the vertical cavity surface emitting terahertz lasers, which have multiple applications in medicine, environment protection, communication technologies and security.
The second group of polariton devices incudes the optical switches, transistors and logic elements essential for optical computation and for information communication technologies. Finally, the non-classical light sources based on exciton-polaritons are of high importance for quantum technologies. Polaritonics brings the quantum coherent phenomena of Bose-Einstein condensation, superfluidity, weak localization, quantum complementarity to everyday life. Altogether, the polariton devices would improve the quality of life in UK and abroad, as they are expected to positively affect such vital areas as medicine, environment protection, security, communications, computing.
Any laser is based on two basic ingredients. The first ingredient is an active medium providing the amplification of light at a given frequency, the so-called optical gain. The second ingredient is an optical cavity which confines the photons in a frequency range where the gain is present. In practice, the active medium is pumped and emits photons by spontaneous emission. The emission is typically broad band and not directional.
Polariton lasers are not the only field of applications of polaritonics. In recent years it was noted that the combination of the peculiar spin and transport properties of polaritons with the possibility of their lateral confinement can form a basis for the creation of novel optoelectronic devices, the so-called spinoptronic devices.
One very fruitful approach is based on the control of the spin degree of freedom of polaritons which offers a way to control the polarisation of light absorbed and emitted by the microcavity. The polarisation of light is rarely used in optical communication because it cannot be conserved in long-range optical fibres. However, short-range optical communication could benefit from polarisation modulation. For this, one should possess optical amplifiers, switches, converters, modulators sensitive to polarisation and stable sources of polarised light. Several functionalities should be combined within the same component with extremely low-power consumption, and possibly integrated with classical electronic elements. In this framework, polaritonic devices based on microcavities look promising. These devices are spin-dependent optoelectronic devices; the so-called spinoptronic devices.
Polaritonics, like electronics and photonics, requires three elements: robust waveform generation, detection, and guidance and control. Without all three, polaritonics would be reduced to just phonon-polaritons, just as electronics and photonics would be reduced to just electromagnetic radiation. These three elements can be combined to enable device functionality similar to that in electronics and photonics.
Phonon-polaritons generated in ferroelectric crystals propagate nearly laterally to the excitation pulse due to the high dielectric constants of ferroelectric crystals, facilitating easy separation of phonon-polaritons from the excitation pulses that generated them. Phonon-polaritons are therefore available for direct observation, as well as coherent manipulation, as they move from the excitation region into other parts of the crystal. Lateral propagation is paramount to a polaritonic platform in which generation and propagation take place in a single crystal. A full treatment of the Cherenkov-radiation-like terahertz wave response reveals that in general, there is also a forward propagation component that must be considered in many cases.
Direct observation of phonon-polariton propagation was made possible by real-space imaging, in which the spatial and temporal profiles of phonon-polaritons are imaged onto a CCD camera using Talbot phase-to-amplitude conversion. This by itself was an extraordinary breakthrough. It was the first time that electromagnetic waves were imaged directly, appearing much like ripples in a pond when a rock plummets through the water’s surface (see Fig. 3). Real-space imaging is the preferred detection technique in polaritonics, though other more conventional techniques like optical Kerr-gating, time resolved diffraction, interferometric probing, and terahertz field induced second-harmonic generation are useful in some applications where real-space imaging is not easily employed. For example, patterned materials with feature sizes on the order of a few tens of micrometres cause parasitic scattering of the imaging light. Phonon-polariton detection is then only possible by focusing a more conventional probe, like those mentioned before, into an unblemished region of the crystal.
Guidance and control
The last element requisite to polaritonics is guidance and control. Complete lateral propagation parallel to the crystal plane is achieved by generating phonon-polaritons in crystals of thickness on the order of the phonon-polariton wavelength. This forces propagation to take place in one or more of the available slab waveguide modes. However, dispersion in these modes can be radically different from that in bulk propagation, and in order to exploit this, the dispersion must be understood.
Control and guidance of phonon-polariton propagation may also be achieved by guided wave, reflective, diffractive, and dispersive elements, as well as photonic and effective index crystals that can be integrated directly into the host crystal. However, lithium niobate, lithium tantalate, and other perovskites are impermeable to the standard techniques of material patterning. In fact, the only etchant known to be even marginally successful is hydrofluoric acid (HF), which etches slowly and predominantly in the direction of the crystal optic axis.
Femtosecond laser micromachining is used for device fabrication by milling ‘air’ holes and/or troughs into ferroelectric crystals by directing them through the focus region of a femtosecond laser beam. . The advantages of femtosecond laser micromachining for a wide range of materials have been well documented. In brief, free electrons are created within the beam focus through multiphoton excitation. Because the peak intensity of a femtosecond laser pulse is many orders of magnitude higher than that from longer pulse or continuous-wave lasers, the electrons are rapidly excited, heated to form a quantum plasma. Particularly in dielectric materials, the electrostatic instability, induced by the plasma, of the remaining lattice ions results in ejection of these ions and hence ablation of the material, leaving a material void in the laser focus region. Also, since the pulse duration and ablation time scales are much faster than the thermalization time, femtosecond laser micromachining does not suffer from the adverse effects of a heat-affected zone, like cracking and melting in regions neighboring the intended damage region.