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Showing posts with label Simone De Liberato. Show all posts
Showing posts with label Simone De Liberato. Show all posts
We report the first observation of epsilon near zero (ENZ) phonon polaritons in an ultrathin AlN film fully hybridized with surface phonon polaritons (SPhP) supported by the adjacent SiC substrate. Employing a strong coupling model for the analysis of the dispersion and electric field distribution in these hybridized modes, we show that they share the most prominent features of the two precursor modes. The novel ENZ-SPhP coupled polaritons with a highly propagative character and deeply sub-wavelength light confinement can be utilized as building blocks for future infrared and terahertz (THz) nanophotonic integration and communication devices.
Nikolai Christian Passler, Christopher R. Gubbin, Thomas Folland, I. Razdolski, D. Scott Katzer, D. F. Storm, Martin Wolf, Simone De Liberato, Joshua D Caldwell, Alexander Paarmann,
We report the first observation of epsilon near zero (ENZ) phonon polaritons in an ultrathin AlN film fully hybridized with surface phonon polaritons (SPhP) supported by the adjacent SiC substrate. Employing a strong coupling model for the analysis of the dispersion and electric field distribution in these hybridized modes, we show that they share the most prominent features of the two precursor modes. The novel ENZ-SPhP coupled polaritons with a highly propagative character and deeply sub-wavelength light confinement can be utilized as building blocks for future infrared and terahertz (THz) nanophotonic integration and communication devices.
Chemical sensing methods based on surface polaritonic resonances stem from their intense near fields and resultant sensitivity to changes in local refractive index. Polar dielectric crystals (e.g. SiC, hBN) support surface phonon polaritons (SPhPs) from the mid-infrared to terahertz range with mode volumes and quality factors exceeding the best case scenario attained by plasmonic counterparts, making them strong candidates for resonant surface-enhanced infrared spectroscopy (SEIRA). We report on the behaviour of SPhP resonances of SiC nanopillars following the incorporation of sub- and nanometric coatings of Al2O3 and ZrO2 obtained by atomic layer deposition. Concurrent anomalous red and blue-shifts of SPhP resonances were observed upon deposition of sub-nanometric Al2O3 films, with shift direction dictated by the mode position relative to the ordinary longitudinal optic (LO) phonon of Al2O3. These concurrent shifts, which are attributed to coupling to the Berreman mode of the Al2O3 layer, persist for thicker films and are correctly predicted by numerical calculations employing the measured Al2O3 permittivity. Deposition of ZrO2, whose phonon resonances are detuned from the SPhPs, also led to anomalous blue-shifts of transverse and longitudinal SPhP resonances around 900 cm-1 for films up to ≈ 1.5 nm, reversing to the canonical red-shift for thicker layers. These anomalous shifts were not reproduced numerically using the measured ZrO2 permittivity and suggest the existence of a localized surface state, which when modelled as a simple Lorentz oscillator, provide semi-quantitative agreement with experimental results. In addition, predicted shifts for thicker ZrO2layers may thus provide a tool for real-time monitoring of ultrathin film growth.
Nanowerk News) Scientists have found that two-dimensional (2D) nanostructures with asymmetric design enable a new quantum mechanism, triggering the emission of tuneable light at terahertz frequencies-with unprecedented efficiency.
The researchers, from the University of Southampton and Imperial College London, found that quantum wells, 2D nanostructures formed of several layers of semi-conductor alloys placed on top of each other like a sandwich, can enhance light emission in a technological challenging spectral range.
It is hoped that the findings will have an impact on photonic and optoelectronic devices across a broad range of applications, including harmless medical imaging and security scanning.
Electrons are trapped in the structure and this confinement can be exploited to enhance their capacity to interact with light at given frequencies much lower than the laser frequency at which they are excited: the system emits light by interacting with "vacuum fluctuations" that permeate space, according to quantum theory.
An optoelectronic device formed of multiple quantum wells, whose design is optimised to maximise the dipole and thus its efficiency, emitting terahertz light.
Nathan Shammah, from the University's Quantum Light and Matter (QLM) group and co-author of the study says: "As the 2D nanostructures can be manufactured with an asymmetric design, this allows light to interact with trapped electrons in a way that is not otherwise allowed. This interaction process, leading to the emission of light at lower frequencies, has not been observed in atoms because those are very symmetrical systems and symmetry rules prevent the transitions that trigger this light emission from happening."
In the paper, which is published in Physical Review B ("Terahertz emission from ac Stark-split asymmetric intersubband transitions"), the researchers predict that by shining light on a 2D asymmetric nanostructure with a laser that is tuned at resonance with the electronic transitions that can occur in the nanostructure, in addition to the scattered laser light, this 2D device would emit light at other frequencies, which can be tuned simply by changing the laser power.
Nathan, who co-authored the paper with Dr Simone De Liberato, from the QLM group, and Professor Chris Phillips from Imperial College London, adds: "Due to the large oscillating dipole and high density of electrons that characterise these 'artificial atoms' formed of asymmetric 2D structures, the control of light-matter coupling can be greatly enhanced, triggering spontaneous light emission, similar to what occurs in LEDs lamps.
"This new mechanism is perfectly suited for the terahertz frequency range, which spans from above the current wi-fi bandwidth to below the visible light spectrum, where the lack of practical light emitters constitutes a serious technological gap."
The high efficiency shown by the simulations suggests that this theoretical result could be exploited in the near future for a broad range of optoelectronic applications-from harmless medical imaging and security scanners, to short-range, ultra-fast wireless communication.
Transitions between the two states of an ac Stark-split doublet are forbidden in centrosymmetric systems, and thus almost impossible to observe in experiments performed with atomic clouds. However, electrons trapped in nanoscopic heterostructures can behave as artificial atoms, with the advantage that the wave-function symmetry can be broken by using asymmetric confining potentials. Here we develop the many-body theory describing the intradoublet emission of a resonantly pumped intersubband transition in a doped asymmetric quantum well, showing that in such a system the intradoublet emission can be orders of magnitude higher than in previously studied systems. This emission channel, which lies in the terahertz range and whose frequency depends upon the pump power, opens the way to the realization of monolithic and tunable terahertz emitters.
Electric dipole transitions between different cavity polariton branches or between dressed atomic states with the same excitation number are strictly forbidden in centro-symmetric systems. For doped quantum wells in semiconductor microcavities, the strong coupling between an intersubband transition in the conduction band and a cavity mode produces two branches of intersubband cavity polaritons, whose normal-mode energy splitting is tunable and can be in the terahertz region. Here, we show that, by using asymmetric quantum wells, it is possible to have allowed dipolar transitions between different polaritonic branches, leading to the emission of terahertz photons. We present a quantum field theory for such a system and predict that high-efficiency, widely tunable terahertz lasing can be obtained.
