Exciton–polariton condensation

We review, aiming at an audience of final year undergraduates, the phenomena observed in, and properties of, microcavity exciton–polariton condensates. These are condensates of mixed light and matter, consisting of superpositions of photons in semiconductor microcavities and excitons in quantum wells. Because of the imperfect confinement of the photon component, exciton–polaritons have a finite lifetime, and have to be continuously re-populated. Therefore, exciton–polariton condensates lie somewhere between equilibrium Bose–Einstein condensates and lasers. We review in particular the evidence for condensation, the coherence properties studied experimentally, and the wide variety of spatial structures either observed or predicted to exist in exciton–polariton condensates, including quantised vortices and other coherent structures. We also discuss the question of superfluidity in a non-equilibrium system, reviewing both the experimental attempts to investigate superfluidity to date, and the theoretical suggestions of how it may be further elucidated.     

Spin-dependent polariton–polariton scattering in planar microcavities

We present a microscopic theory of polariton–polariton (PP) scattering in quantum microcavities, which is developed with allowance for the composite nature of polaritons. Analytical estimations of the effective scattering rate for PP scattering with parallel spin configuration are presented, and the role of dark excitons in the opposite spin configuration is discussed.     

Synthesis and formation mechanism of Ag–Ni alloy nanoparticles at room temperature

Ag–Ni nanoparticles were prepared with a chemical reduction method in the presence of polyvinylpyrrolidone (PVP) used as a stabilizing agent. During the synthesis of Ag–Ni nanoparticles, silver nitrate was used as the Ag⁺ source while nickel sulfate hexahydrate was used as Ni²⁺ source. Mixed solutions of Ag⁺ source and Ni²⁺ source were used as the precursors and sodium borohydride was used as the reducing agent. Five ratios of Ag⁺/Ni²⁺ (9:1, 3:1, 1:1, 1:3, and 1:9) suspensions were prepared in the corresponding precursors. Ag–Ni alloy nanoparticles were obtained with this method at room temperature. Scanning electronic microscope (SEM), energy dispersive spectrum (EDS), high resolution transmission electron microscope (HRTEM) were used to characterize the morphology, composition and crystal structure of the nanoparticles. The crystal structure was also investigated with X-ray diffraction (XRD). In all five Ag/Ni ratios, two kinds of particle structures were observed that are single crystal structure and five-fold twinned structure respectively. Free energy of nanoparticles with different crystal structures were calculated at each Ag/Ni ratio. Calculated results revealed that, with identical volume, free energy of single crystal particle is lower than multi-twinned particle and the difference becomes smaller with the increase of particle size; increase of Ni content will lead the increase of free energy for both structures. Formation of different crystal structures are decided by the structure of the original nuclei at the very early stage of the reduction process.     

Controlled spin pattern formation in multistable cavity–polariton systems

Theoretical studies are performed of planar cavity–polariton systems under resonant optical excitation. We show that if the cavity is spatially anisotropic, the polariton spin is highly sensitive to the pump polarization direction, which can be used to modulate the circular polarization of the output light. In particular, when the right- and left-circular components of the incident wave have equal intensities and mutually opposite angular momenta, the pump has strictly linear yet angle-dependent polarization and as such brings about a periodic angular variation of the polariton spin. Free motion of polaritons is the other factor determining the shape of the cavity-field distribution. Such externally driven and highly tunable spin patterns represent a counterpart of spin shaping in nonresonantly excited Bose–Einstein condensates of cavity polaritons.     

On the possibility of the formation of a NaCl–KCl solid-solution crystal from an aqueous solution at room temperature in small-volume systems

Phase transitions in binary and ternary small-volume systems have been simulated by the methods of equilibrium chemical thermodynamics. Considerable dissimilarities of the equilibrium phase compositions of the systems of macroscopic and microscopic sizes have been revealed. A change in the system’s volume is accompanied by a change in the heterogeneity region in the phase diagram. This can increase considerably the solubility of small systems and lead to the emergence of phases that are thermodynamically unstable in macroscopic systems. Such size effects have been considered by the example of phase transformations in NaCl–KCl–H₂O and NaCl–KCl systems.     

On a new mechanism of polariton–polariton scattering

Condensate states of a two-dimensional exciton–polariton system have been considered under the conditions of direct resonant photoexcitation. It has been theoretically predicted that splitting of eigenmodes with orthogonal polarizations leads to the emergence of a new channel of parametric scattering. A polariton condensate spontaneously decays into a set of states in a finite region of the momentum space, thus leading to a strong inhomogeneity in the distributions of the intensity and polarization even in the case of a strictly constant amplitude and zero in-plane momentum of the external field. The new scattering mechanism makes possible self-oscillating and chaotic states of polariton systems.     

Speed of a ferrite—ferroelectric microwave planar resonator

The speed of a ferrite—ferroelectric microwave planar resonator consisting of a YIG film and a barium strontium titanate plate is investigated. It is shown that the speed of the resonator is governed by the rate of resonance frequency electrical tuning and the tuning is accompanied by a change in the electrical bias. The rate of frequency tuning in the resonator is 1.3–3.3MHz/fus.     

Experimental investigation of the sub-Doppler transmission spectroscopy in a thin vapour layer at room temperature

The sub-Doppler transmission spectrum in a thin vapour layer (about 150 μm) was observed at room temperature using the wavelength modulation technology. The absorption signal and its second-order harmonic were detected with an external-cavity diode laser. A sub-Doppler spectrum corresponding to resonant transitions of the caesium D_2 line (6S_{1/2}→6P_{3/2}) was demonstrated. The dependence of the transmission signal on the intensity of the laser was also investigated.     

Diode-pumped Tm:LuAG laser at room temperature

The lasing characteristics of Tm:LuAG at room temperature are reported.The maximum output power at 2.023-μm wavelength is 4.91 W and the slope efficiency is 25.39%.The mode matching between pump mode and laser mode is optimized by changing the pump beam waist radius and its location.Different output couplers are used to realize the optimal laser output.The relationship between operation temperature and output power is also discussed.     

Fabrication of a bulk silicon p–n homojunction-structured light-emitting diode showing visible electroluminescence at room temperature

We have developed a novel dressed-photon-assisted annealing process, in which the distribution of dopant (boron) domains is modified in a self-organized manner based on the absorption of light having a photon energy higher than the band gap and subsequent stimulated emission. Using this process, we were able to fabricate a bulk silicon p–n homojunction-structured light-emitting diode that showed electroluminescence emission in the visible region at room temperature. A broadband spectrum with three emission peaks at 400 nm, 590 nm, and 620 nm was clearly observed.     

Observation of ammonia dihydrate in the AMH-VI structure at room temperature – possible implications for the outer solar system

Ammonia dihydrate (ADH) is an important constituent of the outer solar system and its high-pressure behaviour is relevant to the modelling of Titan, Uranus and Neptune. Our neutron diffraction studies show that ADH can exist at room temperature in the substitutionally disordered structure of the ammonia monohydrate (AMH) phase VI. This implies that a solid solution may exist between ADH and AMH at high pressure, and this is of probable importance to models of the outer solar system.     

Analysis of surface and guided wave plasmon polariton modes in insulator–metal–insulator planar plasmonic waveguides

Theoretical and experimental study of the surface plasmon–polariton and guided wave plasmon polariton modes is presented for the Sapphire/Ag/Polycarbonate/Air structure. Theoretical results are obtained by solving complex multilayer eigenvalue equations as well as the reflectivity equation for this structure. It is proposed that the mode attenuation can be significantly reduced by inserting a low index dielectric buffer between the metal and the guiding dielectric layer. The dispersion and attenuation curves are generated. Both the surface plasmon and guided wave plasmon polariton modes are studied experimentally. The experimental values of the effective refractive indices agree well with the theoretical values. The electric field profiles are generated and used to examine the nature of modes. After optimization of various parameters the condition for low loss single mode guiding is obtained for the proposed structure. Effect of metal thickness on surface plasmon mode is also discussed. It is inferred that in a properly optimized plasmonic waveguide, the losses can be reduced by a factor of 4.     

Solid–solid transformation route to nanocrystalline sodalite from Al-PILC at room temperature

The present study describes a solid–solid transformation of nanocrystalline sodalite from the solid gel mixture of Al₂O₃-pillared montmorillonite (Al-PILC) with sodium hydroxide at room temperature (25 °C) under an ambient atmosphere. Powder X-ray diffraction (XRD) analysis confirms that the X-ray crystalline sodalite products are crystallized after 12 days, whereas infrared absorption (IR) spectra reveal that diagnostic IR absorption peaks due to single four-membered ring of sodalite framework is observed even after 1 day. Scanning electron microscopy (SEM) shows that Al-PILC is transformed into discrete nano-sized sodalite particles (∼50 nm). Although the induction period, the time elapsing before nucleation, for the solid–solid transformation takes much longer (12 days), the nanocrystalline sodalite is successfully obtained at this extremely mild synthetic condition through solid–solid transformation.     

Diode-pumped high-efficiency Tm:YLF laser at room temperature

High-efficiency continuous-wave (CW) Tm:YLF laser by the dual-end-pumping configuration is presented. Under the total input pump power of 24.0 W,the highest output power reaches 9.8 W in the wavelength range of 1910-1926 nm by use of 10% output coupling,corresponding to optical conversion efficiency of 40.9% and slope efficiency of 51.4%.The free-running laser spectrum of Tm:YLF is measured.     

Fast optical switching and bistability in room temperature CdHgTe at 10.6 μm

We report a study of optical switching in Cd_0.23Hg_0.77Te etalons at room temperature by two-photon excitation at 10.6μm. We compare experimental results of both fast switching using a short-cavity TEA CO₂ laser (FWHM = 30ns) and quasi-CW optical bistability using a hybrid TEA CO₂ laser (FWHM = 1.75μm), with a computer model.     

Reversible magnetic-field-induced phase transformation and magnetocaloric effect above room temperature in a Ni–Mn–In–Fe Polycrystal

A first order structural transformation occurs in the Ni₅₀Mn₃₄In₁₄Fe₂ polycrystalline, associated with magnetic entropy changes of 26.5 and 53.6 J/kg K in an applied magnetic field up to 5 and 8 T, respectively. Moreover, the magnetic entropy change with different applied fields maintains its maximum value within a temperature of 3 K. A phase transformation change of 2 K was obtained under 5000 Oe magnetic field, indicating that it is large enough for inducing a reverse martensitic transformation under large magnetic field. All the results mentioned above are favorable for the application of this intelligent intermetallic material.     

High efficient diode-pumped Tm:YAP laser at room temperature

A high efficient diode-pumped Tm:YAP laser is reported. The maximum output power at 1981 nm is 5.2 W and the slope efficiency is 30%. Unpolarized absorption near 800 nm and unpolarized fluorescence spectra near 1800 nm pumped by laser diode (LD) are measured. In addition, the relationship between operation temperature and output power is discussed.     

Asymmetric nanoparticle may go “active” at room temperature

Using molecular dynamics simulations, we show that an asymmetrically shaped nanoparticle in dilute solution possesses a spontaneously curved trajectory within a finite time interval, instead of the generally expected random walk. This unexpected dynamic behavior has a similarity to that of active matters, such as swimming bacteria, cells, or even fish, but is of a different physical origin. The key to the curved trajectory lies in the non-zero resultant force originated from the imbalance of the collision forces acted by surrounding solvent molecules on the asymmetrically shaped nanoparticle during its orientation regulation. Theoretical formulae based on microscopic observations have been derived to describe this non-zero force and the resulting motion of the asymmetrically shaped nanoparticle.     

Burstein–Moss shift and room temperature near-band-edge luminescence in lithium-doped zinc oxide

Nanopowders of pure and lithium-doped semiconducting ZnO (Zn_1−x Li _x O, where x= 0, 0.01, 0.03, 0.06, 0.09 and 0.15 in atomic percent (at.%)) are prepared by PEG-assisted low-temperature hydrothermal method. The average crystallite size is calculated using Debye–Scherrer formula and corrected for strain-induced broadening by Williamson–Hall (W–H) plot. The peak shift in XRD and the lattice constant of ZnO as a function of unit cell composition are predicted by Vegard’s law. The evolution of ZnO nanostructures from rod-shaped to particle nature is observed from TEM images and the influence of dopant on the morphology is investigated. The optical absorption measurement marks an indication that the incorporation of lithium ion into the lattice of ZnO widens the optical band gap energy from ∼2.60 to ∼3.20 eV. The near band edge (NBE) emission peak centered at ∼3.10 eV is considered to be the dominant emission peak in the PL spectra. Blue emission peak is not observed in doped ZnO, thus promoting defect-free nanoparticles. The Burstein–Moss shift serves as a qualitative tool to analyze the widening of the optical band gap and to study the shape of the NBE luminescence in doped ZnO nanopowders. FT-IR spectra are used to identify the strong metal–oxide (Zn–O) interaction.