Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect.

Topological Photonics

Phase transitions to quantum condensed phases—such as Bose–Einstein condensation (BEC), superfluidity, and superconductivity—have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place. Promising candidate systems are semiconductor microcavities, in which photons are confined and strongly coupled to electronic excitations, leading to the creation of exciton polaritons. These bosonic quasi-particles are 109 times lighter than rubidium atoms, thus theoretically permitting BEC to occur at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for BEC of polaritons. Above a critical density, we observe massive occupation of the ground state developing from a polariton gas at thermal equilibrium at 19 K, an increase of temporal coherence, and the build-up of long-range spatial coherence and linear polarization, all of which indicate the spontaneous onset of a macroscopic quantum phase. Bose–Einstein condensation (BEC), a form of matter first postulated in 1924, has famously been demonstrated in dilute atomic gases at ultra-low temperatures. Much effort is now being devoted to exploring solid-state systems in which BEC can occur. In theory semiconductor microcavities, where photons are confined and coupled to electronic excitations leading to the creation of polaritons, could allow BEC at standard cryogenic temperatures. Kasprzak et al. now present experiments in which polaritons are excited in such a microcavity. Above a critical polariton density, spontaneous onset of a macroscopic quantum phase occurs, indicating a solid-state BEC. BEC should also be possible at higher temperatures if coupling of light with solid excitations is sufficiently strong. Demokritov et al. have achieved just that, BEC at room temperature in a gas of magnons, which are a type of magnetic excitation. This paper presents a comprehensive set of experiments in which polaritons are excited in a semiconductor microcavity. Above a critical density of polaritons, massive occupation of the ground state at 19 K is observed and various pieces of experimental evidence point to a spontaneous onset of a macroscopic quantum phase.

Bose-Einstein condensation of exciton polaritons

Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

(b) Write out the equations for the time dependence of ρ11, ρ22, ρ12 and ρ21 assuming that a light field interaction V = -μ12Ee iωt + c.c. couples only levels |1> and |2>, and that the excited levels exhibit spontaneous decay. (8 marks) (c) Under steady-state conditions, find the ratio of populations in states |2> and |3>. (3 marks) (d) Find the slowly varying amplitude ̃ ρ 12 of the polarization ρ12 = ̃ ρ 12e iωt . (6 marks) (e) In the limiting case that no decay is possible from intermediate level |3>, what is the ground state population ρ11(∞)? (2 marks) 2. (15 marks total) In a 2-level atom system subjected to a strong field, dressed states are created in the form |D1(n)> = sin θ |1,n> + cos θ |2,n-1> |D2(n)> = cos θ |1,n> sin θ |2,n-1>

The Quantum Theory of Light

Optical properties of ZnO-based single nanorods are probed by cathodoluminescence (CL) measurements at T = 5 K. We observe a variation of the ZnO near band edge CL by three orders of magnitude along the nanorod axis, accompanied by a spectral blueshift of 10-30 meV. This indicates a rather poor structural quality of the nanorod bottom part, close to the substrate. ZnO/ZnMgO quantum wells grown on top of ZnO nanorods are found to exhibit much stronger confinement effects as compared to their two-dimensional counterparts, suggesting a reduced spontaneous and piezoelectric polarization effects.

Cathodoluminescence of single ZnO nanorod heterostructures

We present a new method, using amorphous selenium, for inducing a well-controlled 2D–3D transition of a strained CdSe/ZnSe layer, i.e. for initiating self-organization of CdSe quantum dots that does not occur spontaneously during epitaxial growth. X-ray diffraction results evidence the very good crystal quality of the samples obtained using this technique. Optical characterizations by photoluminescence and time-resolved photoluminescence show the interest of this process in the growth of CdSe/ZnSe quantum dots.

https://iopscience.iop.org/article/10.1088/0957-4484/16/8/021/pdf

Control of the two-dimensional–three-dimensional transition of self-organized CdSe/ZnSe quantum dots

We demonstrate three-dimensional optical confinement in CdTe-based microcavities. While the vertical confinement is determined by epitaxially grown CdMnTe/CdMgTe Bragg mirrors, the lateral confinement is achieved by defining pillar structures with electron beam lithography and reactive ion etching. The optical confinement is shown by a size-dependent energy splitting between the fundamental and higher photonic modes.

Optical confinement in CdTe-based photonic dots

The growth of CdMnTe by molecular‐beam epitaxy is described, including an in situ calibration of alloy composition by reflection high‐energy electron diffraction intensity oscillation, which takes advantage of the larger sticking coefficient of Mn with respect to Cd. Layers are studied by photoluminescence, x‐ray diffraction, transmission electron microscopy, and cathodoluminescence imaging. Relaxation of the mismatch strain occurs through different mechanisms, depending on the sign and magnitude of the mismatch. Once identified the characteristic features of the cathodoluminescence images are used to determine the critical thickness of layers of uniform composition or of more elaborate heterostructures. A heuristic criterion for the relaxation of mismatch strain in heterostructures incorporating layers of continuously varying composition is checked.

Growth, structural, and optical properties of II-VI layers: (001) CdMnTe grown by molecular-beam epitaxy

We report on the growth, structural, electronic, and optical properties of Si-doped GaN∕AlxGa1−xN (x=0.11, 0.25) multiple-quantum-well structures grown on SiC by plasma-assisted molecular-beam epitaxy. We have demonstrated that the use of In as a surfactant during growth improves the structural and optical properties of these layers. Photoluminescence studies have made possible the identification of the fundamental and excited electronic levels by comparison with simulations of the electronic structure. Temperature dependence studies reveal an anomalous behavior of the photoluminescence intensity, which is the quenching of the e1−hh1 line, while the e2−hh1 and e3−hh1 transitions become dominant at room temperature in the samples with 11% and 25% Al in the barrier, respectively. This behavior can be explained by the population of the e2 and e3 electronic states by thermally excited carriers, and by the higher oscillator strength of e2−hh1 and e3−hh1 transitions compared with e1−hh1, due to the intense elec...

Le Si Dang

Papers

Cathodoluminescence of single ZnO nanorod heterostructures

Control of the two-dimensional–three-dimensional transition of self-organized CdSe/ZnSe quantum dots

Optical confinement in CdTe-based photonic dots

Growth, structural, and optical properties of II-VI layers: (001) CdMnTe grown by molecular-beam epitaxy

Observation of hot luminescence and slow inter-sub-band relaxation in Si-doped GaN∕AlxGa1−xN (x=0.11, 0.25) multi-quantum-well structures