Excitons In Semiconductors Research Articles

Sculpting oscillators with light within a nonlinear quantum fluid

Polaritons—quasiparticles made up of a photon and exciton strongly coupled together—can form macroscopic quantum states even at room temperature. Now these so-called condensates are imaged directly. This achievement could aid the development of semiconductor-based polariton-condensate devices. Seeing macroscopic quantum states directly remains an elusive goal. Particles with boson symmetry can condense into quantum fluids, producing rich physical phenomena as well as proven potential for interferometric devices1,2,3,4,5,6,7,8,9,10. However, direct imaging of such quantum states is only fleetingly possible in high-vacuum ultracold atomic condensates, and not in superconductors. Recent condensation of solid-state polariton quasiparticles, built from mixing semiconductor excitons with microcavity photons, offers monolithic devices capable of supporting room-temperature quantum states11,12,13,14 that exhibit superfluid behaviour15,16. Here we use microcavities on a semiconductor chip supporting two-dimensional polariton condensates to directly visualize the formation of a spontaneously oscillating quantum fluid. This system is created on the fly by injecting polaritons at two or more spatially separated pump spots. Although oscillating at tunable THz frequencies, a simple optical microscope can be used to directly image their stable archetypal quantum oscillator wavefunctions in real space. The self-repulsion of polaritons provides a solid-state quasiparticle that is so nonlinear as to modify its own potential. Interference in time and space reveals the condensate wavepackets arise from non-equilibrium solitons. Control of such polariton-condensate wavepackets demonstrates great potential for integrated semiconductor-based condensate devices.

Elastic Exciton-Exciton Scattering in Photoexcited Carbon Nanotubes

We report on original nonlinear spectral hole-burning experiments in single wall carbon nanotubes that bring evidence of pure dephasing induced by exciton-exciton scattering. We show that the collision-induced broadening in carbon nanotubes is controlled by exciton-exciton scattering as for Wannier excitons in inorganic semiconductors, while the population relaxation is driven by exciton-exciton annihilation as for Frenkel excitons in organic materials. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems.

Mode interaction and dynamics features of class d lasers

We study a simple model of the class D laser with allowance for the spectral and spatial inhomogeneity of an active medium, whose polarization relaxation rate is much smaller than the field relaxation rate in a resonator. We consider the cases of one-, two-, and four-mode lasing for which the stationary, pulsed, self-modulation, and quasi-chaotic of laser dynamics are numerically studied. The laser parameters are chosen to correspond to the experiments on the Bose–Einstein condensation of dipolar excitons in semiconductor traps with quantum wells, which open up a possibility for creating class D lasers for the first time.

Vibrationally Assisted Polariton‐Relaxation Processes in Strongly Coupled Organic‐Semiconductor Microcavities

AbstractIf a semiconductor with an electronic transition that approximates a two‐level system is placed within an optical cavity, strong coupling can occur between the confined photons and the semiconductor excitons. This coupling can result in the formation of cavity polariton states that are a coherent superposition of light and matter. If the material in the cavity is an organic semiconductor, it has been predicted that interactions between Frenkel excitons, polaritons, and molecular vibrational modes will have a profound role in defining the overall relaxation dynamics of the system. Here, using temperature‐dependent spectroscopy on a microcavity containing a J‐ aggregated cyanine dye, it is shown that a spectrum of localized vibrational modes (identified by Raman scattering) enhances the population of certain polaritonic modes by acting as an energy‐loss channel to the excitons as they undergo scattering. Our work demonstrates that simultaneous control of the optical properties of a cavity and the vibrational structure of a molecular dye could promote the efficient population of k = 0 polariton states, from which lasing and other cooperative phenomena may occur.

One-dimensional polaritons with size-tunable and enhanced coupling strengths in semiconductor nanowires

Strong coupling of light with excitons in direct bandgap semiconductors leads to the formation of composite photonic-electronic quasi-particles (polaritons), in which energy oscillates coherently between the photonic and excitonic states with the vacuum Rabi frequency. The light-matter coherence is maintained until the oscillator dephases or the photon escapes. Exciton-polariton formation has enabled the observation of Bose-Einstein condensation in the solid-state, low-threshold polariton lasing and is also useful for terahertz and slow-light applications. However, maintaining coherence for higher carrier concentration and temperature applications still requires increased coupling strengths. Here, we report on size-tunable, exceptionally high exciton-polariton coupling strengths characterized by a vacuum Rabi splitting of up to 200meV as well as a reduction in group velocity, in surface-passivated, self-assembled semiconductor nanowire cavities. These experiments represent systematic investigations on light-matter coupling in one-dimensional optical nanocavities, demonstrating the ability to engineer light-matter coupling strengths at the nanoscale, even in non-quantum-confined systems, to values much higher than in bulk.

Modulational instability and solitons in excitonic semiconductor waveguides

Nonlinear light propagation in a single-mode micron-size waveguide made of semiconducting excitonic material has been theoretically studied in terms of exciton-polaritons by using an analysis based on macroscopic fields. When a light pulse is spectrally centered in the vicinity of the ground-state Wannier exciton resonance, it interacts with the medium nonlinearly. This optical cubic nonlinearity is caused by the repulsive exciton-exciton interactions in the semiconductor, and at resonance it is orders of magnitude larger than the Kerr nonlinearity (e.g., in silica). We demonstrate that a very strong and unconventional modulational instability takes place, which has not been previously reported. After reducing the problem to a single nonlinear Schr\"odinger-like equation, we also explore the formation of solitary waves both inside and outside the polaritonic gap and find evidence of spectral broadening. A realistic physical model of the excitonic waveguide structure is proposed.

Multiple Exciton Generation in Semiconductor Quantum Dots.

Multiple exciton generation in quantum dots (QDs) has been intensively studied as a way to enhance solar energy conversion by utilizing the excess energy in the absorbed photons. Among other useful properties, quantum confinement can both increase Coulomb interactions that drive the MEG process and decrease the electron-phonon coupling that cools hot excitons in bulk semiconductors. However, variations in the reported enhanced quantum yields (QYs) have led to disagreements over the role that quantum confinement plays. The enhanced yield of excitons per absorbed photon is deduced from a dynamical signature in the transient absorption or transient photoluminescence and is ascribed to the creation of biexcitons. Extraneous effects such as photocharging are partially responsible for the observed variations. When these extraneous effects are reduced, the MEG efficiency, defined in terms of the number of additional electron-hole pairs produced per additional band gap of photon excitation, is about two times better in PbSe QDs than that in bulk PbSe. Thin films of electronically coupled QDs have shown promise in simple photon-to-electron conversion architectures. If the MEG efficiency can be further enhanced and charge separation and transport can be optimized within QD films, then QD solar cells can lead to third-generation solar energy conversion technologies.

Excitons in organic semiconductors

The exciton binding energy ( E b ) is one of the heavily debated issues in conjugated polymer literature. Most of the experimental studies suggest values around 0.3–0.4 eV for PPV. On the other hand, binding energies ranging from 0.06 eV to more than 1 eV have been reported as well. Time Dependent Density Functional Theory (TDDFT) is employed to calculate E b for PPV and polythiophenes and their derivatives in order to (i) to clarify the controversial debate of the magnitude of the exciton in PPV and (ii) to study the influence of the substituents on the exciton binding energy of conjugated polymers.

Model of hopping excitons in GaInNAs: simulations of sharp lines in micro-photoluminescencespectra and their dependence on the excitation power and temperature

The model of hopping excitons in semiconductors proposed by Baranovskii et al (1998 Phys. Rev. B58 13081) has been modified and applied to explain sharp lines observed in micro-photoluminescence(μ-PL) spectra of GaInNAs alloys and their changes with excitation power and temperature.Instead of two types of recombination centres (radiative and nonradiative centres)introduced by Baranovskii et alwe have proposed one kind of localization centre withradiative and nonradiative rates. Such a modification is justifiable due to ourrecent experimental observations for GaInNAs alloys and allows us to explain thefast thermal quenching of localized emission from this alloy. Our simulationsclearly show that the individual sharp PL lines observed at low temperaturesappear for this material due to exciton hopping between localization centres.Taking into account saturation effects and the exciton dissociation phenomenon, ithas been shown that the observed changes in power- and temperature dependentμ-PL spectra can be excellently reproduced by the modified model.

The Scales of Time, Length, Mass, Energy, and Other Fundamental Physical Quantities in the Atomic World and the Use of Atomic Units in Quantum Mechanical Calculations

This article is divided into two parts. In the first part, the atomic unit (au) system is introduced and the scales of time, space (length), and speed, as well as those of mass and energy, in the atomic world are discussed. In the second part, the utility of atomic units in quantum mechanical and spectroscopic calculations is illustrated with examples. It is shown that the system of atomic units can greatly facilitate calculations and unit conversions. In addition, a number of topics that can be fruitfully treated with the au system, including photoelectric effect, hydrogen atom and different Rydberg constants, positronium atom, atomic mass unit, nuclear binding energy, nuclear reaction energetics, annihilation and mass-energy interconversions, femtosecond and attosecond spectroscopies, quantum-size effects, excitons in semiconductors, and so forth are presented and discussed.

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.

Effect of hyperfine interactions on exciton formation in organic semiconductors

Abstract It is still an open question whether the statistical ratio of 1:3 for the formation of singlet vs. triplet excitons in organic semiconductors can be violated. In this work, we use a two-site and a multi-site model to investigate this problem. We show that spin mixing by hyperfine fields leads to a violation of the statistical ratio if singlet and triplet excitons are formed with a different rate. This only occurs if the exciton formation rate is slow enough compared to the hyperfine precession frequency. The violation of the statistical ratio can be suppressed by applying an external magnetic field. In the multi-site model we demonstrate how disorder and Coulomb interactions affect the fraction of singlet excitons formed.

Dimensional and correlation effects of charged excitons in low-dimensional semiconductors

In this paper, we investigate the existence of bound trion states in fractional dimensional nanostructures, in terms of variational calculus. We start with trial states, then we refine the result with the help of the Hartree–Fock approximation and finally we use a partial basis expansion. We show that Hartree–Fock significantly underestimates the trion binding energy and that the correlation energy is comparable with the trion binding energy. Furthermore we calculate the binding energies of positive and negative trions restricted to a large subspace of functions, which we expect to span the low-lying eigenstates of the full Hamiltonian. We find that the difference between the positive and negative trion binding energies varies very little for the electron–hole mass fractions me/mh = σ ∊ [0.8; 1.0] and that the difference between the positive and negative trion energies grows as the dimension decreases. Finally, we compare a cylindrical effective-mass model of a typical carbon nanotube, with a fractional dimensional model with D = 1.71. We find very good agreement between the trion binding energies predicted by the two models.

Semiconductor-metal nanoparticle molecules in a magnetic field: Spin-plasmon and exciton-plasmon interactions

Excitons and plasmons in hybrid semiconductor-metal nanostructures strongly interact via the Coulomb forces. Simultaneously, excitons in semiconductors experience typically strong intrinsic spin-dependent interactions that result in efficient coupling between spins of excitons and dynamic electric fields of photons. A joint action of the exciton-plasmon and spin-dependent interactions leads to a coupling between spins and plasmons. Calculated optical spectra of hybrid nanoparticle molecules reveal such spin-plasmon coupling. The spin-plasmon coupling creates spin-dependent Lamb shifts of excitons. In the presence of the exciton-plasmon interaction, the spin splitting in the exciton spectrum can decrease or even vanish under certain conditions. In a magnetic field, the spin-plasmon interaction strongly alters optical spin-dependent selection rules, leading to forbidden optical lines. Allowed and forbidden optical transitions in the presence of the exciton-plasmon and spin-plasmon interactions typically acquire Fano-type or antiresonance shapes. Plasmon-induced interactions in semiconductor-metal nanocrystal structures suggest interesting opportunities for tailoring of spin and optical spectra of excitons.

Excitons and Photogenerated Charge Carriers in Organic and Inorganic Semiconductors

Excitons and Photogenerated Charge Carriers in Organic and Inorganic Semiconductors

Tailoring light–matter–spin interactions in colloidal hetero-nanostructures

The interplay between light and matter is the basis of many fundamental processes and various applications. Harnessing light-matter interactions in principle allows operation of solid state devices under new physical principles: for example, the a.c. optical Stark effect (OSE) has enabled coherent quantum control schemes of spins in semiconductors, with the potential for realizing quantum devices based on spin qubits. However, as the dimension of semiconductors is reduced, light-matter coupling is typically weakened, thus limiting applications at the nanoscale. Recent experiments have demonstrated significant enhancement of nanoscale light-matter interactions, albeit with the need for a high-finesse cavity, ultimately preventing device down-scaling and integration. Here we report that a sizable OSE can be achieved at substantial energy detuning in a cavity-free colloidal metal-semiconductor core-shell hetero-nanostructure, in which the metal surface plasmon is tuned to resonate spectrally with a semiconductor exciton transition. We further demonstrate that this resonantly enhanced OSE exhibits polarization dependence and provides a viable mechanism for coherent ultrafast spin manipulation within colloidal nanostructures. The plasmon-exciton resonant nature further enables tailoring of both OSE and spin manipulation by tuning plasmon resonance intensity and frequency. These results open a pathway for tailoring light-matter-spin interactions through plasmon-exciton resonant coupling in a judiciously engineered nanostructure, and offer a basis for future applications in quantum information processing at the nanoscale. More generally, integrated nanostructures with resonantly enhanced light-matter interactions should serve as a test bed for other emerging fields, including nano-biophotonics and nano-energy.

Giant Rabi splitting in metal/semiconductor nanohybrids

We present strong coupling regime between localized plasmon in lithographed nanoparticles and excitons in an organic semiconductor. The lithographed nanoparticles allow a very good control of the particle size and environment, thereby avoiding a large inhomogeneous broadening of the plasmonic resonances which could partially mask the plasmon/exciton hybridization. The nanoparticles diameter ranges from 100 to 200 nm. A giant Rabi splitting energy of 450 meV is obtained, and typical behaviors of mixed states, i.e. anticrossing of their energies and crossing of their linewidths, are observed. Three-dimensional finite-difference time-domain simulations and coupled oscillator calculations are used to analyze and corroborate the experimental results.

Excitation induced tunable emission in biocompatible chitosan capped ZnS nanophosphors

Colloidal semiconductor nanomaterials exhibit color modulation which can be useful for various optoelectronic and biolabelling applications. Previous studies on CdSe and CdTe have shown tunable color modulation by varying the size of nanomaterials but toxicity of cadmium has created doubt for its end applications as biosensor. The recent work on ZnS:Mn2+ doped semiconductor has shown some viability for biolabelling but for tunable behavior particles of different doping concentration needs to be synthesized. In the present work all the above problems were considered and viable solution has been given to use ZnS:Mn2+ doped semiconductor using chitosan as capping agent. Here we report deliberate color modulation for chitosan capped ZnS:Mn2+ nanoparticles (NPs) synthesized in single step instead of many samples of different size. The tunable behavior is achieved by varying the excitation wavelength in same sample. Shifting of emission peak from dopant related emission at 590 nm (d states of Mn2+) to 481 nm defect related emission (Sulfur vacancy to Zn2+) and 421 nm (sulfur vacancy to valence band) has been observed. The work illustrates coupling between Frenkel exciton in organic polymers with Wannier exciton in inorganic semiconductors which has been observed for the present nanophosphors. Synthesized chitosan capped ZnS:Mn2+ NPs have been characterized by x-ray diffraction, high-resolution transmission electron microscopy, energy dispersive x-ray, and Fourier transform infrared for structural studies. UV visible absorption spectroscopy and Photoluminescence studies were done to observe optical behavior. Due to nontoxic nature of chitosan capped ZnS:Mn2+ with tunable color properties in single sample, these materials can be used as biolabels. Biocompatibility of chitosan functionalized ZnS:Mn quantum dots has been also verified with bacteria (staphylococcus aureus).

Hydrostatic deformation potentials and the question of exciton binding energies and splittings in aluminium nitride

By comparing a series of optical experiments performed on bulk aluminum nitride crystals and heteroepitaxial films, we determine the hydrostatic excitonic deformation potentials of AlN. The agreement between the whole available experimental data and our analysis consolidates this determination. Using the previously determined values of the valence-band deformation potentials which account for the strain-induced variation in the crystal-field splitting: ${d}_{3}=\ensuremath{-}8.19\text{ }\text{eV}$ and ${d}_{4}=4.10\text{ }\text{eV}$ we obtain values of $\ensuremath{-}6.04$ and 2.15 eV for the hydrostatic excitonic deformation potentials ${a}_{1}$ and ${a}_{2}$ in the context of the quasicubic approximation. This constitutes the first series of values coherent with the whole set of experimental data. The experimental value of $1s\text{\ensuremath{-}}2s$ splitting disagrees with the theory of excitons in anisotropic semiconductors. This disagreement, we attribute it to our poor knowledge of the valence-band dispersion relations of AlN and to the difficulty we face for including in the calculation plausible values for the anisotropic hole effective mass, dielectric constant.

Internal structure of acceptor-bound excitons in wide-band-gap wurtzite semiconductors

We describe the internal structure of acceptor-bound excitons in wurtzite semiconductors. Our approach consists in first constructing, in the context of angular momentum algebra, the wave functions of the two-hole system that fulfill Pauli's exclusion's principle. Second, we construct the acceptor-bound exciton states by adding the electron states in a similar manner that two-hole states are constructed. We discuss the optical selection rules for the acceptor-bound exciton recombination. Finally, we compare our theory with experimental data for CdS and GaN. In the specific case of CdS for which much experimental information is available, we demonstrate that, compared with cubic semiconductors, the sign of the short-range hole-exchange interaction is reversed and more than one order of magnitude larger. The whole set of data is interpreted in the context of a large value of the short-range hole-exchange interaction ${\ensuremath{\Xi}}_{0}=3.4\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{meV}$. This value dictates the splitting between the ground-state line ${\text{I}}_{1}$ and the other transitions. The values we find for the electron-hole spin-exchange interaction and of the crystal-field splitting of the two-hole state are, respectively, $\ensuremath{-}0.4\ifmmode\pm\else\textpm\fi{}0.1$ and $0.2\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{meV}$. In the case of GaN, the experimental data for the acceptor-bound excitons in the case of Mg and Zn acceptors, show more than one bound-exciton line. We discuss a possible assignment of these states.