Exciton Photon Research Articles

Tunable single-photon emission from dipolaritons

We study a system comprising of a double quantum well embedded in a micropillar optical cavity, where strong coupling between a direct exciton, indirect exciton, and cavity photon is achieved. We show that the resulting hybrid quasiparticles - dipolaritons - can induce strong photon correlations and lead to anti-bunched behaviour of the cavity output field. The origin of the observed single photon emission is attributed to unconventional photon blockade. Moreover, we find that the second-order equal time correlation function $g^{(2)}(0)$ can be tuned over a large range using an electric field applied to the structure, or changing the frequency of the pump. This allows for an on-the-flight control of cavity output properties, and is important for the future generation of tunable single photon emission sources.

Phonon-induced effects on exciton dynamics and photon emission from a semiconductor quantum dot microcavity: phonon coherent state representation

A microscopic theory to study the combined dynamics of a quantum dot exciton coupled to a single cavity mode as well as acoustic phonons is presented. By considering the phonon subsystem in the coherent state representation, the phonon’s impact on the emitted photons is investigated. The single photon emission probability and phonon effects on this quantity are studied. On the other hand, we illustrate that while a quantum dot interacts with a single cavity mode, the collapse and revival phenomenon will occur. It is demonstrated that acoustic phonons have a pronounced impact on the collapse and revival of the quantum dot exciton. Our finding reveals that at elevated temperatures the phonon interaction decreases the Rabi frequency. Moreover, the cavity mode possesses non-classical features such as sub-Poissonian statistics.

Terahertz laser based on dipolaritons

We develop the microscopic theory of a terahertz (THz) laser based on the effects of resonant tunneling in a double quantum well heterostructure embedded in both optical and THz cavities. In the strong coupling regime the system hosts dipolaritons, hybrid quasiparticles formed by the direct exciton, indirect exciton and optical photon, which possess large dipole moments in the growth direction. Their radiative coupling to the mode of a THz cavity combined with strong non-linearities provided by exciton-exciton interactions allows for stable emission of THz radiation in the regime of the continuous optical excitation. The optimal parameters for maximizing the THz signal output power are analyzed.

ChemInform Abstract: Nanochemistry and Nanomaterials for Photovoltaics

AbstractReview: 417 refs.

Ultrastrongly Coupled Exciton–Polaritons in Metal‐Clad Organic Semiconductor Microcavities

Ultrastrong exciton–photon coupling of Frenkel molecular excitons is demonstrated at room temperature in a metal‐clad microcavity containing a thin film of 2,7‐bis[9,9‐di(4‐methylphenyl)‐fluoren‐2‐yl]‐9,9‐di(4‐methylphenyl)fluorene. A giant Rabi splitting of Ω ∼ 1 eV is measured using angle‐resolved reflectivity and bright photoluminescence is observed from the lower polariton branch. To obtain the virtual photon and exciton content of the polariton ground state, the results are interpreted in terms of the full Hopfield Hamiltonian, including anti‐resonant terms. Also included is an analytical treatment of the often ignored and sometimes misinterpreted TM‐polarized metal–insulator–metal plasmon–polariton.

Optical nutation in the exciton range of spectrum

Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.

The convergence of longitudinal excitons onto the Γ5 transverse exciton in GaN and the thermal activation energy of longitudinal excitons

The crystal orientation dependence of GaN excitons was investigated via the photoluminescence (PL) technique. The PL emissions at a temperature of 10 K were obtained from two experimental configurations where the emission K vector (the propagation vector) was either parallel (K ∥ c) or perpendicular (K ∥ c) to the crystal c-axis. Longitudinal, transverse and donor-bound excitons were observed in the two configurations. However, the longitudinal excitons converged onto the transverse free exciton Γ5 in the K⊥c emission. This behavior was discussed in terms of electron screening due to the scattering of electrons moving perpendicular to charged dislocation lines. Additionally, the thermal activation energy of the longitudinal excitons was calculated from the temperature dependent PL measurements collected from the K ∥ c emission, and was found to be 5 to 6 times as high as the binding energy of the free excitons. This high energy was interpreted tentatively in view of the creation of polaritons in strong exciton–photon coupling regimes. These findings present fundamental concepts for applications such as vertical cavity surface-emitting lasers (VCSELs) and polariton lasers.

Room-Temperature Polariton Lasing from GaN Nanowire Array Clad by Dielectric Microcavity

Room-temperature polariton lasing from a GaN-dielectric microcavity is demonstrated with optical excitation. The device is fabricated with a GaN nanowire array clad by Si3N4/SiO2-distributed Bragg reflectors. The nanowire array is initially grown on silicon substrate by molecular beam epitaxy. A distinct nonlinearity in the lower polariton emission is observed at a threshold optical energy density of 625 nJ/cm(2), accompanied by significant line width narrowing to 5 meV and a small blue shift of ~1 meV. The measured polariton dispersion is characterized by a Rabi splitting of 40 meV and a cavity exciton detuning of -17 meV. The device described here is a demonstration of exciton-photon strong coupling phenomenon in an array of light emitters and paves the way for the realization of a room temperature electrically injected polariton laser.

Microcavity controlled coupling of excitonic qubits

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton–photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling.

Measurement and modification of biexciton-exciton time correlations

Photons which are generated in a two-photon cascade process have an underlying time correlation since the spontaneous emission of the upper level populates the intermediate state. This correlation leads to a reduction of the purity of the photon emitted from the intermediate state. Here we characterize this time correlation for the biexciton-exciton cascade of an InAs/GaAs quantum dot. We show that the correlation can be reduced by tuning the biexciton transition in resonance to a planar distributed Bragg reflector cavity. The enhanced and inhibited emission into the cavity accelerates the biexciton emission and slows down the exciton emission thus reduces the correlation and increases the purity of the exciton photon. This is essential for schemes like creating time-bin entangled photon pairs from quantum dot systems.

Polariton Bose–Einstein condensate at room temperature in an Al(Ga)N nanowire–dielectric microcavity with a spatial potential trap

A spatial potential trap is formed in a 6.0-μm Al(Ga)N nanowire by varying the Al composition along its length during epitaxial growth. The polariton emission characteristics of a dielectric microcavity with the single nanowire embedded in-plane have been studied at room temperature. Excitation is provided at the Al(Ga)N end of the nanowire, and polariton emission is observed from the lowest bandgap GaN region within the potential trap. Comparison of the results with those measured in an identical microcavity with a uniform GaN nanowire and having an identical exciton-photon detuning suggests evaporative cooling of the polaritons as they are transported into the trap in the Al(Ga)N nanowire. Measurement of the spectral characteristics of the polariton emission, their momentum distribution, first-order spatial coherence, and time-resolved measurements of polariton cooling provides strong evidence of the formation of a near-equilibrium Bose-Einstein condensate in the GaN region of the nanowire at room temperature. In contrast, the condensate formed in the uniform GaN nanowire-dielectric microcavity without the spatial potential trap is only in self-equilibrium.

Nanochemistry and nanomaterials for photovoltaics

Nanochemistry and nanomaterials provide numerous opportunities for a new generation of photovoltaics with high solar energy conversion efficiencies at low fabrication cost. Quantum-confined nanomaterials and polymer-inorganic nanocomposites can be tailored to harvest sun light over a broad range of the spectrum, while plasmonic structures offer effective ways to reduce the thickness of light-absorbing layers. Multiple exciton generation, singlet exciton fission, photon down-conversion, and photon up-conversion realized in nanostructures, create significant interest for harvesting underutilized ultraviolet and currently unutilized infrared photons. Nanochemical interface engineering of nanoparticle surfaces and junction-interfaces enable enhanced charge separation and collection. In this review, we survey these recent advances employed to introduce new concepts for improving the solar energy conversion efficiency, and reduce the device fabrication cost in photovoltaic technologies. The review concludes with a summary of contributions already made by nanochemistry. It then describes the challenges and opportunities in photovoltaics where the chemical community can play a vital role.

Synthesis and applications of organic nanorods, nanowires and nanotubes

One-dimensional (1D) nanostructures, including nanorods, nanowires, nanotubes, etc., exhibit the quantum confinement effects in the other two dimensions. Nanomaterials with 1D coherence are more suitable for the construction of active nanodevices and interconnects rather than zero-dimensional (0D) amorphous nanoparticles. Inorganic 1D nanomaterials have been widely investigated and widely used as building blocks in many kinds of optoelectronic integrations, and it is very reasonable to assume that their organic counterparts can also play an important role in this field. During the past ten years, organic 1D nanomaterials constructed from small functional molecules have obtained more and more attention due to their unique optical and electronic properties as well as their potential applications in nanoscale devices. Their high structural tunability, reaction activity and processability provide great opportunities to miniaturized optoelectronic chips based on organic 1D nanostructures, since they are usually assembled from molecular units with weak intermolecular interactions, such as hydrogen bonds, π–π stacking and van der Waals force. These weak interactions allow for more facile and mild conditions in the fabrication of high quality organic 1D nanostructures rather than those in the construction of their inorganic counterparts. More importantly, very recent studies reveal that the diversity of energy/electron transfer processes in organic semiconductors brings new hopes to break the performance limitations of traditional photonic and electronic devices, thus allowing higher luminescence intensity, more efficient photon confinement, stronger exciton–photon coupling, and so on. Indeed, organic 1D nanomaterials have already emerged to play increasingly an important role in many optoelectronic applications, such as nanolasers, optical waveguides, light-emitting devices, solar cells and sensors. In the past two decades, people have not only witnessed but also taken for granted the rapid development of nanomaterial science, and here we would like to promote awareness of the significance of organic 1D nanomaterials in the field of nanotechnology and optoelectronic nanodevices. This report presents a comprehensive review about recent research in the preparation and applications of 1D nanomaterials from functional low-molecular-weight organic compounds, whose optical and electronic properties are fundamentally different from those of their inorganic counterparts. Here we try to summarize the important breakthroughs from the fabrication of organic nanorods, nanowires and nanotubes, to the application of these nanostructures in integrated photonic elements and optoelectronic nanodevices. We begin with a general summary of the construction strategies (liquid-phase assembly, vapor deposition and template methods) for achieving 1D nanostructures from small organic functional molecules, then provide an overview of the unique optoelectronic properties induced by molecular aggregation in the nanostructures. Special emphasis is put on the luminescent properties of low dimensional sizes that are different from those of the corresponding bulk materials. This offers the materials better photon confinement ability or charge carrier transport property, and hence better optoelectronic performances such as optical waveguiding, multicolor emission, low-threshold nanolasers, light-emitting devices, photon-detecting devices, etc., which are presented one by one in the following section. In the last part of this report, we conclude with our personal viewpoints of the future development of organic 1D nanomaterials and also their great potentials in highly integrated photonic and electronic devices and chips.

Ultracompact Asymmetric Mach–Zehnder Interferometers with High Visibility Constructed from Exciton Polariton Waveguides of Organic Dye Nanofibers

AbstractManipulation of light using subwavelength waveguides is a key technology in the development of miniaturized photonic circuits, which possess various advantages over their electronic counterparts. The novel approach presented for such waveguiding involves the propagation of exciton polaritons (EPs), which are quasi‐particles formed by strong exciton–photon coupling, along organic dye nanofibers. A self‐assembled nanofiber of thiacyanine (TC) with a width of ≈200 nm propagates the EPs created by an optical excitation over a submillimeter‐scale distance and passes through a bend with a micrometer‐scale radius with low bending loss. To demonstrate the remarkable potential of EP‐based miniaturized photonic circuits, asymmetric Mach–Zehnder interferometers (AMZIs) are fabricated with TC nanofibers by micromanipulation. The AMZIs with a footprint of ≈20 μm × 20 μm exhibit a visibility of nearly unity and function as channel drop filters with the considerably high extinction ratio of up to ≈15 dB. Such high‐performance and ultracompact channel drop filters operating in the visible wavelength region have rarely been developed with other waveguide technologies. The coherent properties of the EPs in the nanofibers are investigated using time‐resolved experiments. The coherent properties provide useful information for designing EP‐based photonic circuits and for understanding EP dynamics in a nanofiber.

Diamagnetism of microcavity polaritons induced by spin-dependent polariton–polariton interactions

Diamagnetism of condensed microcavity polaritons in a vertically applied magnetic field is theoretically studied by using the density of free energy of polaritons. The magnetic dependence of polariton–polariton interactions and spin polarization degree of polaritons are derived, and are used to show the diamagnetic behavior of the polariton spin polarization, which is discussed for GaAs-based microcavities. We show that for strong magnetic field the spin polarization of the polaritons is paramagnetic as usual, while around positive exciton–photon detuning and special Rabi splitting, the spin polarization of the polaritons could be diamagnetic. In addition, weak magnetic field and high polariton density are beneficial to observe the polariton diamagnetism.

Chaos and regularity in semiconductor microcavities

Our work presents a study on the nonlinear dynamical behavior for a microcavity semiconductor containing a quantum well. Using an external periodic perturbation in energy level we observe the period-doubling, quasiperiodic, and direct route to chaos as forcing strength is changed. For a particular case the riddled basin for coexisting periodic and chaotic motions are observed. These results suggest that the dynamics of exciton–photon is quite complex in presence of external perturbation.

Saturation behaviour of colloidal PbSe quantum dot exciton emission coupled into silicon photonic circuits

We report coupling of the excitonic photon emission from photoexcited PbSe colloidal quantum dots (QDs) into an optical circuit that was fabricated in a silicon-on-insulator wafer using a CMOS-compatible process. The coupling between excitons and sub-μm sized silicon channel waveguides was mediated by a photonic crystal microcavity. The intensity of the coupled light saturates rapidly with the optical excitation power. The saturation behaviour was quantitatively studied using an isolated photonic crystal cavity with PbSe QDs site-selectively located at the cavity mode antinode position. Saturation occurs when a few μW of continuous wave HeNe pump power excites the QDs with a Gaussian spot size of 2 μm. By comparing the results with a master equation analysis that rigorously accounts for the complex dielectric environment of the QD excitons, the saturation is attributed to ground state depletion due to a non-radiative exciton decay channel with a trap state lifetime ~ 3 μs.

Polarization splitting in polariton electroluminescence from an organic semiconductor microcavity with metallic reflectors

Organic semiconductors have received considerable attention as the active medium in microcavity devices that exploit the regime of strong exciton–photon coupling. The eigenstates of these systems are microcavity polaritons, whose properties are an admixture of the uncoupled exciton and photon. Organic microcavities are particularly interesting due to their large exciton binding energy which permits the electrical excitation of polaritons at room temperature. Measurements of electroluminescence are often facilitated through the use of metallic reflectors that form the optical microcavity and also serve as device electrodes. Here, we demonstrate that such structures exhibit a significant polarization splitting under both optical and electrical excitation. The size of the polarization splitting rivals those observed in strongly coupled microcavities based on distributed Bragg reflectors having a long optical penetration depth.

Erratum: “Guided Bloch surface wave polaritons” [Appl. Phys. Lett. 98, 121118 (2011)

There is a conceptual mistake in the extraction of the Rabi splitting from the simulated data shown in Fig. 2. In fact, the Rabi splitting should be evaluated from the dispersion as a function of wave vector, which, for our structure, yields a corrected value of 5.4 meV instead of 6 meV. Therefore, in the abstract the corrected text should be: “the exciton photon coupling is more than 40% larger as compared to....” The text at the bottom of the first column in pp. 121118–2 should read as “the predicted resonance splitting is about 5.4 meV. This value is more than 40% larger....” In the caption of Fig. 2 we correct as “ b attenuated total reflectance ATR spectrum for the angle of incidence at which the exciton and the photon resonances have the same energy, i.e., dashed line in a .”

Silicon doping effects on optical properties of InAs ultrathin layer embedded in GaAs/AlGaAs:δSi high electron mobility transistors structures

Micro-Raman scattering measurements were used to study the silicon delta-doped layer density variation effect on InAs ultrathin layer embedded in silicon-delta-doped GaAs/AlGaAs high electron mobility transistors (HEMTs) structures properties. These structures were grown by molecular beam epitaxy on GaAs substrates with different silicon (Si) delta-doped layer densities. Two coupled plasmon–longitudinal optical (LO) phonon modes (L− and L+) were observed in the micro-Raman spectra of the Si-delta-doped samples, and both their wave numbers and intensities were dependent on the silicon delta-doped layer density. There is evidence to suggest that the increase of the Si doping level results in the increase of exciton–phonon scattering which is mainly due to the incorporation of Si and the increase of the two-dimensional electron gas (2DEG) in the InAs/GaAs interface. From fitting the temperature-dependence of full width at half maximum (FWHM) of quantum well’s photoluminescence peak (P 1) by the exciton–photon coupling model, it was found that the interaction between exciton and phonon in Si-delta-doped quantum wells was higher than that in the undoped sample. This result was confirmed as resulting from the increase of plasmon–phonon scattering which is attributed to the increase of free carriers donated from implanted Si dopant. The self-consistent Poisson–Schrödinger model calculation results are in good agreement with the experimental results, where the 2DEG densities increase linearly with increasing the Si-delta-doped layer density.