Showing papers on "Spontaneous emission published in 2009"
TL;DR: In this paper, the InGaN based laser diodes (LDs) on semi-polar free-standing GaN substrates were demonstrated under pulsed operation at room temperature.
Abstract: Lasing in pure green region around 520 nm of InGaN based laser diodes (LDs) on semi-polar {2021} free-standing GaN substrates was demonstrated under pulsed operation at room temperature. The longest lasing wavelength reached to 531 nm and typical threshold current density was 8.2 kA/cm2 for 520 nm LDs. Utilization of a novel {2021} plane enabled a fabrication of homogeneous InGaN quantum wells (QWs) even at high In composition, which is exhibited with narrower spectral widths of spontaneous emission from LDs than those on other planes. The high quality InGaN QWs on the {2021} plane advanced the realization of the green LDs.
410 citations
TL;DR: A strictly resonant continuous-wave excitation together with controlling the spontaneous emission lifetime of the single quantum dots via tunable emitter-mode coupling (Purcell) is proven as a versatile scheme to generate close to Fourier transform-limited (T2/(2T1)=0.91) single photons even at 80% of the emission saturation level.
Abstract: Applying continuous-wave pure resonant $s$-shell optical excitation of individual quantum dots in a high-quality micropillar cavity, we demonstrate the generation of post-selected indistinguishable photons in resonance fluorescence. Close to ideal visibility contrast of 90% is verified by polarization-dependent Hong-Ou-Mandel two-photon interference measurements. Furthermore, a strictly resonant continuous-wave excitation together with controlling the spontaneous emission lifetime of the single quantum dots via tunable emitter-mode coupling (Purcell) is proven as a versatile scheme to generate close to Fourier transform-limited (${T}_{2}/(2{T}_{1})=0.91$) single photons even at 80% of the emission saturation level.
256 citations
TL;DR: Cooperative single-photon emission from an atom ensemble will provide insights into quantum electrodynamics and applications in quantum communication and describe the cooperative, spontaneous emission of photons from a collection of atoms.
Abstract: In 1954, Robert Dicke introduced the concept of superradiance in describing the cooperative, spontaneous emission of photons from a collection of atoms. The concept of superradiance can be understood by picturing each atom as a tiny antenna emitting electromagnetic waves. Thermally excited atoms emit light randomly, and the emitted intensity is a function of the number of atoms, N . However, when the atomic “antennas” are coherently radiating in phase with each other, the net electromagnetic field is proportional to N , and therefore, the emitted intensity goes as N 2. As a result, the atoms radiate their energy N times faster than for incoherent emission. It is this anomalous radiance that Dicke dubbed “superradiance” ( 1 – 3 ).
251 citations
TL;DR: In this article, a pump-probe experiment was conducted on photoexcitation by a pump pulse, and the sample was stimulated down to the ground state by a time-delayed probe pulse, the intensity of which was concurrently increased.
Abstract: Fluorescence, that is, spontaneous emission, is generally more sensitive than absorption measurement, and is widely used in optical imaging. However, many chromophores, such as haemoglobin and cytochromes, absorb but have undetectable fluorescence because the spontaneous emission is dominated by their fast non-radiative decay. Yet the detection of their absorption is difficult under a microscope. Here we use stimulated emission, which competes effectively with the nonradiative decay, to make the chromophores detectable, and report a new contrast mechanism for optical microscopy. In a pump-probe experiment, on photoexcitation by a pump pulse, the sample is stimulated down to the ground state by a time-delayed probe pulse, the intensity of which is concurrently increased. We extract the miniscule intensity increase with shot-noise-limited sensitivity by using a lock-in amplifier and intensity modulation of the pump beam at a high megahertz frequency. The signal is generated only at the laser foci owing to the nonlinear dependence on the input intensities, providing intrinsic three-dimensional optical sectioning capability. In contrast, conventional one-beam absorption measurement exhibits low sensitivity, lack of three-dimensional sectioning capability, and complication by linear scattering of heterogeneous samples. We demonstrate a variety of applications of stimulated emission microscopy, such as visualizing chromoproteins, non-fluorescent variants of the green fluorescent protein, monitoring lacZ gene expression with a chromogenic reporter, mapping transdermal drug distributions without histological sectioning, and label-free microvascular imaging based on endogenous contrast of haemoglobin. For all these applications, sensitivity is orders of magnitude higher than for spontaneous emission or absorption contrast, permitting nonfluorescent reporters for molecular imaging.
249 citations
TL;DR: Using fully-vectorial calculations and a comprehensive Fabry-Perot model, it is shown that various realistic nanowire geometries may act as nanoantennas that assist funnelling the emitted photons into a single monomode channel.
Abstract: We design several single-photon-sources based on the emission of a quantum dot embedded in a semiconductor (GaAs) nanowire. Through various taper designs, we engineer the nanowire ends to realize efficient metallic-dielectric mirrors and to reduce the divergence of the far-field radiation diagram. Using fully-vectorial calculations and a comprehensive Fabry-Perot model, we show that various realistic nanowire geometries may act as nanoantennas (volume of ≈0.05 λ3) that assist funnelling the emitted photons into a single monomode channel. Typically, very high extraction efficiencies above 90% are predicted for a collection optics with a numerical aperture NA=0.85. In addition, since no frequency-selective effect is used in our design, this large efficiency is achieved over a remarkably broad spectral range, Δλ=70 nm at λ=950 nm.
234 citations
TL;DR: In this article, a strain-compensated InGaN-AlGaN quantum well (QW) structure consisting of thin tensile-strained AlGaN barriers surrounding the QW was investigated as improved active regions for lasers and light emitting diodes.
Abstract: Strain-compensated InGaN-AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure was calculated by using a self-consistent 6-band kmiddotp formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The spontaneous emission and gain properties were analyzed for strain-compensated InGaN-AlGaN QW structures with indium contents of 28%, 22%, and 15% for lasers (light-emitting diodes) emitting at 480 (500), 440 (450), and 405 nm (415 nm) spectral regimes, respectively. The spontaneous emission spectra show significant improvement of the radiative emission for strain-compensated QW for all three structures compared to the corresponding conventional InGaN QW, which indicates the enhanced radiative efficiency for light emitting diodes. Our studies show the improvement of the optical gain and reduction of the threshold current density from the use of strain-compensated InGaN-AlGaN QW as active regions for diode lasers.
221 citations
TL;DR: In this article, the size dependence of the band gap, spontaneous emission rate, and absorption cross section of quantum dots is systematically investigated over a wide size range, using colloidal CdSe and CdTe QDs as model systems.
Abstract: In this paper, the size dependence of the band gap, of the spontaneous emission rate, and of the absorption cross section of quantum dots is systematically investigated over a wide size range, using colloidal CdSe and CdTe QDs as model systems (diameters ranging from 12 to 8 nm and from 2 to 95 nm, respectively) The size dependence of the band gap is well-described by theoretical models, and is dominated by the quantum confinement contribution (1/d2 scaling) The spontaneous emission rate increases linearly with the emission frequency for both CdSe and CdTe QDs, in good agreement with theoretical predictions By extrapolating the frequency dependence of the emission rates to the bulk band gap values, the exciton radiative lifetime in bulk CdSe and CdTe could be estimated for the first time (viz, 18 and 20 ns, respectively) Comparison between the empirical trends and theoretical predictions provides new fundamental insights into the size dependence of the 1S(e)1S3/2(h) oscillator strengths of QDs, bot
193 citations
TL;DR: In this paper, the quantum-confined Stark effect can be employed to quickly and reversibly switch the dot-cavity coupling simply by varying a gate voltage, with Purcell factors ≥ 7.
Abstract: We report the design, fabrication and optical investigation of electrically tunable single quantum dots—photonic crystal defect nanocavities operating in both the weak and strong coupling regimes of the light–matter interaction. Unlike previous studies where the dot–cavity spectral detuning was varied by changing the lattice temperature, or by the adsorption of inert gases at low temperatures, we demonstrate that the quantum-confined Stark effect can be employed to quickly and reversibly switch the dot–cavity coupling simply by varying a gate voltage. Our results show that exciton transitions from individual dots can be tuned by ~4 meV relative to the nanocavity mode before the emission quenches due to carrier tunneling escape. This range is much larger than the typical linewidth of the high-Q cavity modes (~100 μeV) allowing us to explore and contrast regimes where the dots couple to the cavity or decay by spontaneous emission into the two-dimensional photonic bandgap. In the weak-coupling regime, we show that the dot spontaneous emission rate can be tuned using a gate voltage, with Purcell factors ≥7. New information is obtained on the nature of the dot–cavity coupling in the weak coupling regime, and electrical control of zero-dimensional polaritons is demonstrated for the highest-Q cavities (Q≥12 000). Vacuum Rabi splittings up to ~120 μeV are observed, larger than the linewidths of either the decoupled exciton (γ≤40 μeV) or cavity mode. These observations represent a voltage switchable optical nonlinearity at the single photon level, paving the way towards on-chip dot-based nano-photonic devices that can be integrated with passive optical components.
175 citations
TL;DR: This Letter discusses how linear coupled plasmon particle arrays inspired by radio frequency Yagi-Uda antennas can be used to construct both efficient unidirectional single photon sources and efficient directional single plAsmon sources.
Abstract: This Letter discusses how linear coupled plasmon particle arrays inspired by radio frequency Yagi-Uda antennas can be used to construct both efficient unidirectional single photon sources and efficient directional single plasmon sources. Calculations using an exact multipole expansion method are presented of the spontaneous emission directivity, efficiency, and spontaneous emission decay rates, taking into account material loss in real noble metals. An analysis of the emission properties in terms of the dispersion relation of infinite arrays reveals how one can use guided mode dispersion to achieve desirable figures of merit. The key ingredient is to couple the source to array eigenmodes that are just beyond the light line but still wave vector matched to propagating modes to within the momentum uncertainty set by the inverse antenna length. Finally, this Letter shows that the emission decay rates can be controlled independently of the directionality and without penalty in quantum efficiency.
175 citations
TL;DR: In this article, a three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520-525 nm were grown by metal-organic chemical vapor deposition by employing a graded growth-temperature profile.
Abstract: Three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520–525 nm were grown by metal-organic chemical vapor deposition by employing graded growth-temperature profile. The use of staggered InGaN QW, with improved electron-hole wave functions overlap design, leads to an enhancement of its radiative recombination rate. Both cathodoluminescence and electroluminescence measurements of three-layer staggered InGaN QW LED exhibited enhancements by 1.8–2.8 and 2.0–3.5 times, respectively, over those of conventional InGaN QW LED.
172 citations
TL;DR: A quantum optical model of the system is proposed and fit to the measured spectra, from which the NV(-) zero phonon line coherent coupling rate to the microdisk is estimated to be 28 MHz for a nearly optimally placed nanocrystal.
Abstract: Diamond nanocrystals containing NV color centers are positioned with 100-nanometer-scale accuracy in the near-field of a high-Q SiO2 microdisk cavity using a fiber taper. The cavity modified nanocrystal photoluminescence is studied, with Fano-like quantum interference features observed in the far-field emission spectrum. A quantum optical model of the system is proposed and fit to the measured spectra, from which the NV-zero phonon line coherent coupling rate to the microdisk is estimated to be 28 MHz for a nearly optimally placed nanocrystal.
TL;DR: In this article, a self-consistent 6-band kmiddotp formalism taking into account the valence band mixing, strain effect, and spontaneous and piezoelectric polarizations as well as the carrier screening effect is presented.
Abstract: Staggered InGaN quantum wells (QWs) are analyzed as improved active region for light-emitting diodes (LEDs) emitting at 500 nm and 540 nm, respectively. The calculation of band structure is based on a self-consistent 6-band kmiddotp formalism taking into account the valence band mixing, strain effect, and spontaneous and piezoelectric polarizations as well as the carrier screening effect. Both two-layer staggered InxGa1- xN/InyGa1- yN QW and three-layer staggered InyGa1- yN/InxGa1- xN/InyGa1- yN QW structures are investigated as active region to enhance the spontaneous emission radiative recombination rate (R sp) for LEDs emitting at 500 nm and 540 nm. Analysis of the spontaneous emission radiative recombination rate (R sp) shows significant enhancement for both two-layer staggered InGaN QW and three-layer staggered InGaN QW, in comparison to that of the conventional InzGa1- zN QW. The studies of the carrier lifetime indicate a significant reduction of the carrier lifetime for staggered InGaN QWs, which contribute to the enhancement of the radiative efficiency for both two-layer staggered InGaN QW and three-layer staggered InGaN QW LEDs emitting at 500 nm and 540 nm.
TL;DR: It is shown that the quantum interference between two spontaneous emission channels can be greatly enhanced when a three-level V-type atom is placed near plasmonic nanostructures such as metallic slabs, nanospheres, or periodic arrays of metal-coated spheres.
Abstract: We show that the quantum interference between two spontaneous emission channels can be greatly enhanced when a three-level V-type atom is placed near plasmonic nanostructures such as metallic slabs, nanospheres, or periodic arrays of metal-coated spheres. The spontaneous emission rate is calculated by a rigorous first-principles electromagnetic Green's tensor technique. The enhancement of quantum interference is attributed to the strong dependence of the spontaneous emission rate on the orientation of an atomic dipole relative to the surface of the nanostructure at the excitation frequencies of surface plasmons.
TL;DR: In this paper, the authors demonstrate experimentally bipolar (electrons and holes) current injection into silicon nanocrystals in thin nanocrystalline-Si/SiO2 multilayers.
Abstract: We demonstrate experimentally bipolar (electrons and holes) current injection into silicon nanocrystals in thin nanocrystalline-Si/SiO2 multilayers. These light emitting devices have power efficiency of 0.17% and turn-on voltage of 1.7 V. The high electroluminescence efficiency and low onset voltages are attributed to the radiative recombination of excitons formed by both electron and hole injection into silicon nanocrystals via the direct tunneling mechanism. To confirm the bipolar character, different devices were grown, with and without a thick silicon oxide barrier at the multilayer contact electrodes. A transition from bipolar tunneling to unipolar Fowler–Nordheim tunneling is thus observed.
TL;DR: In this article, the enhanced spontaneous emission factors (Purcell factors) and Lamb shifts from a quantum dot or atom near the surface of a %embedded in a slow-light metamaterial waveguide were investigated.
Abstract: Employing a medium-dependent quantum optics formalism and a Green function solution of Maxwell's equations, we study the enhanced spontaneous emission factors (Purcell factors) and Lamb shifts from a quantum dot or atom near the surface of a %embedded in a slow-light metamaterial waveguide. Purcell factors of approximately 250 and 100 are found at optical frequencies for $p-$polarized and $s-$polarized dipoles respectively placed 28\thinspace nm (0.02\thinspace $\lambda_{0}$) above the slab surface, including a realistic metamaterial loss factor of $\gamma /2\pi =2 \mathrm{THz}$. For smaller loss values, we demonstrate that the slow-light regime of odd metamaterial waveguide propagation modes can be observed and related to distinct resonances in the Purcell factors. Correspondingly, we predict unusually large and rich Lamb shifts of approximately -1 GHz to -6 GHz for a dipole moment of 50 Debye. We also make a direct calculation of the far field emission spectrum, which contains direct measurable access to these enhanced Purcell factors and Lamb shifts.
TL;DR: The surface plasmon-enhanced spontaneous emission based on an organic light-emitting device is reported in this article, where silver nanoparticles were thermally deposited in a high vacuum on cathode that had a 1-nm-thick LiF spacer.
Abstract: The surface plasmon-enhanced spontaneous emission based on an organic light-emitting device is reported in this paper. For surface plasmon localization, silver nanoparticles were thermally deposited in a high vacuum on cathode that had a 1-nm-thick LiF spacer. Since plasmons provide a strong oscillator decay channel, time-resolved photoluminescence (PL) results displayed a 1.75-fold increased emission rate, and continuous wave PL results showed a twofold enhanced intensity. In addition, LiF film/Ag cluster/LiF film structure resolved the carrier injection problem between the cathode and the organic layer. Thus, the suggested design may follow plasmonic applications for a wider organic optoelectronics.
TL;DR: A design for an ultra-high Q photonic crystal nanocavity engineered to interact with nitrogen-vacancy centers located near the surface of a single crystal diamond sample is presented and promises to allow Purcell enhanced collection of spontaneous emission from an NV located more than 50 nm below the diamond surface.
Abstract: A design for an ultra-high Q photonic crystal nanocavity engineered to interact with nitrogen-vacancy (NV) centers located near the surface of a single crystal diamond sample is presented. The structure is based upon a nanowire photonic crystal geometry, and consists of a patterned high refractive index thin film, such as gallium phosphide (GaP), supported by a diamond substrate. The nanocavity supports a mode with quality factor Q>1.5×106 and mode volume V<0.52(λ/nGaP)3, and promises to allow Purcell enhanced collection of spontaneous emission from an NV located more than 50 nm below the diamond surface. The nanowire photonic crystal waveguide can be used to efficiently couple light into and out of the cavity, or as an efficient broadband collector of NV phonon sideband emission. The proposed structures can be fabricated using existing materials and processing techniques.
TL;DR: The two-photon absorption (TPA)-induced room-temperature lasing performance of ZnO nanowires is reported, with a remarkably low threshold for the TPA-induced lasing action, which is of the same order of magnitude as that measured for the linear lasing process.
Abstract: We report in this communication the two-photon absorption (TPA)-induced room-temperature lasing performance of ZnO nanowires. Under femtosecond pulse-excitation at lambda = 700 nm in the infrared regime, a remarkably low threshold of 160 microJ/cm(2) was observed for the TPA-induced lasing action, which is of the same order of magnitude as that measured for the linear lasing process. Time-resolved photoluminescence characterization of two-photon pumped ZnO nanowires reveals the presence of a fast decay (3-4 ps) in the stimulated emission as compared to the slow decay (50-70 ps) for the spontaneous emission. The TPA process in ZnO nanowires was characterized with the nonlinear transmission measurement, which uncovers an enhanced TPA coefficient, about 14.7 times larger than that of bulk ZnO samples. The observed TPA enhancement in ZnO nanowires accounts for the low threshold lasing behavior, and has been attributed to the intensified optical field confined within the nanowire waveguides.
TL;DR: In this article, a self-consistent six-band k·p method was used to investigate the active region for light-emitting diodes (LEDs) emitting at 520-525 nm.
Abstract: Staggered InGaN quantum wells (QWs) are investigated both numerically and experimentally as improved active region for light-emitting diodes (LEDs) emitting at 520–525 nm. Based on a self-consistent six-band k·p method, band structures of both two-layer staggered InxGa1−xN/InyGa1−yN QW and three-layer staggered InyGa1−yN/InxGa1−xN/InyGa1−yN QW structures are investigated as active region to enhance the spontaneous emission radiative recombination rate (Rsp) for LEDs emitting at 520–525 nm. Numerical analysis shows significant enhancement of Rsp for both two-layer and three-layer staggered InGaN QWs as compared to that of the conventional InzGa1−zN QW. Significant reduction of the radiative carrier lifetime contributes to the enhancement of the radiative efficiency for both two-layer and three-layer staggered InGaN QW LEDs emitting at 520–525 nm. Three-layer staggered InGaN QW LEDs emitting at 520–525 nm was grown by metal-organic chemical vapour deposition (MOCVD) by employing graded-temperature profile. Power density-dependent cathodoluminescence (CL) measurements show the enhancement of peak luminescence by up to 3 times and integrated luminescence by 1.8–2.8 times for the three-layer staggered InGaN QW LED. Electroluminescence (EL) output power of the staggered InGaN QW LED exhibits 2.0–3.5 times enhancement as compared to that of the conventional InGaN QW LED. The experimental results show the good agreement with theory.
TL;DR: It is demonstrated that light scattering is highly preferential: more than 99.8% of the photon flux is scattered into the original doubly degenerate cavity modes, an effect that is more typically associated with spontaneous emission.
Abstract: Scattering processes in an optical microcavity are investigated for the case of silicon nanocrystals embedded in an ultra-high-Q toroid microcavity. Using a novel measurement technique based on the observable mode splitting, we demonstrate that light scattering is highly preferential: more than 99.8% of the photon flux is scattered into the original doubly degenerate cavity modes. The large capture efficiency is shown to result from the Purcell enhancement of the optical density of states over the free space value, an effect that is more typically associated with spontaneous emission. The experimentally determined Purcell factor has a lower bound of 171.
TL;DR: In this paper, the decay rate of colloidal CdSe quantum dots in an environment with a controlled local density of states (LDOS) was measured versus frequency and as a function of distance to a mirror.
Abstract: We study experimentally time-resolved emission of colloidal CdSe quantum dots in an environment with a controlled local density of states (LDOS). The decay rate is measured versus frequency and as a function of distance to a mirror. We observe a linear relation between the decay rate and the LDOS, allowing us to determine the size-dependent quantum efficiency and oscillator strength. We find that the quantum efficiency decreases with increasing emission energy mostly due to an increase in nonradiative decay. We manage to obtain the oscillator strength of the important class of CdSe quantum dots. The oscillator strength varies weakly with frequency in agreement with behavior of quantum dots in the strong confinement limit. Surprisingly, previously calculated tight-binding results differ by a factor of 5 with the measured absolute values. Results from pseudopotential calculations agree well with the measured radiative rates. Our results are relevant for applications of CdSe quantum dots in spontaneous emission control and cavity quantum electrodynamics
TL;DR: In this article, a single photon source consisting of an InAs quantum dot coupled to a directional-emission photonic crystal (PC) cavity implemented in GaAs was analyzed, and the cavity design also enabled efficient coupling to a higher-order cavity mode for local optical excitation of cavity-coupled quantum dots.
Abstract: We analyze a single photon source consisting of an InAs quantum dot coupled to a directional-emission photonic crystal (PC) cavity implemented in GaAs. On resonance, the dot’s lifetime is reduced by more than 10 times, to 45ps. Compared to the standard three-hole defect cavity, the perturbed PC cavity design improves the collection efficiency into an objective lens (NA = 0.75) by factor 4.5, and improves the coupling efficiency of the collected light into a single mode fiber by factor 1.9. The emission frequency is determined by the cavity mode, which is antibunched to g^(2)(0) = 0.05. The cavity design also enables efficient coupling to a higher-order cavity mode for local optical excitation of cavity-coupled quantum dots.
TL;DR: Results obtained by an advanced growth of site-controlled quantum dots (SCQDs) on pre-patterned nanoholes and their integration into both photonic resonators and nanoelectronic memories are summarized.
Abstract: Results obtained by an advanced growth of site-controlled quantum dots (SCQDs) on pre-patterned nanoholes and their integration into both photonic resonators and nanoelectronic memories are summarized. A specific technique has been pursued to improve the optical quality of single SCQDs. Quantum dot (QD) layers have been vertically stacked but spectrally detuned for single SCQD studies. Thereby, the average emission linewidth of single QDs could be reduced from 2.3 meV for SCQDs in a first QD layer close to the etched nanoholes down to 600 µeV in the third InAs QD layer. Accurate SCQD nucleation on large QD distances is maintained by vertical strain induced QD coupling throughout the QD stacks. Record narrow linewidths of individual SCQDs down to ~110 µeV have been obtained. Experiments performed on coupled photonic SCQD–resonator devices show an enhancement of spontaneous emission. SCQDs have also been integrated deterministically in high electron mobility heterostructures and flash memory operation at room temperature has been observed.
TL;DR: The ultraviolet (UV) emission from the Au-coated ZnO films was greatly enhanced and the visible emission was significantly suppressed compared with the un- coated ZNO films.
Abstract: The ultraviolet (UV) emission from the Au-coated ZnO films was greatly enhanced and the visible emission was significantly suppressed compared with the un-coated ZnO films. Great changes in photoluminescence of ZnO films are attributed to the electron transfer between conduction band and defect levels through the localized surface plasmons. The increase of electron density in conduction band causes enhanced UV emission, while the decrease in electron density in defect level leads to the suppression of the visible emission. Such ZnO films with enhanced UV emission have potential applications in the highly efficient solid state emitters.
TL;DR: The use of low-gain optical parametric amplification (OPA) as a means of improving temporal contrast to a detection-limited level 10(-10) is demonstrated.
Abstract: We demonstrate the use of low-gain optical parametric amplification (OPA) as a means of improving temporal contrast to a detection-limited level 10−10. 250 μJ, 500 fs pulses of 1053 nm are frequency doubled and subsequently restored to the original wavelength by OPA with >10% efficiency.
TL;DR: In this article, a multiband effective mass theory was used to investigate the optical properties of a quantum-well (QW) light-emitting-diodes with a staggered InGaN layer.
Abstract: Optical properties of staggered 530 nm InGaN/InGaN/GaN quantum-well (QW) light-emitting-diodes are investigated using the multiband effective mass theory. These results are compared with those of conventional 530 nm InGaN/GaN QW structures. A staggered InGaN/InGaN/GaN QW structure is shown to have much larger spontaneous emission than a conventional InGaN/GaN QW structure. This can be explained by the fact that a staggered QW structure has much larger matrix element than a conventional QW structure because a spatial separation between electron and hole wave functions is substantially reduced with the inclusion of a staggered InGaN layer. A staggered QW structure shows that the peak position at a high carrier density (530 nm) is similar to that at a noninjection level.
TL;DR: The enhanced luminescence efficiency and the strong coupling into a limited set of well-defined waveguide modes enables a new class of power-efficient, CMOS-compatible, waveguide-based light sources.
Abstract: We investigate the light emission properties of electrical dipole emitters inside 2-dimensional (2D) and 3-dimensional (3D) silicon slot waveguides and evaluate the spontaneous emission enhancement (F_p) and waveguide coupling ratio (β). Under realistic conditions, we find that greater than 10-fold enhancement in F_p can be achieved, together with a β as large as 0.95. In contrast to the case of high Q optical resonators, such performance enhancements are obtained over a broad wavelength region, which can cover the entire emission spectrum of popular optical dopants such as Er. The enhanced luminescence efficiency and the strong coupling into a limited set of well-defined waveguide modes enables a new class of power-efficient, CMOS-compatible, waveguide-based light sources.
TL;DR: In this article, structural, optical and electrical properties of Si quantum dots formed as multilayers in a SiO 2 matrix with various phosphorus (P) concentrations introduced during the sputtering process were investigated for its potential application in all-silicon quantum dot tandem solar cells.
TL;DR: Using some fundamental symmetries of pulse polarization configurations of nonlinear signals, the authors can construct superpositions of signals designed to better distinguish among various coherent and incoherent exciton transport pathways and amplify subtle variations among different species of the Fenna-Matthews-Olson antenna complex.
Abstract: Linear-spectroscopy is one-dimensional (1D); the absorption spectrum provides information about excitation energies and transition dipoles as projected into a single frequency axis. In contrast, multidimensional optical spectroscopy uses sequences of laser pulses to perturb or label the electronic degrees of freedom and watch for correlated events taking place during several controlled time intervals. The resulting correlation plots can be interpreted in terms of multipoint correlation functions that carry considerably more detailed information on dynamical events than the two-point functions provided by 1D techniques1–7. Correlations between spins have been routinely used in NMR to study complex molecules. The Nobel prize was awarded to Richard Ernst8 for inventing the technique and to Kurt Wuthrich9 for developing pulse sequences suitable for large proteins. Optical analogues of 2D NMR techniques first designed to study vibrational dynamics by Raman or infrared pulses1 and later extended to resonant electronic excitations in chromophore aggregates10 have been made possible thanks to the development of stable femtosecond laser sources with controlled phases11. In an ideal heterodyne-detected 2D experiment (Fig. 1) 3 laser pulses with wavevectors k1, k2, k3 interact sequentially with the molecules in the sample to create a polarization with wavevector k4 given by one of the linear combinations ±k1 ±k2 ±k3. In all other directors the polarization vanishes due to the random phases of contributions from different molecules. The coherent signal is generated in directions close to the various possible k4. The missmatch caused by frequency variation of the index of refraction is optimized (“phase matched”) to generate an intense signal detected by interference with a 4th pulse at the desired wavevector k4. When the radiation field is described quantum mechanically the entire process can be viewed as a concerted 4 photon process. The signal S(t3,t2,t1) depends parametrically on the time intervals between pulses which constitute the primary control-parameters. Other parameters include the direction k4, pulse polarizations, envelope shapes, and even the phases.
Figure 1
Scheme of the time-resolved four-wave-mixing experiment. All calculations are given for the three-band scheme shown on the bottom left.
We shall illustrate the power of 2D techniques and how they work using the three-band model system shown in Fig. 1 which has a ground state (g), a singly excited manifold (e) and a doubly excited manifold (f ). The dipole operator can induce transitions between g to e and e to f . All transitions in the system are stimulated: spontaneous emission is neglected. This three-band model represents electronic excitations in the various physical systems covered in the this article.
Multidimensional signals monitor the dynamics of the system’s density matrix during the time intervals between pulses. Diagonal elements of this matrix ρnn represent populations of various states, while the off diagonal elements ρnm (n ≠ m), known as coherences, carry additional valuable phase information. These signals can be described intuitively using the Feynman diagrams shown in Fig. 2 which display the Liouville space pathways: sequences of interactions with the various fields and the relevant elements of the density matrix during the controlled intervals between interactions6. The two vertical lines represent the ket (left) and the bra (right) of the density-matrix.
Figure 2
Feynman diagrams for two 2D techniques with wavevectors kI and kIII. Incoming and outgoing arrows represent the interaction events, labels indicate states of the system during various intervals between interactions. ESA - excited state absorption, GSB ...
Time runs from bottom to top and the labels mark the density matrix elements during the evolution periods between interactions. The arrows represent interactions with photons and are labelled by their wavevectors. Photon absorption is accompanied by a molecular excitation (g to e or e to f transition) whereas photon emission induces deexcitation (e to g or f to e).
Our discussion will focus on two signals: the photon-echo SkI with kI = −k1+k2+k3 and the double-quantum-coherence SkIII with kIII =+k1+k2−k3. We first present the Feynman diagrams and the quantum pathways relevant for the two techniques for the generic exciton model of Fig. 1. Simulated signals are then presented for three different physical systems: Wannier excitons in semiconductor quantum wells12–15, Frenkel excitons in photosynthetic complexes6,7, and soft x-ray core excitons in molecules16–19. We demonstrate that both techniques provide new insights into the structure and exciton dynamics in semiconductor nanostructures and molecular aggregates and are highly sensitive to the separation between core-shells and the localization of the core-excited states.
The three contributions to the SkI signal depicted in Fig. 2 are known as ground state bleaching (GSB), excited state stimulated emission (ESE) and excited state absorption (ESA)6. In the GSB pathway the system returns to the ground state (and described by the density matrix element ρgg), during the second interval t2, after interacting with the first two pulses. The third interaction is affected by the decrease of the ground state population which reduces (bleaches) the subsequent photon absorption. In the ESE pathway, the system resides in the singly-excited (e) manifold during t2 and the third interaction brings it back to the ground state by stimulated emission. The ESA pathway shares the same t1 and t2 history of the ESE, however the third interaction now creates a doubly-excited state f . The SkI signal is usually displayed as a frequency/frequency correlation plot SkI (Ω3,t2,Ω1) obtained by a double Fourier transform with respect to the time delays t1 and t3, holding t2 fixed. Ω3 and Ω1 reveal the various resonance transitions, as can be anticipated from the diagrams. Only single-exciton ωeg resonances corresponding to optical coherences ρeg show up during t1 and are projected onto the Ω1 axis. The Ω3 axis shows either ωe′g resonances (ESE, GSB) or ωfe (ESA). The t2 evolution reflects exciton populations ρee and intra-band single-exciton coherences ρee′. Population transport, coherence oscillations and spectral diffusion dominate this interval in the ESE and ESA paths6.
Since the molecular frequencies during t1 (ωge) are negative and during t3 (ωeg and ωfe) are positive, the Ω1 frequency axis is reversed in the 2D plots. With this convention uncoupled excitons only show diagonal peaks. Off diagonal cross-peaks are markers of some kind of communication between various excitations which causes their resonance frequency to be different during t1 and t3. This can be attributed either to exciton delocalization or to population transport. A simple interpretation of the signals is possible by using a basis of states localized on the various chromophores. Since the dipole is localized on each chromophore and can only excite one chromophore at a time, cross peaks only appear when the chromophores are coupled. NMR spectra are similarly interpreted in terms of the couplings of localized spin states8. The couplings of chromophores can always be formally eliminated by diagonalizing the single-exciton Hamiltonian and switching to the delocalized exciton basis. However in this representation the dipole operator matrix elements will depend on the details of the eigenstates, which prevents the simple intuitive interpretation of the signal.
The SkIII technique has two ESA-type contributions (Fig. 2). 2D spectra is obtained by either correlating t1 → Ω1 with t2 → Ω2, SkIII (Ω1, Ω2, t3), or t2 → Ω2 with t3 → Ω3, SkIII (t1,Ω2,Ω3). The density matrix evolution during t1 and t2 is identical for the ESA1 and ESA2 diagrams: single-exciton resonances corresponding to ρeg show up during t1. During t2 the system is in a coherent superposition (coherence) ρfg between the doubly-excited state f and the ground-state g. Two-exciton double-quantum-coherence resonances corresponding to the different doubly excited states f are then projected onto Ω2. The t3 evolution is very different: In ESA1 the system is in a coherence between f and e′ (ρfe′) which results in resonances at Ω3 = ωfe′, corresponding to all possible transitions between doubly- and singly- excited states. For ESA2 the system is in a coherence between e′ and g (ρe′g) and reveals single- exciton resonances at Ω3 = ωe′g as t3 is scanned. When the single-exciton states e and e′ do not interact (e.g. when they belong to two uncoupled chromophores), the corresponding two-exciton state is given by a direct product |f〉 = |ee′〉 and the double-excitation energy is the sum ef = ee + ee. In that case ωeg =ωfe′ =ee, the two diagrams exactly cancel and the signal vanishes! The entire SkIII signal is thus induced by correlations and its peak pattern provides a characteristic fingerprint for the correlated doubly excited wavefunctions. This conclusion goes beyond the present simple model. SkIII vanishes for uncorrelated many electron systems described by the Hartree Fock wavefunction and thus provides an excellent background-free probe for electron correlations12,20. The (Ω2,Ω3) correlation plots spread the two-exciton (f state) information along both axes, thus improving the resolution of the two-exciton manifold.
TL;DR: In this article, the authors investigated the performance of doped organic crystal growth at relative high temperatures by the physical vapor transport (PVT) method, which gives the guest molecules high kinetic energy to incorporate into the crystal lattice of the host.
Abstract: Doping an organic crystal such as an inorganic semiconductor without having a bad influence on crystalline qualityisaverydifficulttaskbecauseweakintermolecularinteractionsandlatticemismatchesexistinorganiccondensedstates. Wereportherethesuccessfulgrowthoftetraceneandpentacene-dopedtrans-1,4-distyrylbenzene(trans-DSB)crystalswithhigh crystalline quality, large size, and excellent optical properties. The doped concentration up to 10% can be achieved by controllingthetemperatureofthecrystalgrowthzone.Thefirstkeypointforthecrystalswithahighdopingratioisthechoiceof the host (trans-DSB) and guest (tetracene or pentacene) molecules with comparable crystal lattice structures, which ensure less lattice mismatch. The second key point is crystal growth at relative high temperatures by the physical vapor transport (PVT) method, which gives the guest molecules high kinetic energy to incorporate into the crystal lattice of the host. These doped crystalswithsliceshapeandlargesize(millimeterscale)maintainorderedlayerstructuresandcrystalsurfacecontinuities,which areverifiedbyX-raydiffraction(XRD)andatomicforcemicroscopy(AFM)analysis.Efficientenergytransferfromthehostto the guest and the suppressing of the interaction among the guest molecules lead to color-tunable emission and high luminescent efficiencies (blue forundopedtrans-DSB, η=65(4%;greenfor tetracene-dopedtrans-DSB, η=74(4%;redfor pentacene- dopedtrans-DSB, η=28(4%).Steady-stateandtime-resolvedfluorescencespectroscopyofundopedanddopedcrystals,and their amplified spontaneous emissions, have been investigated. These doped crystals are expected to be of interest for light- emitting transistors, diodes, and electrically pumped lasers.