Electrically Pumped Research Articles

Toward an electrically pumped spaser

The use of surface plasmon polariton (SPP)-based waveguides can significantly reduce the size of optical interconnects, but the propagation length of SPPs is limited by Joule heating losses and does not exceed a few micrometers. In this paper, we present an SPP amplification scheme that utilizes compact electrical pumping and gives a possibility for designing really compact on-chip waveguides. Moreover, we demonstrate here numerically that this approach can be easily used to design an electrically pumped cw or pulsed spaser.

Electrically pumped simultaneous ultraviolet and visible random laser actions from ZnO-CdO interdiffused film

We have demonstrated electrically pumped simultaneous ultraviolet (UV) and visible random laser (RL) actions from ZnO-CdO interdiffused film. The interdiffusion between ZnO and CdO films at 700 °C forms composition-graded CdxZn1−xO alloy within the ZnO-CdO interdiffused film, which is luminescent in both UV and visible regions. A device based on SiO2/ZnO-CdO/SiO2 double-barrier structure on silicon substrate, where SiO2 acts as the barrier, is constructed for electrical pumping of the ZnO-CdO interdiffused film. As the device is applied with sufficiently high forward bias with negative voltage connecting to the silicon substrate, the UV and visible RL actions simultaneously occur. The mechanism for such electrically pumped RL actions has been tentatively elucidated.

Electrically Pumped Vertical External Cavity Surface Emitting Lasers Suitable for Passive Modelocking

Modelocked optically pumped vertical external cavity surface emitting lasers (VECSELs) have generated up to 6.4-W average power, which is higher than for any other semiconductor lasers. Electrical pumping of modelocked VECSELs is the next step toward a higher level of integration. With continuous wave (cw) electrically pumped (EP) VECSELs, an average output power of 900 mW has been demonstrated from the undisclosed proprietary novalux extended cavity surface emitting laser (NECSEL) design. In contrast, modelocked NECSELs have only been demonstrated at 40 mW. Recently, we presented a numerical study of EP-VECSELs suitable for modelocked operation; here, we demonstrate the first realization of this design. Power scaling is achieved with a lateral mode size increase. The competing electrical and optical requirements are, on the electrical side, low ohmic resistance, and on the optical side, low optical losses and low dispersion. Additionally, the device needs to operate in a fundamental mode for stable modelocking. We have fabricated and characterized 60 EP-VECSELs with varying dimensions and compared their lasing performance with our numerical simulations. The tradeoff between good beam quality and output power is discussed with an outlook to the modelocking of these EP-VECSELs. Initial EP-VECSEL devices have generated >;100 mW of cw output power.

Electrically Driven Quantum Dot Micropillar Light Sources

We report on light sources based on electrically pumped quantum dot (QD) micropillar cavities. The low-mode-volume high-quality microstructures feature pronounced cavity quantum electrodynamics (cQED) effects that are exploited for the realization of efficient single-photon sources and low-threshold microlasers. The compact and electrically driven devices are of special interest for applications in the field of quantum communication. In particular, operated as electrically triggered single-photon sources, the QD micropillars can act as building blocks for quantum key distribution and quantum repeaters. On the other hand, the electrically pumped microlasers with in-plane emission via whispering gallery modes or emission normal to the sample's surface are predestinated for integrated light sources in future photonic networks. The devices are based on doped high-quality factor GaAs/AlAs microcavity structures with InGaAs QDs in the active layer. A lateral injection scheme leaves the upper facet of the micropillars free of any absorbing metal and allows for efficient light output under electrical pumping of low-mode-volume micropillars with diameters between 1 and 20 μm. Due to cQED effects, triggered single-photon emission with high photon extraction efficiency up to 62% and a low multiphoton emission probability (g <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> (0) = 0.16) are realized for moderate-quality (Q ) factor samples. The efficient coupling of spontaneous emission into the lasing mode in high- Q micropillars results in ultralow laser threshold currents of less than 10 μA at cryogenic temperatures. Our paper demonstrates the high potential of electrically driven QD micropillars to act as integrated light sources in future communication systems.

Electrically pumped waveguide lasing from ZnO nanowires

Ultraviolet semiconductor lasers are widely used for applications in photonics, information storage, biology and medical therapeutics. Although the performance of gallium nitride ultraviolet lasers has improved significantly over the past decade, demand for lower costs, higher powers and shorter wavelengths has motivated interest in zinc oxide (ZnO), which has a wide direct bandgap and a large exciton binding energy. ZnO-based random lasing has been demonstrated with both optical and electrical pumping, but random lasers suffer from reduced output powers, unstable emission spectra and beam divergence. Here, we demonstrate electrically pumped Fabry-Perot type waveguide lasing from laser diodes that consist of Sb-doped p-type ZnO nanowires and n-type ZnO thin films. The diodes exhibit highly stable lasing at room temperature, and can be modelled with finite-difference time-domain methods.

Electrically Tunable Surface Plasmon Source for THz Applications

In this paper, we propose a design for a widely tunable solid-state optically and electrically pumped terahertz source based on the Smith-Purcell free-electron laser. Our design consists of a thin dielectric layer sandwiched between an upper corrugated structure and a lower layer of thin metal, semiconductor, or high-electron-mobility material. The lower layer is for current streaming, which replaces the electron beam in the Smith-Purcell free-electron laser design. The upper layer consists of two microgratings for optical pumping, and a nanograting to couple with electrical pumping in the lower layer. The optically generated surface plasmon waves from the upper layer and the electrically induced surface plasmon waves from the lower layer are then coupled. Emission enhancement occurs when the plasmonic waves in both layers are resonantly coupled.

(Invited) Characterizations of Direct Band Gap Photoluminescence and Electroluminescence from epi-Ge on Si

Characterizations of direct gap emission from epi-Ge through both optical and electrical pumping were studied for the application of Si compatible light sources such as on-chip optical interconnect. In-situ doping technique with PH3 during the epi-Ge growth was first applied to achieve high n-type doping concentrations. Photoluminescence (PL) measurements show that the direct band emission of Ge increases with higher doping concentration. This confirms that the Ge direct radiative recombination efficiency can be improved by band filling of electrons. A Ge n+/p light emitting diode (LED) on a Si substrate was then fabricated to study the electroluminescence (EL) properties related to the band filling. Direct band gap EL at 1.6 μm was also obtained from this Ge LED. Unlike ordinary electrically pumped devices, this LED shows a superlinear luminescence enhancement at high current. Thermal enhancement effects observed in temperature dependent EL spectra show the capability of this device to be operated at room temperature or above.

Charging Dynamics in Electrically Pumped Quantum Wells

Injection pumping of quantum-well (QW) lasers can lead to charging of the active region due to the different electron and hole masses. To investigate the significance and physical consequences of this effect, the consequences of electrical pumping and the associated charging phenomena are investigated microscopically. The analysis reveals the necessity to include the extended barrier states energetically above the band edge in addition to the quantum-confined well states. It is shown that the barrier-state populations compensate the QW charging. The replenishing of stimulated carrier recombination under CW laser operation causes persistent charge oscillations at the plasma frequency.

Microcavity organic laser device under electrical pumping

Lasing action in an electrically pumped organic laser device is demonstrated with a high quality factor (Q) microcavity structure formed by two high-reflective and low-loss electrical contacts. A 4-(dicyanomethylene)-2-i-propyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran-doped tris(8-hydroxy-quinoline)aluminum (Alq(3)) film serves as the active layer. A single longitudinal lasing cavity mode is obtained at 621.7 nm with a threshold current density of 860 mA/cm(2) under a room temperature pulse operation.

Low Thresholds in Polymer Lasers on Conductive Substrates by Distributed Feedback Nanoimprinting: Progress Toward Electrically Pumped Plastic Lasers

A study was conducted to investigate the effect of optically pumped lasing in a 'super yellow' film cast onto a gate dielectric and gate conducting electrode. It was observed during the study that light-emitting field-effect-transistor (LEFET) architecture with a distributed feedback (DFB) resonator structure nanoimprinted into the gain medium helped to achieve an electrically pumped plastic laser. Nanoimprint lithography was applied to obtain 1D and 2D DFB grating resonator structures in multilayer films. The study showed that the 2D DFB laser provided a lower lasing threshold compare to 1D DFB laser under same conditions. The nanoimprinted 2D DFB laser showed a lasing threshold of 32nJ/pulse and slope efficiency of 1.2%. The 1D grating provided a high degree of chain orientation for the organic semiconducting polymers in the source-drain channel.

MBE growth of active regions for electrically pumped, cw-operating GaSb-based VCSELs

Electrically pumped, cw-operating, single-mode GaSb-based VCSELs are attractive light sources for trace-gas sensing systems using tunable diode laser absorption spectroscopy (TDLAS) [A. Vicet, D.A. Yarekha, A. Pérona, Y. Rouillard, S. Gaillard, Spectrochimica Acta Part A 58 (2002) 2405–2412]. Only recently, the first electrically pumped (EP) devices emitting at 2.325 μm in cw-mode at room temperature have been reported [A. Bachmann, T. Lim, K. Kashani-Shirazi, O. Dier, C. Lauer, M.-C. Amann, Electronics Letters 44(3) (2008) 202–203]. The fabrication of these devices employs the molecular beam epitaxy (MBE) growth of GaSb/AlAsSb-distributed Bragg mirrors, a multi-quantum-well active region made of AlGaAsSb/InGaAsSb and an InAsSb/GaSb-buried-tunnel junction. As VCSELs are usually driven under high injection rates, an optimum electrical design of active regions is essential for high-performance devices. In this paper we present an enhanced simulation of current flow in the active region under operation conditions. The calculation includes carrier transport by drift, diffusion and tunneling. We discuss different design criteria and material compositions for active regions. Active regions with various barrier materials were incorporated into edge emitter samples to evaluate their performance. Aluminum-containing barriers show better internal efficiency compared to active regions with GaSb as the barrier material.

Spectrally narrow emissions at cutoff wavelength from edges of optically and electrically pumped anisotropic organic films

The authors investigated the characteristics of spectrally narrow emissions at the cutoff wavelength from the edges of optically and electrically pumped organic semiconductor films. They estimated the optical properties of 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) films using variable angle spectroscopic ellipsometry, revealing that the BSB-Cz films have uniaxial anisotropy and that BSB-Cz molecules lie nearly parallel to substrate surfaces. The authors discuss here the spectral changes in the edge emissions from neat BSB-Cz and tris(8-hydroxyquinoline)aluminum (Alq3) films of 15 different thicknesses under continuous-wave (cw) optical excitation. The spectrally narrow emissions of transverse electric (TE) and transverse magnetic (TM) modes were observed. The authors showed that the peak wavelengths of these emissions varied according to the cutoff wavelengths of asymmetric slab waveguides and that their spectral widths changed depending on the angular dispersion of the Fabry–Pérot interferometer composed of the organic film and its interfaces. The edge emissions from the BSB-Cz films had narrower bands than those from the Alq3 films and increased with a superlinear dependence on the stripe length of the excitation area, indicating the occurrence of light amplification under cw optical excitation. Finally, the authors demonstrate spectrally narrow emissions from the edges of electrically pumped organic light-emitting diodes (OLEDs) having a fine line-shaped waveguide structure with a silver metal cladding layer. Narrow emissions of TE mode were observed, and the peak wavelengths of the emission spectra corresponded well to the cutoff wavelength of the waveguide with the silver cladding layer. In addition to the narrow emissions, the authors observed some intriguing phenomena suggestive of the occurrence of light amplification in OLEDs under electrical pumping.

On the design of electrically pumped vertical-external-cavity surface-emitting lasers

Vertical-external-cavity surface-emitting lasers (VECSELs) yield an excellent beam quality in conjunction with a scalable output power. This paper presents a detailed numerical analysis of electrically pumped VECSEL (EP-VECSEL) structures. Electrical pumping is a key element for compact laser devices. We consider the optical loss, current confinement, and device resistance. The main focus of our investigation is on the achievement of an adequate radial carrier distribution for fundamental transverse mode operation. It will be shown that a trade off between the conflicting optical and electrical optimization has to be found and we derive an optimized design resulting in guidelines for the design of EP-VECSELs which are compatible with passive mode locking.

Coupled optical and electronic simulations of electrically pumped photonic-crystal-based light-emitting diodes

We use coupled optical and electronic simulations to investigate design trade-offs in electrically pumped photonic-crystal-based light-emitting diodes. A finite-difference frequency-domain electromagnetic solver is used to calculate the spontaneous emission enhancement factor and the extraction efficiency as a function of frequency and of position of the emitting source. The calculated enhancement factor is fed into an electronic simulator, which solves the coupled continuity equations for electrons and holes and Poisson’s equation. We simulate a two-dimensional structure consisting of a photonic-crystal slab with a single-defect cavity and investigate different electrical pumping geometries for such a cavity.

Three-dimensional wavelength-scale confinement in quantum dot microcavity light-emitting diodes

We introduce a microcavity light-emitting diode (LED) structure that uses submicrometer oxide aperture and a quantum dot active region to achieve strong three-dimensional confinement of both the carrier distribution and the optical field. Light–current curves show optical emission for devices as small as 400nm in diameter. Spectroscopy on electrically pumped LEDs, with apertures ranging from 2.5 down to 0.7μm, show several spectral lines corresponding to cavity modes. A strong blueshift of the resonant modes for smaller apertures demonstrates the role of the oxide aperture in confining laterally the optical wave in a volume comparable to (λ∕n)3. Due to the high quality factors and low mode volumes, the devices could be good candidates for the demonstration of the Purcell effect under electrical pumping.

Formation of triplet excimers and dimers in amorphous organic thin films and light emitting devices

Abstract We examine triplet excimers in neat films of the phosphorescent molecule: platinum (II) (2-(4′,6′-difluorophenyl)pyridinato-N,C2) acetyl acetate (FPt1). This molecule was chosen because of its square planar structure, allowing it to facially pack in a crystal with an intermolecular separation of only 3.4±0.1 A, thereby facilitating excimer formation between adjacent molecules. The transient and steady-state photoluminescent and electroluminescent properties of FPt1 are used to understand the formation of triplet excimers ( 3 E 0 * ) and dimers ( 3 D * ) in amorphous films grown by thermal evaporation. We find that 3 E 0 * and 3 D * have peak emission intensities at 2.07 and 1.83 eV, and have lifetimes of (1.6±0.3) μs and (800±80) ns at room temperature, respectively. When films are optically pumped, triplet excitons form on monomers and then diffuse via thermally activated hopping to nearby excimer sites with an activation energy of (37±4) meV. We find that the energy transfer efficiency from the monomer to the excimer state approaches 100% at room temperature, and that the photoluminescence efficiency of the excimer is three times less than that of the monomer. Further, excitation of triplet aggregate states, or metal–metal dimers via the association of cations and anions of electrically pumped FPt1 is achieved in an organic light-emitting device, which achieves a maximum internal quantum efficiency of ∼14%. Electrical pumping also reveals dimer states unresolved in photoluminescent studies. The high efficiency of the triplet excimers of FPt1 is significant since dimeric species, especially triplets, are often associated with concentration quenching and low quantum yields.

Master-equation model of a single-quantum-dot microsphere laser

We present a theoretical model for the electrically pumped single-quantum-dot microsphere laser. We solve the master equation of the system and analyze the steady state and dynamical properties of the optical field, such as output power, photon number fluctuation, and linewidth, for realistic experimental parameters. The laser threshold power is several orders of magnitude lower than is currently possible with semiconductor microlasers. A semiclassical approximation for the output power and laser linewidth is derived and compared to the exact solution. Electrical pumping together with Coulomb blockade effect allows for the realization of regular pumping in the system. We discuss the possibility for the generation of heralded single photons and of sub-Poissonian laser light. @S1050-2947~99!05506-7#

Dynamics of directly created excitons in asymmetric double quantum wells.

In this work, we have studied the dynamics of an electrically pumped exciton that is directly created by hole-assisted electron resonant tunneling in an asymmetric coupled quantum well system. We have numerically integrated in space and time the effective-mass Schr\"odinger equation for the electron-hole pair taking into account the two-particle nature of the tunneling process. We have also included the time evolution of the transition from spatially indirect to direct excitons. Considering a different exciton binding energy for each quantum well, we have obtained a different charge oscillation semiperiod for each binding energy case. In this way, the possibility is shown of having a new kind of terahertz electromagnetic radiation emerging from such a semiconductor quantum well system after electric pumping.

Dynamics of the Nuclear and Electrically Pumped 1.45-μm Atomic Carbon Laser in Mixtures of Helium + CO And Helium + CO2

The first successful modeling of an impurity-type nuclear pumped laser (NPL) (i.e., one that employs trace densities of the lasing species in a noble gas buffer), atomic carbon at 1.45 ..mu..m, was achieved. Such NPLs are important due to their low flux threshold and quasi-steady-state oscillation. The atomic carbon NPL is unique in that time delays up to 5 ms are observed between the laser signal and the excitation pulse in helium + CO/sub 2/ mixtures while no delay is observed in helium + CO. Using a kinetic model in conjunction with an experimental program, we show that this difference in delay arises from slow dissociation of CO/sub 2/ to form CO. Significantly, the model also successfully simulates electrical pumping of He-CO or CO/sub 2/ mixtures.