Sheet Of Quantum Dots Research Articles

Measurement of field match lines in electron beam radiotherapy using carbon quantum dot sheeting enhanced Cherenkov imaging

The aim of this study is to test the utility of optical imaging technique that contains a flexible carbon quantum dot (cQD) sheet and optical camera to measure matching of adjacent radiation fields due to introduced distances delivered during electron radiotherapy treatments. Aqueous solution of cQD is prepared with distilled water at the concentration of 0.1 g/L and the cQD sheet is prepared by spin coating and UV curing with the same concentration. All experiments are performed on the Varian VitalBeam system and optical emission is captured at source to surface distance (SSD) of 100 cm using a tripod mounted camera (Andor iXon ultra-888). The results demonstrate that the captured optical intensity is linear with delivered dose, almost independent of energy and dose rate. The profile in x and y axis show agreement with defined field sizes using maximum slope profile estimate measurement of <3.5 mm. The results of random tests also show that the differences between the measured field match distances calculated by match amplitude of optical imaging and the actual settings are all less than 1.3 mm. This work demonstrated that the optical imaging technique which consist of flexible cQD sheet and optical camera has potential to detect and quantify the field match line in surface tissue during electron beam radiotherapy.

Cluster formation between an oxadiazole derivative with metal nanoclusters (Ag/Au/Cu), graphene quantum dot sheets, SERS studies, and solvent effects

Interaction of an oxadiazole derivative, 5-(3,4-dimethoxyphenyl)-3-(3-methoxyphenyl)-1,2,4-oxadiazole (DPMO) with Ag/Au/Cu and graphene quantum dots with different solvents, is reported theoretically. The adsorption energy is maximum for the Cu6 cluster and minimum for the Ag6 cluster. The asymmetric charge redistribution between DPMO and M6s produces an improvement in dipole moment values. The decrease in energy gaps of complexes increased conductivity and metal clusters can be used as a drug sensor. The solvation energies are more negative in solvent media than in the gaseous media, indicating an enhancement in the solvent medium’s stability. Wave function studies show that there exist significant noncovalent interactions between metal clusters and oxadiazole that facilitate cluster formation. DPMO is found to form stable clusters with graphene which is evident from the enhancement of Raman activity of the system through SERS also enabling it for sensing DPMO in a mixture.

Cultivation of cos lettuce using blue LED and quantum dot wavelength conversion sheets

The aims of this study were to cultivate cos lettuce (Lactuca sativa L. cv. Cos Lettuce) using quantum dot (QD) sheets for wave conversion and to clarify the advantages and disadvantages of this technique for plant cultivation with artificial lighting. Blue LED light was wavelength-converted to green (G-QD) or red (R-QD) light by QD sheets and used for cultivating cos lettuce. Then, lighting with spectral photon flux density distributions similar to those of G-QD and R-QD was designed using green or red LED and blue LED (G-LED and R-LED), respectively, and used for cultivating cos lettuce. Plant growth at 14 days after treatment revealed that the shoot fresh weight was greater under G-QD compared with G-LED, perhaps because the amount of integrated light reception was greater in G-QD owing to the difference in the photosynthetic photon flux density's spatial distribution. Plants showed similar growth levels and morphologies under R-QD and R-LED lights. Plant growth is affected by the light environment, such as spectral photon flux density distributions and its vector field, regardless of the light source. However, the prototype QD sheets used in the present study had low conversion efficiencies and wide directivity angles. If QD products are developed that utilize the advantages of high monochromaticity and long life for plant cultivation at low costs, then they may become widely used.

Mid-Infrared Quantum-Dot Quantum Cascade Laser: A Theoretical Feasibility Study

In the framework of a microscopic model for intersubband gain from electrically pumped quantum-dot structures we investigate electrically pumped quantum-dots as active material for a mid-infrared quantum cascade laser. Our previous calculations have indicated that these structures could operate with reduced threshold current densities while also achieving a modal gain comparable to that of quantum well active materials. Here, we study the influence of two important quantum-dot material parameters, namely inhomogeneous broadening and quantum-dot sheet density, on the performance of a proposed quantum cascade laser design. In terms of achieving a positive modal net gain, a high quantum-dot density can compensate for moderately high inhomogeneous broadening, but at a cost of increased threshold current density. However, by minimizing quantum-dot density with presently achievable inhomogeneous broadening and total losses, significantly lower threshold densities than those reported in quantum-well quantum-cascade lasers are predicted by our theory.

Microchip laser converter based on InGaN laser diode and (Zn)CdSe quantum dot heterostructure

A high-efficiency microchip violet–green laser converter based on a molecular-beam epitaxy grown green-emitting (Zn)CdSe quantum dot (QD) II–VI laser heterostructure, optically pumped by emission of a cheap violet InGaN laser diode (LD) was demonstrated. The active region of the II–VI laser heterostructure consisted of three electronically coupled (Zn)CdSe QD sheets in a single ZnSe quantum well (QW) placed asymmetrically inside a graded-index ZnMgSSe/ZnSe superlattice optical waveguide. The cavity length of the cleaved-facet II–VI laser was 130 μm which is a nearly optimal value for minimizing the excitation power. InGaN LD radiation was focused into a narrow stripe on the surface of the II–VI laser by a microlens optical system. The converter showed laser emission at 541 nm with threshold excitation power of ∼0.5 W. The maximal output pulse power and conversion efficiency of 1.5 W and ∼15%, respectively, were achieved. The device was mounted in a standard TO-18 LD package. Photograph of the operating violet–green microchip laser converter.

High temperature laser diode based on a single sheet of quantum dots

A single sheet of high-density InGaAs quantum dots (QDs) is used as a gain medium of InGaAs–GaAs–AlGaAs lasers. The devices operate at high power in the continuous mode beyond 160 °C with an emission wavelength up to ∼1.27 μm. At short cavity lengths a strong broadening (>300 nm) of the electroluminescence spectrum is observed at high current densities, permitting light sources for broadly wavelength tuneable and multi-wavelength infrared lasers based on a single gain chip, and related frequency conversion devices for the whole visible spectrum range. High power cw operation (>2 W) limited by catastrophic optical mirror damage is realized.

Site-Controlled Growth of Single InP QDs

The MOVPE growth of InP/GaInP quantum dots (QDs) on GaAs substrate “defects” formed by a focused beam of Ga+ ions is studied. It is shown that ordered arrays of QDs with a density of 0.25 (μm)−2 can be obtained in the InP/GaInP system. It is demonstrated that effective luminescence can be obtained by using two QD sheets separated by a GaAs/GaInP buffer layer.

Template method for nano-order positioning and dense packing of quantum dots for optoelectronic device application

In this paper, we present recent progress of our researches on positioning of quantum dot in the field of optoelectronics. The first research is aimed at quantum information device application. As a technology for production of quantum devices, we developed the method for positioning of a single colloidal quantum dot. Oxide lines on a Si substrate drawn by a scanning probe microscope were used as a negative etching mask with controlling their cross section in order to create a nanohole for trapping of a single quantum dot. The other research is aimed at solar cell application. It has been predicted that quantum-dot superlattice solar cell will achieve photoelectric conversion efficiency of more than 70%. After the sedimentation of the colloidal quantum dots into the pyramidal holes processed by anisotropic wet etching on a Si substrate, we observed characteristic photoluminescence from the quantum-dot sheet.

Molecular-beam epitaxy of heterostructures of wide-gap II–VI compounds for low-threshold lasers with optical and electron pumping

The paper presents basic approaches in designing and growing by molecular beam epitaxy of (Zn,Mg)(S,Se)-based laser heterostructures with multiple CdSe quantum dot (QD) sheets or ZnCdSe quantum wells (QW). The method of calculation of compensating short-period ZnSSe/ZnSe superlattices (SLs) in both active and waveguide regions of laser heterostructures possessing the different waveguide thickness and different number of active regions is presented. The method allowing reduction of the density of nonequilibrium point defects in the active region of the II–VI laser structures has been proposed. It utilizes the migration enhanced epitaxy mode in growing the ZnSe QW confining the CdSe QD sheet. The threshold power density as low as P thr ∼ 0.8 kW/cm2 at T = 300 K has been demonstrated for laser heterostructure with single CdSe QD sheet and asymmetric graded-index waveguide with strain-compensating SLs.

Laser characteristics of a heterostructure with a five Cd(Zn)Se QD sheet active region embedded in non‐symmetrical superlattice graded‐index waveguide

AbstractOptically pumped green‐emitting (545–551 nm) ZnSe‐based laser heterostructures grown by molecular‐beam epitaxy have been elaborated and studied. The optimized II–VI laser heterostructure involves a non‐symmetrical Zn(Mg)SSe/ZnSe superlattice (SL) graded‐index waveguide and an active region comprising five CdSe quantum dot sheets each placed in the centre of a 2 nm‐thick ZnSe quantum well. Such a structure design provides both large optical confinement factor and high homogeneity of excitation. Internal laser parameters were determined by measuring the laser threshold and differential quantum efficiency as functions of the cavity length. The values of internal quantum efficiency, internal optical losses, characteristic gain and transparency threshold have been estimated to be ηi = 80.5%, αi = 3.2 cm–1, ΓG0 = 135 cm–1, and IT = 1.35 kW/cm2, respectively. Based on the defined parameters, the values of minimum threshold power and corresponding optimum cavity length have been derived. It is shown that the minimum threshold power decreases significantly (by more than 20%) in comparison with that for the heterostructure with conventional flat‐band SL waveguide (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Superlattice formed by quantum-dot sheets: Density of states and infrared absorption

Low-energy continuous states of electron in a heterosrtucture with periodically placed quantum-dot sheets are studied theoretically. The Green's function of an electron is governed by the Dyson equation with the self-energy function which is determined the boundary conditions at quantum-dot sheets with weak damping in the low-energy region. The parameters of a superlattice formed by quantum-dot sheets are determined using the short-range model of quantum dot. The density of states and spectral dependencies of the anisotropic absorption coefficient under midinfrared transitions from doped quantum dots into miniband states of a superlattice strongly depend on dot concentration and on the period of sheets. These dependencies can be used for the characterization of the multilayer structure and they determine the parameters of different optoelectronic devices exploiting the vertical transport of carriers through quantum-dot sheets.

Cathodoluminescence of ZnSe‐based II‐VI green laser heterostructures

AbstractThe cathodoluminescence technique to study the ZnSe‐based laser heterostructures is described. The heterostructures were grown by molecular beam epitaxy on GaAs (001) substrates and consist of bottom and top ZnMgSSe cladding layers and ZnCdSe/ZnSe quantum well or CdSe/ZnSe quantum dots sheets placed inside Zn(Mg)SSe/ZnSe graded index waveguide. The transport of charge carriers generated by electron beam in heterostructure was investigated using depth resolved cathodoluminescence. For that the dependence of cathodoluminescence intensity of active region on electron beam energy has been simulated using Monte‐Carlo calculation of carrier distribution on depth. It made possible the nondestructive characterization of multilayer heterostructure quality, to estimate both deficiency and carrier transport. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Violet-green electrically pumped laser converters with output power over 150 mW

A violet-green integrated laser converter with a quantum efficiency up to 25.4% and maximum output pulse power of 154 mW at a wavelength of 543 nm has been fabricated on the basis of the II-VI laser heterostructure comprising an active region with five electronically-coupled CdSe/ZnSe quantum dot sheets embedded in a Zn(Mg)SSe/ZnSe superlattice graded-index waveguide. A pulse InGaN/GaN laser diode emitting at 416 nm was used as a pumping source of the laser converter.

ZnMgSSe/ZnSe/CdSe quantum dot heterostructures for green laser applications

Cd(Zn)Se/ZnSSe/ZnMgSSe laser heterostructures with an asymmetrical ZnSSe/ZnSe supperlattice waveguide, and multiple (up to 9) electronically-coupled CdSe quantum dot sheets in the active region were designed and grown by molecular beam epitaxy. Internal quantum efficiency η i, internal loss α i , transparency threshold J T and characteristic gain ΓG 0 were found to be: 58.5%, 7 cm − 1 , 1.78 kW/cm 2, and 194 cm − 1 , respectively, in a laser heterostructure with 9 electronically-coupled CdSe quantum dot active layers. A violet-green integrated laser converter with the output pulse power up to 50 mW was fabricated on the basis of the Cd(Zn)Se/ZnSSe/ZnMgSSe quantum dot laser heterostructure. The heterostructures were optically pumped by emission of a violet ( λ = 416 nm) commercial InGaN/GaN pulsed laser diode. The ways for further laser threshold reduction and increasing the efficiency are discussed.

Wannier-Stark states in a superlattice of InAs/GaAs quantum dots

Electron and hole emission from states of a ten-layer system of tunneling-coupled vertically correlated InAs/GaAs quantum dots (QDs) is studied experimentally by capacitance—voltage measurements and deep-level transient spectroscopy. The thickness of GaAs interlayers separating sheets of InAs QDs was ≈3 nm, as determined from transmission electron microscope images. It is found that the periodic multimo-dal DLTS spectrum of this structure exhibits a pronounced linear shift as the reverse-bias voltage Ur applied to the structure is varied. The observed behavior is a manifestation of the Wannier—Stark effect in the InAs/GaAs superlattice, where the presence of an external electric field leads to the suppression of coupling between the wave functions of electron states forming the miniband and to the appearance of a series of discrete levels called Wannier—Stark ladder states.

Violet–green laser converter based on MBE grown II–VI green lasers with multiple CdSe quantum dot sheets, pumped by InGaN laser diode

AbstractThe violet–green laser converter based on a molecular‐beam‐epitaxy (MBE) grown CdSe quantum dot (QD) laser heterostructure pumped by a commercial InGaN laser diode (LD) emission has been fabricated and studied in detail. The optimized II–VI laser heterostructure consists of asymmetrical ZnSe/ZnSSe superlattice (SL) waveguide and active region comprising five CdSe QD sheets (QDS) placed in the centre of 2‐nm‐thick ZnSe quantum wells. The new laser structure design provides both a high homogeneity of optical pumping of the CdSe QDS due to tunnelling of charge carriers between the QDS separated by 5‐nm‐thick ZnSe/ZnSSe/ZnSe barriers and high optical confinement factor. Optimization of both cavity length of the II–VI laser and parameters of optical focusing system to obtain a narrow stripe with the length slightly exceeding the cavity length has been performed. As a result, the maximum achieved quantum efficiency and pulse output power in green have been as high as 8% and 65 mW, respectively.

10.1007/s11455-008-1001-2

We have studied the room-temperature photoconductivity in the wavelength range 1–2.6 μm in InAs/GaAs heterostructures with quantum dots (QDs). Specific features of these heterostructures grown using the metalorganic vapor phase epitaxy (MOVPE) were an increase in the amount of InAs during the formation of a sheet of QDs and the use of alternating low-and-high-temperature regimes during their overgrowth with a GaAs barrier layer. For the first time, the MOVPE-grown multilayer InAs/GaAs heterostructures with quantum dots exhibited photoluminescence in a wavelength range of up to 1.6 μm and the photoconductivity up to 2.6 μm at room temperature. The heterostructures exhibited a room-temperature voltage sensitivity of 3 × 103 V/W (within a Si-plate filter bandwidth) and a specific detectivity of 9 × 108 cm Hz1/2 W−1.

Room-temperature continuous-wave operation of the In(Ga)As/GaAs quantum-dot VCSELs for the 1.3 µm optical-fibre communication

Efficient room-temperature (RT) continuous-wave (CW) lasing operation of the 1.3 µm MBE (molecular-beam epitaxy) In(Ga)As/GaAs quantum-dot (QD) top-emitting oxide-confined vertical-cavity surface-emitting diode lasers (VCSELs) for the second-generation optical-fibre communication has been achieved. In their design, a concept of a QD inside a quantum well (QW) has been utilized. The proposed In(Ga)As/GaAs QD active region is composed of five groups of three 8 nm In0.15Ga0.85As QWs, each containing one InAs QD sheet layer. In each group located close to successive anti-node positions of the optical standing wave within the 3λ cavity, QWs are separated by 32 nm GaAs barriers. Besides, at both active-region edges, additional single InGaAs QWs are located containing single QD layers. For the 10 µm diameter QD VCSELs, the RT CW threshold current of only 6.2 mA (7.9 kA cm−2), differential efficiency of 0.11 W A−1 and the maximal output power of 0.85 mW have been recorded. The experimental characteristics are in excellent agreement with theoretical ones obtained using the optical-electrical-thermal-recombination self-consistent computer model. According to this, for the 10 µm devices, the fundamental linearly polarized LP01 mode remains the dominating one up to the current of 9.1 mA. The lowest RT CW lasing threshold below 5 mA is expected for 6 µm devices.

Structure optimisation of modern GaAs-based InGaAs/GaAs quantum-dot VCSELs for optical fibre communication

AbstractRoom-temperature (RT) continuous-wave (CW) performance of modern 1300-nm oxide-confined In(Ga)As/GaAs quantum-dot (QD) vertical-cavity surface-emitting diode lasers (VCSELs) taking advantage of many QD sheets is investigated using our comprehensive self-consistent simulation model to suggest their optimal design. Obviously, quantum dots should be as uniform as possible and as dense as possible to ensure high enough optical gain. Besides, our simulation reveals that efficient and uniform current injection into VCSEL active regions necessary to enhance excitation of the desired fundamental LP01 mode is accomplished in the VCSEL configuration with the broad-area bottom contact and the ring upper one as well as with the oxide aperture localized within the first period of the upper p-type DBR. The doping of the DBR mirrors is chosen as a compromise between their high enough electrical conductivity and low enough free-carrier absorption. The oxide aperture is additionally introducing the radial optical waveguiding. Moreover, our analysis has been concluded that VCSEL active regions should be composed of at least 9 QD sheets to acquire efficient RT CW operation. Furthermore, rather longer optical cavities are recommended in this case because localization of QD sheets should be adjusted to the anti-node positions of the optical cavity standing wave.

Effective Green Semiconductor Lasers with Multiple CdSe/ZnSe QD Active Region for Electron Beam Pumping

Electron beam pumped (EBP) semiconductor lasers using ZnSe-based structures as active elements look very promising for the development green lasers because of no requirements of both p-n junction and low-resistivity contacts. Another advantage of EBP lasers is that the maximum conversion efficiency of electron beam energy to light is of ~30%. The possibility of using ZnSe-based quantum-size heterostructures for EBP green lasers demonstrated during last decade. Thus, room-temperature lasing at Ee<5keV and extremely low threshold current density jth=0.4-0.5 A/cm 2 (at Ee=8-10keV) have been demonstrated in laser heterostructures with short-period superlattice (SL) waveguide and single CdSe/ZnSe quantum dot (QD) active region [1]. The structure with three ZnSe/CdSe QD sheets arranged within 0.4um-thick extended SL waveguide has demonstrated laser generation with efficiency (per one facet) as high as ~4% [2]. Here we present the latest results on fabrication and study of ZnSe-based EBP laser heterostructure with multiple CdSe/ZnSe QD sheets. Laser heterostructure has been grown by molecular beam epitaxy pseudomorphically to GaAs (001) substrate and consists of 1.6μmthick ZnMgSSe cladding, active region with ten CdSe/ZnSe QD sheets equally spaced within a 0.6um-thick ZnSe/ZnSSe SL waveguide and cap 20-nm ZnMgSSe layer. The nominal thickness of CdSe QD layers was chosen to ~2.5 monolayers. All measurements were carried out at room temperature in transverse excitation geometry in pulse mode at electron beam energy Ee=10-30 keV. The lasing has been observed at the wavelength of ~542nm (T=300K) with FWHM of the emission line of ~2-3 nm. The maximum achieved values of pulse output power ~12W per facet has been registered at Ee=23keV. Maximum quantum efficiency of ~8.5% per facet has been obtained at the same electron beam energy and electron beam current Ie~5.7mA. The main reason of increase of laser efficiency seems to be more effective utilization of pumping energy by means of adjustment of both structure geometric parameters and electron beam penetration depth into the sample. Further increase of light output power is likely can be achieved by increasing Ie keeping constant electron beam energy Ee. The question on optimum number of active CdSe QD layers will be discussed. This work was partly supported by FASI Contract and ISTC Project No. 3754.