Agus Putu Abiyasa

Enhancement of ultraviolet lasing from Ag-coated highly disordered ZnO films by surface-plasmon resonance

Large improvement in random lasing action at ultraviolet wavelength has been achieved from highly disordered ZnO films with Ag coating. The lasing threshold can be reduced by two times and slope efficiency can be increased by 5.5 times. The improvement is due to the presence of Ag coating, which enhances the surface coupling of lasing emission from the ZnO films by surface-plasmon resonance and reduces the scattering loss experienced by the random cavity modes. Furthermore, the enhancement of lasing efficiency is dependent on the Ag coating’s surface roughness, which can be controlled through the surface morphology of ZnO films.Large improvement in random lasing action at ultraviolet wavelength has been achieved from highly disordered ZnO films with Ag coating. The lasing threshold can be reduced by two times and slope efficiency can be increased by 5.5 times. The improvement is due to the presence of Ag coating, which enhances the surface coupling of lasing emission from the ZnO films by surface-plasmon resonance and reduces the scattering loss experienced by the random cavity modes. Furthermore, the enhancement of lasing efficiency is dependent on the Ag coating’s surface roughness, which can be controlled through the surface morphology of ZnO films.

A bright cadmium-free, hybrid organic/quantum dot white light-emitting diode

We report a bright cadmium-free, InP-based quantum dot light-emitting diode (QD-LED) with efficient green emission. A maximum brightness close to 700 cd/m2 together with a relatively low turn-on voltage of 4.5 V has been achieved. With the design of a loosely packed QD layer resulting in the direct contact of poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (poly-TPD) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) in the device, a ternary complementary white QD-LED consisting of blue component (poly-TPD), green component (QDs), and red component (exciplex formed at the interface between poly-TPD and TPBi) has been demonstrated. The resulting white QD-LED shows an excellent color rendering index of 95.

High-temperature random lasing in ZnO nanoneedles

We report the high-temperature ultraviolet random laser action in ZnO nanoneedles. The characteristic temperature of the ZnO nanoneedle lasers was derived to be 138K in the temperature range from 300to615K. The cavity length of the random lasers as a function of temperature was determined by Fourier transform spectroscopy. The cavity length decreased with an increase in temperature from ∼14μm at 300Kto∼2μm at 550K. The optical gain of the ZnO nanoneedle lasers at high temperature is attributed to a self-compensation mechanism in the cavity length.

Strain dependence of lasing mechanisms in ZnO epilayers

The lasing characteristics of highly disordered ZnO thin films deposited on SiO2∕Si substrates with and without a MgO buffer layer have been investigated. We observed that the emission spectra of the ZnO epilayers with and without a MgO buffer are associated with the radiative recombination of free-exciton (∼380nm) and electron-hole plasma (∼395nm), respectively. The difference in the lasing wavelength is due to the induced compressive (tensile) strain along the c axis of the ZnO epilayers as a result of the presence (absence) of the MgO buffer layer. It is demonstrated that the strain-induced variation of Mott density inside the ZnO epilayers is responsible for the observed lasing characteristics.

Quantum Dot Light-Emitting Diode with Quantum Dots Inside the Hole Transporting Layers

We report a hybrid, quantum dot (QD)-based, organic light-emitting diode architecture using a noninverted structure with the QDs sandwiched between hole transporting layers (HTLs) outperforming the reference device structure implemented in conventional noninverted architecture by over five folds and suppressing the blue emission that is otherwise observed in the conventional structure because of the excess electrons leaking towards the HTL. It is predicted in the new device structure that 97.44% of the exciton formation takes place in the QD layer, while 2.56% of the excitons form in the HTL. It is found that the enhancement in the external quantum efficiency is mainly due to the stronger confinement of exciton formation to the QDs.

Colloidal Quantum Dot Light-Emitting Diodes Employing Phosphorescent Small Organic Molecules as Efficient Exciton Harvesters

Nonradiative energy transfer (NRET) is an alternative excitation mechanism in colloidal quantum dot (QD) based electroluminescent devices (QLEDs). Here, we develop hybrid highly spectrally pure QLEDs that facilitate energy transfer pumping via NRET from a phosphorescent small organic molecule-codoped charge transport layer to the adjacent QDs. A partially codoped exciton funnelling electron transport layer is proposed and optimized for enhanced QLED performance while exhibiting very high color purity of 99%. These energy transfer pumped hybrid QLEDs demonstrate a 6-fold enhancement factor in the external quantum efficiency over the conventional QLED structure, in which energy transfer pumping is intrinsically weak.

Visible red random lasing in Y2O3:Eu3+/ZnO polycrystalline thin films by energy transfer from ZnO films to Eu3+

Visible red random lasing centered at ∼611 nm has been observed in Y2O3:Eu3+/ZnO films at room temperature. Using a 355 nm laser source to excite the ZnO films, ultraviolet (UV) random lasing has been observed. The UV lasing spectrum can be tuned to overlap strongly with the F70-L56 excitation spectrum of Eu3+ ions centered at ∼394 nm by controlling the pump power, leading to very efficient radiative energy transfer from the ZnO films to Eu3+ ions. As a result, a red random lasing centered at ∼611 nm corresponding to the D50-F72 transition of Eu3+ ions was observed.

On the triplet distribution and its effect on an improved phosphorescent organic light-emitting diode

We reported phosphorescent organic light-emitting diodes with internal quantum efficiency near 100% with significantly reduced efficiency roll-off. It was found that the use of different hole transporting layer (HTL) affects the exciton distribution in the emission region significantly. Our best device reaches external quantum efficiency (EQE), current, and power efficiency of 22.8% ± 0.1%, 78.6 ± 0.2 cd/A, 85 ± 2 lm/W, respectively, with half current of 158.2 mA/cm2. This considerably outperforms the control device with N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) (HTL) and 4,4′-N,N′-dicarbazole-biphenyl (host) with maximum EQE, current and power efficiency of 19.1% ± 0.1%, 65.6 ± 0.3 cd/A, 67 ± 2 lm/W, respectively, with half current of only 8.1 mA/cm2.

Edge-Emitting Vertically Aligned ZnO Nanorods Random Laser on Plastic Substrate

Random laser action with in-plane coherent feedback has been realized in vertically aligned ZnO nanorods on plastic substrate. A polycarbonate substrate coated with a thin layer of ZnO template was used to grow the nanorods by an aqueous chemical method. It can be shown that there are some restrictions on the dimensions of the ZnO template and nanorods that can be used to achieve in-plane random lasing action. Furthermore, the lasing characteristics of the ZnO nanorods under deformation (i.e., bending the substrate) are investigated. It is found that the lasing performance of the ZnO nanorods on plastic substrate depends on the direction of bending.

Influence of Surface Roughness on the Lasing Performance of Highly Disordered ZnO Films

The influence of surface roughness on the lasing performance of highly disordered ZnO films is studied. It is found that the highly disordered ZnO films with improvement in surface quality by ion-beam milling can significantly reduce the corresponding scattering loss as well as increase the slope efficiency of the light-light curves even with the presence of MgO cladding layer. Furthermore, it is noted that random lasing action is less dependent on the surface roughness of the ZnO films