We present thermal tuning of air-suspended SU-8 polymer waveguide grating couplers for TE-polarized light. Numerical simulations have been performed to estimate the wavelength shift caused by the change of temperature. Due to the small positive thermal expansion and large negative thermo-optic coefficient of SU-8, a shift toward shorter wavelengths is expected. In the experimental evaluation, a negative wavelength shift from 1542 nm at 20°C toward 1527 nm at 56°C is obtained with approximately −0.42 nm K−1 matching the theoretical considerations.
In order to achieve a high refractive index contrast for air-suspended photonic devices, we present a method for laminating thin polymer films onto structured polymer layers that exhibit an air cavity. By using a flat PDMS stamp, polymer films can be transferred over areas of several hundred square microns. On top of the air-suspended slab a second layer of photoresist can be spun and subsequently every desired photonic structure can be defined by using standard photolithography. Here, to demonstrate the feasibility of our lamination method for polymer photonic devices, we present optical modeling and experimental results of air-suspended single mode rib waveguides. Waveguiding is shown for visible and infrared light and a beam profile for λ = 1550 nm is presented that underpins single mode behavior of the rib waveguide.
Optofluidic devices exploit the characteristics of liquids to achieve a dynamic adaptation of their optical properties. The
use of liquids allows for functionalities of optical elements to be created, reconfigured or tuned. We present an overview
of our work on fluid-control of optical elements and highlight the benefits of an optofluidic approach, focusing on
optofluidic cavities created in silicon photonic crystal (PhC) waveguide platforms. These cavities can be spatially and
spectrally reconfigured, thus allowing a dynamic control of their optical characteristics. PhC cavities are major building
blocks in many applications, from microlasers and biomedical sensor systems to optical switches and integrated circuits.
In this paper, we show that PhC microcavities can be formed by infusing a liquid into a selected section of a uniform
PhC waveguide and that the optical properties of these cavities can be tuned and adapted. By taking advantage of the
negative thermo-optic coefficient of liquids, we describe a method which renders PhC cavities insensitive to temperature
changes in the environment. This is only one example where the fluid-control of optical elements results in a
functionality that would be very hard to realize with other methods and techniques.
We report reconfigurable optofluidic photonic crystal components in silicon-based membranes by controllably
infiltrating and removing fluid from holes of the photonic crystal lattice. Systematic characterizations of our fluidically defined
microcavities are presented, corresponding with the capability to increase or decrease the span of the fluid-filled
regions and thus alter their optical properties. We show initial images of single-pore fluid infiltration for holes of
diameter 265 nm. Furthermore, the infiltration process may employ a large range of optical fluids, adding more
flexibility to engineer device functionality. We discuss the great potential offered by this optofluidic scheme for
integrated optofluidic circuits, sensing, fluorescence and plasmonic applications.
Many optofluidic devices rely on interfacing optical waveguides with microfluidic channels. Often it can be difficult to
realize micron scale waveguides and fluidic channels that are 100 times larger on the same platform. Further, it is often
desirable for an optical waveguide intersection to occur at the vertical centre of a fluidic channel rather than at its top or
at its bottom where the fluid is effectively stationary.
We present a platform for optofluidics which can achieve straightforward integration of large scale fluidic channels and
micron scale waveguides in the epoxy material SU8. A soft imprinting technique is used to define the optical waveguides
as a thin inverted rib core layer between two thick cladding layers. The core is doped with Rhodamine dye to increase the
refractive index and render it optically active for potential use as a lasing material. The fluidic channels are then formed
by a single exposure through the core and both claddings.
At high current densities, the characteristics of organic laser diode structures are strongly influenced by a variety
of loss processes such as bimolecular annihilations, field-induced exciton dissociation and induced absorptions
due to polarons and triplet excitons. Here, we investigate a TE2-mode organic double-heterostructure laser diode
by numerical simulation. The electrical properties are described using a numerical drift-difusion model and the
optical characteristics are modeled using a transfer matrix method. When annihilation processes are included,
a threshold current density of 8.5 kA/cm2 is derived for the considered device. Laser operation is not achieved
when field-induced exciton dissociation is considered. For induced absorptions, maximum relative cross sections
of 9.6 × 10-8 for polarons and 1.4 × 10-4 for triplet excitons have been calculated, which would still allow laser
operation. For higher relative absorption cross sections, laser operation is suppressed for all current densities.
Furthermore, the impact of field quenching is analyzed and the separation of singlet excitons from polarons and
triplet excitons in the time domain is studied.
We demonstrate the feasibility of organic semiconductor lasers as light sources for lab-on-a-chip systems. These lasers
are based on a 1D- or 2D-photonic crystal resonator structure providing optical feedback in the active laser material that
is deposited on top, e.g. aluminum tris(8-hydroxyquinoline) (Alq3) doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). We investigated different fabrication methods for the resonator
structures, like thermal nanoimprint, UV nanoimprint, and laser interference lithography. Different substrate materials
commonly used in lab-on-a-chip systems, e.g. PMMA, Topas, and Ormocer were deployed. By changing the distributed
feedback grating periodicity, we demonstrate a tuning range for a single material system of more than 120 nm.
The investigated organic semiconductor lasers are optically pumped. External optical pumping provides a feasible
way for one-time-use chips. Our recent success of pumping organic lasers with a low-cost laser diode also renders hand-held
As a further step towards the integration of organic lasers in sensor systems, we demonstrate the coupling of an
organic laser into polymeric waveguides which can be combined with microfluidic channels. The integrated organic
lasers and the waveguides are both fabricated on the same polished PMMA substrate using thermal nanoimprint
lithography and deep-UV modification, respectively. We could demonstrate the guiding of the laser light in single-mode
We examine the influence of various annihilation processes on the laser threshold current density of organic semiconductor laser diode structures. A three-layer laser diode structure is systematically investigated by means of numerical simulations. Our self consistent model treats the dynamics of electrons, holes and singlet as well as triplet excitons in the framework of a drift-diffusion model. The resulting particle distributions enter into the optical model. In our approach, we consider the actual waveguide structure and solve the resulting laser rate equation. The various annihilation processes are included as reactions between the different species in the device. We systematically vary the device dimensions and parameters of our singlet exciton annihilation model to identify the dominating quenching process in order to deduce design rules for potential organic laser diode structures. A standard material with typical material properties and annihilation rate coefficients is investigated. Singlet exciton quenching by polarons is identified as the main loss channel. The laser threshold in three layer devices is found to be very sensitive to the thickness of the emission layer.
By employing a combined optical/electronic model, we investigate the effect of electronic properties on the
performance of three layer organic semiconductor structures, which are a potential candidate for future electrically
pumped organic laser diodes. The drift-diffusion equations which describe particle transport are coupled to the
spatially inhomogeneous laser rate equations to solve for the dynamics of the excited state and photon population
in the laser cavity. Due to the high current densities considered, high particle densities occur, which implies that
annihilation processes between the different particle species have to be considered. On the optical side, we take
into account the absorption of the metal electrodes required for current injection to obtain the intensity profiles
of the guided modes.
Our calculations show that the inclusion of annihilation processes leads to a strong dependence of the laser
threshold on the charge carrier mobilities, in contrast to the situation when exciton annihilation is neglected. We
observe optimum values for the charge carrier mobilities in the emission layer regarding the threshold current
and power density. On the other hand, an increase of the mobilities in the transport layers leads to a reduction
of these quantities. The threshold voltage decreases for increasing mobilities, regardless of the layer in which
the mobility is increased. For optimised values, we obtain a threshold current density of jthr = 267 A/cm2 with
annihilation processes taken into account.
The presented results can serve as guidelines in the search for material combinations and devices structures
suitable for electrically pumped organic semiconductor laser diodes.
Compared to well established liquid based dye lasers, amplifying media based on amorphous organic thin films allow the realisation of versatile, cost effective and compact lasers. Aside from that, the materials involved are organic semiconductors, which in principle allow the fabrication of future electrically driven organic laser diodes. A highly promising, low-loss resonator geometry for these lasers is the distributed feedback (DFB) structure, which is based on a periodic modulation of the refractive index in the waveguide on the nanometer scale. By variation of the grating period Λ one may tune the laser emission within the gain spectrum of the amplifying medium. We will demonstrate organic lasers spanning the entire spectral region from 360-715 nm. Tuning ranges as large as 115 nm (λ = 598-713 nm) in the red spectral region and more than 30 nm (λ = 362-394 nm) in the UV render these novel lasers highly attractive for various spectroscopic applications. As the grating period Λ is typically between 100 nm and 400 nm the DFB resonators are fabricated by e-beam lithography. These gratings may, however, be used as masters to obtain an arbitrary amount of copies by nanoimprint lithography into plastic substrates. Therefore these lasers are very attractive even for single-use applications (e.g. in medicine and biotechnology). Today, the key challenge in the field is the realisation of the first electrically driven organic laser. Key pre-requisites are highly efficient amplifying material systems which allow for low threshold operation and charge transport materials that bring about the stability to sustain the necessary current densities, several orders of magnitude higher than in OLEDs. We will demonstrate diode structures operated electrically under pulsed conditions at current densities up to 760 A/cm2 with a product of the current density and the external quantum effciency (J×ηext) of 1.27 A/cm2. Mechanisms deteriorating the quantum efficieny at elevated current densities will be discussed.
The properties of electrically pumped organic laser devices are investigated by the self consistent numerical solution of the spatially inhomogeneous laser rate equations coupled to a drift-diffusion model for the electrons, holes and singlet excitons. By fully taking into account the effect of stimulated emission on the exciton population, we determine the spatial and temporal evolution of the photon density in organic multilayer structures. We apply the model to calculate laser threshold current densities and investigate transient phenomena like the delay of radiation onset. By performing systematic parameter variations, we derive design rules for potential organic laser diode structures.
In this article, a model to calculate the modal gain in organic
laser diode structures is presented. A single layer design is
considered to investigate the dependence of the gain on power
density, charge carrier mobility and thickness of the active layer.
We show that unequal charge carrier mobilities are detrimental and
that there is an optimum active layer thickness of approximately
200 nm, if different devices are compared on the basis of equal
power density. Neglecting all losses, the highest calculated gain
is 0.7/cm for a power density of P=50 kW/cm2 in our MEH-PPV
like model material. Furthermore, the influence of absorption by
polarons is quantified. We show that the cross section for this
process has to be at least 20 times smaller than the cross section
for stimulated emission in order to achieve net gain in the most
favourable case that was considered.
The concept of an AlGaInP thin-film light emitting diode includes a structure of semiconductor layers with low optical absorption on which a highly reflective mirror is applied. After bonding this wafer to a suitable carrier, the absorbing GaAs substrate is removed. Subsequently, electrical contacts and an efficient light scattering mechanism for rays propagating within the chip is provided. To achieve high efficiency operation it is crucial to optimize all functional parts of the device, such as the mirror, contacts, and active layer. Different mirrors consisting of combinations of dielectrics and metals have been tested. New chip designs have been evaluated to reduce the absorption at the ohmic contacts of the device. For efficient light scattering, the surface roughness of the at the emission window has to be optimized.
Using these structures, and a thin active layer consisting of five compressively strained quantum wells, an external quantum efficiency of 40% is demonstrated at 650 nm. Further improvement is expected.
Since the AlGaInP material system can provide only poor carrier confinement for active layers emitting in the yellow wavelength regime, the internal efficiency of these LEDs is comparably low. In order to reduce the problem of carrier leakage, a yellow active region usually consists of some hundred nanometers of active material. To circumvent the problem of this highly absorbing active layer, a separation of the light generation and the area of light extraction is suggested for yellow thin-film LEDs. First results are presented in this paper.
We propose a new process for thin-film surface-textured LEDs that provides uniform current injection for both top and bottom contacts. The structure uses a partially conductive mirror. This eliminates the need for thick epitaxial layers and makes it possible to fabricate very large LEDs. Furthermore, the new process allows to obtain both high external quantum efficiency and high wallplug efficiency. 400 x 400 μm GaInP/AlGaInP LEDs reach maximum external quantum efficiencies of 35% at 12 mA without encapsulation. The wallplug efficiency reaches 34% at 2.6 mA. At an operating current of 60 mA, the devices emit 30 mW of light.
The absorption of lateral guided modes in light emitting diodes is determined by the photocurrent measurement method. A theory for waveguide dispersion is presented and extended by ray-tracing simulations. Absorption coefficients of InGaN-on-sapphire and AlGaInP-based structures is evaluated by comparison with simulation curves. For nitride-based samples with emission wavelengths of 415 nm and 441 nm an absorption of 7 cm-1 is obtained. It is found that scattering is present in the buffer layer and influences the lateral intensity distribution. The investigated AlGaInP-based sample exhibits an absorption of α = 30 cm-1 at 650 nm emission wavelength.
Very high external efficiencies have been reported from surface-textured thin-film light-emitting diodes. We have developed a novel process for the wafer-scale fabrication of surface-textured thin-film LEDs, avoiding the use of wet thermal oxidation and epitaxial lift-off. The LEDs consist of a double-mesa structure with a structured gold reflector serving simultaneously as a p-contact. The light emission occurs on the side of the original GaAs substrate, which is removed by selective etching after glueing the sample with the processed side onto a carrier substrate. The light emission of the devices is fully confined within the diameter of the LED itself. In comparison to our previously reported LEDs, the series resistance has been significantly reduced by the current injection through the mirror. 85(mu) 2m diameter GaAs/AlGaAs LEDs reach maximum external quantum efficiencies of 42% before and 51% after encapsulation. Encapsulated devices reach a maximum wallplug efficiency of 47% at a current of 3.5 mA. At an operating current of 20 mA, they emit 14 mW of light. As a first result on 650 nm GaInP/AlGaInP LEDs we obtained external quantum efficiencies of 28% for un-encapsulated devices with a diameter of 75micrometers . At a drive current of 8 mA the LEDs emit 3.4 mW of light.
We present results on efficient InGaAlP light-emitting diodes using lateral outcoupling taper. This concept is based on light generation in the very central area of a circularly symmetric structure and, after light propagation between two mirrors, outcoupling in a tapered mesa region. We have demonstrated the suitability of this concept on As-based Light-Emitting Diodes emitting at 980 nm. Since the idea is not limited to a certain material system, we fabricated InGaAlP-based LEDs emitting in the red wavelength regime. By adjusting the process flow to the new material system we were able to achieve external quantum efficiencies in the range of 13% for unencapsulated devices. Additionally we present a new concept combining the idea of outcoupling tapers with a waferscale soldering technique. First samples show external quantum efficiencies in the range of 11%.
The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a new type of semiconductor laser structure with the capability of achieving high output power while retaining good beam quality. We demonstrate the first AlGaInP-based red light emitting OPS-TDL structure. The device has been pumped optically with an argon-laser at 514~nm. The device has an epitaxial backside mirror and a multiple quantum well active region, consisting of strained InGaP quantum wells arranged in several groups as a periodic gain structure. A peak single-mode output power of more than 200mW at 660nm has been obtained in pulsed operation. Various designs for the active layer have been investigated.
Usually resonant-cavity light-emitting diodes (RCLEDs) are used as emitters for plastic optical fiber communication. However, there are some arguments that may lead to the introduction of RCLEDs in a much wider range of applications. A typical high-brightness AlGaInP LED consists of a Bragg mirror, the active region and some layers for current spreading and light extraction. The thickness of these layers can add up to several ten microns which causes long epitaxial growth times. The total thickness of a RCLED can be significantly lower. Furthermore, since no lattice mismatched layers such as GaP are involved, the total incorporated strain is low which simplifies wafer handling and device processing. For this reason we studied RCLEDs with a dominant wavelength around 632 nm (superred) and 605 nm (orange). The processes for epitaxial growth and chip fabrication were optimized for homogeneity on 4 inch wafers and suitability for low-cost mass production, respectively. Possible applications for our RCLEDs are optical scanners, indicators, signal lights and other applications which benefit from the enhanced directionality of RCLEDs.