Allan R. Mackintosh

Flexible blue-emitting encapsulated organic semiconductor DFB laser.

Mechanically flexible distributed feedback (DFB) lasers are fabricated by a low-cost approach using soft-lithography from a holographic master grating. The gain material is a star-shaped oligofluorene providing laser emission from 425 to 442 nm with a soft pump threshold at 14.4 μJ/cm (2.7 kW/cm). Encapsulation of the devices enables stable operation in ambient atmosphere at a 1/e degradation energy dosage of 53 J/cm.

New light from hybrid inorganic-organic emitters

We present the highlights of a research programme on hybrid inorganic?organic light emitters. These devices combine recent developments in III?V nitride technology (including UV emitting micro-arrays and specifically tailored quantum wells) with conjugated polymers to access the entire visible spectrum. Two types of devices are studied, those based on down conversion of the quantum well emission by radiative transfer and those based on non-radiative resonant energy transfer. The spectral and operating characteristics of the devices are described in detail. Selectable colour micro-arrays and bar emitters are demonstrated. The nature of the non-radiative energy transfer process has also been studied and we find transfer efficiencies of up to 43% at 15?K, with a 1/R2 dependence on the distance between quantum well and polymer layer, suggesting a plane?plane interaction. The relative importance of the non-radiative resonant energy transfer process increases with temperature to be up to 20 times more efficient, at 300?K, than the radiative transfer process.

Tapered transmission line technique based graded matching layers for thickness mode piezoelectric transducers

Conventionally, in order to acoustically match thickness mode piezoelectric transducers to a low acoustic impedance load medium, multiple quarter wavelength (QW) matching layers are employed at the front face of the device. Typically a number of layers, 2–4 in number, are employed resulting in discrete impedance steps within the acoustic matching scheme. This can result in impedance matching with limited bandwidth characteristics. This paper investigates the application of tapered transmission line filter theory to implement a graded impedance profile, through the thickness of the matching layer scheme, to solve the impedance mismatch problem whilst accounting for enhanced transducer sensitivity and bandwidth.

Flexible distributed-feedback colloidal quantum dot laser

By fabricating a submicron-scale gratingstructure on a bendable polymer substrate, we demonstrate a flexible distributed-feedback colloidal quantum dot laser. This laser uses cadmium selenide/zinc sulfide core-shell nanostructures, operating in transverse electric polarized multiple-modes, and has a typical threshold pump fluence of ∼4 mJ/cm2.

Hybrid inorganic/organic microstructured light-emitting diodes produced using photocurable polymer blends

Light-emitting diodes (LEDs) in the form of a one-dimensional array of microstripes emitting at 370nm were fabricated from AlInGaN inorganic semiconductor. These microlight sources were then used to “directly write” microstructures in photocurable blends of organic light-emitting polymers (LEPs) spin coated onto the LED surface. In this way, thin microstripes of LEP as narrow as 50μm have been fabricated and integrated with the micro-LEDs. These “self-aligned” polymer microstripes serve as wavelength downconverters under further excitation by the UV micro-LEDs, producing hybrid inorganic/organic microstructured LEDs.

Investigating the thermal stability of 1-3 piezoelectric composite transducers by varying the thermal conductivity and glass transition temperature of the polymeric filler material

The thermal behaviour of a number of 1-3 piezoelectric composite transducers is discussed. In particular, devices manufactured from a polymer filler with a relatively high glass to rubber transition temperature (T/sub g/), and from polymer systems with increased thermal conductivity, are evaluated. The mechanical properties of the various filler materials were obtained via ultrasonic measurements, with the thermal properties extracted using dynamic mechanical thermal analysis (dmta), differential scanning calorimetry (dsc) and laserflash studies. A range of ultrasonic transducers were then constructed and their thermal stability studied using a combination of impedance analysis and laser surface displacement measurement.

Novel polymer systems for deep UV microlens arrays

We report for the first time a UV curable polymer with effective optical transmission below 300 nm. Through careful control of kinetics, various viscosities can be generated to optimize the film forming properties via spin coating. The transmission of the monomers and films is investigated over a spectral range which spans the 240–370 nm output of ultraviolet AlInGaN light-emitting diodes. The refractive index of the polymer has been measured by ellipsometry to give a value of 1.57 at 280 nm. Using standard lithography techniques with reactive ion etching, arrays of microlenses have been fabricated in this polymer with diameters of 30 µm and below and are characterized by atomic force microscopy and confocal microscopy.

Dip-pen nanolithography of nanostructured oligofluorene truxenes in a photo-curable host matrix

We report the controlled patterning of nano-sized oligofluorene truxenes onto silicon dioxide by dip-pen nanolithography (DPN) using a UV-curable pre-polymer as a carrier fluid. In this technique, a sharp atomic force microscope (AFM) cantilever tip is used to transfer the liquid ink onto a surface using piezo-controlled movements and excellent spatial registry. The photo-curable carrier fluid is then exposed to UV-light to produce a cross-linked, host matrix while retaining the photoluminescent properties of the truxenes and providing protection against photo-oxidation. The chemical composition of the composite structures deposited by DPN was characterised by Raman microspectroscopy and microphotoluminescence to demonstrate successful incorporation of the photoluminescent truxenes in the polymer matrix.

Light emitting polymer blends and diffractive optical elements in high-speed direct laser writing of microstructures

In this paper, we describe a series of improvements that have been made to our direct laser writing waveguide/microfluidic fabrication technology. We demonstrate significant increases in the writing speed (measured in micrometres of written structure per second) by both the use of customized photopolymers containing light emitting polymer and the inclusion of a diffractive optical element to enable the writing of multiple channels in a single pass.

Low-threshold organic semiconductor lasers: moving out of the laboratory

The recent, rapid development of organic semiconductor lasers builds on that of organic LEDs, which are now commercially available in displays. It opens up the prospect of compact, lowcost (even disposable) visible lasers suitable for a range of uses from point-of-care diagnostics to sensing.1 However, practical application of organic semiconductor lasers faces two main challenges. The first concerns the ability to use ultracompact pump sources with sufficient output (spectral, power) above the lasing threshold of a particular gain material and resonator. Second, lasing tests are typically conducted in vacuum to inhibit photo-oxidative degradation by excluding oxygen and moisture. Consequently, we must emulate these conditions by having adequate encapsulation. The choice of materials for lasing has mainly focused on available light emitters designed for organic LEDs (OLEDs). It is desirable to design organic semiconductors from the outset that will address the main challenges. The correct material will have excellent solution-processing properties, address the issue of photo-excitation by having a low lasing threshold and low optical losses, and spectrally match the output of the pump source. Furthermore, it can be expected that residual catalyst will accelerate degradation, so a very pure and stable material is required for the laser. Star-shaped macromolecules are branched materials consisting of linear oligomeric arms joined together by a central core. Combining such an architecture with -electron-conjugated arms would result in new electrical, optical, and morphological properties. In addition, because of the monodispersed nature of the materials, the product of the synthesis is pure and Figure 1. (top left) Oligofluorene truxene T4, with R D C6H13. (top right) Laser emission from T4 spin-coated on a 2D distributedfeedback grating. (Photo courtesy of Georgios Tsiminis, University of St Andrews, UK.) (bottom) Flip-chip CMOS micro-LED with 368nm emission.