Gregor Langer

Two-photon absorption-induced photoacoustic imaging of Rhodamine B dyed polyethylene spheres using a femtosecond laser

In the present paper we demonstrate the possibility to image dyed solids, i.e. Rhodamine B dyed polyethylene spheres, by means of two-photon absorption-induced photoacoustic scanning microscopy. A two-photon luminescence image is recorded simultaneously with the photoacoustic image and we show that location and size of the photoacoustic and luminescence image match. In the experiments photoacoustic signals and luminescence signals are generated by pulses from a femtosecond laser. Photoacoustic signals are acquired with a hydrophone; luminescence signals with a spectrometer or an avalanche photo diode. In addition we derive the expected dependencies between excitation intensity and photoacoustic signal for single-photon absorption, two-photon absorption and for the combination of both. In order to verify our setup and evaluation method the theoretical predictions are compared with experimental results for liquid and solid specimens, i.e. a carbon fiber, Rhodamine B solution, silicon, and Rhodamine B dyed microspheres. The results suggest that the photoacoustic signals from the Rhodamine B dyed microspheres do indeed stem from two-photon absorption.

Refractive Index Changes in Polymers Induced by Deep UV Irradiation and Subsequent Gas Phase Modification

Poly(4-vinylbenzyl thiocyanate) (PVBT) and a copolymer of styrene and 4-vinylbenzyl thiocyanate (PST-co-VBT) were investigated with regard to changes of the refractive index under UV irradiation (λ = 254 nm). After irradiation, the refractive index n D of PVBT films increased from 1.629 to 1.660. In the case of the copolymer PST-co-VBT n D increased from 1.616 to 1.630. The change of the refractive index mainly resulted from the photoinduced isomerization of thiocyanate groups (SCN) to the corresponding isothiocyanates (NCS). The NCS groups formed in the irradiated zones were selectively modified with gaseous amines (ammonia, propylamnie, ethylenediamine, and hydrazine) to give thiourea derivatives vias an addition reaction. The gas phase modification induced further changes of the refractive index without any loss of the film quality (e.g. propylamine, Δn up to -0.026, an hydrazine, Δn up to +0.035). In addition, the thickness h of the polymer films increased by up to 21% as a result of the gas phase modification. In contrast, wet chemical treatment with bulky amines (1-methylnaphthyl-amine, 1-pyrenemthylamine) caused a considerable deterioration of the film quality. The variation of the refractive index in polymers such as PVBT and PST-co-VBT is of potential interest for holographic and the recordings and the setup of polymer based DFB lasers.

Optoelectronic printed circuit board: 3D structures written by two-photon absorption

The integration of optical interconnects in printed circuit boards (PCB) is a rapidly growing field due to a continuously increasing demand for high data rates, along with a progressive miniaturization of devices and components. For high-speed data transfer, materials and integration concepts are searched for which enable high-speed short-range connections, accounting also for miniaturization, and costs. Many concepts are discussed so far for the integration of optics in PCB: the use of optical fibers, or the generation of waveguides by UV lithography, embossing, or direct laser writing. Most of the concepts require many different materials and process steps. In addition, they also need highly-sophisticated assembly steps in order to couple the optoelectronic elements to the optical waveguides. An innovative approach is presented which only makes use of only one individual inorganic-organic hybrid polymer material to fabricate optical waveguides by two-photon absorption (TPA) processes. Particularly, the waveguides can be directly integrated on pre-configured PCB by in situ positioning the optical waveguides with respect to the mounted optoelectronic components by the TPA process. Thus, no complex packaging or assembly is necessary, and the number of process steps is significantly reduced, where the process fits ideally into the PCB fabrication process. The material properties, the TPA processing of waveguides, and the integration concept will be discussed. Recent experiments employing vertical-cavity surface-emitting lasers demonstrated data rates exceeding 6 Gbit/s.

Remote mid-infrared photoacoustic spectroscopy with a quantum cascade laser.

We demonstrate non-contact remote photoacoustic spectroscopy in the mid-infrared region. A room-temperature-operated pulsed external-cavity quantum cascade laser is used to excite photoacoustic waves within a semitransparent sample. The ultrasonic waves are detected remotely on the opposite side of the sample using a fiber-optic Mach-Zehnder interferometer, thereby avoiding problems associated with acoustic attenuation in air. We present the theoretical background of the proposed technique and demonstrate measurements on a thin polystyrene film. The obtained absorption spectrum in the region of 1030-1230  cm(-1) is compared to a spectrum obtained by attenuated total reflection, showing reasonable agreement.

The impact of the non-linearity of the radiant flux on the thermal load of the color conversion elements in phosphor converted LEDs under different current driving schemes.

For a systematic approach to improve the reliability and the white light quality of phosphor converted light-emitting diodes (LEDs) it is imperative to gain a better understanding of the individual parameters that affect color temperature constancy and maintenance. By means of a combined optical and thermal simulation procedure, in this contribution we give a comprehensive discussion on the impact of different current driving schemes on the thermal load of the color conversion elements (CCEs) of phosphor converted LEDs. We show that on the one hand a decreasing duty cycle under pulse width modulation driving conditions may cause a notable temperature variation and on the other hand also effects due to the non-linearity between the blue radiant flux and the current have to be considered for the thermal load of the CCEs.

Organic surface emitting laser based on a deep-ultraviolet photopolymer containing thiocyanate groups

Gratings for laser applications are being optically recorded in poly(styrene-co-4-vinylbenzyl thiocyanate) (PST-co-VBT) by deep-ultraviolet lithography (λ<300 nm). The gratings writing configuration is based on interference patterning of this holographic material. Grating structures can be further stabilized and modified by selective amine reaction from the gas phase. Gratings based on refractive index contrast (Δn∼+0.02) and on thickness modulation (Δd up to 70 nm) are obtainable from this photoreactive polymer. Holographically recorded structures in films of PST-co-VBT doped with a laser dye are designed for second order distributed feedback operation resulting in low threshold organic laser devices (wL,th=0.25 μJ cm−2) with laser emission output (TE-mode: λL=632 nm) directed perpendicular to the film plane.

Polymer-based red, green, and blue emitting devices fabricated by reductive photopatterning

Using a color tuning approach reliant on reductive photopatterning, we present red-green-blue electroluminescence from a single layer polymer light-emitting device. To be able to cover the full color range, we employ a single emissive layer consisting of a blue emitter (the host polymer), as well as green and red emitting guest polymers. The energy transfer between the host and the various guest compounds is tuned via a reductive photoinitiated process in the presence of gaseous hydrazine. This process is compatible with regular film casting techniques such as spin coating, and therefore can be regarded as a promising alternative to the more complex, traditional patterning approaches.

Frequency domain photoacoustic and fluorescence microscopy.

We report on simultaneous frequency domain optical-resolution photoacoustic and fluorescence microscopy with sub-µm lateral resolution. With the help of a blood smear, we show that photoacoustic and fluorescence images provide complementary information. Furthermore, we compare theoretically predicted signal-to-noise ratios of sinusoidal modulation in frequency domain with pulsed excitation in time domain.

Formation of Images and Surface Relief Gratings in Photosensitive Polymers Containing SCN Groups

Poly(4-vinylbenzyl thiocyanate) (PVBT) and a copolymer of styrene and 4-vinylbenzyl thiocyanate (PST-co-VBT) were assessed as dual-tone imaging systems. These polymers change their refractive index under deep UV irradiation as a result of the photoisomerization of SCN to NCS groups. Subsequent modification of the illuminated zones with gaseous amines grave thiourea groups via an addition reaction. The post-exposure modification of the film with gaseous amines produced large increasese of the film thickness (formation of negative images). This effect was utilized to produce surface relief gratings. Using contact lithography with λ + 193 and 248 nm, gratings with periods Λ = 300 and Λ = 600 nm were obtained. Under selected experimental conditions, a nucleophilic substitution (S N ) of the SCN units in the unirradiated zones by amines was achieved (formation of positive images). Gratings of these types are of general interest for the setup of organic lasers based upon the principle of distributed feedback (DFB) as well as for holographic recordings.

All-optical tunability of holographically multiplexed organic distributed feedback lasers.

We report the fabrication and characterization of optically pumped multiple grating distributed feedback lasers in dye doped organic thin films. Each multiplexed laser structure is inscribed at a different angle in the sample plane and possesses a unique emission wavelength. The polarization sensitivity of these structures with respect to the pumping light is exploited to enable simple and high-speed switching of the device emission wavelength.