Low threshold organic thin-film laser devices

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In this work, low threshold organic semiconductor lasers emitting throughout the entire visible wavelength range are presented. Organic semiconductor lasers (OSLs) are a fascinating class of laser devices that have a huge potential for sensing and display applications. Their ease of fabrication and tuneability across the full visible wavelength range are only a few of their advantageous properties which fueled intense research towards organic laser devices. For future electrically pumped organic lasers, as well as for compact (laser) diode-pumped devices, reduction of the organic laser threshold is of crucial importance, since low optically pumped thresholds translate into lower current densities required for injection lasing.

With blue-emitting one-dimensional first- and second-order distributed feedback (DFB) lasers based on the copolymer BN-PFO , laser operation in a wavelength range from 438 to 459 nm was realized. For an optimized second-order laser, we obtained a very low threshold energy of 280 pJ/pulse, which could be further reduced to 160 pJ/pulse by employing first-order feedback. These very low threshold values render BN-PFO a very promising material for future organic semiconductor laser diodes. Furthermore, we have investigated DFB lasers based on a mixed-order resonator concept and the polyfluorene derivative F8DP . We showed that this improved resonator concept is a very promising design which combines the advantages of first- and second-order DFB resonators. By varying the grating parameters, organic solid-state lasers with customized properties can be fabricated. Optimizing the polymer film parameters led to a very low laser threshold of 45 pJ/pulse ( 36 nJ/cm2), which is among the lowest values ever reported for organic semiconductor lasers.
These DFB lasers have been optically pumped by frequency-tripled Nd:YVO4 lasers or complex optical parametric oscillator (OPO) systems, resulting in versatile but expensive and bulky laser sources. For many applications, e.g. for laser-based analytical techniques and sensors, much more compact and inexpensive all solid-state laser sources are desirable. Whilst an organic injection laser doesn’t exist, it might prove useful for numerous applications to adopt an indirect electrical pumping scheme using efficient and compact electrically driven light sources to pump an OSL optically. The recent evolution of blue-violet emitting inorganic (In)GaN laser diodes renders them attractive as such a pump source. During the course of this work, a very compact all solid-state laser system using a low-cost pulsed (In)GaN laser diode has been realized. Laser emission spanning the complete visible wavelength range was achieved by employing a variety of organic materials and resonator geometries. As a future asset, these hybrid organic/inorganic lasers could be made mechanically tuneable by either using a wedge-shaped organic thin-film or by spatially varying the lattice period. Both concepts alter the emission wavelength when the organic laser is moved mechanically in front of the focussed pump laser diode. These hybrid laser systems could provide the basis for innovative portable analysis systems, e.g. for medical point-of-care sensor systems. An even lower-cost pumping scheme based on LEDs could lead to extremely low-cost and versatile laser sources emitting throughout the entire visible wavelength range.
But the ultimate goal remains the realization of an electrically pumped organic laser diode. In the course of this thesis, a self-consistent numerical simulation tool was employed to carry out comprehensive investigations of the influence of various parameters on the laser threshold in electrically pumped multilayer OSLs. It could be shown that the threshold current densities necessary for lasing in an organic laser diode structure will be of the order of 500-1000 A/cm2. The main reasons for these high threshold values are:
• waveguide losses
• excited state absorptions
• bimolecular annihilation processes
In order to reduce the waveguide losses, two concepts have been discussed: either using thin active layers in combination with low-loss transparent conductive oxide (TCO) electrode materials, or using thick (doped) multilayer devices with metal electrodes. The threshold current density is also negatively influenced by polaron and excited state absorption. The dimensionless quantity, ς has been introduced to quantify the effect of polaron and excited state absorption in the device. It saturates at increasing current densities, implying that polaron- and triplet-triplet absorption might prevent electrically pumped devices from lasing for all current densities, depending on the respective absorption cross sections. It was shown that ς does not strongly depend on the device geometry. For the studied devices, an increased charge carrier mobility in the transport layers does not reduce polaron absorption significantly, but if the mobilities in the emission layer and in the transport layers could be increased simultaneously, the effect of polaron absorption would be reduced. We also investigated the influence of bimolecular annihilation processes on the threshold current density using numerical simulations. For a set of typical annihilation and material parameters, the threshold current density was calculated to be ≈560 A/cm2. It was found to depend critically on the emission layer thickness. Singlet-polaron and singlet-triplet annihilations were identified as the dominating quenching processes for the investigated parameter range.
According to the presented numerical simulations, organic laser diodes will require very high current densities, hence the current durability of organic materials will be an important issue. Dielectric discharges and thermal breakdown were identified as the major causes for catastrophic device failure under high excitation conditions. Thus, thermal management was identified as a key element to improve device stability. To reduce the thermal load of the devices, high thermal conductivity substrates or pulsed operation can be employed. We were able to demonstrate that organic materials can indeed sustain the required current densities. In high current excitation experiments, more than 550 A/cm2 could be passed through a thick photocrosslinked hole-transport layer in pulsed mode. This very encouraging and significant result shows that photocrosslinked all-polymer devices might be the proper choice to realize an organic injection laser.

Apart from the concepts elucidated above, further approaches to an organic injection laser exist. Recently, ambipolar light-emitting organic field-effect transistor (OFET) geometries were discussed in the context of OSLs. But to date, the achieved current densities are typically about one order of magnitude too low. Another promising novel approach is to induce capacitively coupled lasing action in OSLs. An organic active material is sandwiched between two dielectric-clad electrodes and excited via an AC voltage. This is advantageous as it avoids the optical losses associated with injecting electrodes close to the active layer. This concept will be evaluated in the near future in our group at the LTI .