Improved operational lifetime of semiconducting polymer lasers by encapsulation

Abstract

We report polymer distributed feedback lasers with dramatically extended operational lifetimes by using a simple encapsulation process. The lasers are configured as surface emitting, two-dimensional distributed feedback lasers based on the polymer poly͓2-methoxy-5-͑2Ј-ethylhexyloxy͒-1,4-phenylene vinylene͔. The microstructure is transferred to the polymer surface through solvent assisted micromolding. Once encapsulated, a 2500-fold improvement in lifetime is demonstrated under ambient conditions, compared to the unencapsulated device. A blueshift of the emission wavelength observed during operation is characterized by absorption and ellipsometry measurements and attributed to a change in effective index due to a loss of conjugation in the polymer.Since the demonstration of lasing from a semiconducting conjugated polymer, 1 the development of lasers based on these fascinating materials has been a vigorous research topic. Their large gain cross sections coupled with broad emission spectra and ease of processing from solution, make conjugated polymers particularly suited to the fabrication of photonic devices. [2][3][4] The most common resonators now used are diffractive structures that incorporate wavelength scale microstructure to form distributed feedback ͑DFB͒ lasers. The corrugation can be etched into a silica substrate 5 or patterned directly into the polymer film using a soft lithographic method such as solvent assisted micromoulding 6,7 ͑SAMiM͒. In both cases, the high refractive index contrast of the microstructured interface can allow for very strong feedback in a compact device. 8 A significant challenge for the application of these devices is to develop configurations which achieve adequate stability. One limitation of these materials is their susceptibility to degradation in the presence of oxygen and water. To date, there is little quantitative data on the operating lifetimes of organic semiconductor lasers. 9-11 While a few polymer lasers have included a thin passive polymer cover layer, 12,13 the quantitative effect on lifetime was not discussed. Using encapsulation to extend operational lifetime is commonplace in the field of organic light emitting diodes ͑OLEDs͒, protecting the emissive polymer layer and metal contacts from oxidation. In the case of OLEDs, encapsulation is typically applied on top of the metal cathode so properties of the device are largely unaffected with little impact on the external quantum efficiency or the photoluminescence spectrum. 14 For DFB lasers, more care must be exercised when selecting encapsulating materials. Changes to the refractive index contrast between layers can have a dramatic effect on the waveguide modes of the device. The encapsulating layers should also be optically transparent and not react adversely with the polymer while still acting as an effective atmospheric barrier.In this letter, we show that a polymer laser can be encapsulated without degrading its performance when compared to an unencapsulated laser. Absorption and ellipsometry data are used to q...

Organic solid‐state integrated amplifiers and lasers

“…The longest operating lifetime reported for a solid-state organic laser to date exceeds 2 × 10 7 pulses and was obtained from DFB lasers based on the fluorene copolymer Dow Red F [177]. Degradation can be mitigated by encapsulating the gain medium into other materials [187][188][189][190], a method that is commonly used to extend the operational lifetime of OLEDs by protecting from oxidation the emissive polymer layer and the metal contacts. The operating lifetime of lasers based on semiconducting conjugated polymers has been significantly prolonged by encapsulating the gain medium in either adhesives [187,188] or photopolymerizable resins [189] or other polymers [190].…”

“…Degradation can be mitigated by encapsulating the gain medium into other materials [187][188][189][190], a method that is commonly used to extend the operational lifetime of OLEDs by protecting from oxidation the emissive polymer layer and the metal contacts. The operating lifetime of lasers based on semiconducting conjugated polymers has been significantly prolonged by encapsulating the gain medium in either adhesives [187,188] or photopolymerizable resins [189] or other polymers [190]. The effectiveness of this approach was convincingly demonstrated in an experiment, in which the lifetime of a DFB MEH-PPV polymer laser increased from the 6 × 10 3 pulses to 1.6 × 10 7 pulses after encapsulation [187].…”

All-polymer organic semiconductor laser chips: Parallel fabrication and encapsulation

“…After this experiment, lasing could be observed as soon as the pump energy was increased for all the devices. As in the case of epoxy encapsulation [15] a blue-shift in the emission wavelength occurs during operation. The wavelength change was ~0.02 nm/10 5 pulses for the PMMA laser and ~5.2 nm/10 5 pulses for the COC lasers.…”

“…Bulović et al reported on organic semiconductor microcavity lasers encapsulated by evaporated organic and metal layers [14]. Richardson et al demonstrated encapsulation with an optical adhesive on top of organic semiconductor lasers [15]. Persano et al presented encapsulation utilizing a rapid prototyping technique and additionally gave an overview over present encapsulation schemes for organic photonic devices [16].…”

Highly-photostable and mechanically flexible all-organic semiconductor lasers

“…One possible way to achieve this is to embed the OS emitters within an appropriate polymeric host matrix, creating (when the OS gain molecules are of appropriate size) a nanocomposite. OS lasers have been reported using such an approach, showing extended photostability but to the detriment of the laser threshold performance [8,19]. Another way to improve the lifetime of an organic laser is to 'encapsulate', i.e.…”