Nanoimprinted distributed feedback lasers of solution processed hybrid perovskites

Hybrid perovskite materials have considerable potential for light emitting devices such as LEDs and lasers. We combine solution processed CH3NH3PbI3 perovskite with UV nanoimprinted polymer gratings to fabricate distributed feedback (DFB) lasers. The lead acetate deposition route is shown to be an effective method for fabricating low-loss waveguides (loss coefficient ~6 cm-1) and highly compatible with the polymer grating substrates. The nanoimprinted perovskite exhibited single-mode band-edge lasing, confirmed by angle-dependent transmission measurements. Depending on the excitation pulse duration the lasing threshold shows a value of 110 μJ/cm2 under nanosecond pumping and 4 μJ/cm2 under femtosecond pumping. We demonstrate further that this laser has excellent stability with a lifetime of 10 pulses. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation,


Introduction
Hybrid organic-inorganic halide perovskites have recently emerged as an important new class of optoelectronic materials [1].Perovskite solar cells have shown by far the most rapid growth in performance of any photovoltaic technology with efficiencies now exceeding 20% [2,3].Most research has focused on methylammonium lead halides (CH 3 NH 3 PbX 3 where X=Cl, Br or I).By altering the halide constituent, the band-gap may be tuned from nearinfrared to deep-blue [4,5].The CH 3 NH 3 PbX 3 thin films can be solution processed by depositing precursors onto a substrate followed by a thermal annealing process.There is now considerable interest in light-emitting devices as well as solar cells.Light-emitting diodes (LEDs) [6][7][8] have been demonstrated using CH 3 NH 3 PbX 3 thin films, while fully inorganic CsPbX 3 colloidal nanocrystals have shown photoluminescence quantum yields (PLQY) of up to 90 % [9,10].
The high brightness and band-gap tuneability has made CH 3 NH 3 PbX 3 perovskites attractive candidates as optical gain media for a new family of low-cost semiconductor lasers.Combined with their high ambipolar charge mobility they have future potential to be used as visible wavelength tuneable diode lasers.Perovskite lasers have been demonstrated in several configurations: resonators including a Fabry-Perot cavity formed with parallel edge facets [11]; ring resonators in microspheres or nanoplatelets [9,[12][13][14]; and random lasing in scattering films [15].These structures have supported multimode lasing spanning the full amplified spontaneous emission (ASE) bandwidth (5-10 nm).For many applications (spectroscopy, sensing, communications) laser action at user-defined wavelengths and singlemode operation is very desirable [16].Photonic crystal single-mode perovskite lasers have recently been demonstrated using both SiO 2 and Si photonic crystals fabricated by electron beam lithography [17] and holographic lithography [18].Here we present distributed feedback (DFB) perovskite lasers fabricated on UV nanoimprint lithography (UV-NIL) polymer gratings.This is a simple, high-throughput and fully solution processable method for DFB grating fabrication which we demonstrate to be compatible with perovskite solution processing.
To achieve high performance operation in perovskite lasers, it is important to create films with good optical performance and low scattering losses.As such, deposition methods which produce large crystallites such as thermal evaporation are less suitable for making DFB lasers [19].Here we demonstrate DFB lasing by using the lead acetate deposition method to form a low loss optical waveguide on top of high fidelity polymer micro pillar arrays made by UV-NIL where sub-nanometre lasing spectra are observed at the band edge of the photonic dispersion.We compare laser thresholds under nano-and femto-second optical pumping and show that the perovskite lasers are very stable compared with organic semiconductor lasers, even at high repetition rates of 20 kHz, dropping to half their initial output after ~ 10 8 pulses.

CH 3 NH 3 PbI 3 solution deposited waveguides
Perovskite solutions were prepared by combining methylammonium iodide and lead acetate trihydrate, at a 3:1 molar ratio (dissolved in dimethylformamide (DMF) at 400 mg/ml).Hypophosphorous acid was also added to the solution for improved film quality and increased PLQY [20] (0.3% of the total volume) prior to deposition.Films were fabricated inside a N 2 glovebox by spin-coating solution onto substrates pre-treated by oxygen plasma ashing.DFB laser samples had an additional encapsulating layer of CYTOP spin-cast on top of the perovskite surface.Planar waveguide samples were initially fabricated by using glass substrates without encapsulation.These were used to measure the amplified spontaneous emission as well as the perovskite waveguide loss.The 450 nm output from an OPO (4 ns pulse duration, 20 Hz) (Continuum Panther) was focused to a stripe of dimensions 4 x 0.5 mm using a cylindrical lens onto a glass/perovskite film.When pumping above ASE threshold, the stimulated emission signal was collected from the edge of the film and passed through a 100 μm slit into a fibre coupled CCD spectrograph.Using a motorised stage the pump stripe was scanned across the 2.5 cm x 2.5 cm films, and the ASE signal collected at each position.
Figure 1(a) shows the measured ASE spectra centred at 788 nm and the reduction in ASE intensity detected as the stripe was moved away from the edge of the film.The reduction is due to loss in the waveguide and the decay was fitted to the equation , where is the initial intensity, is the loss coefficient and is the distance between the excitation stripe and the detection edge, giving the waveguide loss to be 6 ± 0.3 cm -1 .arises from t using a J.A. W grating area, a hows the angle esonance, the w he coupling of ng of this reson m Fig. 4(a) we note that the e enough to dire gle dependent trans on coefficients of C ellipsometry.

Discuss
The he pulses the lasing g over the esholds of pumping y periodic te surface hieved by by spindeposition methods such as evaporated crystals [26], creating relatively low loss waveguides with loss coefficient, α = 6 ± 0.3 cm -1 [Fig.1(a)], much smaller than some previously reported perovskite waveguide losses (19-21 cm -1 ) [27] comparable with others for CH 3 NH 3 PbI 3 perovskite films deposited from DMF (6.7 cm -1 ) [28].In comparison to more established DFB lasers, these perovskite films still have very high surface roughness as shown in Fig. 2(b)-2(d).If smoother films could be fabricated with improved deposition methods we would expect the lasing threshold to be lowered, increasing the future applicability of perovskite lasers.The negative effect of the surface roughness can be seen in Fig. 3(b); at high pump intensities random lasing modes can be seen to emerge in the background of the DFB laser spectrum.

Conclusion
In conclusion, we have demonstrated simple fabrication of a solution-processed CH 3 NH 3 PbI 3 distributed feedback laser using nanoimprinted polymer gratings.The laser shows single frequency lasing at the stop band edge of the photonic structure.SEM images of the perovskite surface and a cross-sectional image of the waveguide demonstrate that the solution processed CH 3 NH 3 PbI 3 formed via the lead acetate route can fill sub-micron structures well and that the surface is unperturbed by the presence of the grating underneath.Under femtosecond excitation the perovskite laser exhibits a low lasing threshold of 4 μJ/cm 2 and high stability with a half-life of 10 8 pulses.These results show that perovskite materials provide a promising new route to low-cost fully solution-processed lasers.

Fig. 1 .
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3. Nano-im
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