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. 2023 Jan 31;15(6):8436–8445. doi: 10.1021/acsami.2c22920

Distributed Feedback Lasers by Thermal Nanoimprint of Perovskites Using Gelatin Gratings

Isabel Allegro †,*, Víctor Bonal ‡, Emil R Mamleyev §, José M Villalvilla ‡, José A Quintana ∥, Qihao Jin †, María A Díaz-García ‡,*, Uli Lemmer †,§,*
PMCID: PMC9940720  PMID: 36720173

Abstract

graphic file with name am2c22920_0005.jpg

To date, thermal nanoimprint lithography (NIL) for patterning hybrid perovskites has always involved an intricate etching step of a hard stamp material or its master. Here, we demonstrate for the first time the successful nanopatterning of a perovskite film by NIL with a low-cost polymeric stamp. The stamp consists of a dichromated gelatin grating structured by holographic lithography. The one-dimensional grating is imprinted into a perovskite film at 95 °C and 90 MPa for 10 min, resulting in a high quality second-order distributed feedback (DFB) laser. The laser exhibits an excellent performance with a threshold of 81 μJ/cm2, a line width of 0.32 nm, and a pronounced linear polarization. This novel approach enables cost-effective fabrication of high-quality DFB lasers compatible with different perovskite compositions and photonic nanostructures for a wide range of applications.

Keywords: nanoimprint lithography, distributed feedback lasers, perovskite films, polymer gratings

Introduction

Metal halide perovskite semiconductors recently emerged as attractive laser gain media due to their excellent optoelectronic properties and rapid progress in photovoltaics.1−4 These perovskites combine the benefits of organic and inorganic semiconductors: Like organic semiconductors, they are compatible with solution-processing methods and possess high gain, enabling low threshold lasing. Moreover, due to the high charger carrier mobilities, they offer the potential for electrical pumping, similar to inorganic semiconductors.5,6 Recently, Qin et al. demonstrated stable continuous-wave lasing at room temperature in novel two-dimensional (2D) perovskite materials, a major milestone toward electrically pumped, solution-processed lasers.7 So far, there have only been a few reports on CW amplified spontaneous emission (ASE) and lasing in perovskites, mostly relying on second-order distributed feedback (DFB) resonators.7−11 DFB lasers have the advantages of sustaining single-mode lasing with low thresholds and narrow line widths. In addition, the DFB resonator offers a mirror-free cavity that is compatible with inexpensive fabrication techniques and can be integrated in thin-film diode structures.5,12−19

Several approaches for fabricating DFB gratings for perovskite lasers have been reported. These include often an expensive electron beam lithography (EBL) step and always a final etching step of a hard material usually realized by reactive ion etching (RIE) or ion beam etching (IBE). The etching is performed to pattern either a substrate,7,9,20−22 a stamp for imprint lithography,11,17,23,24 or a master grating to serve as a template.13,25−31 Alternative methods include the etching of the perovskite layer itself32−34 and a template-assisted crystallization with a soft stamp.33,35−40 However, these methods typically result in lower film quality and additional fabrication challenges, such as the difficulty to control the crystallization process, the film thickness, and achieving an even contact between the template and film.

Thermal nanoimprint lithography (NIL) is a low-cost, high-throughput technique that is used for transferring nanopatterns from a harder stamp to a sample material by direct mechanical deformation.41,42 The stamp is typically a hard, resistant material that withstands the process conditions (pressure and temperature), such as silicon (oxide), nickel, or a hard curable polymer like OrmoStamp.43,44 NIL has been widely used to transfer patterns to perovskite films as it was shown to be beneficial for the perovskite layer: During imprinting, the perovskite film recrystallizes, resulting in a film with improved crystal quality and higher stability against degradation.24,45 Furthermore, it has been established that the NIL process results in fewer surface and structural defects in the film that serve as nonradiative recombination centers and are detrimental for lasing.24,45,46 Next to the defect-assisted nonradiative losses, an essential parameter to consider for perovskite thin-film lasers is the waveguide propagation loss. This is strongly influenced by the surface roughness of the film, which can be greatly improved by imprinting with a smooth stamp.24,45,46 Additional benefits of NIL are a precise control of the lasing wavelength due to a better control of the film thickness and the possibility to reuse a single stamp for fabricating multiple samples.

Herein, we fabricate gratings by holographic lithography (HL) on dichromated gelatin (DCG) and directly use them as stamps in a thermal NIL process applied to perovskite thin films. This is the first demonstration of DCG gratings as NIL stamps. The novelty of this approach consists in the fabrication and the direct use of polymeric stamps that, unlike other stamps used so far, require neither an expensive EBL nor intricate etching steps of a hard material.14,47,48 This results in a cost-effective and versatile approach, relevant for both commercial and research applications and a wide range of possible photonic applications. Metal halide perovskites are relatively easy to deform due to their softness.46 Therefore, the stamp material used in NIL can be softer than typically required.44 By using a DCG stamp for NIL, we combine (1) the facile and cost-effective fabrication of polymeric gratings and (2) the benefits of NIL for an improved quality of the perovskite thin film. Besides, large samples (centimeter size) can be prepared.

Results and Discussion

Design and Fabrication of Dichromated Gelatin Gratings

We fabricated one-dimensional (1D) gratings to serve as stamps to imprint, by thermal NIL, perovskite films, thus conforming DFB lasers. The perovskite has a triple cation composition (Cs0.1 (MA0.17FA0.83)0.9Pb0.84(I0.8Br0.2)2.68) spin-coated on a glass substrate with a thickness of ca. 230 nm and a photoluminescence (PL) spectrum centered around 760 nm (Figure S1). The gratings are fabricated by HL using the Lloyd configuration (Figure 1a) on a DCG film. The process involves exposing a spin-coated DCG film followed by a washing and a dry-development step.49 More details on fabrication procedures are explained in the Experimental Section.

Figure 1.

Figure 1

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Dichromated gelatin (DCG) gratings with a 1D nanopattern fabricated by holographic lithography. (a) Schematic of the setup for holographic lithography with (1) laser, (2) microscope objective (40×), (3) pinhole (10 μm), (4) collimator (20 cm focal length), (5) mirror, and (6) DCG film coated on a fused silica substrate (2.5 × 2.5 cm2). The angle between the mirror and the DCG film is 90°, and the period Λ of the interference pattern in this configuration is given by Λ = λ/(2 sin δ). (b) SEM images, (c) AFM image, and (d) corresponding height profile of the resulting DCG gratings with a period of 344 nm. The scale bars correspond to a length of 500 nm.

The main parameters considered for the grating design are the grating period, the grating depth, and the duty cycle. To determine the grating period Λgrating, we follow the resonance condition for a surface-emitting, second-order DFB laser50−52

graphic file with name am2c22920_m001.jpg 1

where neff is the effective refractive index of the respective waveguide mode. We chose grating periods of Λgrating ≈ 345 nm, assuming an effective refractive index of the TE0 mode of neff ≈ 2.19 (more details in the Experimental Section). The periodicity of the grating was controlled by changing the angle between the two interfering beams impinging onto the DCG film (Figure 1a). The target wavelength (760 nm) is the center of the PL spectrum. Potential fabrication errors can shift the lasing peak by changing either the period or the effective refractive index, which depends on several parameters such as perovskite film thickness, duty cycle, and grating depth.40,53,54

The grating depth is determined by the initial DCG film thickness and the development time.14 For devices with no DCG residual layer, such as the ones prepared here, the grating depth corresponds to the DCG thickness, which can be tuned by simply varying the gelatin concentration. Typical depth values reported in the literature for perovskite DFB lasers are between 60 and 140 nm.7,26,27,30,46 Here, the grating depth was chosen to be around 100 nm, which is optimal to simultaneously obtain high laser slope efficiency and low threshold and to have some margin to account for any fabrication errors.14,25 Because the grating is imprinted onto the perovskite thin film, the resulting samples consist of a residual waveguide with grating relief on the top surface (Figure 2a). The thickness of the bottom residual waveguide is given by the difference between the total spin-coated thickness and the grating depth. The effective index is mainly influenced by the residual waveguide as most of the waveguide mode is confined to this region. The grating depth affects predominantly the coupling strength of the resonator and consequently the lasing threshold.14 Both duty cycle and grating depth influence the coupling coefficient of the DFB laser, and it is important to design and study these parameters attentively to achieve a good lasing performance.50,55 In a second-order DFB resonator, a duty cycle of 0.25–0.35 (0.75–0.65) results in maximum feedback, whereas a duty cycle of 0.5 has minimal coupling strength and reflectivity (Figure S8). For the grating design we choose a rectangular profile with duty cycle of 0.75 (0.25 gap) that gives high-quality DCG gratings and should result in wider imprinted perovskite lines due to the soft edges of the stamp. Such a design demonstrated to be successful to optimize the efficacy of the DCG gratings in DFB lasers with a top-layer resonator configuration.48 The goal is that the final grating duty cycle is in the range 0.15–0.40 to have a high coupling coefficient while preserving reasonable output coupling.55−58

Figure 2.

Figure 2

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Thermal nanoimprint lithography of DCG gratings on perovskite films to form perovskite DFB resonators. (a) Schematic of the NIL process and the resulting perovskite grating. (b) Perovskite film with transferred grating pattern, evident from the bright red square originated from diffraction of ambient light through the 1 cm2 imprinted area. (c) SEM images of the resulting perovskite film with transferred pattern, after four imprint procedures. (d) AFM image and (e) corresponding height profile of the resulting perovskite grating. The roughness of the perovskite grating (average RMS = 4.4 nm) is calculated by averaging over the ridges and grooves of the grating separately (of the scanned area of 1 μm × 2 μm). The scale bars corresponds to a length of 500 nm.

The fabricated DCG gratings were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) to study their morphology, roughness, and height. The roughness of the pristine DCG stamps (Figure 1b–d) is 2.2 nm (RMS), which is a good value for a polymer film. Despite it being rougher than a flat silicon stamp (0.6 nm26), it is still possible to achieve a smooth perovskite grating surface with good lasing performance. With regard to period and duty cycle, both parameters are verified with SEM measurements (Figure 1b) that show accurate and homogeneous values over large areas. The depth of the DCG stamp is approximately 120 nm (Figure 1d), which is slightly higher than the target value (100 nm) for the perovskite grating: We expected the grating groove not to be completely filled during imprinting and that the final perovskite grating depth is slightly lower and approximately matches the designed 100 nm.45

Thermal Nanoimprint Lithography on Perovskite Films

The fabricated DCG layer with engraved gratings was used as a stamp to transfer the grating nanopattern to the polycrystalline perovskite films via thermal NIL. Prior to imprinting, the grating surface was treated with trichloro(1H,1H,2H,2H-perfluorooctyl)silane which forms a strong hydrophobic layer on the stamp (Figure S2). This silanization enables a smooth release of stamp and perovskite after the imprint step. A schematic of the NIL setup is presented in Figure 2a. The grating stamp was placed on top of the perovskite film between the two plates of the hot embossing machine. The imprinting was performed at 90 MPa and 95 °C for 10 min (Figure S3). The successful transfer of the grating pattern becomes apparent by visual inspection of the sample due to the efficient diffraction of ambient light through the 1 cm2 imprinted area (Figure 2c). We noticed some defects at the edges that can have different origins: inhomogeneities in the antiadhesion layer (Figure S2), potential tweezer damage from the splitting of stamp and substrate, and damaged (not sharp) stamp edges. However, the edges of the 1 cm2 imprinted area have no influence on the lasing process because it originates from the spot of the excitation laser (with an area of 3 × 10–4 cm2) focused on the central part of the 1 cm2 imprinted area. This is confirmed by the good lasing performance of the laser devices.

The morphology of the nanoimprinted perovskite is analyzed with SEM. The images validate that the grating pattern on the perovskite layer (Figure 2c) accurately matches the negative pattern of the stamp (Figure 1b). We further investigated the duty cycle and depth of the final DFB resonator in detail. As expected, the perovskite grating exhibits a larger duty cycle (of approximately 0.37, Figure 2c) than the 1 – 0.75 = 0.25 from the geometry of the stamp. AFM measurements (Figure 2e) reveal a grating depth of d ≈ 100 nm, which corresponds to the intended value, accounting for the “underfilling” of the grating grooves.45 In addition, we calculated the roughness of the imprinted perovskite. After the recrystallization process during NIL, the RMS roughness is reduced by 3-fold, to 4.4 nm, compared to the pristine film (RMS = 12.8 nm, Figure S4). This value is quite good, given the reported roughness values for perovskite lasers (0.75–12 nm).11,20,29,31 These improvements agree with previously observed values for different perovskites, indicating the compatibility of this approach with other perovskite compositions.46

The reusability of the grating is evident from SEM measurements. Both, the DCG grating stamp and the imprinted perovskite grating exhibit a good quality after multiple imprints. The morphology of the DCG grating before imprint and after multiple imprints remains intact (Figures 1b and S5) as well as the morphology of the perovskite sample imprinted by the same stamp (Figure 2c). Finally, the optical performance (in more detail below) also confirms the reusability of the DCG stamps for NIL (Figure S6).

Overall, the discussed results demonstrate that the perovskite layer is soft enough to be deformed by the DCG during nanoimprinting and that high-quality nanostructures can be replicated. Furthermore, the reproducibility and reusability of the DCG grating for NIL were verified along with the beneficial effects of the NIL process for perovskite films. In our experiments, we imprinted a total of six DCG gratings (period of 344 nm), four of which showed good lasing, resulting in a process yield of the NIL of 67%. In general, these imprinted structures do not have to be limited to 1D second-order DFB gratings but can also be used in future works for different patterns, such as the recently reported first-order DFB perovskite lasers25 or 2D photonic crystals.11,23,46 This future potential is supported by prior work with DCG resonators used as separate layers. Particularly, the DCG has demonstrated no resolution limitations down to grating values of 200 nm and also has demonstrated its used in 2D DFB lasers.48,59 Moreover, the use of DCG gratings as NIL stamps is relevant not only for laser applications but also for other optoelectronic devices60 that make use of nanostructured perovskite films such as solar cells,61,62 photodetectors,63,64 and light-emitting diodes.36,38 It is also envisaged that the DCG stamps have the potential to imprint other materials different than perovskites, such as thermoplastic polymers hosting functional compounds.65

Lasing Performance of Perovskite Second-Order DFB Lasers

The performance of the imprinted perovskite DFB lasers is investigated by exciting the samples with a pulsed laser focused onto the film surface and collecting the emission from the top. The top excitation was revealed to be the most efficient for second-order DFB lasers.25Figure 3a illustrates the emission spectra of the laser at different excitation densities. At low fluence, we observe a broad PL spectrum centered at 760 nm, identical with the emission of the unpatterned perovskite film (Figure S1). The emission intensity increases with increasing excitation fluence, and after a certain threshold, a narrow lasing peak emerges. Occasionally, at high excitation fluences, a small broader shoulder on the low-energy side of the lasing peak is observed that originates from background ASE. This is not unusual due to perovskites’ strong ASE, and it has been observed previously in other perovskite lasers.11,27,31 One approach used to suppress parasitic ASE that is compatible with our fabrication method could be the expansion of the 1D grating to a 2D pattern.15,59

Figure 3.

Figure 3

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Lasing emission from the perovskite DFB structure. (a) Emission spectra below and above the lasing threshold, (b) input–output characteristics of lasing emission (red spheres) with a threshold Pth,lasing ≈ 81 μJ cm–2 and of the ASE emission (black spheres) from an unpatterned film (without grating) and a threshold of Pth,ASE ≈ 166 μJ cm–2. (c) High-resolution spectrum of the lasing peak, with a line width of Δλ = 0.32 nm. (d) Polar plot of the polarization of the laser emission (red spheres and red shaded area) and ASE of an unpatterned film (black spheres and gray shaded area), showing that the laser exhibits a pronounced linear polarization and that the ASE emission is unpolarized. The gray shaded area denotes the average ASE amplitude, which fluctuates over time.

The threshold behavior of the laser device is presented in Figure 3b which shows the maximum output intensity as a function of the pump fluence. The lasing onset is recognized by slope change in the lasing input–output characteristics due to the transition from spontaneous to stimulated emission.66 We define the lasing threshold at the intersection of the two linear fits of the below and above threshold emission, which gives a threshold of 81 μJ/cm2 (Figure 3b). This threshold is an acceptable value compared to the reported values so far (in the range of approximately tenths of μJ/cm2) and even outperforms some DFBs relying on more complex fabrication techniques including EBL, RIE, or IBE (Table S1 and Figure S7). When investigating and comparing laser thresholds, it is crucial to take the pump source parameters (wavelength, pulse width, and spot size) into account since they influence the observed threshold values (Table S1 and Figure S7).20,27,67,68

As a reference, we measured the ASE threshold in an unstructured film to get an estimation of the gain threshold in these perovskite thin films. The ASE threshold of the pristine samples is 166 μJ/cm2, approximately twice as high as the laser threshold. The slope efficiency η of the ASE is almost half as high as that of the lasing input–output characteristics ηASE = 0. 56ηlasing. This result is in alignment with the prediction that laser emission has an improved threshold and slope efficiency compared to the ASE due to the enhanced light amplification through additional optical feedback provided by the resonator. To further improve the threshold, material engineering approaches compatible with our fabrication route could be investigated. These include passivation methods and quantum confinement strategies using low-dimensional perovskites.7,21,46 Additionally, innovative resonator designs to lower the lasing threshold could be investigated, such as mixed-order gratings or substructured gratings.55,69 This would allow to improve the ratio of feedback (gain) to outcoupling losses in the resonator, which is the main parameter determining the lasing threshold.

Next, we investigate the quality of the laser emission by analyzing the peak line width (Figure 3c). The line width of the lasing peak Δλ = 0.32 nm is determined from the higher resolution spectrum (Figure 3c), and it is an order of magnitude lower than the ASE line width (Figure S1). However, this value is still limited by the resolution of the spectrometer so it gives an upper limit of the actual laser line width. Typical line widths for perovskite DFB lasers are in the range 0.2–2 nm, placing our result among the best reported so far.17

The polarization results of the laser and ASE emission are depicted in Figure 3d. The graphic shows the emission intensity as a function of the angle of a linear polarizer placed between DFB laser and detector. The laser exhibits a strong linear polarization. Emission maxima occur at 10° and 190° and minima at 100° and 280° (Figure 3d). Because the polarizer is roughly aligned with the DFB grating lines, these results indicate that a linear polarization with a maximum at 0° corresponds to an emission polarized along the grating lines. The samples are placed on the holder without any alignment aid; hence, the 10° deviation from the 0° line is a reasonable offset. In contrast to the lasing emission, ASE of the unstructured film is unpolarized, and the emission intensity does not depend on the polarizer angle (Figure 3d). The perovskite film (with a thickness of approximately 230 nm) only supports two fundamental modes: TE0 and TM0. Given the polarization direction, the lasing peak corresponds to the TE0 mode. This agrees with our expectations because TE modes have a better mode confinement and thus a higher gain-to-loss ratio resulting in lower lasing thresholds.70,71 The emission wavelength also agrees with the second-order DFB resonance condition (eq 1) when using the effective index value of the TE0 mode.

To avoid misinterpretation and show a clear proof of lasing, it is necessary not only to have a clear threshold behavior with a narrow and polarized emission but also to investigate the resonator’s influence on the emission peak.72,73 Therefore, we fabricated DFB resonators with different grating periods to test the cavity’s influence on the lasing mode. We expect that in a second-order DFB laser, the lasing peak λlasing can be tuned by changing the grating period Λgrating, following the resonance condition in eq 1. The samples fabricated with a grating Λgrating = 344 nm result in a lasing peak at the high-energy side of the ASE spectrum. For this reason, we further increase the grating period to shift the laser emission to even longer wavelengths. Figure 4 shows the laser emission peak for three different grating periods. Indeed, the laser peak moves to longer wavelengths with increasing grating period.

Figure 4.

Figure 4

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Higher resolution spectra of lasing peaks for different grating periods, showing the wavelength tunability of the DFB lasers. The laser emission exhibits a red-shift for increasing period.

One important thing to notice is that this shift (Figure 4) can only be explained by taking the dispersion of the refractive index into account, and it does not correspond to what is expected from eq 1 assuming a constant effective index.13,30 Perovskite semiconductors have a strong negative dispersion around the PL wavelength range,74−76 resulting in a negative dispersion of the effective refractive index dneff/dλ < 0. The theoretical calculation of the effective refractive index of each lasing mode is not straightforward. For a similar triple cation perovskite composition as in this work, the refractive index changes by Δn ≈ 0.03 (in the range 755–770 nm).76 This results in a change of neff of about 0.04. The index neff is influenced not only by the lasing mode profile but also by the film thickness (and eventually by photoinduced changes of refractive index77,78). Even the small thickness variation of a spin-coated sample of ca. 15 nm can give a significant change in effective index of Δneff > 0.07. All these influences play a role in the laser wavelength. We can calculate the effective refractive index of our observed lasing modes by taking the measured lasing peak wavelength and dividing it by the grating period (Figure S9). The results in Figure S9 show the decrease in effective refractive index with wavelength, as expected from the negative dispersion of the refractive index in this wavelength range. We observe a shift of ca. 4 nm in the laser peak wavelength for an increase in grating period of ca. 10 nm. An identical shift was previously observed in similar perovskite DFB lasers.13 Consequently, it is crucial to consider the strong dispersion of perovskite semiconductors when designing resonant structures for this class of materials. Furthermore, a detailed discussion would need to include an analysis of the interplay of gain and index coupling in our DFB structures.

Conclusion

In conclusion, we successfully implemented low-cost DCG gratings as stamps to pattern nanostructures into a perovskite film via thermal nanoimprint lithography (at 95 °C and 90 MPa). To the best of our knowledge, this is the first demonstration of a high-quality imprinted perovskite nanostructure without the need of any hard stamp etching step during the fabrication process. To demonstrate the quality of the patterned structures, we analyzed the optical performance of the resulting second-order distributed feedback (DFB) resonator on the perovskite layer. The perovskite DFB laser exhibits a single-mode operation with a threshold of 81 μJ/cm2, pronounced linear polarization, and a narrow emission of Δλ < 0.32 nm. These results demonstrate the excellent quality of the imprinted laser. Remarkably, both the morphology analysis and the laser performance show that the perovskite DFB grating quality is maintained after using the same stamp for the nanoimprint process several times. This new fabrication method offers a cost-effective alternative to transfer high-quality nanostructures to perovskite films for a wide range of photonic applications. Moreover, it is envisaged that the DCG gratings have the potential to imprint not only perovskites but also other functional materials used in optoelectronic devices.

Experimental Section

Fabrication of Dichromated Gelatin (DCG) Gratings

Gratings were fabricated by HL using dichromated gelatin (DCG) as a negative photoresist. DCG was deposited over fused silica substrates by spin-coating at 4000 rpm from a 40 °C water solution containing 2.2 wt % of gelatin (Russelot, 200 bloom) with respect to water and 35 wt % of the sensitizer ammonium dichromate (Merck) with respect to gelatin. After that, one-dimensional gratings were recorded by HL using the Lloyd configuration (Figure 1a) with an average exposure of 45 mJ cm–2 from a continuous Ar laser emitting at 364 nm (Coherent Innova 308C argon ion laser). Samples were desensitized in a cold–water bath at 15 °C, and last, the surface-relief gratings were dry-developed in an oxygen plasma from the surface treatment machine (Diener Zepto). The grating periods were calculated from the direction of the diffracted light. Further details on the fabrication process can be found elsewhere.49

Perovskite Thin-Film Fabrication

The perovskite solution was prepared by mixing the precursors PbI2 (TCI Chemicals), PbBr2 (TCI Chemicals), CH(NH2)2I (FAI, GreatCell Solar), CH3NH3Br (MABr, GreatCell Solar), and CsI (Alfa Aesar) in a 3:1 (vol %) N,N-dimethylformamide (DMF, anhydrous, Sigma-Aldrich):dimethyl sulfoxide (DMSO, anhydrous, Sigma-Aldrich) ratio with a concentration of 0.8 M. The precursors were added with the following ratios: Cs0.1(MA0.17FA0.83)0.9Pb0.84(I0.8Br0.2)2.68. The resulting film thickness of the triple cation perovskite is ca. 220 nm. The perovskite films were deposited on the glass substrates by a two-step spin-coating process: In the first step, the perovskite solution was spin-coated dynamically at 1000 rpm for 10 s. In the second step, at 6000 rpm, for 20 s, chlorobenzene (anhydrous, Sigma-Aldrich, 100 μL) was dripped onto the sample after 12 s. The films were annealed at 100 °C for 1 h to complete the crystallization process. The complete process is performed in a nitrogen-filled glovebox.

Thermal Nanoimprint Lithography (NIL)

The thermal NIL process was performed with a self-built hot embossing machine.79,80 The substrate with the perovskite film was placed on the bottom plate of the NIL machine and the DCG grating used as a stamp is placed on top of it with the grating teeth facing down. In a first step, the top and bottom plates are pressed together to get a first contact between sample and stamp. After first contact, the temperature was increased up to 95 °C, and once this temperature was reached, the pressure was increased to 90 MPa for a duration of 10 min. Once the imprint was finished, the temperature was cooled to 30 °C; after cooldown the pressure was released, and the stamp was separated from the sample (progress depicted in Figure S3).

For the antiadhesion treatment of the stamp, its surface was first activated via a light oxygen plasma treatment (20 W, 2 min) and subsequently placed in a closed Petri dish with trichloro(1H,1H,2H,2H-perfluorooctyl)silane (ca. 10 μL) droplets. The Petri dish was placed on a hot plate at 150 °C for 1 h in a nitrogen-filled glovebox.

Optical Characterization

The emission properties of the devices were measured using a frequency-doubled Nd:YLF laser (InnoLas Laser GmbH) with a wavelength of 532 nm, a pulse width of 0.8 ns, and a repetition rate of 1 kHz as a pump laser. The samples were excited perpendicularly from the DFB side, and the emission was collected perpendicular to the sample’s surface through a dichroic mirror and coupled to a spectrograph (Acton SpectraPro SP-2300, Princeton Instruments) connected to an intensified CCD camera (PI-MAX 4, Princeton Instruments). The spectrograph has three gratings with 150, 600, and 1800 lines/mm. The laser spot size at the sample position was determined by the moving knife edge method to be 3 × 10–4 cm2. The polarization measurements were performed by placing a linear polarizer on a motorized rotation mount (K10CR1, Thorlabs) at the spectrograph’s entrance slit. During all measurements, the samples were placed in a nitrogen-filled chamber.

Scanning Electron Microscopy (SEM)

The surface morphology of stamps and perovskite films was studied with a Carl Zeiss AG-SUPRA 60VP SEM. The scans were performed using secondary electrons with the maximum acceleration voltage of 2 kV. For better imaging, all samples were coated with a thin silver layer (≈17 nm).

Atomic Force Microscopy (AFM)

AFM measurements were performed with a NanoWizard3 (JPK) with silicon tips (r < 8 nm, f = 320 kHz, Fk = 42 N/m) in an intermittent contact mode, with a scan rate of 2 μm/s. The investigation of the grating profile and surface roughness was performed using the open-source Gwyddion software.

Effective Refractive Index Calculation

To obtain the effective refractive index, we applied the transfer matrix method for simulating the three-layer stack of a substrate with infinite thickness (nglass = 1.51), a 180 nm perovskite layer (nperovskite = 2.53), and air with infinite thickness. The waveguide modes were extracted by finding the poles in the transfer matrix.81

Acknowledgments

We are grateful to Dr. Simon Ternes, Dr. Mohamed Hussein, and Henning Mescher for their fruitful discussions and encouragement. Moreover, we acknowledge the support of the Karlsruhe School of Optics & Photonics (KSOP) financed by the Ministry of Science, Research and the Arts of Baden-Wrttemberg as part of the sustainability financing of the projects of the Excellence Initiative II. We also acknowledge financial support through Germany’s Excellence Strategy via the Excellence Cluster 3D Matter Made to Order (3DMM2O, EXC-2082/1-390761711) and the Karlsruhe Nano Micro Facility (KNMFi). The group at the University of Alicante acknowledges financial support from the “Ministerio de Ciencia e Innovación/Agencia Estatal de investigación” (MCIN/AEI) of Spain, the European Regional Development Fund, European Social Funds (project PID2020-106114GB-I00), and the Conselleria de Innovación, Universidades y Sociedad Digital de la Comunidad Valenciana (Grant No. AICO/2021/093). V.B. acknowledges financial support from the University of Alicante to perform a research stay at KIT in the frame of his PhD.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.2c22920.

  • Characteristics of pristine film, contact angle measurements, nanoimprint process, SEM images and lasing emission after several imprints, summary of reported DFB lasers in the literature (PDF)

The authors declare no competing financial interest.

Supplementary Material

am2c22920_si_001.pdf (784.7KB, pdf)

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