In this work, we investigate the properties of an organic distributed feedback laser as the concentration of the gain material in the waveguide core is varied across two orders of magnitude, from 5% down to 0.025%. The laser dye DCJTB (4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-9-enyl-vinyl)-4H-pyran) incorporated into a PVK (poly(9-vinylcarbazole)) host matrix provided the gain. The composite layer of PVK:DCJTB was spin-cast onto a silica grating with second order periodicity, and upon nanosecond optical excitation lasing was generated in the wavelength range of 600 nm. The threshold pulse energy for achieving lasing increased as the concentration of DCJTB was reduced, however the threshold excitation density quantified in terms of number of excited molecules per unit area remained nearly constant at 1.3×1013 molecules/cm2. In contrast, the relative slope efficiency for lasing decreased considerably as the gain concentration was reduced. We show that this effect can not be explained by a standard 4-level lasing model, but rather that it is due to optically induced charge separation for the DCJTB molecules situated in the PVK host matrix. Our findings suggest that fast charge separation and long back recombination times can be a significant factor in limiting further reduction of the gain concentration in organic DFB lasers.
© 2016 Optical Society of America
The first distributed feedback (DFB) laser was reported by Kogelnik and Shank in  and it was based on the organic dye Rhodamine 6G dissolved in gelatin as the gain material. Shortly afterwards, an inorganic GaAs based DFB laser was fabricated by Nakamura  which opened the path to numerous commercially available laser devices. Since then, a large number of studies have been performed on organic based DFB devices, using different active materials such as conjugated polymers, e.g. ladder-type poly(p-phenylene) , poly(paraphenylene vinylene)-based [4, 5], and polyfluorene in . Host-guest systems using Alq3 as the host material [7, 8], dye-doped polymers , dye-doped metal oxide solgels  and others. Different aspects of DFB lasers have been investigated such as the influence of the excitation spot size , grating depth, and waveguide layer width , the effect of the duty cycle , and other emission properties . The influence of dye concentration on the lasing threshold of an organic DFB and the impact of dye concentration on relative lasing efficiency above threshold are questions that have not been discussed extensively in the literature.
In general, a DFB laser consists of a waveguide layer which is modulated periodically in space either by index variation, by gain variation or by both. In such devices, light propagating in the waveguide layer is reflected from the periodic structure multiple times forming a photonic band gap for wavelengths satisfying the Bragg condition of [Eq. (1)]. In a second order DFB, when the waveguided light scatters from the grating, it is emitted in the normal direction relative to the plane of the waveguide. The distributed optical feedback mechanism together with optical gain in the waveguide layer make it possible to achieve lasing in such structures with relatively low threshold. Moreover, since light propagates within a thin layer of material, it is possible to reach high optical gain even for a small total volume of gain media.
In this work, we explore the behavior of an organic DFB laser as the concentration of gain material is varied over more than 2 orders of magnitude. The gain material we use is the laser dye 4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-9-enyl-vinyl)-4H-pyran (DCJTB) (see Fig. 1) incorporated into a poly(9-vinylcarbazole) (PVK) polymer matrix. In previous studies of similar systems , the concentration was fixed at 3 wt. %. Usually the concentration of gain material in a host matrix is of the same order of magnitude, namely a few percent, such as the 2.3% DCM doping in an Alq3 matrix , 3% DCM2 in Alq3 , and 0.5% for PDI-C6 doped into a PS film . An example of a study using low gain concentration is the 0.05 wt.% of Rhodamine 6G reported in . Since the laser dyes used in most organic DFB’s are 4-level laser systems, dye concentration is typically adjusted higher in order to minimize the lasing threshold in terms of incoming energy, with an upper limit of concentration being dictated by concentration quenching, which reduces the fluorescence quantum yield when the dye molecules are too close to each other.
2. Sample design and fabrication
Here we show the effects of changing the concentration of the laser dye DCJTB over the range between 5 wt.% to 0.025 wt.%. We chose the laser dye DCJTB because it is known to exhibit relatively low aggregation and hence low concentration quenching . In the current work, the DCJTB is mixed with PVK, a high refractive index polymer (n = 1.69) that serves as the host matrix “core” for the asymmetric waveguide. The gratings for the DFB were fabricated to have a resonant wavelength near the peak of the fluorescence spectrum of DCJTB (Fig. 1), at about 600 nm. To construct a second order DFB, the grating periodicity must satisfy the Bragg condition:12]. In this model, we assume the effect of the grating on the waveguide mode thickness is negligible and consider only the average height of the waveguide layer. The electrical field equations have the following form:
- and inside the SiO2 layer:
We show numerical solution of this equations in Fig. 2(c). For the thicknesses under consideration, there are two possible solutions to the above set of waveguide equations, which correspond to the TE0 and TM0 modes. In addition, the lasing wavelength depends on nature of the grating modulation. For pure gain modulation, lasing occurs at the wavelength corresponding to the center of the bandgap, but at the long wavelength edge of the bandgap for pure refractive index modulation.
The grating was fabricated via electron beam lithography. The sample consisted of a silicon substrate with a 1 μm thick layer of thermally grown SiO2 on top. The electron beam resist, PMMA 950K A4, was spin coated on top of the substrate at 4000 rpm to form a layer of 200 nm in thickness, followed by baking at a temperature of 180°C for 90 sec. The grating was patterned over an area of 0.5 mm × 2 mm using the RAITH eLINE Plus electron beam lithography system. The grating was designed to have a period of 400 nm, with a line width of 190 nm and space width of 210 nm. The sample was exposed to the electron beam using Fixed Beam Moving Stage (FBMS) technology, in which the stage moves on a defined trajectory with a constant velocity while the electron beam does not change position, but just slightly deflects to expose the line and to achieve the desirable line width. This technique enabled us to achieve zero stitching error in the grating writing process. The accelerating voltage of the e-beam was 30KV, the beam current was 300 pA, the stage speed was 0.2 mm/sec. After developing the photoresist, a 20 nm thick film of Cr was deposited by e-beam evaporation which served as a hard mask for the etching process. After lift-off, inductively coupled plasma (SPTS, ICP) of CF4 gas was used to etch the SiO2 layer. The gas was flowed at 50 sccm, with a pressure of 10 mTorr, ICP power of 400W, and platten power of 50W. An etch time of 1 minute resulted in a 50 nm deep trench in the SiO2 layer. The remaining Cr was removed via wet etch solution.
The gain layer was prepared by separately dissolving PVK and DCJTB in chlorobenzene and mixing them in different ratios in order to achieve different concentrations of dye in the host matrix. The concentration of PVK in chlorobenzene for the mix was kept constant, since the matrix concentration affects the viscosity and hence the thickness of the thin film formed during spin coating. To produce films of 200 nm, we used a concentration of 24 mg/ml of PVK in chlorobenzene, and the solution was spin coated on the grating at 1000 rpm. The film thickness corresponds to neff = 1.52 for TE0 mode and a resonant wavelength of 608 nm. The actual center of the bandgap was at about 600 nm.
The DFB structures were optically excited using 532 nm wavelength laser (Spectra-Physics, Explorer) that was configured to provide 8 ns pulses at a repetition rate of 1 Hz. The excitation spot was 300 μm in diameter and was formed by a 10× objective lens (NA = 0.3). In general, a bigger spot size can lead to a reduction of the DFB lasing threshold in terms of energy density, however the total energy in the pulse is limited by the excitation laser. In  it was found that for spot sizes with a diameter above 300 μm, any further reduction in threshold energy is very small. Emitted light was collected via the same objective lens and was separated from the excitation by a dichroic mirror and then routed into a spectrometer (Princeton Instruments SP-2500i) equipped with a cooled CCD. Fluorescence spectra were analyzed using a 150 g/mm for broadband measurements and a 1800 g/mm grating for high spectral resolution. The polarization of the incoming beam was adjusted to maximize the lasing output. The pulse energy was controlled by λ/2 waveplate and polarizer.
Lasing from the devices was observed for the concentrations of DCJTB dye in PVK from 5% to 0.025%. It was manifest by threshold-like behavior of the output energy as a function of input energy (Fig. 3(a) and 3(c)) and the appearance of a narrow spectral line above threshold (Fig. 4(b)). The lasing was observed in the wavelength range between 605 nm to 610 nm, depending on the sample. The shift towards the longer wavelength edge of the DFB bandgap is typical for mixed gain and refractive index modulation.
As the concentration of DCJTB was reduced, the value of the energy fluence needed to achieve threshold rose according to the reduction of the light absorption in the gain layer (Fig. 5): A = I(1 − e−αd), where α is the absorption coefficient, which is proportional to concentration of dye molecules. Nevertheless, the threshold absorbed energy density remains nearly constant from 0.025% up to 1%, as expected for lasing from organic dyes which typically behave as 4-level lasing systems. For dye ratios above 1%, the threshold excitation density rises because concentration quenching reduces the quantum yield of fluorescence.
However, the relative slope efficiency, which we define as the slope efficiency of the device above and below lasing depends strongly on dye concentration (Fig. 5(b)). This dependence cannot be explained by gain saturation or absorption of excited states since the above threshold linearity of output energy to input energy extends at least up to 3Eth for doping ratio of 0.025% (Fig. 3). Triplet state absorption, which is a key factor in hindering CW lasing, cannot have a considerable influence on the system since it would require an unrealistically high triplet formation rate  as we will show further on. The possible change in the coupling factor - β, which can affect slope efficiency, is not compatible with our observation that the lasing threshold is nearly constant in terms of excitation density. Likewise, annihilation processes such as singlet-singlet, singlet-triplet, and triplet-triplet annihilation are only observed at high exciton density, while in our work we are far from this regime. The model for an ideal 4-level system predicts a small decrease in relative slope efficiency at low concentration, but it cannot explain the experimental data (Fig. 5(b)).
The observed results can be explained by considering charge separation of the excited DCJTB molecules. PVK is a known p-type material while DCJTB is n-type. It was observed in systems using a similar laser dye, DCM2 doped into the hole transport material TPD  that indeed excitons generated on the DCM2 molecules dissociate into separated electrons and holes at the dye-host matrix interface with electrons being trapped on the dye molecules. Geminate recombination  of such states typically occurs on the timescale of about 100 ns  as observed in organic photovoltaics. High hole mobility in PVK may further promote dissociation of the charges and thus lead to non-geminate recombination which can have lifetimes up to 1 μs . In either case, recombination times of the separated charges are expected to be long compared to the spontaneous emission time - τsp and excitation pulse duration.
For our model, we derived rate equations that are similar to the formalism in  which describes lasing in the presence of electron transfer from excited singlet to triplet states. Here, focusing on the possibility of charge separation, we assume the DCJTB molecules, by themselves act as a 4-level lasing system. Namely, transitions from the excited state of absorption to the excited state of emission and likewise from the ground state of emission to the ground state of absorption occur fast enough to be considered as immediate. In addition, as we explained above, upon charge separation, the recombination time of the separated charges is much longer than the pulse duration, so no significant relaxation from these states is observed during the excitation pulse. We define N0, N3 as ground and excited states of the absorption transition and N1, N2 as ground and excited states of the emission transition. Since the transition from N3 to N2 as well as the transition from N1 to N0 is on picosecond scale, which is much faster when any other dynamics we can assume N3 = N1 = 0.
Solving the rate equations for different concentrations of dye molecules, we were able to describe reasonably well the experimental data using τCS/τsp = 0.79. This additional relaxation pathway can also be expected to reduce the fluorescence quantum yield in PVK, as it competes with spontaneous emission. Indeed, we evaluated the quantum yield of fluorescence for DCJTB in PVK compared to PMMA, ΦPVK and ΦPMMA, respectively. We found from fluorescence and absorption measurements that ΦPVK/ΦPMMA = 0.55. According to , the quantum yield of DCJTB in PMMA is ΦDCJTB = 0.68 which gives ΦPVK = 0.38. This reduction is consistent with our model that in PVK additional relaxation pathways are present that reduce the quantum yield.
It can be seen in Fig. 5(c) that the percentage of dye molecules trapped in the charge separated states becomes close to 100% for low concentrations of DCJTB. Charge separated states population mostly builds up during the lasing while it remains small before the lasing threshold is reached even for low concentrations of DCJTB. Hence the charge separation mechanism affects primarily the relative slope efficiency for lasing and not the threshold excitation density. Thus lasing occurs for dye concentrations that are as small as 0.025%, even though the slope efficiency has been reduced by charge separation. However, since the number of molecules needed to be excited is already about 50% of the total available molecules in the gain layer, it is hardly possible to see lasing at further reduced levels of dye doping.
Very recently, excitonic lasers utilizing monolayers of inorganic transition-metal dichalcogenides were reported in . The concentration reached in this work suggests the possibility of a similar DFB architecture based on a monomolecular organic layer. In such a device, all of the active material would be confined into a single film, one molecular layer thick, inside of the waveguiding layer. For instance, if all of the DCJTB molecules in the 0.025% doped gain layer were concentrated in a single monolayer, the distance between them would be about 3.4 nm. This is the same spacing as for concentrations of about 2% of organic dye in a bulk host matrix. According to  at this distance, concentration quenching is small and quantum efficiency is close to its maximum value. Furthermore, since in the monomolecular layer, each molecule is surrounded by a lower number of neighboring dye molecules than in bulk, for the same distance between molecules, we can expect a lower non-radiative energy transfer rate. Hence, a lower degree of concentration quenching can be expected, which can enable efficient lasing even for higher local concentrations.
We investigated the lasing behavior in a DCJTB based DFB across two orders of dye concentration. The lasing threshold was found to be inversely proportional to the concentration of DCJTB, but nearly constant in terms of absorbed excitation density. Below a DCJTB concentration of 0.025%, lasing was not observed indicating a lower limit. Further dye molecules concentration reduction is not possible since the density of molecules becomes comparable to the excitation density at the lasing threshold. Varying the dye concentration had a significant impact on the relative slope efficiency for the DFB lasing output. We demonstrated that the decrease in efficiency as the gain concentration is lowered can be attributed to rapid charge separation that results in long lifetime charge separation states. Therefore, the influence of the electrical properties of the host and active materials, while sometimes not evident at high concentrations of gain, can have considerable impact on the lasing dynamics in the low concentration limit.
We want to thank Dr. Basanth S. Kalanoor for helping with the experiment. We gratefully acknowledge the financial support from the Israel Science Foundation (ISF 206738) for funding this research and the Israel National Nanotechnology Initiative for the spectral measurement equipment, made possible by a Focal Technology Area grant (FTA 458004).
References and links
1. H. Kogelnik and C. Shank, “Stimulated emission in a periodic structure,” Appl. Phys. Lett. 18(4), 152–154 (1971). [CrossRef]
2. M. Nakamura, A. Yariv, H. Yen, S. Somekh, and H. Garvin, “Optically pumped GaAs surface laser with corrugation feedback,” Appl. Phys. Lett. 22(10), 515–516 (1973). [CrossRef]
3. C. Kallinger, M. Hilmer, A. Haugeneder, M. Perner, W. Spirkl, U. Lemmer, J. Feldmann, U. Scherf, K. Müllen, A. Gombert, and W. Valker, “A flexible conjugated polymer laser,” Adv. Mater. 10(12), 920–923 (1998). [CrossRef]
4. R. Gupta, M. Stevenson, A. Dogariu, M. McGehee, J. Park, V. Srdanov, A. Heeger, and H. Wang, “Low-threshold amplified spontaneous emission in blends of conjugated polymers,” Appl. Phys. Lett. 73(24), 3492–3494 (1998). [CrossRef]
5. G. Turnbull, T. Krauss, W. Barnes, and I. Samuel, “Tuneable distributed feedback lasing in MEH-PPV films,” Synthetic Metals 121(1–3), 1757–1758 (2001). [CrossRef]
6. G. Heliotis, R. Xia, D. Bradley, G. Turnbull, I. Samuel, P. Andrew, and W. L. Barnes, “Blue, surface-emitting, distributed feedback polyfluorene lasers,” Appl. Phys. Lett. 83(11), 2118–2120 (2003). [CrossRef]
7. D. Schneider, S. Hartmann, T. Benstem, T. Dobbertin, D. Heithecker, D. Metzdorf, E. Becker, T. Riedl, H.-H. Johannes, W. Kowalsky, T. Weimann, J. Wang, and P Hinze, “Wavelength-tunable organic solid-state distributed-feedback laser,” Appl. Phys. B 77(4), 399–402 (2003). [CrossRef]
8. M. Berggren, A. Dodabalapur, R. Slusher, A. Timko, and O. Nalamasu, “Organic solid-state lasers with imprinted gratings on plastic substrates,” Appl. Phys. Lett. 72(4), 410–411 (1998). [CrossRef]
9. T. Voss, D. Scheel, and W. Schade, “A microchip-laser-pumped DFB-polymer-dye laser,” Appl. Phys. B 73(2), 105–109 (2001). [CrossRef]
10. X.-L. Zhu, S.-K. Lam, and D. Lo, “Distributed-feedback dye-doped solgel silica lasers,” Appl. Opt. 39(18), 3104–3107 (2000). [CrossRef]
11. X. Liu, S. Klinkhammer, Z. Wang, T. Wienhold, C. Vannahme, P.-J. Jakobs, A. Bacher, A. Muslija, T. Mappes, and U. Lemmer, “Pump spot size dependent lasing threshold in organic semiconductor DFB lasers fabricated via nanograting transfer,” Opt. Express 21(23), 27,697–27,706 (2013). [CrossRef]
12. V. Navarro-Fuster, I. Vragovic, E. M. Calzado, P. G. Boj, J. A. Quintana, J. M. Villalvilla, A. Retolaza, A. Juarros, D. Otaduy, S. Merino, and Díaz-García, “Film thickness and grating depth variation in organic second-order distributed feedback lasers,”.
13. S. S. Yang, Y.-C. Chang, P.-C. Yen, and Y.-C. Chou, “Effects of duty cycle on the characteristics of a composite surface-emitting organic distributed feedback laser,” JOSA B 24(8), 1857–1861 (2007). [CrossRef]
14. G. Heliotis, R. Xia, G. A. Turnbull, P. Andrew, W. L. Barnes, I. D. W. Samuel, and D. D. Bradley, “Emission Characteristics and Performance Comparison of Polyfluorene Lasers with One-and Two-Dimensional Distributed Feedback,” Adv. Func. Mater. 14(1), 91–97 (2004). [CrossRef]
15. X. Liu, P. Stefanou, B. Wang, T. Woggon, T. Mappes, and U. Lemmer, “Organic semiconductor distributed feedback (DFB) laser as excitation source in Raman spectroscopy,” Opt. Express 21(23), 28,941–28,947 (2013). [CrossRef]
16. J. Wang, T. Weimann, P. Hinze, G. Ade, D. Schneider, T. Rabe, T. Riedl, and W. Kowalsky, “A continuously tunable organic DFB laser,” Microelectron. Eng. 78, 364–368 (2005). [CrossRef]
17. W. Horn, S. Kroesen, and C. Denz, “Two-photon fabrication of organic solid-state distributed feedback lasers in rhodamine 6G doped SU-8,” Appl. Phys. B 117(1), 311–315 (2014). [CrossRef]
18. A. P. Green and A. R. Buckley, “Solid state concentration quenching of organic fluorophores in PMMA,” Phys. Chem. Chem. Phys. 17(2), 1435–1440 (2015). [CrossRef]
19. A. Taleb, B. T. Chiad, and Z. S. Sadik, “Spectroscopic study of DCM as an active medium for luminescent solar concentrators,” Renewable Energy 30(3), 393–398 (2005). [CrossRef]
20. S. H. Kang, T. Crisp, I. Kymissis, and V. Bulović, “Memory effect from charge trapping in layered organic structures,” Appl. Phys. Lett. 85(20), 4666–4668 (2004). [CrossRef]
21. G. Dennler, M. C. Scharber, and C. J. Brabec, “Polymer-Fullerene bulk-heterojunction solar cells,” Adv. Mater. 21(13), 1323–1338 (2009). [CrossRef]
22. H. Mangold, A. A. Bakulin, I. A. Howard, C. Kästner, D. A. Egbe, H. Hoppe, and F. Laquai, “Control of charge generation and recombination in ternary polymer/polymer: fullerene photovoltaic blends using amorphous and semi-crystalline copolymers as donors,” Phys. Chem. Chem. Phys. 16(38), 20,329–20,337 (2014). [CrossRef]
23. S.-L. Chua, B. Zhen, J. Lee, J. Bravo-Abad, O. Shapira, and M. Soljačić, “Modeling of threshold and dynamics behavior of organic nanostructured lasers,” J. Mater. Chem. C 2(8), 1463–1473 (2014). [CrossRef]
24. Y. Ye, Z. J. Wong, X. Lu, X. Ni, H. Zhu, X. Chen, Y. Wang, and X. Zhang, “Monolayer excitonic laser,” Nature Photon. (2015). [CrossRef]