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A tunable surface-emitting dual-wavelength laser emitted from the blended organic gain layer based on a holographic polymer dispersed liquid crystal (HPDLC) transmission grating feedback structure was reported. The organic blended gain layer was formed from Poly (2-methoxy-5-(2’-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and Poly (2-methoxy-5-(3′, 7’-dimethyloctyloxy)-1, 4-phenylenevinylene) (MDMO-PPV) with a weight ratio of 2:1. The dual-wavelength laser located at 629.9 nm and 640 nm was obtained in a single beam. The optical characteristics of these two organic semiconducting materials and the dual-wavelength laser performance under an electric field are investigated and illustrated. This simple tunable dual-wavelength laser shows the potential to extend the development of organic lasers.
© 2016 Optical Society of America
1. Introduction
Dual-wavelength lasers (DWLs) have drawn much scientific attention due to their potential for various applications including Raman spectroscopy, wavelength-division multiplexing (WDM), two-wavelength interferometry, THz frequency generators and others [1–4]. Researchers have proposed a series of methods to achieve DWLs in the past decades and in general their studies can be divided into two aspects: inorganic and organic DWLs [5–7]. For achieving inorganic DWLs, they put attention to Nd-doped crystals or inorganic semiconducting materials and created various device frames to establish a discrete feedback path for each laser wavelength. They mainly focus on designing a V-shaped cavity or two detached F-P feedback cavities, while both of these two structures are requesting by many other fittings, such as Brewster window, FP etalon, grating, filter and optically coated mirrors [6, 8–10]. These additional optical elements would inevitably induce optical instability and a bulky configuration for DWLs device.
On the other hand, thanks to the appearance of organic semiconducting materials, these materials combine the photoelectric properties of semiconductors with the advantages of simple process, good flexibility and low cost [11]. They also exhibit strong absorption, large Stokes shift and wide fluorescence bands, which are very suitable for fabricating organic DWLs [12, 13]. Moreover, a holographic polymer dispersed liquid crystal (HPDLC) grating structure fabricated by holographic polymerization technique and composed of alternating LC-rich and polymer-rich lamellae has been developed [14, 15]. The HPDLC grating acts as laser feedback cavity and provides low threshold and single mode emission for the long gain path and effective wavelength selectivity [16, 17]. Organic DWLs with a holographic grating feedback structure have been reported [18, 19]. The DWL was emitted from the laser dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) through seventh and eighth-order Bragg diffraction or the DWL was generated from two laser media like DCM and Poly (2-methoxy-5-(2’-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) through second-order Bragg diffraction. Furthermore, these organic semiconducting DWLs don’t need to add other additional elements, so they retain a compact configuration which shows the potential to be integrated in all optical devices. These two DWLs have not shown tunable lasing behaviors with external force like electric field.
In this paper, we present a tunable dual-wavelength surface-emitting laser from the blended organic active layer based on an external holographic feedback layer. The two organic semiconducting materials adopted here for the first time are MEH-PPV and Poly (2-methoxy-5-(3′, 7’-dimethyloctyloxy)-1, 4-phenylenevinylene) (MDMO-PPV). A dual-wavelength emission at 629.9 nm and 640 nm was obtained from MEH-PPV and MDMO-PPV respectively but in a single laser beam. In this work, the DWL was generated from the same organic gain layer and showed tunable laser behaviors under electric field.
2. Experiments
The organic semiconducting MEH-PPV (Mw~105) and MDMO-PPV (Mw~105) were obtained from Xi’an P-OLED Material Corporation and used as received. The chemical structures of these two organic semiconducting materials and the schematic configuration of our organic dual-wavelength laser are shown in Fig. 1. The organic gain films were spin-coated from MEH-PPV, MDMO-PPV or MEH-PPV:MDMO-PPV with the weight ratio of 2:1 solution in chlorobenzene (CB) with the same concentration of 0.8 wt%, and the film thickness was fixed at 80 nm by the spin-coating rate and confirmed by the Dektak profilometer. Then an empty cell was fabricated by combining the organic gain film coated indium tin oxide (ITO) glass substrate with the other polyimide (PI) film coated ITO glass substrate and the cell gap was controlled at 6 um by Mylar spacers. The PI film was rubbed unidirectionally to control LC molecules aligned along the groove direction of HPDLC grating (z axis). The HPDLC grating is formed on the top of the organic gain film between two 30 nm ITO coated glass substrates.
Fig. 1 Chemical structures of (a) MEH-PPV and (b) MDMO-PPV and (c) the schematic structure of organic dual-wavelength laser in this work.
The HPDLC structure was made from a prepolymer mixture composed of LC TEB30A (no = 1.522, ne = 1.692, Slichem, 28 wt%), difunctional acrylate monomer phthalicdiglycoldiacrylate (PDDA, Sigma-Aldrich, 60 wt%), photo-initiator Rose Bengal (RB, Sigma-Aldrich, 0.5 wt%), coinitiator N-phenylglycine (NPG, Sigma-Aldrich, 1.5 wt%), N-vinylpyrrolidone (NVP, Sigma-Aldrich, 10 wt%). The uniform mixture was injected into the empty cell and exposed to the interference patterns generated by two coherent s-polarized laser beams from a 532 nm Nd:YAG laser [20, 21]. The light intensity of each recording beam was 8 mW/cm2 and the exposure time was 5 minutes. The writing and diffraction efficiency measuring setup are illustrated in Fig. 2. The HPDLC grating period () can be controlled according to
by changing the intersection angle (θ) between the two coherent lasing beams. The HPDLC grating period was 405.5 nm for all the samples in order to obtain light feedback through the second-order Bragg diffraction for organic gain layer.To determine the LC molecules orientation and ensure all the samples have similar morphologies, the diffraction efficiency (η) of each sample was measured by a He–Ne laser (circular polarization, 632.8 nm) at the incident Bragg angle onto the sample and the first order diffracted beam was measured by the Detector 2. Both the Detector 1 and the Detector 2 are the Model JG2 laser power meters, which were made in Peking University. The diffraction efficiency is defined as the incident light intensity (Iin) divided by the first order diffracted light intensity (Iχ), here χ is the s or p polarization state obtained by rotating the polarizer in the light path. The diffraction efficiency value of each sample is shown in Table 1. Moreover, we notice that s-polarized and p-polarized light parallel to z axis and x axis, respectively.
Table 1. The gain films and diffraction efficiencies of different samples in the experiment.
In order to ascertain the scope of dual-wavelength laser, the absorption and photoluminescence (PL) spectra of organic gain film was measured by a UV-Vis spectrophotometer (Shimadzu UV-3101; Shimadzu Corp., Kyoto, Japan) and fluorescence spectrophotometer (Hitachi F-7000; Hitachi, Ltd., Tokyo, Japan), respectively. The schematic optical setup for the absorption and PL spectra measurements are shown in Fig. 3. The light source in Fig. 3(a) and (b) are the tungsten lamp and the xenon lamp, respectively, and the detectors are the photomultiplier tube.
Meanwhile, a Q-switched Nd:YAG pulsed laser operating at 532 nm (repetition rate: 2 Hz and pulse duration: 8.6 ns) was used as the optical pumping source in our experiment. A cylindrical lens and an adjustable slit were used to shape the pump beam into the dimension of 4 mm × 0.1 mm and focused onto the sample with the incident angle of 45° correspond to the normal of the substrate. The emitted dual-wavelength laser was collected along the normal of the sample by an optical fiber spectrometer and the pump laser energy was measured by an energy meter (NIM-E1000; China). A square-wave signal generator with the voltage frequency of 1 KHz was used to control the LC orientation. The schematic setup of the lasing experiment is shown in Fig. 4.
3. Results and discussion
3.1 Optical properties of organic gain films
The optical properties of organic gain films are demonstrated in Fig. 5. The absorption spectra between these three gain films share the similar shape and located close to each other and this phenomenon is the same to their PL spectra. The absorption and PL peaks of MEH-PPV film, MEH-PPV:MDMO-PPV blended film and MDMO-PPV film located at 498 nm and 593.2 nm, 499 nm and 596 nm, 502 nm and 599.8 nm, respectively. The energy-band gaps of MEH-PPV and MDMO-PPV molecules are 2.2ev and their energy levels are almost consistent with each other [22, 23]. Different from other reports on blended materials, they mainly aimed to take advantage of energy transfer between the guest and the host for improving the energy transfer efficiency or quantum yield [24–26]. While in this work, the emission band of MEH-PPV separates well from the absorption spectra of MDMO-PPV suggests that Förster energy transfer can’t be obtained from the blended organic gain film. Moreover, these two organic materials also exhibit other analogous optical properties, such as anisotropic refractive indices of thin film, exciton diffusion lengths and fluorescence lifetime [27–31]. These significant properties implied that dual-wavelength laser can probably be formed from the blended organic gain film as Förster energy transfer doesn’t exist in the blended system. Besides, the overlap between the absorption and the PL spectral from 525 nm to 600 nm indicates that dual-wavelength laser can only be achieved above 600 nm due to self-absorption effect.
Fig. 5 Normalized intensity for absorption and PL spectra of MEH-PPV film, MEH-PPV:MDMO-PPV blended film and MDMO-PPV film.
3.2 Dual-wavelength laser performance
3.2.1 Laser emission
When the samples were optical pumped above the lasing threshold, dual-wavelength lasers were excited and emitted from the normal direction of glass substrate in a single beam. The spectra of dual-wavelength laser from blended sample b was measured at 0.8 uJ/pulse and shown in Fig. 6(a). The two lasers were separated surpass 10 nm from each other and located at 629.9 nm and 640 nm with the spectral widths (full width at half maximum, FWHM) of 0.3 nm and 0.6 nm. For distinguishing the two organic semiconducting lasers, the sample a and sample c with the same parameters were fabricated. The lasing spectra of sample a located at 629.5 nm and sample c located at 640.1 nm was collected at 2.2 uJ/pulse and illustrated in Fig. 6(b).These results implied that the laser located at 629.9 nm was generated from MEH-PPV and the other laser was induced from MDMO-PPV. The physical mechanism of organic semiconducting laser has been reported early and each of dual-wavelength lasers is according with waveguide theory [15, 19, 32]. Therefore we believe that the two organic materials exhibit distinct anisotropic refractive indices values in blended organic film, thus the dual-wavelength laser can be formed without Förster energy transfer between them. The dual-wavelength laser follows the Bragg equation:
here, Λ is the grating period, neff is the effective refractive index of the laser mode and m is the Bragg order, which was selected as 2 in this work [33]. According to Eq. (2), the neff of the two lasers emitted from MEH-PPV and MDMO-PPV were 1.553 and 1.578, respectively. The results are reasonable because the refractive indexes (TE mode) are 1.78 for MEH-PPV and 1.86 for MDMO-PPV [27, 28].Fig. 6 Spectra of (a) dual-wavelength laser of sample b measured at 0.8 uJ/pulse and (b) lasers of sample a emitted from MEH-PPV and sample c emitted from MDMO-PPV measured at 2.2 uJ/pulse.
3.2.2 Laser threshold
As an important factor of laser action, the dual-wavelength laser threshold behavior was investigated. Figure 7 shows the dependence of lasing intensity on the pump energy of sample b. The solid squares are the experimental values and the colored lines are the fitting curves. An abrupt increase in the slope of curves was obtained when pumped above the lasing threshold and the output energy intensity increased linearly with the increase of the pump energy. The intersection between the two linear lines and the pump energy axis provide laser thresholds. The output laser energy thresholds are 0.28 uJ/pulse for the 640 nm MDMO-PPV laser and 0.38 uJ/pulse for the 629.9 nm MEH-PPV laser. While considering that partial pump energy was reflected and transmitted from the sample, the actual thresholds of the two lasers were lower than measured values. In addition, laser performance of MDMO-PPV was better than MEH-PPV in the DWL, even though the 629.9 nm output laser intensity was higher than 640 nm laser when pumped above 0.52 uJ/pulse, because the organic gain film was formed from the MEH-PPV: MDMO-PPV with the weight ratio of 2:1. And this result is according with the output lasing intensity shown in Fig. 6.
3.3 Tunability by electric field
The polarization states of the dual-wavelength laser have been detected in the experiment and both lasers were TE polarized waves [15, 19]. That is to say the dual-wavelength laser and s-polarized light share the same polarization state parallel to z axis (as shows in Fig. 1) The diffraction efficiency of s-polarized light would change when applied an external electric field, as LC molecules in LC-rich layer reorientation from z axis to the field direction along y axis. Figure 8 demonstrated the diffraction efficiency of s-polarized light as a function of the driving electric field for sample b. The diffraction efficiency of s-polarized light was monotonically decreases with the increasing of electric field until the electric field increased to 9 V/um. Eventually, the diffraction efficiency of s-polarized light decreased to 1.1% which was the same as p-polarized light. The result illustrates that the saturated electric field was 9 V/um and s or p-polarized light have the same refractive index (n0) of LC molecules.
In this work, the dual-wavelength laser tunable behaviors under electric field were investigated. It can be found that organic laser wavelength can be modulated according to the Bragg equation by changing Bragg order, the effective refractive index of the laser mode or the HPDLC grating period. In order to maintain the surface-emitting property of dual-wavelength laser the Bragg order fixed at 2, we modulate the effective refractive index of the laser mode, which was associated with the refractive index of HPDLC grating layer (ngrating), and the ngrating can be infected by the orientation of LC-rich layer under external electric field [34, 35]. The DWL wavelengths can be also changed with HPDLC grating period.
Figure 9 shows the dual-wavelength laser tunable behaviors on the dependence of electric fields and grating periods. The dual-wavelength laser could be modulated continuously, as LC molecule orientation changed with the increase of the external electric field in Fig. 9(a). The lasers from MEH-PPV and MDMO-PPV blue shift 1.3 nm and 1.6 nm respectively with the electric field increased to 9 V/um. The result was in good according with the change of diffraction efficiency of s-polarized light shown in Fig. 8. When the electric field increased to 9 V/um, the orientation of LC molecules aligned along the electric field (y axis) and s-polarized light have the refractive index (n0) of LC molecules. The decrease of the refractive index of HPDLC grating layer leads to the blue shift of lasers. The slight blue shift reveals the good stability of dual-wavelength laser and the experimental result was similar with the other report [36]. Also the lasing wavelength modulated range could be further expanded by inducing other LC molecules with larger refringence. Figure 9(b) shows lasing wavelength varying with the grating periods. The lasers from MEH-PPV and MDMO-PPV red shift 18.5 nm and 19 nm respectively with the period of HPDLC grating changed from 393.7 nm to 405.5 nm. While the larger lasing wavelength modulation could be achieved by varying grating period but the period can’t be changed continuously in a single sample, so multiple samples must be used in stack in practice.
Fig. 9 Spectra of the organic dual-wavelength laser on the dependence of (a) electric fields and (b) grating periods.
Finally, the output dual-wavelength lasing intensity and lasing thresholds on the dependence of electric field of sample b were also investigated and shown in Fig. 10. The pumping intensity kept constant at 0.9 uJ/pulse in Fig. 10(a). During the electric tuning process, the dual-wavelength lasing intensity decrease linearly from 2700 to 320 (arb.units) for laser emitted from MEH-PPV and 2037 to 190 (arb.units) for laser from MDMO-PPV. Similarly, as expected, the lasing thresholds rise linearly from 0.38 to 0.89 (uJ/pulse) and 0.28 to 0.83 (uJ/pulse) for lasers from MEH-PPV and MDMO-PPV, respectively, with the increase of electric field. These results proved that the decrease of refractive index modulation between LC-rich and polymer-rich lamellae under electric field would lead to a lower lasing intensity and a higher lasing threshold for the reduction of the amount of feedback power provided by the HPDLC structure [33]. Thus, we can control the organic dual-wavelength laser performance by electric field.
4. Conclusion
In conclusion, we have presented a surface-emitting dual-wavelength laser from a blended gain layer with a HPDLC grating feedback structure. The dual-wavelength laser located at 629.9 nm and 640 nm with the FWHM of 0.3 nm and 0.6 nm and lasing thresholds of 0.38 uJ/pulse and 0.28 uJ/pulse, respectively, was obtained in a single beam. We notice that Förster energy transfer can’t be existed in the blended organic gain film which ensures the formation of dual-wavelength laser.
We further investigated the dual-wavelength laser behaviors under electric field and lasing wavelength tunable behavior under grating period. The lasers from MEH-PPV and MDMO-PPV blue shift 1.3 nm and 1.6 nm with the electric field and red shift 18.5 nm and 19 nm with the grating period varied from 393.7 nm to 404.5 nm. Besides, the dual-wavelength lasing intensity decrease linearly and the lasing thresholds rise linearly with the increase of electric field, which show the possibility of electrical control of organic dual-wavelength laser. We think that this work would give insight to tunable organic DWL applications and more work to further enhance the tunable range is ongoing.
Funding
National Natural Science Foundation of China (Grant Nos. 61475152, 61377032, 61378075, and 61405194).
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