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We experimentally demonstrate and characterize an organic octagonal quasicrystal slab with a single-defect microcavity at low-index contrast. The gain medium is the conjugated-polymer, composed by two PPV derivatives, a BEHP-PPV and a MEH-PPV. By optical pumping, the lasing action is achieved at 607 nm with a FWHM of 1nm. The threshold of lasing is 9μJ/cm2. The intensity of the lasing peak depends linearly on the pump energy above the threshold.
©2013 Optical Society of America
1. Introduction
Since the first demonstration of a polymer light-emitting diode in the green-yellow spectral region was reported in 1990, much research has been done to develop new polymers and the corresponding organic optoelectronic devices. Organic semiconductors have the advantage over the novel optoelectronic properties with larger third-order nonlinearity, ultrafast time response, easy structure tailor, and superior waveguide properties. It makes them attractive candidates as light-emitting materials and laser gain media, as well as for the other applications from diagnostics to sensing [1–3]. In 1996, Tessler et al first reported an optically-pumped organic solid-state microcavity laser with structure consisting of a 100nm thick layer of PPV between a pair of mirrors [4]. Since then, various kinds of organic semiconductor lasers have been demonstrated with different structures, like Fabry-Perot [5,6], microring [7] or distributed feedback photonic crystal (PhC) resonator [8]. The PhC microcavity as a kind of artificial array has a predominant ability to confine the light in a small volume, producing a strong localized electromagnetic field inside, which makes it possible for the lasing or trapping [9–12]. Triangular, square and hexagonal periodic lattices PhCs have been researched [13]. Compared with periodic PhCs, quasi-periodic photonic crystals (QPCs) have their distinctive properties, like rotational symmetry and long-range order. QPCs, therefore offer more flexibility in modifying optical properties and allow smaller dielectric constant necessary for the complete photonic bandgaps (PBGs). For instance, Notomi et al. have fabricated QPC lasers with Penrose lattice, and observed coherent lasing action due to the optical feedback from quasi-periodicity, exhibiting a variety of 10-fold symmetry lasing spot patterns [14]. Baba et al. have fabricated a 12-fold symmetry QPC point defect laser and obtained the lasing action at room temperature [15].
In this article, we propose a new conjugated-polymer QPC microcavity and describe the fabrication and lasing property. In contrast to small molecules organic materials, like aluminum tris commonly used in PhC lasers [9,11], the conjugated-polymers, as long-chain molecules with alternating single-double bonds give simpler fabrication of devices (such as spin-coating or ink-jet printing), higher optical nonlinearity and better thermal stability [16,17]. Although the dielectric constant of organic materials is smaller than that of inorganic semiconductors, the quasi-period structure with high-level symmetry may offer excellent PBGs at low-index contrast. Therefore, the combination of conjugated polymers and quasi-periodic structures would provide better active layers, more efficient and much more uniform in-plane confinement in all directions. It is beneficial for optoelectronic devices to achieve ultralow lasing threshold and higher slope efficiency.
In this work, we select a kind of composed PPV derivatives as a gain medium:poly{[2-[2´,5′-bis(2´´-ethylhexyloxy)phenyl]-1,4-phenylenevinylene]-co-[2-methoxy-5-(2´-ethylhexyloxy)-1,4-phenylenevinylene]}.This polymer consists of two PPV derivatives, BEHP-PPV and MEH-PPV, in a 60:40 proportion, respectively. The chemical structure of (BEHP-PPV)-co-(MEH-PPV) is shown in Fig. 1. Compared with its component polymers, the composed material has been reported to be more resistant to photodegradation during the preparation of the polymer solution and presents higher solubility in chloroform, tetrahydrofuran, and xylene at room temperature [2]. However, there are few reports of organic PhC laser diodes based on. In preparation of organic films, (BEHP-PPV)-co-(MEH-PPV) powder was diluted in the mixed solution of chloroform and tetrahydrofuran with a 4:3 proportion in a 20mg/ml, and agitated for 10 min in an ultrasonic agitator. Then it was stirred 1 hour with a magnetic stirrer, followed by filtration through a 10μm prefilter and a 0.45μm filter. Subsequently, we deposited it by spin coating for 20 seconds on the quartz substrate at room temperature. Different spin speeds, times and spin eccentricity were tested to get best uniformity and repeatability. The film thickness is controlled within a region of 400-500nm. The corresponding refractive index of this compounded polymer is n = 1.44 [18].
Fig. 1 Absorption spectrum of (BEHP-PPV)-co-(MEH-PPV) thin films measured by spectrophotometer (Lamda 950) and photoluminescence spectrum excited by Nd: YAG laser at wavelength 355nm and width 30ps. Inset: Chemical structure of (BEHP-PPV)-co-(MEH-PPV).
2. Designing and optimizing the microcavity structure
The absorption spectrum and photoluminescence (PL) spectrum of (BEHP-PPV)-co-(MEH-PPV) are depicted in Fig. 1. The absorption spectrum is measured by a Lamda 950 UV-Visible spectrophotometer over from 300 nm to 1100nm. Excited by frequency-tripled Nd:YAG lasers at wavelength 355 nm, the PL signal is collected by a grating spectrometer. Figure 1 shows the maximum absorption peak of the polymer around 435 nm and the fluorescence covering from 550 nm to 625nm, which corresponds to a value between the blue spectrum of BEHP-PPV and yellow spectrum of MEH-PPV.
Based on this conjugated polymer, we use MIT Electromagnetic Equation Propagation (Meep) software to simulate electromagnetic wave propagation in PhC slabs. This software is based on a finite-difference time-domain (FDTD) method. The incident beam is set as a TE-like Gaussian wave with a light source placed at the left boundary of the sample (see Fig. 2(a)). The absorbing boundary condition (perfectly matched layer) is used as boundary condition. How to achieve an effective photonic bandgap consistent with the PL spectrum is important, especially at low-index contrast. We try different quasiperiod symmetry (8, 10, 12-fold) and adjust various physical parameters, including the lattice constant, air-rod radius and slab thickness. The simulation shows that the following parameters of 8-fold quasiperiod are better: the lattice constant a = 270 nm, air-rod radius r = 0.26a (70 nm), slab thickness d = 500 nm. In addition, the 8-fold structure compared with other symmetries has proper air-filling factor, so that adjacent rods do not easily connect together during nano-etching in experiment. Secondly, we introduce a single-point defect into QPC in order to obtain high-Q resonant modes close to the PL peak. However, the one-hole-missing microcavity has little effect on the bandgap. Therefore, we construct a larger microcavity by taking the central nine air-rods off. It shows that the bandgap becomes wider covering from 554 nm to 625 nm and the resonance mode arises at the wavelength 589 nm close to the PL peak (see Fig. 2(b)). The resonance mode of 589 nm is well localized inside with comparatively high quality factor. Considering the resolution of focused ion beam (FIB) etching is 10nm, we further fine-tune the structure parameters within limits and find all resonant modes are well localized inside the cavity, although the resonant frequencies are shifted around 589 nm.
Fig. 2 (a) Schematic geometry of an organic octagonal QPC witha9-hole-missing defect microcavity, (b) transmission spectrum of TE-like Gaussian light in QPC microcavity with the slab thickness 500nm. (c) field pattern of the resonant mode at 589nm.
3. Fabricating the QPC structure
According to the simulation, we adopt FIB etching system (Model DB235, FEI Company, USA) as a micro- and nano-machining tool to transfer the designed pattern onto the (BEHP-PPV)-co-(MEH-PPV) film. The Ga+ ion beam generated by a Canion ion gun was connected to the ultra-high vacuum chamber, where the sample is placed. A spot current of 30pA was obtained from a weak emission current of 1μA at 25KeV. A weak current can reduce the sample damage and the Gaussian wings of the ion beam. The scanning electron microscopy image of the PhC microcavity is shown in Fig. 3. The total etched area is about 5μm × 5μm with 516 air-rods inside.
4. Optical properties of organic QPC microcavity
The experimental setup used to optically pump the QPC microcavity is shown in Fig. 4. The structure is pumped by tripled Nd: YAG lasers (355nm, 30ps, 10Hz), irradiating vertically on the lower surface of the sample. The output signals coupled out of the QPC plane are collected by an objective lens, and then sent to a multi-mode optical ðber by using a second objective lens. The emission spectrum is obtained and analyzed by coupling signals from a QPC cavity into a grating spectrometer (Ocean Optics USB 2000 + miniature fiber optic spectrometer with optical resolution 1nm). We place the sample at the back focus of Nd: YAG lasers with the spot diameter 100μm to avoid destroying the organic film.
Fig. 4 (a) Experimental setup for the detection of an organic QPC microcavity and (b) distribution of refractive index in microcavity domain
Figure 5 demonstrates the measured emission spectra from the microcavity area and the unpatterned area, respectively. A lasing peak from the microcavity area is observed at 607nm with a FWHM of 1nm, which is limited by the resolution of the spectrometer. The interface roughness and non-uniformity inevitably occur in the fabrication of PhCs, especially when crystals are of micrometer and submicrometer sizes. The resolution of the microstructure processed by FIB lithography technology is about 10nm, which also leads to the deviation in the rods’ location and size. These factors consequently cause the mismatch between the experiment and simulation. Figure 6 shows the emission spectra collected at various pump pulse energy densities. Below 9μJ/cm2, the light coupled-out perpendicular to the microcavity has a broad spectrum, which is a characteristic of spontaneous emission from the conjugated organic BEHP-co-MEH-PPV film. At low intensities, the shape of the spectrum is almost independent of the pump intensity. At 9μJ/cm2, the light output changes from a broad spectrum below threshold to a narrow spectrum above threshold. The lasing action from the resonant mode in the microcavity can be achieved at 607 nm. With the pump pulse energy further increasing, the spectrum is completely dominated by the lasing peak. Meanwhile, we measured the change of fluorescence spectra from the unpatterned film by increasing the pump energy. The output signal does not present any spike in the spectra. The relationship between the lasing light and pump energy is depicted in Fig. 7. The output laser intensity is nonlinearly dependent on the input pump power, with a kink at the lasing threshold. The threshold of irradiated energy density is 9μJ/cm2, which is lower than that previously reported from Baba et al. [15] with GaInAsP slab quasicrystal lasers and Chakaroun et al. [11] with Alq3:DCJDB periodic PhC microcavity.
Fig. 5 Excitation spectra from the microcavity area in a 8-fold QPC pattern and from the unpatterned area.
Fig. 6 Excitation spectra of the BMPPVPhC microcavity at different pump energy densities. The spectrum at the bottom is multiplied by a factor of 10.
The lasing principle of the QPC microcavity can be further explained as follows. The PL spectrum of (BEHP-PPV)-co-(MEH-PPV) is in a range of 550nm-625nm. If the pump light irradiates the organic film, a broad spontaneous emission can be observed in the unpatterned area. Due to the modulation of microstructure, the distribution of photonic density of states is low within the bandgap. However, in the single-point microcavity, the density of states at the resonant frequency supported by the defect is observed high. It is proved that, if the PL light modes and the resonant modes supported by the microcavity are well matched, those photons may all be trapped into the defect zone, no matter where the PL light is. By reflecting from the upper surface of organic film and the bottom of quartz substrate, which serves as a resonator, the light amplification is stimulated in the gain medium. Of course, the coupling efficiency for this PhC microcavity laser is surely lower than that of the conventional DFB lasers. The lasing action is achieved and propagates along the waveguide formed in the defect region. Consequently, the lasing light emits from the surface in the vertical-cavity direction.
5. Conclusion
In summary, based on the conjugated-polymer (BEHP- PPV-co-MEH-PPV) membrane, we fabricated 8-fold QPC microcavity with a 9-hole-missing point-defect at room temperature. By optical pumping, the lasing action is observed at 607 nm with a FWHM of 1nm. The threshold of lasing is 9μJ/cm2. The intensity of the lasing peak depends linearly on the pump energy above the threshold.
Acknowledgments
This work is supported by the National Natural Science Foundation of China with Grant No. 11204387, the Key Project of Science and Technology from Ministry of Education of China (212205), the “985 Project” (Grant No. 98507-010009, 98504-012004), “211 Project” of the Ministry of Education of China
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