A distributed-feedback laser, operating in the spectral range of 590-670 nm is reported. The laser device is based on surface relief grating providing a second order DFB structure. The surface relief grating is inscribed in an azobenzene-containing material and an active layer realized in high refractive index poly(phenylquinoxaline) matrix. It is shown that the design of laser device provides for the possibility of simple replacement of an active layer. This allows, in particular, laser wavelength tuning without rewriting of the surface relief grating.
©2011 Optical Society of America
In the last 15 years azobenzene-containing materials attract an attention for the generation of surface relief gratings (SRG) through photo-induced mass transport [1–4]. The SRGs are promising for application in optics and photonics, e.g. diffractive optics, optical data storage and communications [3–7]. The formation of SRG is advantageous due to its efficiency, all-optical nature and reversibility. Recently, a number of new types of azobenzene-containing materials with additional properties and/or higher efficiency were reported. For instance, several types of effective easy made azobenzene-containing materials, different from traditional side-chain polymers, were developed [8–21]. The majority of the materials were of supramolecular type [4,8,15–21]. For instance these new material concepts led to a record modulation depth of 1.8 µm  in ionically bound material and high thermal stability in polyelectrolyte material PAZO . A record inscription rate was registered in oligomers obtained through epoxy ring opening [10,13]. Also high inscription rate was observed in thin layer LC materials with photo-triggered mechanism [16,21]. Colorless SRG were obtained in polyurea based oligomers , and an effective inscription (also with red light) in extremely thin films of three-armed low molecular weight materials [12,14] has been observed. Thus, second important class would be amorphous low molecular weight materials (see  for review). Some of them have relatively high SRG thermal stability .
For the last 10 years there were also few attempts to use holographic gratings in azobenzene-containing materials as distributed feedback (DFB) structures for lasing. The most obvious approach would be using the SRG in azobenzene-containing layer just as a master for replication of structures into other polymers [24–26]. This is a general approach for the utilization of the SRG in azobenzene-materials aiming on different applications, particularly suitable because original gratings are colored and not always temporally, temperature and light stable. The approach has been employed for DFB lasing [24–26]. Few reports, however, on the direct application of SRG in azobenzene-containing layers are also available [27–30]. Direct application of SRG is not that easy due to above mentioned instability and high refractive index of azobenzene-containing materials. According to published results and general considerations following device designs (Fig. 1 ) are plausible in this case:
As demonstrated by Kogelnik and Shank  the DFB laser modes being able to propagate must satisfy the following equation:24–29]. Recently short period gratings in azobenzne-containing material have been also used as lithographic mask . However, the inscription of relatively deep SRG with such short period (ca 400 nm) is not that obvious and usually requires special investigation for particular azobenzene-containing material.
Figure 1 shows some possible configuration for DFB structures based on SRG. In the first configuration (Fig. 1a) the sequence of layers in waveguide is following: a substrate, an azobenzene-containing layer, and an active layer, where the value of refractive index of the active layer is smaller than that of the azobenzene-containing layer. This is the most obvious design as it is easy to choose a matrix material for the active layer with refractive index smaller than the refractive index of the azobenzene-containing layer. The lasing in this configuration has been realized for SRG of the 2nd and 4th diffraction order, respectively [27,30]. The peculiarity of the configuration is that the waveguiding in the active layer only is not possible, because the refractive index of this layer is smaller than that of the azobenzene-containing layer. The guided wave propagates in this case through both active and azobenzene-containing layers, being totally reflected on the interfaces active-layer-air and azobenzene-containing-layer-substrate. It should be noted, that the refractive index of the active layer should be higher than the index of the substrate to enable such propagation. The wave propagation through the azobenzene-containing layer decreases the gain of this wave per unit length, because no amplification occurs in this layer. This is the main disadvantage of this configuration (Fig. 1a) for DFB structures.
The second type of DFB structure involves another layer sequence: a substrate, an active layer, and an azobenzene-containing layer (see Fig. 1b) [28,29]. The refractive index of the active layer in this case is also smaller than the index of the azobenzene-containing layer and greater than that of the substrate. The guided wave propagates through both the active and the azobenzene-containing layers as in the previous case. The difference is that the azobenzene-containing layer is on the top of the waveguide and the SRG can be rewritten to change the grating period in the case of reversible azobenzene-containing material. This allows a wavelength tuning of the DFB lasing in this structure and seems to be an advantage of this DFB configuration (Fig. 1b). However, its realization is difficult, because azobenzene-containing material with good SRG light and thermal reversibility would not be stable and, hence, the number of rewriting cycles is limited. Also the active layer in this configuration cannot be changed in the case of laser dye bleaching, which is a typical problem of the use of organic laser dyes .
In this paper we present a new third design for a DFB structure (Fig. 1c), which uses the same sequence of layers as the one for the first type (Fig. 1a). However, the refractive index of the active layer in this case is higher than that of the azobenzene-containing layer. This provides waveguiding in the active layer only, which results in a higher gain for the guided wave. Using previous experience in the construction of stacking structures with SRG in azobenzene-containing materials [33–36] we decided to use the same azobenzene-containing polyelectrolyte PAZO  for the fabrication of DFB laser devices of this design (Fig. 1c). PAZO shows good SRG inscription properties, good compatibility with other polymers and high thermal SRG stability. To achieve this goal SRG with small period and sufficient modulation depth has to be written and a polymer matrix material with refractive index higher than that of PAZO has to be found. Both obstacles have been overcome and a laser generation with different laser dyes has been created.
A solution of an azobenzene derivative for film preparation was prepared according to a previously published procedure [33–36]. Films of ca 1.5µm thickness were prepared by spin-coating on glass substrates (refractive index 1.52).
The experimental set-up for an inscription of the SRG used was described earlier [8–13]. The films were irradiated with a polarization interference pattern formed by two linearly orthogonally polarized beams with polarization planes at ± 45° to the incidence plane. Irradiation wavelength was 488 nm (Ar+ laser Coherent, Innova 90), and the angle of incidence was in the range of 45-24° resulting in the periods of 345-600 nm. Intensities of interfering beams were set to ca 250 mWcm−2. The samples were mounted on a special BK7 glass prism using an immersion liquid to eliminate parasitic reflection of the interfering waves from the interface substrate-air. This parasitic reflection disturbs the interference pattern and hence decreases the quality of the SRG. The inscription kinetics was monitored by measuring the intensity of the 0th order diffraction signal of a probe laser beam with the wavelength of 633nm. The final SRGs were imaged by Atomic Force Microscopy with a Solver P47H Smena (NTMDT, Russia) in non-contact mode and a Veeco CPII in intermittent-contact mode. AFM images were treated using WSxM 5.0 software . The refractive index of polyelectrolyte PAZO films has been obtained by the ellipsometry using a Sentech SE-400 ellipsometer.
For active layers, DCM (Radiant Dyes Laser Accessories GmbH), DCM2 (Luminescence Technology Corp., Taiwan) and pyromethene 580 (Exciton) laser dyes have been used. Poly(phenylquinoxaline) (PPQ) was synthesized as described earlier . The dyes were introduced in concentrations 5-10% w/w into PPQ solution in 1,1,2-trichloroethane. The solutions were spin-coated onto the SRG area of the azobenzene-containing films. The used active layer could be easily removed from the SRG area by soaking in a chlorinated solvent and another active layer could be applied, what is an advantage of the used approach .
A schematic presentation of the experimental setup for the lasing measurements is shown in Fig. 2a . A Q-switched frequency doubled Nd:YAG pulsed laser (Surelite I, Continuum, Inc., with OPO) operating at 532 nm with a pulse duration of 6 ns and repetition rate of 10 Hz was used as a pump source. The energy from the laser was measured using laser power and pulse energy meters (pyroelectric sensor Ophir PE25). A cylindrical lens with a focal length of 40 mm was used to shape the pump beam into a narrow stripe of 5 mm length and 0.4 mm width. The light stripe was oriented along the grating vector. The pump light was linearly polarized in the direction orthogonal to the grating planes. The output signal was collected by an optical fibre which was coupled to a CCD-based spectrometer Polytec Berlin AG (spectral resolution is ca. 1.5 nm). Using transversal pumping, the pump stripe covers more than ten thousand grating periods and allows an effective distributed feedback to be obtained. An image of the generated light was obtained using CCD-camera (WinCamD, Gentec-EO Inc.) with orange-glass filter to remove the pumping light. The DFB gratings were stored and tested under ambient conditions.
3. Results and discussion
The polyelectolyte azobenzene-containing material PAZO has been investigated quite in detail before. It has shown very good film building properties with a film roughness less than 2 nm  and exhibited nice compatibility with polymers of non-polar nature [33–36]. The holographic properties have been investigated in a wide range of periods and the polymer formulation has been improved to augment inscription rate [33–36]. However, the holographic inscription rate was still moderate in comparison to e.g. fast epoxy based materials [10,13], and the material was not investigated, in the high frequency grating range suitable for DFB structures of 2-4 order, since the previous interest lay in SRG with µm periods [9,33–36]. So at the beginning we had to test the material capability in the high frequency grating range. Figure 3 exhibits a typical writing kinetics for the SRG of 400nm period. The distinctive (ca 5%) fall of the 0th order diffraction efficiency in time indicates clearly the SRG formation.
Quantitative estimation of the amplitude modulation from diffraction efficiency kinetics is quite difficult, so final amplitudes were measured with AFM. Typical AFM images of small period SRGs written in times of 20-60min are presented in Fig. 4 , which shows that an amplitude modulation of 55 nm was achieved for the SRG of 345 nm period and of 120 nm for the SRG of 580 nm period. Thus, in spite of a small decrease of the 0th order diffraction efficiency, the quality and deepness of the SRG is sufficiently high. These SRGs are suitable as DFB structures.
The azobenzene-containing material PAZO (according to ellipsometric measurements) has a rather high refractive index of 1.67. A matrix material with refractive index higher than 1.67 is therefore required for the active layer in order to realize the DFB design, presented in Fig. 1c. A suitable material with refractive index of 1.75 is a poly(phenylquinoxaline) PPQ, which was investigated earlier [38,40–43]. Besides having high refractive index PPQ is well soluble in chlorinated solvents. PAZO layers, however, are only soluble in polar alcohol type solvents. Hence, both polymers are well-suited for subsequent layer deposition as required for the device design described above. By introduction of different laser dyes into the PPQ matrix and by changing the grating frequency, different lasing wavelength could be realized.
Figure 5 presents the spectra of laser generation in three DFB structures with different active layers and different SRG periods. In each case, a value of the SRG period was chosen to set the resonance wavelength of the SRG (see Eq. (1)) close to the position of the luminescence maximum of the active layer. Spectra of amplified spontaneous emission (ASE) for active layers, obtained in the same structures without SRG, are also presented in Fig. 5. In all cases the generated light was linearly polarized in the direction parallel to the direction of grating grooves. This means that only TE-modes were excited in the investigated structures.
The spectra in Fig. 5 clearly indicate laser generation in the range 590-670 nm. It is evident that the generation rises on the ASE background. Laser threshold energy was found to be in the range of 15-25 µJ/pulse. The corresponding threshold energy density (0.75-1.3 mJ/cm2) is comparable with those for laser designs presented both in Fig. 1b (0.4 mJ/cm2 ) and in Fig. 1a (0.2 mJ/cm2  – 1.8 mJ/cm2 ). Seems an advantage of a higher gain for laser designs of Fig. 1b and Fig. 1c is not clear. This is because the threshold depends also on other parameters such as SRG quality and shape, attenuation of waveguide, dye quantum efficiency and concentration, etc. Further increase of quality of SRG and of active layer film surface as well as an optimization of the pump wavelength, of the confinement of laser mode into gain volume will lead to the decrease of the laser threshold.
Unfortunately, the resolution of the employed spectrometer did not allow measuring correctly the bandwidth of laser lines presented in Fig. 5. The values of this bandwidth (full-width-at-half-maximum, FWHM), estimated from spectra presented in Fig. 5, are 3-5 nm. Figure 5 shows also that laser wavelengths can differ from the wavelength of the ASE spectrum maximum (Figs. 5a and 5c) or can almost coincide with it (Fig. 5b), depending on the SRG period. However, in all cases ASE was detected at lower energy density (ca 10µJ/pulse). Its spectral bandwidth was usually more than 10 nm (FWHM), which is significantly higher than that of laser generation. A typical generated laser beam registered with the CCD camera is shown in Fig. 2b.
It should be noted that in the considered DFB structure, an active layer, based on PPQ polymer, can be easily removed by soaking of the device in chlorinated solvent and then another active layer can be spin-coated on top of the grating. This allows not only the quick replacement of a bleached active layer, but also slight laser wavelength tuning without rewriting of SRG. Indeed, the laser wavelength depends on the effective refractive index (see Eq. (1)), which can be calculated with the Ray Model method  as following:
Equations (3) and (4) show that the angle θ and the effective refractive index neff depend on thickness of the active layer. Therefore, the resonance wavelength of a DFB structure depends also on the active layer thickness. Figure 6 shows the dependence of the resonance wavelength on the active layer thickness, calculated by solving of Eqs. (1)–(4) numerically for the following parameters: n = 1.75, nAZO = 1.67, Λ = 345 nm, m = 2, which correspond to our experiment (Fig. 5c). A solution of these equations exists only for l equal to 0. Figure 6 shows that an increase of active layer thickness leads to an increase of laser operation wavelength.
The possibility of wavelength tuning was demonstrated also experimentally. Recoating of SRG with an active layer of thickness of ca 600nm instead of a layer with ca 500nm thickness resulted in a generation wavelength shift of 3 nm (from 590 nm to 593 nm) as it is shown in Fig. 5c and Fig. 6 (squares). Figure 6 shows that this value is in good agreement with the results of calculations, where the increase of layer thickness caused a wavelength shift of 3 nm, too. The small deviation of calculated data from experimental data is explained by uncertainty of values for refractive indices and layer thickness. The possibility of wavelength tuning by recoating of the active layer is a further advantage of the used DFB design.
Operation of a DFB laser consisting of a second order SRG inscribed in an azobenzene-containing material and a PPQ-based active layer is presented. It is shown that the design of the DFB laser device, proposed in this work, allows an easy recoating of the active layer. This makes not only quick replacement of an active layer in case of its bleaching possible, but also a laser wavelength tuning without rewriting of the SRG.
The authors would like to thank Dr. V. Ksianzou (University of Applied Science Wildau) for carrying out ellipsometric measurements.
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