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Random lasing emission from FTO and glass substrates coated with dye doped SU-8 epoxy based polymer
Optics and Laser Technology 119 (2019) 105602
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Random lasing emission from FTO and glass substrates coated with dye doped SU-8 epoxy based polymer
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Peymaneh Rafieipoura, Abbas Ghasempour Ardakania, , Gholam-Mohammad Parsanasabb ⁎
a b
Department of Physics, Shiraz University, Shiraz 71454, Iran Department of Electrical Engineering, Shahid Beheshti University, Tehran 19839-69411, Iran
HIGHLIGHTS
B dye doped SU-8 polymer is used as the gain medium in random lasers. • Rhodamine of FTO layer and roughness existing in the glass substrate can play as scattering centers in the proposed RL. • Grain characteristics of proposed RLs are studied. • The • When glass and FTO substrates are coated with solution of dyes in alcohol no random lasing emission is observed. ABSTRACT
The demand for improving dyeing efficiency and photochemical/photo-physical properties of organic-dyes-based random lasers while focusing on eco-friendly procedures, cost effective production and reduced toxicity has been grown for the past few years. In order to fulfill these requirements, an organic dye dissolved in the solution of SU-8 polymer is proposed as the gain medium in a typical random laser structure. Considering adhesion properties of the SU-8 resin, we observe random lasing action in a glass substrate and also fluorine doped tin oxide (FTO) layer covered by the solution of rhodamine B dye doped SU-8 polymer. Line-width narrowing and drastic increase of the emitted intensity confirm the onset of random lasing emission in the proposed structures. When we use alcohol solution as the dye solvent instead of SU-8 polymer, no random laser emission is observed in these structures. It is also shown that due to larger grains of the FTO substrate compared to the roughness of the glass substrate, RL emission occurs more efficiently in the FTO substrate. Obtained results may stimulate facile fabrication of cost effective, intensive and low threshold RLs based on dye doped SU-8 polymer as the gain medium.
1. Introduction Lasing emission in a diffusive amplifying medium was thought to be impossible for the past 60 years. However, it is widely accepted to date that lasing emission might be achieved in a strongly scattering medium relying on multiple light scattering rather than two well-aligned mirrors. The pioneering work on random lasers (RLs) has been performed by Letokhov who solved a diffusive equation for light photons in a random gain medium, regardless of the interference effects [1]. However, the interest in RLs blossomed by the experimental realization of Lawandy et al. in 1994, based on a solution of TiO2 nanoparticles and Rhodamine 640 perchlorate dye [2]. The strength of light multi-scattering inside the random amplifying medium plays an important role in defining the RL emission characteristics. Due to the absence of light interference effects for a random gain medium in a diffusive regime, the obtained lasing emission is named as RL emission with non-resonant feedback [2,3]. One of its well-known characteristics is the appearance
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of a laser-like emission peak with linewidth between 4 and 9 nm in the emission spectrum, for pump intensities above a certain threshold. It is attributed to a large number of oscillating modes mutually coupled together, which results in the formation of a single lasing peak with a wider linewidth [4,5]. On the other side, if the scattering strength enhances much further or the random medium falls within the Anderson localization regime, light photons undergo multiple scattering and amplification inside a highly randomized gain medium. So recurrent light scattering may then occur and light traps inside and confines to some closed paths in the random gain medium and then amplifies. In this case, very narrow spikes with sub-nanometer linewidths appear in the emission spectrum, which are called the RL modes. Their frequency is determined by the constructive interference condition of light, recalling that of a Fabry-Perot cavity [6,7]. The first experimental realization of this RL with resonant feedback was reported in 1998, based on a compact disc of ZnO powders act as both scattering elements and gain medium [8]. During the past few years, various materials have been
Corresponding author. E-mail address: [email protected] (A. Ghasempour Ardakani).
https://doi.org/10.1016/j.optlastec.2019.105602 Received 23 December 2018; Received in revised form 24 March 2019; Accepted 20 May 2019 0030-3992/ © 2019 Published by Elsevier Ltd.
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Fig. 1. (a) SEM image and (b) XRD pattern of the FTO layer coated on the glass substrate. The scale bar is 500 nm.
Section 4.
developed as the gain or feedback part of a RL, resulting in the fabrication of plasmonic RLs [9], quantum dot RLs [10], fiber RLs [11], Raman RLs [12] and electrically pumped RLs [13]. RLs have been found numerous potential applications in spectroscopic applications, bioimaging, sensing and other fields of science reported in literature [14]. The most striking advantage of RLs as light sources over conventional lasers and also light emitting diodes is their feasibility of manufacturing process and low cost fabrication. Organic dyes such as RhB and Rh6G are the most common choices as the RL gain medium. Regardless of their low price, two main drawbacks for implementing RLs based on organic dyes in real life applications are either lack of stability and dramatic reduction of efficiency or irreversible photo-degradation of dye molecules under intense radiation. Dye containing polymers are presumably one of the most promising candidates to improve dyeing efficiency or photochemical/ photo-physical properties. A lot of research works involving dyepolymer conjugates have been investigated for improving the performance and efficiency of RLs [15–24]. However, the feasibility of manufacturing processes while also focusing on reduction of production costs is still lacking in many cases, which results in limitations and uncertainties in potential applications. SU-8, a well-known epoxy based polymer, has been extensively used in MEMS [25]. Benefiting from high chemical resistivity, mechanical stability and biocompatibility as well as excellent adhesion properties, using dye doped SU-8 resin as the gain medium is promising in RLs. Using this polymer instead of other solvents such as alcohols resolve problems including instability, settling down of scattering particles and evaporation of solvents. In this paper, an organic dye (RhB) dissolved in the solution of SU-8 polymer is introduced as the gain medium in a typical random laser structure. Herein, random lasing action in fluorine doped tin oxide (FTO) layer as the scattering medium is demonstrated experimentally. A drastic increase in the emitted intensity as well as the decrease in its linewidth suggests the occurrence of random lasing action in the proposed structure. Interestingly, random lasing action is also observed in a glass substrate as the scattering medium. This phenomenon which is ascribed to the scattering of light from the roughness of the glass substrate happens due to the strong adhesion properties of the SU-8 resin. To the best of our knowledge, random lasing action in FTO layer as well as glass substrate has not yet been reported. Comparing their corresponding emissive properties, random lasing action with a low threshold and high efficiency is obtained in the FTO layer. This novel work confirms both the feasibility and reliability of using organic dyes doped SU-8 resin as the gain medium in RLs for potential technical and industrial applications. The remaining parts of this paper are organized as follows: an illustration of the experimental procedure is given in Section 2. The results of the experiment along with the related discussions are described in Section 3. The paper is then finished with some conclusions in
2. Experimental method Here, we aim to examine the RL emission from the following samples: (1) A FTO layer coated with RhB dye doped SU-8 polymer (2) A glass microscope slide coated with RhB dye doped SU-8 polymer (3) A glass microscope slide coated with RhB dye dissolved in ethylene glycol (4) A suspended layer of RhB dye doped SU-8 polymer To prepare sample 1, a 100 × 100 mm plate consists of FTO layer coated on a glass substrate was purchased from Geartcellsolar Company. We cut it into 20 × 10 mm pieces. One of the pieces was washed with detergent, deionised water and other solvents such as ethanol and acetone in sequence, and was then dried in air before performing the experiment. Scanning electron microscope (SEM), X-ray diffraction (XRD) pattern and atomic force microscope (AFM) analysis were used to examine the surface morphology and structural properties of the FTO layer. SEM image of the FTO layer is shown in Fig. 1(a). It was carried out on a TESCAN-Vega 3 scanning electron microscope (TESCAN, Czech Republic). As it is observed, the surface of the FTO layer contains irregular shaped grains with an average size of 92.437 nm (measured using ImagJ software). XRD pattern corresponding to the FTO layer is also depicted in Fig. 1(b). XRD spectrum of the FTO layer was performed on a D8-ADVANCE X-ray Diffractometer (Bruker, German). Several peaks located at 2θ = 26.55°, 33.8°, 37.8°, 51.5°, 61.65° and 65.7° are assigned to diffractions from (1 1 0), (1 0 1), (2 0 0), (2 1 1), (3 1 0), and (3 0 1) basal planes, respectively. The significant difference in the intensity of the peak appearing at 2θ = 37.8° is characteristic of the preferred plane of crystallite growth. The topography image of the FTO layer is presented in Fig. 2. It was characterized with a WITec Atomic Force Microscope (AFM) alpha300, working in the active mode. Fig. 2 shows the grain height histogram of the FTO layer while the inset displays its corresponding height map (top view, 24.80 µm2). The topography of the sample shows a rough surface with an average grain height of ∼50.2 nm, FWHM of 27.25 nm and the standard deviation of ∼11.57 nm. To prepare sample 2, we needed a glass substrate which was a glass microscope slide with 76.45 mm length, 25.05 mm width and 1.05 mm thickness. It was washed and cleaned before the experiment. The topography of this substrate was examined with AFM analysis and the height histogram of the glass surface is depicted in Fig. 3. Inset shows the height map (top view, 24.80 µm2) of the glass surface. It is clearly seen that the surface of the glass substrate is much smoother than the surface of the FTO layer. However, the existence of some ripples with 2
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polymer solution obtained. The schematic illustration of drop coating method is presented in Fig. 4(a). In this method, a few droplets of the gain medium with the total volume of ∼50 µl were transferred onto the FTO surface by a small plastic dropper and spread over the entire surface. We then left the prepared sample in the ambient temperature for 24 h such that the polymeric solution deeply penetrated into the holes of the surface of the FTO layer and was then dried in air at room temperature. The sample was kept in a closed box during the drying process, to be protected from the pollutants and environmental dust. The obtained layer on the FTO substrate was nearly uniform and had an average thickness of ∼0.05 mm. The sample 2 composed of the glass substrate coated with RhB dye doped SU-8 epoxy based polymer was prepared in the same way. The final thickness of the gain layer which was spread over the glass surface was ∼0.05 mm. In order to prepare sample 3, a glass microscope slide was coated with the solution of RhB dye in ethylene glycol by the same drop coating method described above. The thickness of the gain layer on the glass surface was ∼0.05 mm. Since efficiency of the gain medium based on the solution of RhB dye in ethylene glycol decreases after evaporation of the solvent, it should be prepared just before performing the RL experiments. To prepare sample 4, we dropped coated a thick layer of RhB dye doped SU-8 polymer solution with the final thickness of ∼1.1 mm onto a glass substrate. Then, it was kept in a closed box for 24 h in air at room temperature. Then it was sliced and pressed mechanically to obtain rectangular pieces with thickness of 0.75 mm and size of 5.80 mm × 6.70 mm, consisting of RhB dye doped SU-8 epoxy based polymer. RL experiments were performed by applying the second harmonic of a home-made Q-switched Nd-YAG pulsed laser (532 nm, 10 Hz, 10 ns) as the excitation source. We used a collimator after the pump laser such that the cross section of the pump light was adjusted to be a circle with diameter of 10 mm. An aperture diaphragm was applied for adjusting the cross section of the pump light and so the excitation area on the sample. A schematic representation of the experimental set up is shown in Fig. 4(b). The pump light passes through an aperture diaphragm and a spherical lens with 10 cm focal length, respectively. Then it focused to a circular spot of diameter ∼2.4 mm on the sample with normal incidence. An optical fiber is placed in such a direction to record the reflection spectra of the excited sample. Whenever the sample is pumped, its corresponding random lasing emission is coupled to an optical fiber through a fiber coupler and guided to an ocean optics HR4000 spectrometer (resolution of 0.1 nm). The emission spectrum is then recorded by using a computer. The energy of the pump pulse was also measured using a joule-meter (1 micro-joule measurement accuracy) of Gentec Company.
Fig. 2. Atomic force microscope height histogram of the FTO layer. Inset displays height map of the FTO layer, scale bar is 1 µm.
Fig. 3. Atomic force microscope height histogram of the glass surface. Inset shows height map of the glass surface, scale bar is 1 µm.
the average height of ∼1.94 nm, FWHM of ∼1.07 nm and standard deviation of ∼0.45 nm are the remarkable characteristics of the glass surface. The gain medium in sample 1, 2 and 4 was Rhodamine B (RhB) dye doped in SU-8 epoxy based polymer while in sample 3 the solution of RhB in ethylene glycol was used as the gain medium. RhB dye was purchased from Sigma Aldrich. An ethylene glycol solution of RhB consisting of 21 mg RhB in 5 cc ethylene glycol (purchased from Merck with 99.5% purity) was prepared after 60 min stirring. To produce the dye-doped polymer gain medium, we dissolved 21 mg of RhB dye in 5 cc of SU8-2002 (MicroChem Inc.) as in Ref. [26]. The mixture was stirred for 120 min at room temperature. In order to prepare sample 1, we dropped coated RhB dye doped SU8 polymeric solution onto the FTO layer till a thin layer of dye doped
Fig. 4. (a) Schematic illustration of drop coating the polymeric solution of RhB dye doped SU-8 onto the FTO layer. (b) Schematic representation of the experimental set up. 3
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Fig. 5. (a) Variation of the emission spectrum as a function of pump intensity corresponding to the FTO layer coated with RhB dye doped SU-8. (b) Corresponding peak emitted intensity and linewidth (FWHM) versus pump intensity. (c) Normalized emission spectra corresponding to the FTO layer coated with RhB dye doped SU8 polymer at different values of pump intensity.
3. Results and discussions
the threshold pump intensity is clearly observed in Fig. 5(c). Furthermore, some ripples are apparent in the normalized emission spectra which result from the background noise and intensity fluctuations of the pump light. They are dominant for very low pump intensities. It is because the intensity of the emission peak is comparable with the background noise at low pump intensities. According to RL theory with non-resonant feedback, light photons entered a random amplifying medium undergo multiple scattering events. The optical path length of light then increases inside the gain medium and the light gains more optical amplification. Hence, light multiple scattering by the irregularly shaped FTO grains inside the gain medium is considered to provide the essential optical feedback for random lasing emission. When the obtained gain balances with the natural losses of the lasing system, the lasing condition fulfills and then random lasing emission occurs at the threshold pump intensity. As a result, the broad-band spontaneous emission of the gain medium narrows toward the central frequency of the gain line-shape and its peak emitted intensity increases rapidly by further increasing the pump intensity. Due to non-resonant nature of the phenomena, this lasing behavior is called RL emission with nonresonant feedback. Fig. 6 shows the evolution of emission spectra corresponding to the glass substrate coated with RhB dye doped SU-8 polymer, as a function of pump intensity. The plots of maximum emitted intensity and FWHM versus pump intensity are depicted in Fig. 6(b). The estimated laser threshold is around 3.5 MW/cm2. The linewidth and the central wavelength of the emitted peak are ∼6.1 and ∼635.5 nm, respectively for the pump intensity of 15.15 MW/cm2. Furthermore, the central
We first study the random lasing emission characteristics corresponding to the FTO layer coated on a glass substrate (called FTO substrate). The evolution of emission spectra as a function of pump intensity is illustrated in Fig. 5. As shown in Fig. 5(a), a broad emission spectrum is observed for very low pump intensity. It is attributed to the spontaneous emission of the gain medium. Since increasing the pump intensity provides more optical gain, the peak emitted intensity increases and the linewidth decreases by increasing the pump intensity. Plots of maximum emitted intensity and linewidth (FWHM) of the emission peak versus pump intensity are shown in Fig. 5(b). One can see that the peak emitted intensity increases monotonically by increasing the pump intensity before an abrupt change in the slope occurs. Then, the monotonic increase of the emitted intensity continues rapidly after the threshold pump intensity. The lasing threshold is the pump intensity at which the intensity of the emission peak increases drastically. The laser threshold is obtained as ∼1.7 MW/cm2 from Fig. 5(b). The threshold behavior, demonstrating the onset of RL operation in the structure, is clearly seen in Fig. 5(b). At the pump intensity of 15.15 MW/cm2, the linewidth and the central wavelength of the emitted peak are ∼5.1 and ∼637.5 nm, respectively. On the contrary, the central wavelength and linewidth of the emitted peak are around 635.7 nm and 64 nm for the pump intensity of 1.1 MW/cm2, respectively. Normalized RL emission spectra from the FTO substrate coated with RhB dye doped SU-8 polymer at different values of pump intensity is also shown in Fig. 5(c). Linewidth collapse occurring around 4
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Fig. 6. (a) Variation of the emission spectrum as a function of pump intensity corresponding to the glass substrate coated with RhB dye doped SU-8. (b) Corresponding peak emitted intensity and linewidth (FWHM) versus pump intensity. (c) Normalized emission spectra corresponding to the glass substrate coated with RhB dye doped SU-8 polymer at different values of pump intensity.
wavelength and linewidth of the emitted peak are around 638.5 nm and 23 nm, respectively for the pump intensity of 2.16 MW/cm2. Linewidth collapse around the threshold pump intensity is also illustrated in Fig. 6(c) which represents the normalized emission spectra corresponding to the glass substrate at different pump intensities. Fig. 6 constitutes direct evidence for the onset of random lasing action in the glass substrate, the same phenomenon as that of in the FTO substrate. The argument developed before is true in this case again. In spite of the similarity of the phenomenon, we interpret this finding (random lasing emission from the glass substrate) to be caused by light multi-scattering from the roughness of the glass substrate. The same observations involving surface roughness induced RL emission are also demonstrated by others [27–30]. To further verify the role of SU-8 resin, we study the pump dependent variation of emission spectra corresponding to the glass substrate coated with the solution of RhB dye dissolved in ethylene glycol (sample 3). The evolution of its emission spectra as a function of pump intensity is presented in Fig. 7(a). The broadband spontaneous emission of the gain medium with linewidth (FWHM) of ∼49.2 nm and the central wavelength of ∼592 nm are observed for the pump intensity of 2.16 MW/cm2. We see neither significant spectral narrowing nor drastic increase of the emitted intensity by increasing the pump intensity. The linewidth and central wavelength of the emission spectrum are approximately 48.9 nm and 596.4 nm, respectively at the pump intensity of 33.57 MW/cm2. Obtained results confirm the occurrence of
photoluminescence emission (PL) from the glass substrate coated with the solution of RhB dye in ethylene glycol. There is a red-shift in the emission wavelength in the sample containing SU-8 compared to the sample involving ethylene glycol. This may be because of higher refractive index of SU-8 compared to ethylene glycol. In addition, plots of peak emitted intensity versus pump intensity for the glass substrate coated with RhB dye doped SU-8 resin and RhB dye doped ethylene glycol are compared with each other in Fig. 7(b). In contrast to the case including the solution of RhB dye in ethylene glycol, a threshold behavior demonstrating the random lasing action is clearly seen for the case in which the glass substrate is coated with RhB dye doped SU-8 resin. It should be noted that when rhodamine dyes dissolved in alcohol alone are coated on the FTO substrate, no random lasing emission occurs as shown in our previous work [31]. Hence, notable differences existing between the emission spectra of the FTO and glass substrates in the cases involving organic dyes alone and organic dyes doped SU-8 resin as the gain medium, convince us to speculate that the adhesion properties of the cross-linked polymer network come into play and hence the scattering of light happens more strongly. The surface of a substrate is never truly smooth, consisting of several valleys and peaks. So the penetration of SU-8 polymeric solution in voids of the FTO or glass substrates results in the light multiple scattering induced random lasing emission in both cases. According to wetting theory which describes the adhesion phenomenon by implementing molecular contact and surface forces existing between two dissimilar surfaces, complete 5
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Fig. 7. (a) Variation of the emission spectrum as a function of pump intensity corresponding to the glass substrate coated with RhB dye doped ethylene glycol. (c) Peak emitted intensity versus pump intensity for the glass substrate coated with RhB dye doped SU-8 polymer and RhB dye doped ethylene glycol.
wetting is favored when the adhesive has a lower surface tension than the substrate [32,33]. Glass is known as a low surface energy substrate. In contrast, metallic surfaces possess high surface energies. As a result, it is expected that higher bond strength achieves in the case of metals being in connection with epoxy polymers. FTO is a notable n-semiconductor material which has a lot of conduction electrons. So we conclude that SU-8 resin exhibits excellent adhesion to the FTO surface, in comparison with the glass surface. This phenomenon leads to a strong joint as well as more complete filling of the voids or pores of the FTO surface. When light encounters scattering centers (surface roughness) with larger scattering cross sections, it will scatter more strongly. As a result, its dwell time and optical path length inside the gain medium increases. Consequently, more optical gain is provided and one can expect an increase in the peak emitted intensity and a decrease in the corresponding laser threshold. As the grains of the FTO layer are larger than the roughness of the glass substrate, its corresponding random lasing emission would then be more efficient, intensive and occurs at a lower laser threshold. Furthermore, due to larger refractive index of SU-8 resin compared to ethylene glycol, more waveguide confinement occurs in the case of dye doped SU-8 resin on the glass or FTO substrate and this effect can help lasing action in addition to multiscattering. The effects of waveguide formation on the laser action in the polymer based RLs have been reported in recent years [17,34]. Hence strong adhesion of SU8 resin to the glass or FTO in one hand and forming the waveguide confinement in the other hand lead to RL emission in samples including dye doped SU-8 resin as the gain medium compared with ones including dye alone. Therefore, obtained results in this paper provide inspiration for designing and manufacturing simple, cost effective and easily handled RLs based on dye doped SU-8 polymer as a potential candidate for the gain medium. In the following, we investigate the effects of total internal reflections (TIR) at the polymer/dye-air interface or waveguide confinement on the RL emission spectra and RL threshold corresponding to the FTO and glass substrates. Fig. 8(a) depicts the evolution of emission spectra corresponding to the suspended layer of dye doped SU-8 polymer alone (sample 4) as a function of pump intensity. The peak appearing at 532 nm corresponds to the excitation wavelength. The reason for appearance of this peak can be the reflection of the pump light from the sample holder and the surface of the sample. The emitted peak is observed around 635 nm in the emission spectra. However, the emission peak has lower intensity compared to one in sample 1 and 2 at the same pump intensity. In addition, a slow increase of the emitted intensity versus pump intensity is clearly seen in Fig. 8(b). After the pump intensity of approximately 16.5 MW/cm2, the increase of the emitted intensity versus pump intensity continues with a sudden positive change in the slope. The linewidth and the central wavelength of the
emitted peak are around 17.7 and 637.7 nm, respectively for the pump intensity of 15.15 MW/cm2. Also, the central wavelength and linewidth of the emitted peak are approximately 635 nm and 7.5 nm, respectively for the pump intensity of 30.65 MW/cm2. Regarding the linewidth value of the emission peak, amplified spontaneous emission (ASE) occurs in the suspended dye-polymer layer or sample 4. Obtained results demonstrate the occurrence of ASE in the layer of dye-polymer solution alone at a very high pump threshold of 16.5 MW/cm2. Similar observation was also reported based on a thin film layer of polymethyl methacrylate (PMMA) doped with 3-(1,1-dicyanoethenyl)-1-phenyl4,5-dihydro-1H-pyrazole (DCNP) dye [35]. We compare random lasing emission from the suspended layer of dye doped SU-8 polymer with that of FTO and glass substrates in Fig. 9. Consequently, the waveguide confinement alone cannot provide the optical feedback for random lasing emission in the SU-8 on the glass or FTO substrate and multiple scattering due to grain boundary of FTO and roughness of glass is necessary for lasing action. However, waveguide confinement facilitates the lasing action and decrease the laser threshold. It seems important to mention here that the RL emission spectra of the FTO and glass substrates are highly dependent on the excitation point and area on the sample. It is one of the specific features of a RL system. Hence, comparing RL emission characteristics under the same experimental conditions is of great importance. Hence, we have held the position of the optical fiber with respect to the sample as well as the pump area and position on the sample fixed during all measurements. Fig. 9(a) and (b) represent emission spectra and normalized emission spectra corresponding to the sample 1, 2 and 4 at the fixed pump intensity of 15.15 MW/cm2. Ripples existing in the emission spectrum of the suspended layer of dye dope SU-8 polymer are attributed to the background noise and intensity fluctuations of the pump light. One can observe that the random lasing emission from sample 1 is more intensive and has a narrower linewidth with respect to that of sample 2 and 4. The linewidths of the emitted peak are approximately 5.1, 6.1 and 17.7 nm for sample 1, 2 and 4, respectively. Plots of peak emitted intensity versus pump intensity corresponding to sample 1, 2 and 4 are also represented in Fig. 9(c). The respective laser thresholds are ∼1.7, 3.7 and 16.5 MW/cm2. It is obvious in Fig. 9(c) that random lasing action in the FTO substrate occurs at a lower laser threshold compared with that of the glass substrate and ASE in dye-polymer layer. Also, we notice the higher slope efficiency corresponding to random lasing operation in the FTO substrate. The present results suggest that the obtained low threshold and intensive random lasing emission in the FTO and glass substrates is caused by light multi-scattering from the grains of the FTO layer and the roughness of the glass substrate, respectively. Light scattering by either dye-polymer complexes or total internal reflections at the dye/polymer-air interface (polymer waveguide effect) 6
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Fig. 8. (a) Variation of the emission spectrum as a function of pump intensity corresponding to a layer of RhB dye doped SU-8 polymer alone. (b) Peak emitted intensity and linewidth (FWHM) corresponding to the layer of RhB dye doped SU-8 polymer alone versus pump intensity.
would then play less significant role in the lower threshold and more intensive random lasing emission from both the FTO and glass substrates.
glass substrate as the scattering media in proposed RLs. Obtained results demonstrate the onset of random lasing action in FTO layer as well as glass substrate. Due to larger grains of the FTO layer compared with the roughness of the glass surface, intensive random lasing emission is obtained at a lower threshold in the FTO layer. Our present experiments may regard as a possible starting point for the potential facile fabrication of intensive and low threshold RLs well-suited to be used in future RL based technologies.
4. Conclusion In summary, we have reported on the facile fabrication of RLs based on RhB dye doped SU-8 resin as the gain medium. Strong adhesion properties of the SU-8 resin enables employing the FTO layer and also
Fig. 9. (a) emission spectra and (b) Normalized emission spectra corresponding to the suspended layer of SU-8 polymer as well as FTO and glass substrates coated with RhB dye doped SU-8 polymer for the constant pump intensity of 15.15 MW/cm2. (c) Peak emitted intensity versus pump intensity corresponded to the suspended layer of SU-8 polymer as well as FTO and glass substrates coated with RhB dye doped SU-8 polymer. 7
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Appendix A. Supplementary material
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Random lasing emission from FTO and glass substrates coated with dye doped SU-8 epoxy based polymer
T
Peymaneh Rafieipoura, Abbas Ghasempour Ardakania, , Gholam-Mohammad Parsanasabb ⁎
a b
Department of Physics, Shiraz University, Shiraz 71454, Iran Department of Electrical Engineering, Shahid Beheshti University, Tehran 19839-69411, Iran
HIGHLIGHTS
B dye doped SU-8 polymer is used as the gain medium in random lasers. • Rhodamine of FTO layer and roughness existing in the glass substrate can play as scattering centers in the proposed RL. • Grain characteristics of proposed RLs are studied. • The • When glass and FTO substrates are coated with solution of dyes in alcohol no random lasing emission is observed. ABSTRACT
The demand for improving dyeing efficiency and photochemical/photo-physical properties of organic-dyes-based random lasers while focusing on eco-friendly procedures, cost effective production and reduced toxicity has been grown for the past few years. In order to fulfill these requirements, an organic dye dissolved in the solution of SU-8 polymer is proposed as the gain medium in a typical random laser structure. Considering adhesion properties of the SU-8 resin, we observe random lasing action in a glass substrate and also fluorine doped tin oxide (FTO) layer covered by the solution of rhodamine B dye doped SU-8 polymer. Line-width narrowing and drastic increase of the emitted intensity confirm the onset of random lasing emission in the proposed structures. When we use alcohol solution as the dye solvent instead of SU-8 polymer, no random laser emission is observed in these structures. It is also shown that due to larger grains of the FTO substrate compared to the roughness of the glass substrate, RL emission occurs more efficiently in the FTO substrate. Obtained results may stimulate facile fabrication of cost effective, intensive and low threshold RLs based on dye doped SU-8 polymer as the gain medium.
1. Introduction Lasing emission in a diffusive amplifying medium was thought to be impossible for the past 60 years. However, it is widely accepted to date that lasing emission might be achieved in a strongly scattering medium relying on multiple light scattering rather than two well-aligned mirrors. The pioneering work on random lasers (RLs) has been performed by Letokhov who solved a diffusive equation for light photons in a random gain medium, regardless of the interference effects [1]. However, the interest in RLs blossomed by the experimental realization of Lawandy et al. in 1994, based on a solution of TiO2 nanoparticles and Rhodamine 640 perchlorate dye [2]. The strength of light multi-scattering inside the random amplifying medium plays an important role in defining the RL emission characteristics. Due to the absence of light interference effects for a random gain medium in a diffusive regime, the obtained lasing emission is named as RL emission with non-resonant feedback [2,3]. One of its well-known characteristics is the appearance
⁎
of a laser-like emission peak with linewidth between 4 and 9 nm in the emission spectrum, for pump intensities above a certain threshold. It is attributed to a large number of oscillating modes mutually coupled together, which results in the formation of a single lasing peak with a wider linewidth [4,5]. On the other side, if the scattering strength enhances much further or the random medium falls within the Anderson localization regime, light photons undergo multiple scattering and amplification inside a highly randomized gain medium. So recurrent light scattering may then occur and light traps inside and confines to some closed paths in the random gain medium and then amplifies. In this case, very narrow spikes with sub-nanometer linewidths appear in the emission spectrum, which are called the RL modes. Their frequency is determined by the constructive interference condition of light, recalling that of a Fabry-Perot cavity [6,7]. The first experimental realization of this RL with resonant feedback was reported in 1998, based on a compact disc of ZnO powders act as both scattering elements and gain medium [8]. During the past few years, various materials have been
Corresponding author. E-mail address: [email protected] (A. Ghasempour Ardakani).
https://doi.org/10.1016/j.optlastec.2019.105602 Received 23 December 2018; Received in revised form 24 March 2019; Accepted 20 May 2019 0030-3992/ © 2019 Published by Elsevier Ltd.
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Fig. 1. (a) SEM image and (b) XRD pattern of the FTO layer coated on the glass substrate. The scale bar is 500 nm.
Section 4.
developed as the gain or feedback part of a RL, resulting in the fabrication of plasmonic RLs [9], quantum dot RLs [10], fiber RLs [11], Raman RLs [12] and electrically pumped RLs [13]. RLs have been found numerous potential applications in spectroscopic applications, bioimaging, sensing and other fields of science reported in literature [14]. The most striking advantage of RLs as light sources over conventional lasers and also light emitting diodes is their feasibility of manufacturing process and low cost fabrication. Organic dyes such as RhB and Rh6G are the most common choices as the RL gain medium. Regardless of their low price, two main drawbacks for implementing RLs based on organic dyes in real life applications are either lack of stability and dramatic reduction of efficiency or irreversible photo-degradation of dye molecules under intense radiation. Dye containing polymers are presumably one of the most promising candidates to improve dyeing efficiency or photochemical/ photo-physical properties. A lot of research works involving dyepolymer conjugates have been investigated for improving the performance and efficiency of RLs [15–24]. However, the feasibility of manufacturing processes while also focusing on reduction of production costs is still lacking in many cases, which results in limitations and uncertainties in potential applications. SU-8, a well-known epoxy based polymer, has been extensively used in MEMS [25]. Benefiting from high chemical resistivity, mechanical stability and biocompatibility as well as excellent adhesion properties, using dye doped SU-8 resin as the gain medium is promising in RLs. Using this polymer instead of other solvents such as alcohols resolve problems including instability, settling down of scattering particles and evaporation of solvents. In this paper, an organic dye (RhB) dissolved in the solution of SU-8 polymer is introduced as the gain medium in a typical random laser structure. Herein, random lasing action in fluorine doped tin oxide (FTO) layer as the scattering medium is demonstrated experimentally. A drastic increase in the emitted intensity as well as the decrease in its linewidth suggests the occurrence of random lasing action in the proposed structure. Interestingly, random lasing action is also observed in a glass substrate as the scattering medium. This phenomenon which is ascribed to the scattering of light from the roughness of the glass substrate happens due to the strong adhesion properties of the SU-8 resin. To the best of our knowledge, random lasing action in FTO layer as well as glass substrate has not yet been reported. Comparing their corresponding emissive properties, random lasing action with a low threshold and high efficiency is obtained in the FTO layer. This novel work confirms both the feasibility and reliability of using organic dyes doped SU-8 resin as the gain medium in RLs for potential technical and industrial applications. The remaining parts of this paper are organized as follows: an illustration of the experimental procedure is given in Section 2. The results of the experiment along with the related discussions are described in Section 3. The paper is then finished with some conclusions in
2. Experimental method Here, we aim to examine the RL emission from the following samples: (1) A FTO layer coated with RhB dye doped SU-8 polymer (2) A glass microscope slide coated with RhB dye doped SU-8 polymer (3) A glass microscope slide coated with RhB dye dissolved in ethylene glycol (4) A suspended layer of RhB dye doped SU-8 polymer To prepare sample 1, a 100 × 100 mm plate consists of FTO layer coated on a glass substrate was purchased from Geartcellsolar Company. We cut it into 20 × 10 mm pieces. One of the pieces was washed with detergent, deionised water and other solvents such as ethanol and acetone in sequence, and was then dried in air before performing the experiment. Scanning electron microscope (SEM), X-ray diffraction (XRD) pattern and atomic force microscope (AFM) analysis were used to examine the surface morphology and structural properties of the FTO layer. SEM image of the FTO layer is shown in Fig. 1(a). It was carried out on a TESCAN-Vega 3 scanning electron microscope (TESCAN, Czech Republic). As it is observed, the surface of the FTO layer contains irregular shaped grains with an average size of 92.437 nm (measured using ImagJ software). XRD pattern corresponding to the FTO layer is also depicted in Fig. 1(b). XRD spectrum of the FTO layer was performed on a D8-ADVANCE X-ray Diffractometer (Bruker, German). Several peaks located at 2θ = 26.55°, 33.8°, 37.8°, 51.5°, 61.65° and 65.7° are assigned to diffractions from (1 1 0), (1 0 1), (2 0 0), (2 1 1), (3 1 0), and (3 0 1) basal planes, respectively. The significant difference in the intensity of the peak appearing at 2θ = 37.8° is characteristic of the preferred plane of crystallite growth. The topography image of the FTO layer is presented in Fig. 2. It was characterized with a WITec Atomic Force Microscope (AFM) alpha300, working in the active mode. Fig. 2 shows the grain height histogram of the FTO layer while the inset displays its corresponding height map (top view, 24.80 µm2). The topography of the sample shows a rough surface with an average grain height of ∼50.2 nm, FWHM of 27.25 nm and the standard deviation of ∼11.57 nm. To prepare sample 2, we needed a glass substrate which was a glass microscope slide with 76.45 mm length, 25.05 mm width and 1.05 mm thickness. It was washed and cleaned before the experiment. The topography of this substrate was examined with AFM analysis and the height histogram of the glass surface is depicted in Fig. 3. Inset shows the height map (top view, 24.80 µm2) of the glass surface. It is clearly seen that the surface of the glass substrate is much smoother than the surface of the FTO layer. However, the existence of some ripples with 2
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polymer solution obtained. The schematic illustration of drop coating method is presented in Fig. 4(a). In this method, a few droplets of the gain medium with the total volume of ∼50 µl were transferred onto the FTO surface by a small plastic dropper and spread over the entire surface. We then left the prepared sample in the ambient temperature for 24 h such that the polymeric solution deeply penetrated into the holes of the surface of the FTO layer and was then dried in air at room temperature. The sample was kept in a closed box during the drying process, to be protected from the pollutants and environmental dust. The obtained layer on the FTO substrate was nearly uniform and had an average thickness of ∼0.05 mm. The sample 2 composed of the glass substrate coated with RhB dye doped SU-8 epoxy based polymer was prepared in the same way. The final thickness of the gain layer which was spread over the glass surface was ∼0.05 mm. In order to prepare sample 3, a glass microscope slide was coated with the solution of RhB dye in ethylene glycol by the same drop coating method described above. The thickness of the gain layer on the glass surface was ∼0.05 mm. Since efficiency of the gain medium based on the solution of RhB dye in ethylene glycol decreases after evaporation of the solvent, it should be prepared just before performing the RL experiments. To prepare sample 4, we dropped coated a thick layer of RhB dye doped SU-8 polymer solution with the final thickness of ∼1.1 mm onto a glass substrate. Then, it was kept in a closed box for 24 h in air at room temperature. Then it was sliced and pressed mechanically to obtain rectangular pieces with thickness of 0.75 mm and size of 5.80 mm × 6.70 mm, consisting of RhB dye doped SU-8 epoxy based polymer. RL experiments were performed by applying the second harmonic of a home-made Q-switched Nd-YAG pulsed laser (532 nm, 10 Hz, 10 ns) as the excitation source. We used a collimator after the pump laser such that the cross section of the pump light was adjusted to be a circle with diameter of 10 mm. An aperture diaphragm was applied for adjusting the cross section of the pump light and so the excitation area on the sample. A schematic representation of the experimental set up is shown in Fig. 4(b). The pump light passes through an aperture diaphragm and a spherical lens with 10 cm focal length, respectively. Then it focused to a circular spot of diameter ∼2.4 mm on the sample with normal incidence. An optical fiber is placed in such a direction to record the reflection spectra of the excited sample. Whenever the sample is pumped, its corresponding random lasing emission is coupled to an optical fiber through a fiber coupler and guided to an ocean optics HR4000 spectrometer (resolution of 0.1 nm). The emission spectrum is then recorded by using a computer. The energy of the pump pulse was also measured using a joule-meter (1 micro-joule measurement accuracy) of Gentec Company.
Fig. 2. Atomic force microscope height histogram of the FTO layer. Inset displays height map of the FTO layer, scale bar is 1 µm.
Fig. 3. Atomic force microscope height histogram of the glass surface. Inset shows height map of the glass surface, scale bar is 1 µm.
the average height of ∼1.94 nm, FWHM of ∼1.07 nm and standard deviation of ∼0.45 nm are the remarkable characteristics of the glass surface. The gain medium in sample 1, 2 and 4 was Rhodamine B (RhB) dye doped in SU-8 epoxy based polymer while in sample 3 the solution of RhB in ethylene glycol was used as the gain medium. RhB dye was purchased from Sigma Aldrich. An ethylene glycol solution of RhB consisting of 21 mg RhB in 5 cc ethylene glycol (purchased from Merck with 99.5% purity) was prepared after 60 min stirring. To produce the dye-doped polymer gain medium, we dissolved 21 mg of RhB dye in 5 cc of SU8-2002 (MicroChem Inc.) as in Ref. [26]. The mixture was stirred for 120 min at room temperature. In order to prepare sample 1, we dropped coated RhB dye doped SU8 polymeric solution onto the FTO layer till a thin layer of dye doped
Fig. 4. (a) Schematic illustration of drop coating the polymeric solution of RhB dye doped SU-8 onto the FTO layer. (b) Schematic representation of the experimental set up. 3
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Fig. 5. (a) Variation of the emission spectrum as a function of pump intensity corresponding to the FTO layer coated with RhB dye doped SU-8. (b) Corresponding peak emitted intensity and linewidth (FWHM) versus pump intensity. (c) Normalized emission spectra corresponding to the FTO layer coated with RhB dye doped SU8 polymer at different values of pump intensity.
3. Results and discussions
the threshold pump intensity is clearly observed in Fig. 5(c). Furthermore, some ripples are apparent in the normalized emission spectra which result from the background noise and intensity fluctuations of the pump light. They are dominant for very low pump intensities. It is because the intensity of the emission peak is comparable with the background noise at low pump intensities. According to RL theory with non-resonant feedback, light photons entered a random amplifying medium undergo multiple scattering events. The optical path length of light then increases inside the gain medium and the light gains more optical amplification. Hence, light multiple scattering by the irregularly shaped FTO grains inside the gain medium is considered to provide the essential optical feedback for random lasing emission. When the obtained gain balances with the natural losses of the lasing system, the lasing condition fulfills and then random lasing emission occurs at the threshold pump intensity. As a result, the broad-band spontaneous emission of the gain medium narrows toward the central frequency of the gain line-shape and its peak emitted intensity increases rapidly by further increasing the pump intensity. Due to non-resonant nature of the phenomena, this lasing behavior is called RL emission with nonresonant feedback. Fig. 6 shows the evolution of emission spectra corresponding to the glass substrate coated with RhB dye doped SU-8 polymer, as a function of pump intensity. The plots of maximum emitted intensity and FWHM versus pump intensity are depicted in Fig. 6(b). The estimated laser threshold is around 3.5 MW/cm2. The linewidth and the central wavelength of the emitted peak are ∼6.1 and ∼635.5 nm, respectively for the pump intensity of 15.15 MW/cm2. Furthermore, the central
We first study the random lasing emission characteristics corresponding to the FTO layer coated on a glass substrate (called FTO substrate). The evolution of emission spectra as a function of pump intensity is illustrated in Fig. 5. As shown in Fig. 5(a), a broad emission spectrum is observed for very low pump intensity. It is attributed to the spontaneous emission of the gain medium. Since increasing the pump intensity provides more optical gain, the peak emitted intensity increases and the linewidth decreases by increasing the pump intensity. Plots of maximum emitted intensity and linewidth (FWHM) of the emission peak versus pump intensity are shown in Fig. 5(b). One can see that the peak emitted intensity increases monotonically by increasing the pump intensity before an abrupt change in the slope occurs. Then, the monotonic increase of the emitted intensity continues rapidly after the threshold pump intensity. The lasing threshold is the pump intensity at which the intensity of the emission peak increases drastically. The laser threshold is obtained as ∼1.7 MW/cm2 from Fig. 5(b). The threshold behavior, demonstrating the onset of RL operation in the structure, is clearly seen in Fig. 5(b). At the pump intensity of 15.15 MW/cm2, the linewidth and the central wavelength of the emitted peak are ∼5.1 and ∼637.5 nm, respectively. On the contrary, the central wavelength and linewidth of the emitted peak are around 635.7 nm and 64 nm for the pump intensity of 1.1 MW/cm2, respectively. Normalized RL emission spectra from the FTO substrate coated with RhB dye doped SU-8 polymer at different values of pump intensity is also shown in Fig. 5(c). Linewidth collapse occurring around 4
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Fig. 6. (a) Variation of the emission spectrum as a function of pump intensity corresponding to the glass substrate coated with RhB dye doped SU-8. (b) Corresponding peak emitted intensity and linewidth (FWHM) versus pump intensity. (c) Normalized emission spectra corresponding to the glass substrate coated with RhB dye doped SU-8 polymer at different values of pump intensity.
wavelength and linewidth of the emitted peak are around 638.5 nm and 23 nm, respectively for the pump intensity of 2.16 MW/cm2. Linewidth collapse around the threshold pump intensity is also illustrated in Fig. 6(c) which represents the normalized emission spectra corresponding to the glass substrate at different pump intensities. Fig. 6 constitutes direct evidence for the onset of random lasing action in the glass substrate, the same phenomenon as that of in the FTO substrate. The argument developed before is true in this case again. In spite of the similarity of the phenomenon, we interpret this finding (random lasing emission from the glass substrate) to be caused by light multi-scattering from the roughness of the glass substrate. The same observations involving surface roughness induced RL emission are also demonstrated by others [27–30]. To further verify the role of SU-8 resin, we study the pump dependent variation of emission spectra corresponding to the glass substrate coated with the solution of RhB dye dissolved in ethylene glycol (sample 3). The evolution of its emission spectra as a function of pump intensity is presented in Fig. 7(a). The broadband spontaneous emission of the gain medium with linewidth (FWHM) of ∼49.2 nm and the central wavelength of ∼592 nm are observed for the pump intensity of 2.16 MW/cm2. We see neither significant spectral narrowing nor drastic increase of the emitted intensity by increasing the pump intensity. The linewidth and central wavelength of the emission spectrum are approximately 48.9 nm and 596.4 nm, respectively at the pump intensity of 33.57 MW/cm2. Obtained results confirm the occurrence of
photoluminescence emission (PL) from the glass substrate coated with the solution of RhB dye in ethylene glycol. There is a red-shift in the emission wavelength in the sample containing SU-8 compared to the sample involving ethylene glycol. This may be because of higher refractive index of SU-8 compared to ethylene glycol. In addition, plots of peak emitted intensity versus pump intensity for the glass substrate coated with RhB dye doped SU-8 resin and RhB dye doped ethylene glycol are compared with each other in Fig. 7(b). In contrast to the case including the solution of RhB dye in ethylene glycol, a threshold behavior demonstrating the random lasing action is clearly seen for the case in which the glass substrate is coated with RhB dye doped SU-8 resin. It should be noted that when rhodamine dyes dissolved in alcohol alone are coated on the FTO substrate, no random lasing emission occurs as shown in our previous work [31]. Hence, notable differences existing between the emission spectra of the FTO and glass substrates in the cases involving organic dyes alone and organic dyes doped SU-8 resin as the gain medium, convince us to speculate that the adhesion properties of the cross-linked polymer network come into play and hence the scattering of light happens more strongly. The surface of a substrate is never truly smooth, consisting of several valleys and peaks. So the penetration of SU-8 polymeric solution in voids of the FTO or glass substrates results in the light multiple scattering induced random lasing emission in both cases. According to wetting theory which describes the adhesion phenomenon by implementing molecular contact and surface forces existing between two dissimilar surfaces, complete 5
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Fig. 7. (a) Variation of the emission spectrum as a function of pump intensity corresponding to the glass substrate coated with RhB dye doped ethylene glycol. (c) Peak emitted intensity versus pump intensity for the glass substrate coated with RhB dye doped SU-8 polymer and RhB dye doped ethylene glycol.
wetting is favored when the adhesive has a lower surface tension than the substrate [32,33]. Glass is known as a low surface energy substrate. In contrast, metallic surfaces possess high surface energies. As a result, it is expected that higher bond strength achieves in the case of metals being in connection with epoxy polymers. FTO is a notable n-semiconductor material which has a lot of conduction electrons. So we conclude that SU-8 resin exhibits excellent adhesion to the FTO surface, in comparison with the glass surface. This phenomenon leads to a strong joint as well as more complete filling of the voids or pores of the FTO surface. When light encounters scattering centers (surface roughness) with larger scattering cross sections, it will scatter more strongly. As a result, its dwell time and optical path length inside the gain medium increases. Consequently, more optical gain is provided and one can expect an increase in the peak emitted intensity and a decrease in the corresponding laser threshold. As the grains of the FTO layer are larger than the roughness of the glass substrate, its corresponding random lasing emission would then be more efficient, intensive and occurs at a lower laser threshold. Furthermore, due to larger refractive index of SU-8 resin compared to ethylene glycol, more waveguide confinement occurs in the case of dye doped SU-8 resin on the glass or FTO substrate and this effect can help lasing action in addition to multiscattering. The effects of waveguide formation on the laser action in the polymer based RLs have been reported in recent years [17,34]. Hence strong adhesion of SU8 resin to the glass or FTO in one hand and forming the waveguide confinement in the other hand lead to RL emission in samples including dye doped SU-8 resin as the gain medium compared with ones including dye alone. Therefore, obtained results in this paper provide inspiration for designing and manufacturing simple, cost effective and easily handled RLs based on dye doped SU-8 polymer as a potential candidate for the gain medium. In the following, we investigate the effects of total internal reflections (TIR) at the polymer/dye-air interface or waveguide confinement on the RL emission spectra and RL threshold corresponding to the FTO and glass substrates. Fig. 8(a) depicts the evolution of emission spectra corresponding to the suspended layer of dye doped SU-8 polymer alone (sample 4) as a function of pump intensity. The peak appearing at 532 nm corresponds to the excitation wavelength. The reason for appearance of this peak can be the reflection of the pump light from the sample holder and the surface of the sample. The emitted peak is observed around 635 nm in the emission spectra. However, the emission peak has lower intensity compared to one in sample 1 and 2 at the same pump intensity. In addition, a slow increase of the emitted intensity versus pump intensity is clearly seen in Fig. 8(b). After the pump intensity of approximately 16.5 MW/cm2, the increase of the emitted intensity versus pump intensity continues with a sudden positive change in the slope. The linewidth and the central wavelength of the
emitted peak are around 17.7 and 637.7 nm, respectively for the pump intensity of 15.15 MW/cm2. Also, the central wavelength and linewidth of the emitted peak are approximately 635 nm and 7.5 nm, respectively for the pump intensity of 30.65 MW/cm2. Regarding the linewidth value of the emission peak, amplified spontaneous emission (ASE) occurs in the suspended dye-polymer layer or sample 4. Obtained results demonstrate the occurrence of ASE in the layer of dye-polymer solution alone at a very high pump threshold of 16.5 MW/cm2. Similar observation was also reported based on a thin film layer of polymethyl methacrylate (PMMA) doped with 3-(1,1-dicyanoethenyl)-1-phenyl4,5-dihydro-1H-pyrazole (DCNP) dye [35]. We compare random lasing emission from the suspended layer of dye doped SU-8 polymer with that of FTO and glass substrates in Fig. 9. Consequently, the waveguide confinement alone cannot provide the optical feedback for random lasing emission in the SU-8 on the glass or FTO substrate and multiple scattering due to grain boundary of FTO and roughness of glass is necessary for lasing action. However, waveguide confinement facilitates the lasing action and decrease the laser threshold. It seems important to mention here that the RL emission spectra of the FTO and glass substrates are highly dependent on the excitation point and area on the sample. It is one of the specific features of a RL system. Hence, comparing RL emission characteristics under the same experimental conditions is of great importance. Hence, we have held the position of the optical fiber with respect to the sample as well as the pump area and position on the sample fixed during all measurements. Fig. 9(a) and (b) represent emission spectra and normalized emission spectra corresponding to the sample 1, 2 and 4 at the fixed pump intensity of 15.15 MW/cm2. Ripples existing in the emission spectrum of the suspended layer of dye dope SU-8 polymer are attributed to the background noise and intensity fluctuations of the pump light. One can observe that the random lasing emission from sample 1 is more intensive and has a narrower linewidth with respect to that of sample 2 and 4. The linewidths of the emitted peak are approximately 5.1, 6.1 and 17.7 nm for sample 1, 2 and 4, respectively. Plots of peak emitted intensity versus pump intensity corresponding to sample 1, 2 and 4 are also represented in Fig. 9(c). The respective laser thresholds are ∼1.7, 3.7 and 16.5 MW/cm2. It is obvious in Fig. 9(c) that random lasing action in the FTO substrate occurs at a lower laser threshold compared with that of the glass substrate and ASE in dye-polymer layer. Also, we notice the higher slope efficiency corresponding to random lasing operation in the FTO substrate. The present results suggest that the obtained low threshold and intensive random lasing emission in the FTO and glass substrates is caused by light multi-scattering from the grains of the FTO layer and the roughness of the glass substrate, respectively. Light scattering by either dye-polymer complexes or total internal reflections at the dye/polymer-air interface (polymer waveguide effect) 6
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Fig. 8. (a) Variation of the emission spectrum as a function of pump intensity corresponding to a layer of RhB dye doped SU-8 polymer alone. (b) Peak emitted intensity and linewidth (FWHM) corresponding to the layer of RhB dye doped SU-8 polymer alone versus pump intensity.
would then play less significant role in the lower threshold and more intensive random lasing emission from both the FTO and glass substrates.
glass substrate as the scattering media in proposed RLs. Obtained results demonstrate the onset of random lasing action in FTO layer as well as glass substrate. Due to larger grains of the FTO layer compared with the roughness of the glass surface, intensive random lasing emission is obtained at a lower threshold in the FTO layer. Our present experiments may regard as a possible starting point for the potential facile fabrication of intensive and low threshold RLs well-suited to be used in future RL based technologies.
4. Conclusion In summary, we have reported on the facile fabrication of RLs based on RhB dye doped SU-8 resin as the gain medium. Strong adhesion properties of the SU-8 resin enables employing the FTO layer and also
Fig. 9. (a) emission spectra and (b) Normalized emission spectra corresponding to the suspended layer of SU-8 polymer as well as FTO and glass substrates coated with RhB dye doped SU-8 polymer for the constant pump intensity of 15.15 MW/cm2. (c) Peak emitted intensity versus pump intensity corresponded to the suspended layer of SU-8 polymer as well as FTO and glass substrates coated with RhB dye doped SU-8 polymer. 7
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Appendix A. Supplementary material
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