1. Introduction

Since the demonstrations of the semiconducting (conjugated) polymer light emitting diodes [1, 2], semiconducting (conjugated) polymer also draws lots of attentions as promising gain materials in lasing application [3, 4]. Lasing emission from solid films containing semiconducting polymer can be realized using scattering off random nanostructures [5, 6], Fabry-Perot cavity [7], distributed feedback (DFB) structures [8, 9], photonic crystals [10, 11], plasmonic nanostructures [12]. The main target in this area is to realize electrically pumped organic semiconductor laser, in which the trade of between the high conductivity and high quantum efficiency of the organic semiconductors needs to be well balanced [13]. In a planar wave guide, consisting of organic semiconductor polymer sandwiched by air and glass substrate cladding layers, the Q factor is very low. Gain narrowing phenomena were observed in the amplified spontaneous emission (ASE) process [14]. ASE is very similar to lasing phenomenon; both of the two physical processes have the typical characteristics: gain narrowing, directional emission and threshold of pumping energy. However, the main difference is the coherence length of ASE is much shorter than that of laser emission [14]. ASE has been extensively used to investigate the gain and absorption coefficients of active materials [15–18].

In this work, we demonstrate a spatially variant color light source by using the directional ASE processes from a cascaded organic thin films, consisting of blue and green emission organic semiconductors. When the cascaded films were pumped by a pulsed UV laser (λ = 355 nm), the directional ASE signals (blue and green light) from the edges of two cascaded thin films are projected to far field with partially spatial overlap. Then the fiber coupled detector is used to measure the position dependent the emission spectra of the thin film light source. It is shown that the color of light emission can be tuned almost linearly in the CIE coordinates from (0.42, 0.55) to (0.18, 0.11) by spatially gating the emission signals. It means the position sensitive color of thin film light source is from blue, to white and finally to green. Usually, the narrow band emission from conventional broadband light source can be realized by using grating, Fabry-Perot cavity, narrowband filters and so on. In comparison, continuous color emission from the ASE processes of cascaded organic thin film can be obtained by simply shifting the spatial position of the collection fiber. We can expect the method we show here can be used to dynamically modulate the polychrome emission of a thin film light source for display, bio-imaging and related applications.

2. Experimental details

Figure 1 show the working principle of the spatially variant color light source. The blue emission polymer PFO (Poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) and green emission polymer F8BT (Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo [2,1,3]thiadiazol-4,8-diyl)]) were used as the active materials in this work. These materials were bought from Sigma-Aldrich. The molecule weight and polydispersity index of PFO are Mw $\underset{¯}{\underset{¯}{<}}$ 20000 and ~ 3.7. The averaged molecule weight of F8BT is ~10000-20000 and polydispersity index is < 3. Both of them were dissolved in toluene solution with concentration of 16 mg/mL and 23 mg/mL respectively. The polymer films were fabricated by spin-coated onto the 1 mm thick pre-cleaned fused silica substrates with thickness of 120 nm and 250 nm, respectively. As shown in Fig. 1a, the cascaded thin film system includes PFO and F8BT, which are bounded together via 60 μm thick optically clear adhesive (OCA) [19]. The intention of adding the OCA is to totally separate the two materials preventing any energy transfer between the interface of PFO and F8BT. In addition, the cascaded thin films cannot be spin-coated sequentially as both materials are solved in the same solvent, toluene, and they will be mixed in the spin-coating processes. Most importantly, the refractive index of interfacial layer OCA and silica substrate is around 1.48, which is less than that of both PFO and F8BT [20]. Then both PFO and F8BT layers act as a planar waveguide, which is necessary for the generation of amplified spontaneous emission under optical pumping.

 

Fig. 1 (a) Cross-section of cascaded films with PFO (120 nm), F8BT (250 nm), optically clear adhesive (60 µm), and silica (1 mm); (b) ASE measurement configuration, the cascaded films is pumped with Nd:YAG laser (355 nm, 10 Hz) and the excitation area is 5 mm x 300 µm; and the ASE output is collected from the edge of the sample by the fiber coupled spectrometer.

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In the ASE experiment (Fig. 1b), the PFO/F8BT cascaded films were pumped by third harmonic generation of Nd:YAG laser at wavelength: λ = 355 nm with repetition rate of 10 Hz and pulse width of 5 ns. The laser beam is firstly diverged by a concave lens, and then squeezed into stripe beam by a cylindrical lens. Finally, the stripe beam was focused on to the device after cutting the non-uniform edges of the stripe laser spot using adjustable slits. The area size of the rectangular laser spot is 5 mm x 300 µm. The spontaneous emission and ASE signals from PFO and F8BT were collected from the edge of the cascaded thin film by using an objective lens and analyzed by fiber coupled spectrometer (Ocean Optics, USB 4000). In Fig. 1b, D1 and D2 represent the highest intensity positions of ASE from PFO and F8BT respectively. The pulse energy of pumping laser is monitored by a photodiode, then we can measure the power dependent ASE from the thin film light source.

3. Results and discussion

In Fig. 2, we characterized the transmission efficiency of the optical clear adhesive/silica, PFO/silica, F8BT/silica and silica/PFO/OCA/F8BT/silica cascaded films. It shows the OCA/silica device has a high transmittance for the pumping laser (355 nm). The transmission efficiency for both the PFO/silica and F8BT/silica samples is around 10% at the wavelength of pumping laser. However, the PFO/OCA/F8BT cascaded thin films is a good ultraviolet filter.

 

Fig. 2 Transmittance of (i) PFO/silica (120 nm, filled squares), (ii) F8BT/silica (250 nm, filled circles), (iii) Optically Clear Adhesive (60 µm, open squares) and (iv) Cascaded PFO (120 nm)/OCA/F8BT (250 nm) films sandwiched by two silica substrate (open circles).

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In ASE experiment, the stripe type Nd:YAG laser described in last section was normally incident onto the cascaded thin films from F8BT to PFO direction. In fact, the absorption efficiency of F8BT and PFO have been balanced by controlling film thickness, so it doesn't matter if the pumping laser is incident from PFO to F8BT direction. The ASE signals from the edges of PFO (ASE_PFO) and F8BT (ASE_F8BT) thin films are separated by the OCA at the beginning, then the two directional emissions have overlaps when they propagate to the far field which is determined by the numerical aperture of each planar waveguide. The ASE signals were collected by an objective lens (numerical aperture: 0.25) and imaged in the far field through a tube lens with focal length of 30 mm. The spatially variant color emission of ASE_PFO and ASE_F8BT signal was then relayed to the far field which are determined by the imaging system we used. We scanned the position of the collection fiber (600 micron in diameter) in the x direction and the ASE spectra under pumping density: 247 μJ /cm² is shown in Fig. 3(a).It can be found that the total ASE spectra strongly depend on the spatial location of the fiber tip. For example, at position D1, the ASE_PFO (peak position: 450 nm) is dominant while ASE_F8BT (peak position: 575 nm) is dominant at D2. The ASE intensities show an inverse proportion when fiber position deviates from D1 and D2 points. Figure 3(b) shows the power dependent ASE from PFO and F8BT respectively in cascaded films. At low pumping density, the spontaneous emission in both PFO and F8BT dominates the fluorescence signals. By increasing the pumping density, the emission spectra shows a typical threshold phenomena: 174 μJ /cm² for PFO and 203 μJ /cm² for F8BT. These threshold values are comparable to the previous reported results [8, 9].

 

Fig. 3 (a) Normalized ASE spectra in different positions from edge of the sample with same pumping energy density at 247 μJ/cm²; D1 and D2 are defined in Fig. 1, the white light represented the white light ASE with CIE (0.32, 0.35). (b) Energy dependence of PFO (circles) and F8BT (triangles), and symbols represent the peak value of the ASE spectra under various energy density; showing the clear threshold at 174 μJ/cm² and 203 μJ/cm² respectively.

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The color of the output ASE changes when the collection fiber is moving from D1 to D2. We can use Eq. (1) to calculate the Commission Internationale d’Eclairage (CIE) coordinates [21] in color space based on the measured spectra.

As shown in Fig. 4, the circle symbols represent the CIE coordinate of the specific spectrum. It is found that the CIE coordinates can be continuously tuned from (0.42, 0.55) to (0.18, 0.11) with a nearly linear relationship. The synthesized color of the ASE emission from cascaded organic thin films could be tuned to CIE coordinates at (0.32, 0.35), which is very close to the center of CIE coordinates (0.33, 0.33) representing a white light emission.

 

Fig. 4 The color coordinates (circle symbols) mapping onto the CIE chart; the linear relationship of CIE shift is observed when fiber position is moved to D1 to D2 position.

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4. Conclusion

We have demonstrated a spatially variant color light source made of cascaded organic semiconducting films. The amplified spontaneous emissions from the two organic thin films can be simultaneously obtained by a single pumping laser beam. The directional output of ASE signals from the cascaded organic thin films forms a position dependent color emission, with CIE coordinates being continuously tuned from blue to green color and passing through the center (white light). Such kind of device has great potential in providing polychrome light source in display, bio-imaging applications, etc. For practical application, the lifetime of the color light source depends on the pumping density and surrounding environment.