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We investigate the emission spectra from the edge of optically pumped waveguide. The waveguide is based on vacuum-deposited thin films of small molecular, 2,5,2',5′-tetrakis(2,2-diphenylvinyl)biphenyl (TDPVBi). Narrowed emissions are observed both at high (> 6 KW/cm2) and low (< 1 W/cm2) pump power sources, which are attributed to two different propagation modes in the asymmetric slab waveguides, guided mode and cutoff mode, respectively. The peak wavelengths of the guided mode appear at the maximum of the photoluminescence (PL) spectrum of the TDPVBi film. In contrast, both the peak wavelength and polarization of the cutoff mode are thickness dependent. The optical gains of the two modes are measured by the variable stripe length (VSL) method. The amplification with an exceptional low threshold for the cutoff mode has been demonstrated. Our results suggest that the cutoff mode is a promising route for the reduction of lasing threshold.
©2011 Optical Society of America
1. Introduction
Since the first report of optically pumped lasing from polymers in the solid state [1–3], amplified spontaneous emission (ASE) and lasing action have been demonstrated from many organic materials [2–5]. To reduce ASE threshold and obtain optical amplification, many types of structures have been used, for example, asymmetric slab waveguide [2,6], one-dimensional distributed feedback [7], two dimensional distributed feedback [8–10], microdisk [11], microdroplet [12], microring [13], and distributed bragg reflectors [11] and microgoblet [14]. For the asymmetric slab waveguide structure, the optical amplification generally occurs in the guided mode [2,6]. Recently, the optical gain has been reported in a special mode in the asymmetric slab waveguide [15,16]. The propagating wavelength of the special mode is close to the cutoff wavelength of the asymmetric slab waveguide, thus is termed as cutoff mode [17,18]. Up to date, all the works about the ASE in the asymmetric slab waveguide are either the guided modes or the cutoff modes, there is no work that reports the optical gain in both the guided mode and cutoff mode, simultaneously.
In this work, we simultaneously observed the optical amplification behaviors of the guided mode and cutoff mode in the same sample, and investigated them in detail. The small molecular of 2,5,2',5′-tetrakis(2,2-diphenylvinyl)biphenyl (TDPVBi) [19] was used as the core layer of the asymmetric slab waveguide structure and its molecular structure is shown in the insert of Fig. 2 . The biphenyl core which is a relatively free rotation centre can provide proper flexibility, and the large substituted groups make the rotation of the biphenyl core partly restrained. Thus, the flexibility benefits the molecule to form a dense film during vacuum-depositing process. Meanwhile, the TDPVBi film shows enhanced solid-state fluorescence quantum efficiency (80%) due to the aggregation-induced emission (AIE) phenomena [20].
Fig. 2 Absorption and photoluminescence spectra of TDPVBi film. The narrowed spectra of a waveguide with a 109 nm-thick TDPVBi film at high (green line) and low (blue line) pump power sources are also shown.Insert: the molecule structure of the TDPVBi.
2. Experimental Section
The TDPVBi films, with the thickness ranging from 50 to 180 nm, were thermal evaporation on cleaned glass substrates in a vacuum of < 5 × 10−4 Pa. To perform the edge emission measurement, a Nd:YAG laser (λ = 355nm, pulse width: 10ns, repetition rate: 10Hz) and a pulse diode laser (λ = 375 nm, pulse width: 65 ps, repetition rate: 10 MHz) were used for the high (>6 KW/cm2) and low (< 1W/cm2) power excitation sources, respectively. The excitation power was adjusted by a set of neutral density filters. A cylindrical lens and an adjustable slit were used to shape the pump beam into a stripe with a width of 0.5 mm and a varied length. The edge emission of samples was directly collected by an optical fiber (Ф1mm without the lens) which was connected to the spectrometer (Maya2000-pro, Ocean Optics).
3. Results and discussion
The TDPVBi film (n~1.8) together with the glass substrate (n~1.5) on one side and air (n~1) on the other side forms an asymmetric slab waveguide. Figure 1 shows the scheme of the waveguide in which ligh t propagates in the two optical modes, guided mode (green line) and cutoff mode (blue line). If the incident angle of the propagating light is larger than the critical angle θc of the interface between the TDPVBi film and glass substrate, the light can propagate inside the core layer without leakage. This is the guided mode in which the peak wavelength of the narrowed spectrum should locate at the maximum of the gain, generally, the maximum of the photoluminescence (PL) spectrum of the TDPVBi film. On the other hand, if the incident angle is slightly smaller than the θc, some of the light will be leaked into the glass substrate and propagates in a direction nearly parallel to the interface between the TDPVBi film and glass substrate. This is the cutoff mode in which the peak wavelength of the narrowed spectrum is very close to the cutoff wavelength of the waveguide. The cutoff wavelength can be calculated by the following equations [15]:
Fig. 1 Side elevation of the waveguide structure in which light propagates in the guided (green line) and cutoff mode (blue line). θc is the critical angle at the TDPVBi/glass interface.
Where d is the thickness of the TDPVBi film, and n1, n2,and n3 are the refractive index of the glass, TDPVBi film and air, respectively, m is the mode number (m = 0, 1, 2,…).
The absorption spectrum of TDPVBi film is shown in Fig. 2. We can see it has one peak located at 364 nm. This suggests the lasers both at λ = 355 nm and λ = 375nm are suitable for optical excitation. We have observed two different narrowed emission spectra in the sample with a 109 nm-thick TDPVBi film (see Fig. 2), and the PL spectrum of the TDPVBi film is as the reference. The Nd:YAG laser and the diode laser were used as high (>6 KW/cm2, green line) and low (< 1W/cm2, blue line) pump power sources, respectively. The peak of the narrowed emission excited by the high pump power source is at 488 nm, corresponding to the maximum of the PL spectrum of the TDPVBi film. On the other hand, the narrowed emission excited by the low pump power source has two peaks locating at 473 nm and 599 nm corresponding to the TM and TE mode, respectively.
The peak wavelengths of the narrowed spectra excited by the high and low pump power sources versus the thickness of the TDPVBi film are plotted in Fig. 3 (a) and (b) , respectively. The calculated cutoff wavelengths by Eqs. (1) and (2) are also shown in Fig. 3 (b). For the high pump power source, the peak wavelengths locate between 486nm and 492 nm, which is near the PL maximum, λ = 488 nm. This can be understood in terms of the guided mode. The peak wavelength of the narrowed emission is determined by the wavelength with the highest density of states, that is, at the peak of the PL spectrum. On the other hand, for the low pump power source, both the peak wavelengths of TE and TM agree well with the calculated cutoff wavelength, and depend on the thickness of the TDPVBi film, thus should be ascribed to the cutoff modeo
Fig. 3 Peak wavelengths of the narrowed spectra for the guided mode and cutoff mode as a function of the TDPVBi film thickness, the calculated cutoff wavelengths (blue line) are also shown.
The emissions spectra of the two modes as a function of the viewing angle were measured. Figure 4 shows the dependence of the FWHMs of the guided mode (green squares) and cutoff mode (blue squares) on the viewing angle θ in the range from −10° to 15°. The FWHMs of the guided mode is kept constant with the θ between −6°and 8°and gradually increase at larger θ. This indicates the guided mode is only observed in a small range of the θ, and for the larger θ, the observed spectra are not from the guided mode but the leaky mode. On the other hand, the FWHMs of cutoff mode show a minimum value of 9 nm at 0°and increase to 100 nm at higher angle.
Fig. 4 FWHMs of the narrowed spectra of the sample with a 109-nm-thick TDPVBi film for the guided mode (green squares) and cutoff mode (blue squares) as a function of the viewing angle θ.
Figure 5 shows the dependence of the FWHMs of the narrowed spectra for the guided mode (green squares) and cutoff mode (blue squares) on the pump power. For the guided mode, the FWHMs decrease with the pump power and show a well defined threshold of ~6 KW/cm2. For the cutoff mode, the FWHMs are pinned at 16 nm even at the pump power down to 200 mW/cm2, and we observed no clear threshold characteristic within the whole measuring range.
Fig. 5 FWHMs of the narrowed spectra of the sample with a 109-nm-thick TDPVBi film for the guided mode (green squares) and cutoff mode (blue squares) as the function of the pump power.
To investigate the optical amplification of the narrowed emissions from the edge of the waveguide structure, we measured the optical gain by using the variable stripe length (VSL) method [6]. Figure 6(a) shows the dependence of the peak intensities of the guided mode (green squares) and cutoff mode (blue squares) on the pump stripe length. Both the intensities of the two modes show a superlinear increase, which is a direct evidence of the presence of the positive gain. We fit the curves by the following equation:
Where I is the peak intensity, AP0 is the spontaneous emission intensity which is proportional to the pump intensity P0, g is the net gain, and l is the pump stripe length. The net gain g of the guided mode is extracted to be 18.5 cm−1 at high pump power of 12 KW/cm2 and that of the cutoff mode is 9.4 cm−1 at low pump power of 461 mW/cm2.Fig. 6 (a) Dependence of the peak intensities of the narrowed spectra for the guided mode (green squares) and cutoff mode (blue squares) on the pump stripe length, the TDPVBi film thickness of the sample is 109 nm. (b) Dependence of the peak intensities of the narrowed spectra for the guided mode (green squares) and cutoff mode (blue squares) on the distance between the pump stripe and the edge of the sample
The exceptional low pump power (0.46 W/cm2) for the optical gain in the cutoff mode may be attributed to the cavity enhancement effect [15]. The incident angle of light in the cutoff mode is slightly smaller than the critical angle between the TDPVBi film and glass substrate, in such a case, the reflection at this interface is close to 100%. On the other hand, the reflection at the interface between TDPVBi and air should be 100% due to total reflection at this interface. Then a high-Q cavity composed of the two interfaces is formed. Therefore the light in the cutoff mode can be amplified in the cavity when it ropagates in the direction nearly parallel to the interface between the TDPVBi film and glass substrate.
To fully characterize the two different modes, we measured the loss coefficient α for both modes in the sample with a 109 nm-thick TDPVBi film. The pump stripe is kept constant and moves away from the edge of the sample. Assuming that the emission from the end of the pump stripe I0 is constant, the emission from the edge of sample should decrease as , where x is the distance between the end of the pump stripe and the end of the sample, and the α is the loss coefficient. Figure 6(b) shows the dependence of the peak intensity on the x. By fitting the curves, we obtained the loss coefficient of only 6.8 cm−1 for the cutoff mode, compared to 18.6 cm−1 for the guided mode. The exceptional low pump power for the optical gain in the cutoff mode can partly contributed to the smaller loss.
In conclusion, the narrowed emissions from both the guided mode and cutoff mode were observed under high and low pump power sources, respectively. The characteristics of the two modes for the same sample were compared in detail. The threshold and loss coefficient of the cutoff mode are much lower than those of the guided mode. The optical gain of the cutoff mode can be obtained at an exceptional low pump power of less than 1 W/cm2, which is one of the lowest values ever reported. It is believed that utilizing the cutoff mode is a promising route to reduce the threshold of the electric injection laser in future.
Acknowledgments
We are grateful for financial support from National Natural Science Foundation of China (grant numbers 60878013).
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