Compared with conventional lasers, the random laser is realized through strong multiple scatterings in disordered gain system. In this paper, random lasing (RL) in one-dimensional metal surface plasmon (SP) waveguide with gold-plated self-formed silicon pyramids was investigated comprehensively. Consequently, the emission intensity of RL was enhanced dramatically and the RL threshold was reduced significantly through Au-coated Si spiky tips. Meanwhile, one-dimensional metal SP channel waveguides confined the emitting light in a certain direction with a small divergence angle. Using FDTD simulations, it was found that the enhancement effect for RL is likely attributed to the localized surface plasmon (LSP) field. In addition, the LSP field nearby the spiky tips can enhance field-molecule interaction, which was benefit for lasing in small scale. The results in this letter supplied a feasible method to realize the application of RL in subwavelength optical elements.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Unlike conventional lasers that require a resonator cavity composed of two aligned mirrors to create coherent optical feedback, random lasers (RLs) require a different feedback mechanism, for which the feedback is provided by multiple scattering [1,2]. Multiple scattering is a general phenomenon in daily life, so random lasers have attracted plenty of attention in the past two decades [2–4]. Random lasers were predicted by Letokhov  in 1968 and were first reported by Lawandy et al.  Then Cao et al. observed random lasing emission in disordered powder media [7,8]. Due to the unique properties and potential applications such as speckle-free full-field imaging and biological probe , several results about the applications of RL have been reported [10–17].
Generally, the output direction of RL is usually uncontrollable and presents a large divergence angle, which partly restricts its practical application in some degree . Recently, optical waveguide structure was proposed to confine the emission light in a stripe, which was expected to overcome the weakness of large divergence angle [19–22]. Although the RL in subwavelength scale will undergo a high lasing threshold, fortunately the strong localized field enhancement effect can provide a solution to reduce the threshold substantially .
In this letter, it was demonstrated that enhanced RL in one-dimensional channel waveguides can be realized through self-formed disordered Si pyramids coated with gold film. The feedback of RL was constructed by strong scattering from Si pyramid scatterers in waveguides. Meanwhile, the radiation was enhanced by the localized field at the Si/Au spiky tips through LSP effect. Our results also indicated that metal-coated channel waveguides can efficiently confine the emitting light in a small divergence angle for RLs. The proposed method based on gold-coated channel waveguides in the paper will promote the applications of RL in a wider field.
2. Experimental details
One-dimensional silicon channel waveguides were fabricated by wet etching. The width (depth) of waveguides was designed to be about 25 μm (15 μm), as shown in Fig. 1(a). It can be seen from scanning electron microscopy (SEM) that silicon pyramids are distributed randomly in waveguides, which can be attributed to the non-uniform wet etching process. The enlarged image of individual pyramid illustrates eight smooth top surfaces [Fig. 1(b)]. Detailed description about the fabrication process of channel waveguides are given in Supporting Information. Figure 1(c) displays the SEM image of cross-section for the channel waveguide, which exhibits a typical ladder-like shape. To reduce the absorption loss from silicon template, a silicon oxide interlayer was inserted. Rhodamine 6G (R6G) (Alfa Aesar) dissolved in SU-8 (Micro Chem) was used as gain media, and the concentration of R6G/SU-8 mixture was controlled to be 5 mM (mmol/L). The composite gain media were spin-coated onto the Si plate and etched channel waveguides at the speed of 3000 rp/s respectively. The thickness of the active media film amounts to ~5 μm for each sample. For comparability, all the samples were prepared on the same condition. The schematic diagram of RL measurement is illustrated in Fig. 1(d), a 532 nm pulse light from Q-switched frequency-doubled Nd:YAG laser (repetition rate 5 Hz, pulse duration 17 ns) was used to excite samples. The pumping beam was focused on samples in stripe shape by a cylindrical lens. The width of focused stripe is about 20 ± 5 μm, which approaches the width of a channel waveguide. The emission signal from the sample was collected by an ultraviolet objective lens (14 × ) and then analyzed by a Princeton spectrometer (Acton SP750i) equipped with a silicon charge-coupled device (CCD).
3. Results and discussions
For bare silicon plate that coated with gain media, the intensity of fluorescence peaks grows nonlinearly and the full-width at half-maximum (FWHM) of peaks narrows with increasing of the excitation power from 132 to 616 kW cm−2 [Fig. 1(e)]. Although a kink behavior was observed from light-in versus light-out (L-L) curve of active film [Fig. S1], the distinct lasing spikes were absence in spectra. In contrast, the emissions of R6G on silicon channel waveguides show different spectrum characteristics. As the exciting power rises, the dominated spontaneous emission band in spectra is replaced by a series of separated narrow spikes with full width at half maximum (FWHM) less than 0.5 nm. It is noted that the number of these sharp spikes is enhanced dramatically with increasing of excitation power. These features above are consistent with random lasing emission characteristics [Fig. 1(f)] [2,7,18]. The intensity of spikes as a function of exciting power displays a superlinear behavior [Fig. 1(g)] and the FWHM of peaks is decreased dramatically from 15 nm to 0.1 nm. From the L-L curve of the sample, a pumping threshold (Pth) of about 390 kW cm−2 is extracted. The emergence of sharp spikes above the kink-point of L-L curve confirms that the channel waveguide supports lasing actions. Considering the disordered silicon pyramids in the channel, the optical feedback is resulted from strong multiple scatterings induced by it [24,25]. Since the shape and size of localized paths for photons are random, a series of distinct lasing modes can be observed in the spectra.
Generally, the lasing threshold will increase significantly as the scale of gain media reduces gradually. Although random lasing modes have been observed from the bare channel waveguide structure with thick gain film [Fig. 1(f)], no lasing spikes are detected from the same structure as thickness of gain media is less than 2 μm due to the large radiation losses . For the thinner active film on bare channel waveguide, the emission peak was not observed until the pumping power is larger than 540 kW cm−2 [Fig. 2(a)]. The L-L curve shows that the pumping threshold for the thinner active film in bare channel waveguide is as high as 540 kW cm−2 [Fig. 2(d)]. In order to enhance the optical field confinement and radiation efficiency, the localized surface plasmon (LSP) is proposed and the lasing in subwavelength scale based on metal-coated channel waveguides has also been reported . Here, a uniform 20 nm-thick metal film (Ag and Au) was deposited onto Si channel waveguides structure by electron beam evaporator. Furthermore, a thin silicon-dioxide dielectric interlayer was introduced to suppress the metal internal loss . Subsequently, the thickness of spin-coated R6G/SU-8 composite films on the waveguides was controlled to be about 2 μm. It is noted that the emission spectra of R6G on silver-plated waveguide exhibits a typical lasing action, as shown in Fig. 2(b). As pumping power density rises from 380 to 616 kW cm−2, the broad spontaneous emission band is replaced by a few dominated sharp spikes  and the FWHM of the peaks decreases from 7 to 0.2 nm. Particularly, the emission intensity of Ag-coated sample is about 1.2 times magnitude than that of the bare waveguide structure. Furthermore, the lasing threshold Pth is reduced to 460 kW cm−2 as shown in the L-L curve [Fig. 2(e)], which is smaller than that of the bare sample. Although the Ag can enhance the radiation efficiency of R6G, the intrinsic emission band of sample (centered at 575 nm) is not located at the optimum LSP resonance peak of Ag. In order to enhance radiation efficiency further, Au is proposed due to its LSP resonance peak locates at about 550 nm.
The lasing behavior of Au-coated structures with the same thickness of gain media was investigated in detail. Figure 2(c) plots the typical room-temperature emission characteristics of the Au-coated sample under the excitation of pulse laser. A broad spontaneous emission band around 576 nm can be seen at low excitation intensity. However, some narrow emission peaks with FWHM less than 0.8 nm superimpose on the spontaneous emission band when the pump density is larger than 420 kW cm−2. With increasing of excitation density further, the intensity of spikes increases superlinearly and the linewidth decreases to 0.1 nm. Meanwhile, more narrow spikes occur in the lasing spectra. Surprisingly, the emission intensity is enlarged 7 times than that of Ag-plated sample and the threshold at kink point is extracted to be 400 kW cm−2 from L-L curve of the sample [Fig. 2(f)]. In addition, the fast Fourier transform (FFT) of lasing spectra was used to analyze the formation of localized random cavities. The localized cavity length Lc in metal surface-plasmon waveguide was extracted to be about 28 μm, which is longer than the bare channel waveguide structure (15 μm). Above result indicates that localized surface-plasmon resonance at Au-coated tips is benefit to low-threshold random lasing in the channel waveguide. Given the above, Au is the better candidate than Ag to realize the radiation enhancement of R6G. The unique properties such as low-threshold and strong emission intensity induced by gold will be benefit to practical applications of RL.
Since the spatial radiation pattern is a critical characteristic of RL for applications, the near- and far-field emission patterns of samples have been measured. CCD images of near-field patterns (NFP) for emissions from the end facet of bare Si waveguide excited by different intensities were given in Fig. 3(a). Under low excitation condition (0.5 Pth), weak spontaneous emissions could hardly be observed. The green background light was originated from the residual scattering signals of excited laser. As the pumping power exceeded the threshold (1.5 Pth), a bright emission spot emerged in the image (bottom part of Fig. 3a). Meanwhile, the far-field pattern of RL emission presented a Gaussian-type spatial distribution above threshold [Fig. 3(b)].
Compared with bare Si waveguides, the near-field photograph of Au-coated channel waveguide exhibited a fine spot at low exciting power (0.5 Pth), as shown in the top part of Fig. 3(c). Furthermore, a brighter speckle with weak green background light could be seen from the facet of the waveguide as pumping power raised to 1.5 Pth [bottom part of Fig. 3(c)]. Accordingly, a Gaussian-type spatial distribution of the far-field pattern for RL based on metal coated waveguide was displayed clearly in Fig. 3(d). The linewidth of peak for metal-plated channel waveguide was much narrower than that of bare waveguide, which indicated that the RL from metal SP waveguide has better coherence.
In addition, the dependence of emission intensity on angles was examined to investigate the divergence angle of RL from channel waveguide. In this experiment, a collection fiber with diameter of 200 μm was placed towards the output facet of one of the channels. And the fiber was connected to Ocean Optics spectrometer, then it was moving along the direction of x-axis to record the spectra [inset of Fig. 3(e)]. It was found that the divergence angle is about 4° and 6° for metal SP and bare waveguide in x-axis respectively [Fig. 3(e)]. Here, the intensity of spectra has been normalized at the angle of 0°. For the direction of y-axis, the divergence angle is about 9° for both metal SP and bare waveguide [Fig. 3(f)]. Hence, it was demonstrated that the metal coated channel waveguide can effectively confine the propagation of light in one dimension with a small divergence angle.
To explore the essential mechanism of RL through LSP, FDTD simulations had been carried out. The excitation source light (at 532nm) incident onto the Si plate normally. For the bare plate structure (with/with Au coating) and channel waveguide without Si pyramids, it was found that no localize spatial optical fields was constructed at the interface between metal and gain media [Fig. S4 and Fig. S5]. Figure 4(a) describes the schematic modal for the calculation, where the tips represent the disordered Si pyramids in waveguide and the interlayer indicate the silicon-dioxide dielectric material. In the longitudinal direction (z-axis), only part of light can propagate in the waveguide, which is revealed by the fuzzy optical field as shown in Fig. 4(b). The cross-section image (from x-y plane) indicates the confinement effect of bare channel waveguide is very weak, only a few residual lights can be reserved at the corner of waveguide [Fig. 4(c)]. The enlarged pattern of individual Si tip illustrates precisely the optical field distribution nearby the tip [Fig. 4(d)]. It is obviously that the localized field is absence near the tips without metal.
In contrast, the Au-plated channel waveguide can support confinement for the optical field [Fig. 4(e)]. It is shown clearly that long-range propagating modes will be constructed at the bottom of waveguide as illustrated in Fig. 4(f). In other words, disordered Au-coated tips can supply effective scattering feedback for RL along z-axis in channel waveguide. On the other hand, the optical field confined in the metal SP waveguide can also be revealed in the cross-section of the structure [Fig. 4(g)]. Although a portion of light is scattered into free space, part of the optical field associated with local surface plasmon still conserved nearby the disordered Au-coated tips. Figure 4(h) presents the simulation result of the optical field distribution nearby individual Au-coated tip. In comparison with bare Si pyramids, Au-plated structures have significant LSP fields on tips [30,31], which leads to a remarkable increase on the emission intensity [32,33]. The strong LSP field is considered to provide localization enhancement for gain media so the radiation efficiency is largely increased and lasing threshold will be decreased simultaneously [34–36].
In summary, we demonstrate the enhanced RL in one-dimensional metal surface plasmon channel waveguide with gold-coated self-formed disordered silicon pyramids. The randomly distributed Si tips provide strong scatterings which form localized paths for light stimulated amplification. Compared with the bare waveguide, the emission intensity of R6G in Au-plated structure is enhanced by one order of magnitude. According to the FDTD simulations, the low lasing threshold is benefit from localized surface-plasmon enhancement nearby Au/Si tips. Moreover, the one-dimensional channel waveguide can confine the propagation of stimulated emitting effectively with a small divergence angle (5°). Our results present a feasible method to enhance RL in microscale, which will promote the applications of RL in broader areas.
The Supporting Information is available free of charge on the website. Experimental details about the fabrication of waveguide and random lasing properties were given.
National Natural Science Foundation of China (NO. 61504172, NO. 51232009, NO. 61574063); Guangdong Natural Science Funds for Distinguished Young Scholars (NO. 2016A030306044); Science and Technology Program of Guangzhou (No. 201707020014).
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