1. Introduction

Since Lawandy et al. reported the laser action in strongly scattering media in 1994 [1], random lasers have attracted worldwide attention for their unique properties, such as randomness instead of mirrors, miniature in volume and simplicity in structures [2–4 ]. Several types of random lasers based on semiconductor nanoparticles [5,6 ], conjugated polymer films [7], organic dye-doped gel films [8], have been reported. Of particular interest, the metal nanoparticles or nanostructures provide the plasmonic scattering or plasmonic feedback, enhancing the electric field in the vicinity of the surface through localized surface plasmon resonance [9–12 ]. The physical effects accompanying light propagation in random media are important for practical engineering applications such as laser radars, biomedical imaging, remote sensing, communications, ranging and distance measuring and optical astronomy [13–17 ]. However, the development of biosensors and optical information processing requires the miniature of random lasers. A new kind of random laser was reported, performing as gain medium to provide both optical amplification and distributed feedback for trapping light [18,19 ]. A random distributed feedback fiber laser has efficiency and performance that are comparable to and even exceed those of similar conventional fiber laser [20, 21 ].

Recently, we demonstrated a tuneable plasmonic feedback random laser that consisted of randomly distributed Ag nanowires (NWs) in polydimethylsiloxane (PDMS) [22]. In this paper, we provided a new design that the plasmonic random fiber laser on an optical fiber facet. An optical fiber was dipped into the blender solution of PDMS, Ag NWs and Rhodamine (R6G), and then the plasmonic random laser on optical fiber end face was made after the cross-linking polymerization and drying processes. The method reported here offers a simple and straightforward way to fabricate plasmonic random lasers on the optical fiber facet. And the random laser can be pumped in a flexible way for the bendable properties of the optical fiber, showing the applications in telecommunications and distributed remote sensors.

2. Operating principle and experimental results

The uniform bicrystalline Ag nanowires were synthesized by a soft solution-phase approach [23, 24 ], and the Ag NWs in an ethanol solution was obtained with a 3.56 mg/mL concentration. The Ag NWs ink was the blender of Ag NWs and R6G ethanol solution (3 mg/mL) with a volume ratio of 1:10. Then the Ag nanowire ink and PDMS were mixed at a volume ratio of 1:5 in an ultrasonic tank (250 W, 40 kHz) for 10 minutes for homogeneity. An optical fiber with a fiber core diameter of 400 μm and a length of 10 cm was used as the substrate of the random laser. An end face of optical fiber was dipped into the blender solution of PDMS and Ag NWs ink, and then the plasmonic random laser on the optical fiber facet was fabricated after the cross-linking polymerization and drying processes.

Figure 1 shows the schematic and close-up view of the plasmonic random laser on the optical fiber facet. The PDMS layer forms a hemisphere coating on the optical fiber facet. The PDMS film is about 200 nm thick on the top of the hemisphere. The PDMS film doped with the Ag NWs and R6G is thick enough to provide the waveguiding plasmonic gain channel. The pump laser irradiates from the left and propagates in the optical fiber. Then the random laser emits to the right direction.

 

Fig. 1 (a) Schematic of a plasmonic random laser on the optical fiber facet. (b) Optical micrograph of the front view of the random laser on the optical fiber facet. (c) Optical micrograph of the side view of the random laser on the optical fiber facet. The scale bars represent 200 nm.

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Figure 2 shows the optical micrograph, SEM images of the plasmonic random laser on the optical fiber facet. The mean diameter and mean length of Ag NWs are about 70 nm and 10 μm, respectively. In Fig. 2(a), parts of the Ag NWs are obscure under the current depth of focus. The Ag NWs are immersed in the PDMS hemisphere and many ends of the Ag NWs stretch out in Fig. 2(b). And as shown in the Fig. 2 (c), an Ag NW embedded in the section of PDMS layer.

 

Fig. 2 (a) The optical micrograph of Ag NWs embedded in the PDMS film. The scale bar is 50 μm. (b) The SEM image of the side view of the random laser on the optical fiber facet. The scale bar indicates 500 μm. (c) Ag NW SEM micrograph on a section of the plasmonic random laser. The scale bar denotes 5 μm.

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Figure 3 shows the extinction of Ag NWs, the photoluminescence spectra of the R6G, Ag NWs and R6G doped in PDMS, respectively. The extinction and photoluminescence spectra were measured using a spectrometer (Maya 2000 Pro, Ocean Optics). Generally, in order to achieve a random laser, the plasmonic resonance from the metal nanostructures should overlap the emission of the polymer in spectrum as largely as possible [9]. In this paper, a frequency-doubled neodymium-doped yttrium aluminum garnet (Nd: YAG) pulsed laser was employed as the pump source, with a wavelength of 532 nm, 30-ns laser pulses at a repetition rate of 10 Hz and with pulse energy up to 50 mJ. When irradiated by the pump laser, the R6G molecules absorbed the photons of scattering pump light. The radiated photons of dye are observed as photoluminescence spectrum. Lighted by the external pump laser, the plasmon resonance spectrum of Ag NWs enables strong pump light scattering inside the waveguide of PDMS film, improving the pump efficiency of the random laser.

 

Fig. 3 Extinction of Ag NWs, the photoluminescence spectra of the R6G, Ag NWs and R6G doped in PDMS, respectively.

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The R6G-doped PDMS film is the active waveguide layer to provide the plasmonic feedback mechanism. The Ag NWs are distributed in the active layer of PDMS, providing a three-dimensional (3D) plasmonic feedback. The pump laser light enters the active layer, and is scattered by the Ag NWs which are distributed in the PDMS film. Most of the scattered light is confined within the active waveguide layer, leading to high efficiency amplification. For the excitation of the localized plasmon resonance of the Ag NWs, the R6G dye is radiated and the photoluminescence spectrum is enhanced by several orders of magnitude.

The plasmonic random laser on the optical fiber facet is irradiated by the pump laser pulses with a radius of approximately 2 mm, which is focused to propagate in the fiber and pump the random laser. The random laser is measured by a Maya 2000 Pro spectrometer. The Spectra, intensity and full width at half maximum (FWHM) of the random laser emissions at various pump power densities are shown in Fig. 4 . The emitted random laser is centered at approximately 571 nm, with FWHM of about 1 nm, working in the resonant feedback regime [23]. The line width of emission in this paper is smaller than that of our previous report [22]. However, the emission from the R6G-dopped PMMA nanofiber has a linewidth of 0.3nm [25]. The difference of the line widths reported is related to the different geometry of the waveguide assisted amplified spontaneous emission, which directly affects the spectral properties of the resulting optical amplification [26]. In this paper, the plasmonic random laser on the optical fiber facet has a pump threshold of about 4.2 nJ/cm². The interaction between the plasmon resonance of Ag NWs and the photoluminescence spectrum of R6G provides the low threshold of random laser on the optical fiber facet.

 

Fig. 4 (a) Spectra of the random laser emissions at various pump power densities. (b) The intensity and FWHM of the output laser as a function of the pump power density, indicating a pump threshold of about 4.2 nJ/cm².

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Generally, the thickness of waveguiding plasmonic gain channel has an influence on the intensity of the emitted random laser. In Fig. 1, the PDMS layer forms a hemisphere coating on the optical fiber end face, and the thickness of waveguide layer gradually decreases from the top to the edge of the hemisphere. In Fig. 5(a) , $\alpha$ is defined as the angle between the receiving direction of intensity detector and the axis of the plasmonic random laser. The intensity of the emitted random laser is the largest when $\alpha$ equals to 0°, as shown in Fig. 5(b). For example, the percentage of the emitted intensity in 45° direction is about 36% compared to that of the zero direction. Generally, the random laser emits without intensity preferred. In this paper, this configuration works as a lens and the most output energy of random laser concentrates in a small angle range along the axis of the optical fiber. So this plasmonic random laser on the optical fiber facet provides a design of controlling the emission directions of random laser.

 

Fig. 5 (a) Schematic of the the intensity measurement at different angles. (b) The intensity of the output laser as a function of the angle.

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3. Conclusion

A plasmonic random laser on the optical fiber facet was fabricated based on a waveguiding plasmonic gain channel. The R6G-doped PDMS film is the active waveguide layer, and the Ag NWs strongly scatter the pump light inside the PDMS film, providing a 3D plasmonic feedback. The plasmon resonance of Ag NWs reduced the threshold of the random laser significantly, achieving a pump threshold of about 4.2 nJ/cm². The plasmonic random laser on the fiber facet has a hemispherical shape, working as a lens and concentrating the most emitted intensity in a small angel range along the axis of the optical fiber. Thus, this kind of configuration provides an efficient way to obtain the directional output of random lasers. This work not only demonstrates a simple and straightforward method for fabricating a plasmonic random laser on fiber facet but also proposes an efficient way to control the directional output from such random lasers. The individual random lasers may serve as pixels in an illumination system and could be addressed individually for both spatial and color effects.