ASE from polymer films embedded with silver nanowires

Silver nanowires (AgNWs) were developed on glass substrate using polyol method at 160 °C. Centrifuge at 1500 rpm for different ratio of ethanol was done to extract the AgNWs. Rhodamine 6G (R6G) was then applied at different thicknesses. Enhanced emission from R6G was observed when the concentrations used are at 40 µL and 20 µL. Overall results indicate promising potential for enhancing plasmon resonances in optical emission by utilising silver nanowires as scatterers and R6G as the gain medium.


Introduction
A random laser is a type of laser that occurs when photons remain in a semiconductor long enough to achieve gain without the use of mirrors.The photons are contained by a set of mirrors in a conventional laser configuration.However, a random laser has no reflectors.They confine photons by maintaining internal scattering from nanowire structures long enough within the gain medium for photons to achieve gain for lasing.This is possible by altering the refractive index within the gain medium; in this case using silver nanowires (AgNWs).Due to the optical cavity provided by nanowires, lasing is achievable despite the limited quantity of gain material.Since its first presentation in 2001, significant scientific effort has been put into the development of semiconductor nanostructures, which are sometimes referred to as nanowire lasers [1].Polymer films containing nanoparticles of silver and weakly scattering polymers have produced random lasers having coherent feedback [2,3].A random laser was made by placing silver nanoflowers on a polymer membrane and placing the membrane on a flexible substrate [4].Using a fluoresecent laser dye (especially DCM)-doped polymer sheet with Zinc oxide (ZnO) micro-cabbages as scatterers, a system with low-threshold continuous-wave random laser is reported [5].It was reported that the threshold for random lasing may be greatly lowered by using a plasmonic hybrid structure.An example is using gold-silica (Au core-SiO2) shell nanocubes coated on silver (Ag) film (Au@SiO2 NCs-Ag film) [6].However, all these works rely on liquid-based samples.In this paper, solid-state gain medium of Rhodamine 6G with the AgNWs is attempted towards making a solid-based random laser device suitable for integration with current laser systems.
In this work, AgNWs was synthesized with a polyol method.The structural and optical properties of AgNWs coated with R6G as the gain medium are discussed.The AgNWs developed using polyols as a simple, rapid, and environmentally friendly approach is proposed.This method produces AgNWs with substantial aspect ratio.For the purpose of finding the best gain medium thickness, reaction parameters on the synthesis process, such as temperature range, molecular of the polyvinylpyrrolidone (PVP) molecular weight, Sodium chloride (NaCl) concentration, and mixing rate of the solution, were kept constant in the preparation of the silver nanowires.The goal of this study was to investigate the effect of the dye amount thickness prepared with same concentration of dye drop-casted on to AgNWs on the emission of R6G.

Preparation Method
The Ag NWs was synthesized using a modified polyol procedure.Solutions of 0.45 molarity Polyvinylpyrrolidone (PVP), 0.2 molarity Sodium chloride (NaCl), 0.2 molarity Potassium bromide (KBr), as well as 0.095 molarity of silver nitrate (AgNO3) were prepared.In a 50 mL flask, 12 mL of ethylene glycol (EG) was added and warmed in an oil bath for 10 minutes prior to the addition of 0.02 g of NaCl, and 0.04 g of KBr.Then with magnetic stirring at 250 rpm, 0.35 g of the PVP was added.After 10 minutes of gentle stirring, g of the silver nitrate was added into the solution.After the completion of the added, the reaction was kept till the color was changed to a bluish green.Deionized water (DI) was used to dilute the samples at a ratio of 1:1 and left to cool at room temperature.The speed of centrifuge was at 1500 rpm to purify the AgNWs whereby the heavy particles were precipitate, and the light nanowires would be on the top of the liquid (supernatant).The supernatant was washed 3 times with acetone as well as ethanol before being reconstituted in 10 ml of ethanol for further analysis.
The Ag nanowire suspended in ethanol is coated on glass substrate and dried on a hot plate at approximately 60 ºC.Next, R6G prepared in ethanol with concentration of 2.088mM was mixed with (4.4 mg/ml) of polyvinylpyrrolidone polymer (PVP) and coated on the glass substrates above the silver nanowires at different thicknesses corresponds to each concentration of 20µL to 120µL and left to dry at room temperature for 24 hours.
For information on morphology and structural characteristics on the sample, field emission scanning electron microscope (FESEM) measurements were conducted.The Optical properties of R6G on AgNWs were investigated by photoluminescence measurements.A 355 nm Nd:YAG laser source with 0.6 ns pulse width and 20 kHz repetition rate was employed to excite the samples.The emission was recorded using Ocean optics fibre spectrometer and displayed the spectrum was using standard Ocean View software on standard PC.

Morphology and structural characteristics
Fig. 1. shows FESEM images of the surface form for all samples with different amount of R6G drop casted on the AgNWs.The average diameter and length of AgNWs were analysed using ImageJ software.The average length and diameter of AgNWs was about 4.6 μm, and 16 nm respectively, before and after inclusion of the dye.No changes of the length and diameter was observed.However, the amount of AgNWs observed on the surface decreases as the thickness of the dye increased.

R6G emission
Photoluminescence measurements were performed on all samples at a constant pump power of 31 mW and is displayed in Fig. 2. The highest peak marked as yellow represent the lowest thickness of R6G drop cast onto the sample.As the thickness amount increases, the peaks decrease.However, by measuring the peak's full width at half maximum (FWHM), the green peak showed the lowest FWHM of 57.7 nm as compared to 74.2 nm obtained from the yellow peak.This indicates spectral narrowing of the green peak when the R6G was 40µL, which indicates an onset of amplified spontaneous emission (ASE) [7].

Fig.2. Intensity Vs Wavelength samples of AgNWs with different dye thicknesses.
Table .1,summarizes key parameters such as the FWHM, peak centre wavelength and peak height for all samples starting from highest peak to lowest peak indicated by Yellow (G) -Green(E)-Red(C)-Blue(D) -Purple(F) -Black(B) curves in Fig. 2. The yellow peak represent the lowest thickness corresponds to amount of 20µL of dye and black is the thickest dye corresponds to amount of 120 µL.As shown in table.1, the centre peak wavelength is slightly shifted in two thicknesses of dye whereby the yellow peak is shifted to higher wavelength and the purple is shifted to lower wavelength.This is consistent with reported work due to the different amounts of dye [8].By comparing the FESEM images in Fig. 1, the thickness of R6G influenced the amount of AgNWs appearing on the surface.The amount of NWs present at 40 µL of R6G is adequate to create ASE.To improve the emission, more work is required to refine the concentration and thickness of R6G so as to allow more AgNWs surface but at the same time provide sufficient gain from R6G to observe ASE and also lasing.

Table 1 . R6G with different thicknesses to measure FWHM, peak center and, peak height.
Using AgNWs as metal particles for enhancing luminescence emission by means of plasmonic resonances with various dye thicknesses demonstrates ASE at 40 µL of R6G.Shifts in wavelength occurs when thickness is below of concentration 40 µL and above 100 µL.Low thickness of dyes showed enhanced luminescence due to the amount of AgNWs present at the surface of the sample.