This report dominantly focused on employment of natural micro-pillars, embedded on the surface of bambusa tulda leaves, as scattering centres for achieving a single mode random laser (RL) at ∼582 nm with a lower line width (∼1.8 nm) and lasing threshold (132 W/cm2) in Rhodamine-B dye gain medium. The stability in performances is checked over 2 months of duration and scattering activities of the natural micro-pillars are confirmed via numerical simulation using COMSOL and power Fourier transform (PFT) analyses. The demonstration of speckle-free imaging established the low coherence of the RL light. The plant-extricated, handy, low-cost, and simple RL system is proposed to be a new platform having diverse future photonic applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Right after the invention of laser source in 1960 by Maiman in a ruby crystal , laser technology has brushed up over the last a few decades and played a significant role in advancement of modern day technology. However, Letokhov et al.  and Markushev et al.  demonstrated the concept of random lasing (RL) with a non-resonant feedback (lsc>>λ) system. And recently the field of RL has got a new impetus with the recent advancements in disordered nano and micro structures having application in photonics. The bulky and complex structure of a conventional laser system has been replaced by the handy RL system which is easy to prepare, low cost, has low input laser threshold and broad emission spectrum to choose etc. Unlike in conventional laser sources, in RL system, feedback is provided by disordered induced scattering events in a gain medium rather than reflection from the cavity formed by the end-mirrors . For a disordered active medium where optical scattering is strong, multiple scattering of the light wave from the scatterer may form a closed loop path and when the gain along such a loop exceeds the loss, the RL emission is achieved [1–3]. Also, if photons propagate long enough within an active medium, it will also have some opportunities to be amplified and thus, we can observe RL emission even in weakly scattering systems, as proposed by Mujumder et al. . RL emissions in different suitable gain media by using different types of scatterer starting from metal nanoparticles [5,6], carbon nanostructures [7,8], bimetallic nanoparticles [9,10], perovskite nanoparticles , organic–inorganic nano-composites , different organic nanostructures , human tissues , silk nano-fibers , micro-fluidic channels , bacterial cellulose  etc. have been reported in recent times. Random lasers are now finding applications in a far-reaching technological areas, including in medical sectors like cancer cell detection [18,19], speckle free full frame imaging in bovine mellanosome  etc.
Although in the afore-mentioned earlier reports various researchers have demonstrated the RL emissions by employing nano-structures/ different feedback or gain media, but the materials used either as scatterer or as gain media are not bio-degradable. Thus, utilization of biologically formed natural scatterer for the generation of RL has been an up-to-date trend emerged over the couple of years. Zhang et al.  have reported RL emission employing cicada wing micro-pillars. Li et al.  have reported RL by making replicas of micro-papilla implanted on lotus leaves. Also recently our research group has successfully achieved RL in bio-extracts [22,23] in CW regime. Kim et al. have also reported CW lasing by using Ar-ion laser (λ = 514.5 nm) and green He-Ne laser (λ = 543.5 nm)  and this trend has gained a considerable attention in the scientific community.
To tackle this problem of CW laser pumped RL, at the same time which can be an option for easy to make, non-hazardous and compact system, in this report we have demonstrated how the naturally fabricated micro-pillars on the front surface of bambusa tulda leaves can be employed as scattering centres for amplification of light to generate RL emission. To develop a plant-extricated, handy, low-cost and simple RL system the front surface of these bamboo leaves are coated with specific concentrations of Rhodamine-B dye layers which act as gain medium. The schematic diagram of an uncoated and dye coated bamboo leaf systems are shown in Figs. S1 (a-b). Those micro-pillars on bamboo leaf surface have acted as mirror-less cavity system to provide sustainable feedback to amplify the emitted light from the gain molecules under excitation of 532 nm CW laser. The used micro-pillar scatterer is stable and shown to generate RL over a time span of 2 months to retain its characteristic performance. Although at low pump power, many modes are seen to appear, at higher saturated pump power, a single mode at ∼584 nm is sustained, confirming the lasing which has occurred due to the amplification of the emitted light in multiple scattering centres within the micro-pillars. The angle dependent or polar variation of emission from the front of Rhodamine-B coated bamboo leave surface has shown that the RL emission has mostly occurred at ∼60° and 120° with respect to the incident pump direction. The lasing performances of the micro-pillar system have been characterized by determination of the cavity length of ∼16.2 µm and Q factor of ∼1000, which suggest the construction of an efficient random laser system. Here, we have further established the low spatial coherence of the developed RL system by demonstration of almost speckle free imaging of a lab based microscopic substrate with ∼93% reduction in the spatial coherency as compared to that of the conventional illumination sources, like a CW laser. Thus, it is envisioned that the proposed RL system will set a new platform for handy, low-cost and simple RL system to be employed in diverse fields of photonic applications, such as in bio-medical imaging and sensing devices.
2. Sample preparation and experimental setup
At first fresh bamboo leaves have been picked up from local scrubland of NIT Durgapur campus. Then leaves are rinsed thoroughly with 99% distilled water and those are left to become dried under room temperature to make it free from any kind of contaminants. After that, a small portion from the bamboo leave has been cut from leaf lamina leaving midrib of side regions, by scissor to take a square part (∼ 2 ×2 cm) from it. Rhodamine-B dye (99% pure) ([9-(2-carboxyphenyl)-6-diethylamino-3-xanthenylidene]-diethylammonium chloride) has been used as-purchased from Sigma Aldrich. After bamboo leaves are dried, those are coated with 3 different concentrations of aqueous Rhodamine-B solutions [∼60 µM (S1), ∼80 µM (S2), ∼100 (S3) µM] by drop-casting method. After that, each of these 3 pieces of the bamboo leaves coated with Rhodamine-B are attached to microscopic cover slides on the drop-casted surface to form a compact system and made ready for pumping it with a 532 nm CW laser. After usage of them in RL experiment, the S3 sample has been kept under ambient condition for ageing study over next 2 months to examine its characteristics in retaining RL performance. From the date of first experiment done with this S3 setup, data have been recorded for each 1 month interval for a span of 2 months. After that, the image of a transmission electron microscopic grid (TED PELLA, PELCO 300 Mesh Grid) has been collected with the produced RL illumination. A digital camera having 13 Megapixel resolutions in association with a handy paper made microscope, named as Foldscope, , has been used to capture the image.
3. Results and discussions
Figure 1(a) shows a digitally captured image of a fresh bamboo leaves, whereas a Field Emission Scanning Electron Micrograph (FESEM) image, captured using a ZEISS Sigma microscope operating at a voltage of 3KV, is shown in Fig. 1(b). The sample surface has been coated with gold before FESEM characterization. The top-view of the FESEM image revealed the presence of natural micro pillars system, distributed unevenly over the front surface of bamboo leaves, as shown in Fig. 1(b). For better understanding of the distribution of those micro-pillars, the average micro-pillars distance on the bamboo leaves surface is calculated from Fig. 1(b) and it is found to be ∼3.27 µm as shown in the histogram plot (inset of Fig. 1(b)). An interactive surface 3-D plot of the marked area, as shown in Fig. 1(b), has been made by using imageJ software, for better visualization of those micro pillars, and it is shown in Fig. 1(c). Also, the UV-visible (UV-vis.) absorption and photoluminescence (PL) emission spectrum of aqueous solution of Rhodamine-B dye are measured by using a double beam spectrophotometer (Hitachi, U3010) and a fluorescence spectrometer (PerkinElmer LS-55), respectively.
Figure 1(d) shows the normalized spectra of UV-Vis. absorption (red coloured), PL emission (yellow coloured) of aqueous solution of Rhodamine-B dye and the spectrum of the pump laser (green coloured). The pump laser is a CW laser source (HOLMARC) having a peak power of ∼5 mW and emitting a wavelength of 532 nm. The RL spectra have been collected by using a spectrometer (Avaspec) having a spectral resolution of 0.6 nm. It can be seen from Fig. 1(d) that the peak absorption of Rhodamine-B is centred at ∼550 nm and the PL emission is occurring at ∼580 nm. The used pump radiation being at 532 nm lies well within the absorption band of Rhodamine-B, and so it can cause excitations in Rhodamine-B gain molecules which have further led to the fluorescence emission at ∼580 nm.
The emission spectra from S1, S2 and S3 systems have been recorded with various input pump intensities (Pin) and those data are further analysed by calculating the integrated intensity and full width at half maxima (FWHM) of each spectra. The normalized emission spectra, obtained from the S1 and S2 at different Pin are shown in Fig. S2 and it is showing the occurrence of amplified spontaneous emission (ASE) modes in these samples. Figure 2(a) shows the normalized plot of the emission spectra obtained from S3 system, where ∼100 µM concentration of aq. Rhodamine-B solution has been employed, under various Pin. In Fig. 2(b) although a broad fluorescence emission band of Rhodamine-B is found at low Pin, but as the Pin is increased, some sharp peaks do appear over the fluorescence band of Rhodamine-B due to the amplification of the produced light via scattering in bamboo leaf. The inset of Fig. 2(a) is showing the presence of a mode at ∼582 nm, which has appeared due to the scattering of amplified light on the micro pillars present on the front surface of bamboo leaf.
In Fig. 2(b) the plot of integrated intensity vs. Pin is shown and from this plot it is evident that, although for S1 and S2 systems, the integrated intensity is increased almost continuously after a certain value of Pin thus only ASE has occurred in these sample. However, in S3 sample, there is signature of saturation in the integrated intensity vs. Pin curve after the critical value of Pin of ∼132 W/cm2 which is an indication of RL . Thus the threshold lasing intensity (ThPin), which is the transition point from ASE to RL, corresponding to the S3 system has been found to be 132 (±1) W/cm2. The emission spectra of S1, S2 and S3 are showing the presence of FWHM narrowing very clearly. However, in S3 system, the observed line width narrowing is significant and a narrowed emission peak due to RL is obtained at ∼582 nm with FWHM of ∼1.8 (±0.6) nm only. During RL experiments, when the pump intensity is increased, a mode competition between the different RL modes or spikes occurs as a result of photon hopping effect i.e., the jump of a photon from one mode to the other modes occurs, and dominant modes begin to appear and interact with the side modes [26,27]. Also, the threshold and other characteristics of RL depend on the pump pulse regime and the presence of spontaneous emission (SE) strongly inﬂuences the threshold behaviour of RL, particularly in the case of CW pumped RL [26,27]. Therefore, the threshold condition of RLs is not very distinct in the present case. However, a significant improvement in the mode pattern is observed in 1 month and 2 months aged S3 sample as shown below in Figs. 3(a) and 3(b), respectively.
From Fig. 3(a) it can be seen that a sharp RL emission peak at ∼584 nm is appeared whereas in Fig. 3(b) it can be seen that the emission peak has appeared at the same position but the lasing mode has become more prominent than before (inset of Fig. 3(b)). The change in the spectral position of modes may be a direct consequence of non-uniformity of scattering centres. It can be seen from FESEM image in Fig. 1(b) that the micro-pillars of bamboo leaves have different sizes and distances. As a result the amplified light traversed different paths before reaching to the detector. The strongest mode appeared in the emission which can be a possible reason of appearance of such different modes. The electric field distributions around the micro-pillars are shown in Fig. 3(c), as obtained by a numerical simulation using commercially available COMSOL Multiphysics software. To simulate the random structure and the corresponding scattering phenomena, we have built the model similar to the disordered geometry based upon the interactive 3-D surface plot as shown in Fig. 1(c). The inherent disordered structure of micro-pillars helps in generation of RL through multiple scattering of the incident light.
The emission spectra, collected at three different critical intensities in the 2 month old aged S3 sample are shown in Fig. S3 which are classified into three regions named as, Spontaneous emission (SE), ASE and RL [28,29]. Fig. S3 showed that the number of lasing modes those appeared during RL emission is less in number in compared to that during ASE. This is due to the possibility of (i) generation of multiple stochastic modes during ASE, and (ii) decrease in the number of lasing modes during the RL emission, after the mode competition. As depicted in Fig. 3(d), when the input power Pin has crossed the 1st threshold of ∼56 W/cm2, transition from SE to ASE has occurred. However, upon further increase in the value of Pin, a 2nd kink at ∼133 W/cm2 has occurred, which corresponds to the transition from ASE to RL mode. A clear transitions from SE to ASE and finally to RL action followed by gain saturation can be seen from two kinks, marked in Fig. 3(d). The second kink during RL emission is due to the gain saturation of a particular lasing mode leading to the appearance of a ‘S’ like behaviour in log-log plot of integrated intensity with Pin (Fig. 3(d)). The appearance of such a ‘S’ like behaviour in log-log plot of integrated intensity with Pin (Fig. 3(d)) is a key recognition to the transition from SE to ASE and then to RL .
In this context, when the input pump intensity has surpassed the threshold, the obvious narrowing of FWHM has also been occurred as shown in Fig. 3(d). While calculating the FWHM in ASE and RL regimes, the top most spikes in emission band has been considered for FWHM calculation, by subtracting the SE background [30,31] followed by a Gaussian curve fitting approach. The nonlinear change of FWHM with Pin is a fundamental character of RL emission. However, for the values of incident pump intensity at above the first threshold (i.e. ∼56 W/cm2), the FWHM has been dramatically dropped from 42 to 16 nm (Fig. 3(d)), due to the occurrence of ASE. By further increase in Pin to the second threshold (∼133 W/cm2), the FWHM has decreased to 1.9 (±0.6) nm (Fig. 3(d)), implying the appearance of RL emission .
The sharp changes in the integrated intensity around a threshold pump intensity and subsequent bandwidth narrowing are important key signatures of RL . Also, as shown in Fig. 3(e), it can be concluded that due to SE, many modes have appeared at low Pin of the pump laser. But at high Pin, a single mode at ∼584 nm is sustained, confirming RL generation which has occurred due to the amplification of the emitted light in multiple scattering centres within the micro-pillars on the surface of bamboo leaf. The image shown in Fig. 3(f) showed the digital image of RL emission from the S3 system.
Moreover, a statistical data has been collected in (2 months old) S3 sample to record the variation of the highest mode positions coming at particular Pin of the pump laser. From Fig. 4(a), it can be seen that for Pin lower than ThPin, the arbitrary appearance of modes have increased which generally occurs in case of SE. It is also evident from Fig. 4(a) that for values of Pin which is higher than ThPin, the arbitrary variation of the peak position of the mode has decreased drastically and the lasing has occurred at ∼584 nm wavelength.
Also, an angle dependent emission spectra or polar variation of emission from the front of Rhodamine-B coated bamboo leave surface has also been recorded for the angular range from 0° to 180° with respect to the incident pump light direction. As shown in Fig. 4(b), the polar variation of emission constitutes a butterfly like emission spectra from the Rhodamine-B coated bamboo leaf system confirming that the maximum lasing emission is occurring only in some particular direction from bamboo leaf surface. Here, the emission is happening at ∼60° and 120° with respect to the incident pump beam direction and such variation has been also reported earlier .
To analyse the type of optical cavity formation within those micro pillars, a Power Fourier Transform (PFT) analysis of the obtained RL spectrum is performed. Figure 4(c) shows the result of the PFT analysis done on the highest intensity emission spectrum (square marked) of Fig. 3(b). From the PFT spectra one can determine the optical cavity path length (lc) by using the expression ,2(b)). This result leads to a conclusion that in this system the amplified light traverses 5 times the average micro pillars distance for stimulation of the gain molecules to generate RL emission. Also, here the condition lsc>>λ is satisfied, thus the system works in the weakly scattering regime [4,32,33].
The quality factor (Q-value) is one of the crucial parameter of RL which varies inversely with the energy loss rate of a particular mode and we can write ,
One of the major utilization of RL emission can be in obtaining ‘speckle free imaging’ to reveal image with less distortion and better clarity. Redding et al. have reported almost no speckle formation by RL illumination . However, here in this report, imaging has been done directly from the RL emission and Fig. 5 shows the schematic diagram of the process taken to capture the images of the TEM grid. Light coming from RL emission is passed through a 632 nm filter before it goes to a collimating lens to form a parallel beam and then it goes to the convex lens (focal length, f of 3 cm) to directly incident upon TEM grid before it goes through the Foldscope. After that a 13 megapixel (MP) mobile camera has been used to capture the direct enlarged images of that TEM grid from the Foldscope. TEM grid is taken as an object to illuminate for confirmation of low coherence of the RL emission. Three different illumination sources, a 632 nm He-Ne CW laser, the ASE from this Rhodamine-B coated bamboo leaves system and the RL emission itself have been employed and the obtained images of the TEM grids are compared.
Now, under the illumination of conventional laser sources, the formation of speckles is possible because of interference among scattered photons, and those speckles can corrupt the image beyond recognition. However, when illuminating with a low-spatial-coherence source such as RL, interference among scattered photons is prohibited, leading to a uniform background signal. From Figs. 6(a-c), it can be seen that the digital pictures captured become more prominent and clearer as we go from 632 nm CW laser illumination to ASE illumination and further towards RL illumination.
To analyze the spatial coherence, the speckle contrast is deﬁned as,$\left\langle I \right\rangle$ are the standard deviation and average intensity of the image, respectively. The light for which C = 1 is totally spatial coherent, and that in which C = 0, is spatially incoherent . In this regards, we have measured the exact values of speckle contrast from the illumination of 632 nm CW laser source, amplified spontaneous emission (ASE) and RL and it has been already given in inset of Figs. 6(a-c). The values of σI and $\left\langle I \right\rangle$ have been estimated from the pixel intensity distribution of the digital images taken under the illumination of 632 nm CW laser, ASE and RL and is shown in Figs. 6(d-f). The speckle contrast (C) has been measured from the portion red rectangular box shown in the inset of Figs. 6(a-c) respectively. In both cases of ASE and RL, the value of C<<1, and it suggests low spacial coherency of the as-designed RL system. The value of C is ∼12 times and ∼16 times less in the ASE and RL system respectively, than CW laser. Whereas, the value of C of the image captured under the RL illumination is found to be ∼93% lowered in compared to that of 632 nm CW laser. In the Table 1, the values of the speckle contrast of those 3 different sources of illumination is summarized:
The speckle contrast is an important parameter to indicate the spatial coherence of the source of illumination [15,33–35]. The lower value of C indicate more spatially incoherent source. Here from the Table 1 it can be concluded that C is the lowest (=0.030) for RL illumination, and so it is the most spatially incoherent source of light. As a result the image quality is improved by averaging out all the noises. The lower speckle contrast of the RL emission is a well establishment of its use in noiseless imaging as done here.
In summary, we have successfully provoked the use of natural micro-pillars on the bamboo leaves surface for demonstration of RL action in presence of a laser dye as gain medium. The RL action has been appeared due to the formation of multiple scattering centres in the micro-pillars on the bamboo leaves as shown by a COMSOL simulation. The used micro-pillar scatterer can help in generation of RL over a time span of 2 months, as inspected from the RL emission characteristics in aged bamboo leaves. Moreover, the developed RL system has been used as a cost effective light source for imaging of a lab based microscopic substrate with ∼93% reduction in the spatial coherency in compared to that of the conventional illumination sources like CW laser. The present study has not only describes a simple and unique method to synthesize environment friendly, non-hazardous, plant-extricated RL system via a simple-route technique, but the interplay of scattering by the ‘micro-pillar and amplified light’ duo can open up huge opportunity to set new platforms for handy, low-cost and non-complex RL system to be employed in diverse fields of photonic applications, such as bio-medical imaging and sensing devices.
Department of Science & Technology and Biotechnology, Government of West Bengal (332(Sanc.)/ST/P/S&T/16G-24/2018).
The authors are grateful to the Department of Science & Technology and Biotechnology, Government of West Bengal for the partial financial support through project Grant No.: Grant-in-Aids no. 332(Sanc.)/ST/P/S&T/16G-24/2018 dt. 19.03.2019. AD is grateful to Ministry of Education, Government of India, NIT Durgapur for providing the maintenance fellowship. Also authors are grateful to CoE (Advanced Materials) under TEQIP-III, Ministry of Education, Govt. of India and NIT Durgapur for FESEM measurement.
Authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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