- Email: [email protected]
Waveguiding performance of rhodamine 6G doped polymer nanowire
Accepted Manuscript Title: Waveguiding Performance of Rhodamine 6G Doped Polymer Nanowire Author: Hanyang Li Yan Wang Jin Li Shuangqiang Liu Jun Yang PII: DOI: Reference:
S0030-4026(16)30474-0 http://dx.doi.org/doi:10.1016/j.ijleo.2016.05.041 IJLEO 57667
To appear in: Received date: Revised date: Accepted date:
6-1-2016 17-2-2016 10-5-2016
Please cite this article as: Hanyang Li, Yan Wang, Jin Li, Shuangqiang Liu, Jun Yang, Waveguiding Performance of Rhodamine 6G Doped Polymer Nanowire, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.05.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Waveguiding Performance of Rhodamine 6G Doped Polymer Nanowire Hanyang Li1, Yan Wang1, Jin Li2, Shuangqiang Liu1, and Jun Yang1* 1
Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering
University, Harbin 150080, China 2
College of Information Science and Engineering, Northeastern University, Shenyang 110819,
China *Email address: [email protected]
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Abstract We report waveguiding performance and guide loss both of straight and bent dye doped polymer nanowires. We also demonstrate Rhodamine 6G Doped Polymer nanowires with diameter of 500 nm, which were fabricated into self-coupled resonator. Single-mode lasing peak centered at 542.2 nm was obtained in the spectrum. The self-coupled resonators perform better mode selection compared with the linear structure nanowires, which can be utilized to realize tunable laser systems and enhance the coupling efficiency of emission in ultra-small resonators. Keywords: R6G-doped, nanowire, waveguiding, self coupled resonator.
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1 Introduction Recently, considerable progress was achieved in developing sub-wavelength sized optically pumped laser based on nanowires (NWs) [1-3]. The optical and optoelectronic properties of one-dimensional organic nanomaterials are fundamentally different from their inorganic counterpart [4], particularly the case for single dye-doped polymer nanowire (PNW), because their high photoluminescence efficiency and large stimulated emission cross-sections enhances the performance of corresponding devices [5]. Organic solid-state NWs [6-10] or semiconductor NWs [11]-[15] were controlled assembly and manipulated into arbitrary geometries, optical pumping of those NWs exhibit performance metrics such as narrow linewidths, high Q factors and low thresholds, which already show great promise applications for integrated, narrow band filters and chemical and biological sensing [16,17]. When optical path length of a ring-resonator is exactly a whole number of wavelengths, resonance occurs, such as micro-knot [18], micro-ring [19], and loop mirrors microcavity [20], which supports multiple resonances, the free spectral range (FSR) depends on the resonator length [21]. One method to increase mode selection ability of resonator is to use series-coupled method, because the series-coupled resonators can generate a mode selection mechanism by the Vernier effect [22]. The extinction ratio and the modulation can both be significantly enhanced. When both rings can be tuned individually, it becomes possible not only to move the resonance wavelength around but also switch the resonance on or off. However, complicated manipulation and careful balancing the coupling structure is needed if the double ring resonator constituted by a single NW, which is difficult to achieve in experiment. Additionally the coupling losses cause a decrease in Q factor. In our previous work [23], three Rhodamine6G PNWs with different diameter and were respectively folded into three laser cavities, which have similar geometry but different size, and laser output at different wavelengths. In this paper, we report the preparation methods of R6G-PNWs, both straight waveguide and bent waveguide were fabricated, the relationship between guide loss and propagation distance is described. Due to the coupling efficiency and the Vernier effect, the coupling microcavity constituted by dye-doped PNW could be used to realize tunable laser and single-mode lasing emission. 2 Preparation Methods R6G-PNWs were fabricated by directly drawing from fluorescent dyes doped molten PMMA. 3 / 12
Experimentally, 4 mg of R6G (Alfa Aesar) and 0.5 g methyl methacrylate (MMA) are dissolved into 20 ml of ethanol, the mixed solution is stirred at 60°C, 0.005 wt% of azodiisobutyronitrile (AIBN) is added into the mixed solution , heated at 75°C for pre -polymerization 2h, decreased to 60 °C for polymerization 2h, drying at 110 °C for 6h. The bulk R6G-PMMA with R6G is 0.79 wt% was obtained. We use a heating plate to melt R6G-PMMA (melt temperature 240°C-270°C) and keep temperature at around 270°C during the NWs drawing. A probe with the tip of several micrometers is immersed into the molten R6G-PMMA, and then draw the probe out of the melting quickly, the extended R6G-PMMA wire is quickly quenched in air, the process of the drawing is shown in Figure 1(a).
Fig. 1. (a) Schematic of R6G-PNWs fabrication by direct drawing process from molten R6G-PMMA. (b) Schematic of the instrumental setup for the R6G-PNWs lasing experiments.
Experiments were performed with the setup shown in Figure 1(b), the R6G-PNWs were deposited on a low-index MgF2 substrate and pumped with by frequency-doubled pulses of a Q-switched Nd:YAG laser (λ=532nm, pulse duration is 10ns). Pump laser was focused to a spot diameter of ~70 µm through a 40× objective (NA=0.65) which allows for lasing-emission from the NW and pump light power density measurements at the same sample spot, because there are different energies above and below the beam focus. The pulse energies were measured and averaged using an optical energy meter. Reflection of the pump light was removed by a 532 nm filter, lasing-emission of the R6G-PNWs was split by a beam splitter to a spectrometer (spectral resolution = 0.1 nm) and CCD. 3. Experimental Results and Analysis The waveguiding of the NW is a important parameter for the laser cavity, and the relationship between guide loss and propagation distance is of crucial importance in the paper. A single bent 4 / 12
R6G-PNW with diameter of 500 nm length of 40 µm is excited by the probe tip, and we keep the laser power at 0.2 mW and locate the excited spot along the bent R6G-PNW by adjusting the XYZ translation stage, as shown in Figure 2(a).
Fig. 2. (a) Dark-filed PL microscope images of a 500 nm diameter 40 µm length bent R6G-PNW excited by the probe tip at different spot along the R6G-PNW. The insets are the gray images of the excited and output spots. (b) Bent R6G-PNW, normalized intensity of the output PL dependent the guided distance, the red line is fitted using a first-order exponential function.
To quantitatively analyze the waveguide efficiency of the R6G-PNW, we calculate the PL intensity of the excited spot and the output spot by studying the image brightness, as used in previous [24]. The spot images are converted from RGB to gray styles in Adobe Photoshop, whose gray values are calculated by Matlab to characterize the corresponding intensity. The PL intensity of the output spot is normalized against the excited spot, then the decay of the guided normalized PL intensity dependent propagation distance (d) is obtained, as shown in Figure 2(b). Upon increasing d, the PL intensity decreases as ≈ exp (-αd), with a loss coefficient α = 605 cm-1. The exponential decay of light along the R6G-NW indicates the predominance of losses due to absorption in the organic dye R6G. The loss coefficient is comparable to those reported for electrospun polymer nanofibers. The loss can be decreased by doping polymer waveguides with donor/acceptor in a dual-dye system.
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Fig. 3. (a) Dark-filed PL microscope images of a 800 nm diameter 30 µm length straight R6G-PNW excited by the probe tip at different spot along the R6G-PNW. (b) Straight R6G-PNW, normalized intensity of the output PL dependent the guided distance.
We also studied the waveguide efficiency of the straight R6G-PNW, a single straight R6G-PNW with diameter of 800nm length of 30 µm is excited by the same probe tip, and the same laser power. Excited spot along the straight R6G-PNW is shown in Figure 3(a). PL intensity dependent propagation distance of this straight R6G-PNW is obtained, the loss coefficient of straight R6G-PNW is α = 525 cm-1, as shown in Figure 3(b). The evanescent field around the NW allows light guiding through sharp bends (micrometer level) with low optical loss, both for semiconductor NWs and polymer NWs. The transmission of a 300-nm-diameter PNW (n=1.49) guiding a broadband supercontinuum and monochromatic lasers. The range of the transmittance is from 60% to 80% in visible light. At the wavelength far from the absorption band of the dopants, measured optical loss of the PMMA NW is typically lower than 0.1 dB/mm [25]. The bending loss decreases with the increasing index due to stronger optical confinement. For example, with a refractive index of 1.38, the bending loss of the 450 nm diameter NW is 3.5 dB/90°; when the refractive index increases to 2.0, the bending loss decreases to a negligible value of 0.1 dB/90° [26]. Generally, the real refractive index of the gain material will be a little higher than that measured from the passive hybrid material since absorption of laser dye will add additional contribution to dispersion at the lasing wavelength. Due to the molecular relaxation process, the 6 / 12
refractive index change is on the order of 10-4.
Fig. 4. (a) SEM image of R6G-PNW with diameter of 900 nm and length of 36 µm, the R6G-PNW was cut into two segments, the length of the shorter one is 14 µm and the longer one is 22 µm. Inset: Dark-filed PL microscope image of the NW. (b) The spectra with the multiple peaks for the two segments straight R6G-PNW. The pump energy density is 66.4µJ/cm2.
To study the optical properties of the R6G-PNW, a R6G-PNW with 900 nm diameter and 50 µm length, which was cut into two segments then brought into together as shown in Figure 4(a). We had measured the emission of this straight geometry structure R6G-PNW. The pump energy density is 66.4µJ/cm2. The spectra with the multiple peaks were recorded and the result is shown in Figure 4(b). It is difficult to realize single-mode operation by solely increasing the pumping intensity in this straight geometry structure NW.
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Fig. 5. (a) SEM images of the SCR cavity fold by a 500 nm diameter 80 µm length R6G-PNW. Inset: Dark-filed PL microscope image of the NW. (b) The acquired single mode emission spectrum for (a) at the pump energy density of 113 µJ/cm2.
Another single R6G-PNW with 500 nm diameter 80 µm length was bent to a self-coupled resonator (SCR) geometry structure by two micromanipulators as shown in Figure 5(a). A single-mode lasing emission was obtained at the lasing peak centered at 545.2 nm as shown in Figure 5(b). The pump energy density is 113 µJ/cm2. Note that the small peak at wavelength of 532.5 nm is part of scattering pumping light, collected by the spectrograph through the filter. The SCR exhibited single emission peaks because of the particular geometry structure. The two inherent resonance paths provided by a single NW to realize the mode selection, this structure is a consequence of geometric optimization. Considering the NWs coupling with the evanescent field, we speculate that there are two propagation directions at the top junction — a forward direction and a backward direction. Therefore, the two ring resonators are formed two closed loops in the left and right, corresponding two resonant modes will also exit in the system, one circulating clockwise (CW mode) and the other circulating counterclockwise (CCW mode). When the left and right of the loop length are not equal, the left mode and right mode have different resonant 8 / 12
wavelength. According to the Vernier effect, only when the resonant wavelength satisfies Mλ0 = nL1 and Nλ0 = nL2 (where M and N are integers, L1 and L2 are the left and right loop length), the resonance can be amplified in the SCR. 4. Conclusion To summarize, we fabricated high light coupling micocavity with micromanipulation of a single R6G-PNW. The dye-doped NWs act as both gain media and laser cavity. The relationship between guide loss and propagation distance of straight R6G-PNW and bent R6G-PNW were measured, which indicated that R6G-PNW is a typical characteristic of active waveguides. The behavior of the single nanowire resonator was explained by comparison with the laser spectrum, as well as experimental evidence, the evanescent field around the NW allows light guiding through sharp bends (micrometer level) with low optical loss. Acknowledgements This work was funded by the National Natural Science Foundation of China (Grants No.61227013,61422505, 61307104,61405044), the Program for New Century Excellent Talents in University (NCET-12-0623), National Key Scientific Instrument and Equipment Development Project (No. 2013YQ040815), and Specialized Research Fund for the Doctoral Program of Higher Education (No.20122304110022).
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References [1] J. Clark, and G. Lanzani, Organic photonics for communications, Nature Photonics, 4 (2010) 438-446. [2] A. Camposeo, F. Di Benedetto, R. Stabile, A. A. R. Neves, R. Cingolani, and D. Pisignano, Laser Emission from Electrospun Polymer Nanofibers, Small, 5 (2009) 562-566. [3] L. Y. Cao, P. Y. Fan, E. S. Barnard, A. M. Brown, and M. L. Brongersma, Tuning the Color of Silicon Nanostructures, Nano Letters, 10 (2010) 2649-2654. [4] H. W. Xu, J. B. Wright, A. Hurtado, Q. M. Li, T. S. Luk, J. J. Figiel, K. Cross, G. Balakrishnan, L. F. Lester, I. Brener, and G. T. Wang, Gold substrate-induced single-mode lasing of GaN nanowires, Applied Physics Letters, 101(2012) 221114. [5] Q. M. Li, J. B. Wright, W. W. Chow, T. S. Luk, I. Brener, L. F. Lester, and G. T. Wang, Single-mode GaN nanowire lasers, Optics Express, 20 (2012) 17873-17879. [6] H. Yu and B. Li, Wavelength-converted wave-guiding in dye-doped polymer nanofibers, Scientific Reports, 3 (2013) 1674. [7] J. Chen, Y. Chen, W. Luo, J. Kou, F. Xu and Y. Lu, Multifunctional optical nanofiber polarization devices with 3D geometry, Optics Express, 22 (2014) 17890-17896. [8] D. O'Carroll, I. Lieberwirth, and G. Redmond, Melt-processed polyfluorene nanowires as active waveguides, Small, 3 (2007) 1178-1183. [9] A. Noy, A. E. Miller, J. E. Klare, B. L. Weeks, B. W. Woods, and J. J. DeYoreo, Fabrication of luminescent nanostructures and polymer nanowires using dip-pen nanolithography, Nano Letters, 2 (2002) 109-112. [10] F. Quochi, F. Cordella, A. Mura, G. Bongiovanni, F. Balzer, and H. G. Rubahn, Gain amplification and lasing properties of individual organic nanofibers, Applied Physics Letters, 88 (2006) 041106. [11] J. C. Johnson, H. J. Choi, K. P. Knutsen, R. D. Schaller, P. D. Yang, and R. J. Saykally, Single gallium nitride nanowire lasers, Nature Materials, 1 (2002) 106-110. [12] D. Vanmaekelbergh, and L. K. van Vugt, ZnO nanowire lasers, Nanoscale, 3 (2011) 2783-2800. [13] A. L. Pan, W. C. Zhou, E. S. P. Leong, R. B. Liu, A. H. Chin, B. S. Zou, and C. Z. Ning, Continuous Alloy-Composition Spatial Grading and Superbroad Wavelength-Tunable Nanowire Lasers on a Single Chip, Nano Letters, 9 (2009) 784-788. [14] L. K. Van Vugt, S. Ruhle, and D. Vanmaekelbergh, Phase-correlated nondirectional laser emission from the end facets of a ZnO nanowire, Nano Letters, 6 (2006) 2707-2711. [15] R. Chen, T. T. D. Tran, K. W. Ng, W. S. Ko, L. C. Chuang, F. G. Sedgwick, and C. Chang-Hasnain, Nanolasers grown on silicon, Nature Photonics, 5 (2011) 170-175. [16] J. Giblin, V. Protasenko, and M. Kuno, Wavelength Sensitivity of Single Nanowire Excitation Polarization Anisotropies Explained through a Generalized Treatment of Their Linear Absorption, Acs Nano, 3 (2009) 1979-1987. [17] C. Meng, Y. Xiao, P. Wang, L. Zhang, Y. X. Liu, and L. M. Tong, Quantum-Dot-Doped Polymer Nanofibers for Optical Sensing, Advanced Materials, 23 (2011) 3770-3774. [18] X. S. Jiang, Q. H. Song, L. Xu, J. Fu, and L. M. Tong, Appl. Phys. Lett. 90 (2007) 233501. [19] D. Perron, M. Wu, C. Horvath, D. Bachman, and V. Van, All-plasmonic switching based on 10 / 12
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thermal nonlinearity in a polymer plasmonic microring resonator, Optics Letters, 36 (2011) 2731-2733. Y. Xiao, C. Meng, P. Wang, Y. Ye, H. K. Yu, S. S. Wang, F. X. Gu, L. Dai, and L. M. Tong, Single-Nanowire Single-Mode Laser, Nano Letters, 11 (2011) 1122-1126. M. Sumetsky, Y. Dulashko, and R. S. Windeler, Super free spectral range tunable optical microbubble resonator, Optics Letters, 35 (2010) 1866-1868. K. Oda, N. akato, and H. Toba, A wide-FSR waveguide double-ring resonator for optical FDM transmission systems, Journal of Lightwave Technology, 9 (1991) 728-736. H. Li, J. Li, L. Qiang, Y. Zhang, S. Hao. Single-mode lasing of nanowire self-coupled resonator, Nanoscale, 5(2013) 6297-6302. B. Yan, L. Liao, Y. M. You, X. J. Xu, Z. Zheng, Z. X. Shen, J. Ma, L. M. Tong, and T. Yu, Single-Crystalline V2O5 Ultralong Nanoribbon Waveguides, Advanced Materials, 21 (2009) 2436-2440. F. X. Gu, L. Zhang, X. F. Yin, and L. M. Tong, Polymer single-nanowire optical sensors, Nano Letters, 8 (2008) 2757-2761. H. K. Yu, S. S. Wang, J. Fu, M. Qiu, Y. H. Li, F. X. Gu, and L. M. Tong, Modeling bending losses of optical nanofibers or nanowires, Applied Optics, 48 (2009) 4365-4369.
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Figure Captions: Fig. 1. (a) Schematic of R6G-PNWs fabrication by direct drawing process from molten R6G-PMMA. (b) Schematic of the instrumental setup for the R6G-PNWs lasing experiments.
Fig. 2. (a) Dark-filed PL microscope images of a 500 nm diameter 40 µm length bent R6G-PNW excited by the probe tip at different spot along the R6G-PNW. The insets are the gray images of the excited and output spots. (b) Bent R6G-PNW, normalized intensity of the output PL dependent the guided distance, the red line is fitted using a first-order exponential function.
Fig. 3. (a) Dark-filed PL microscope images of a 800 nm diameter 30 µm length straight R6G-PNW excited by the probe tip at different spot along the R6G-PNW. (b) Straight R6G-PNW, normalized intensity of the output PL dependent the guided distance.
Fig. 4. (a) SEM image of R6G-PNW with diameter of 900 nm and length of 36 µm, the R6G-PNW was cut into two segments, the length of the shorter one is 14 µm and the longer one is 22 µm. Inset: Dark-filed PL microscope image of the NW. (b) The spectra with the multiple peaks for the two segments straight R6G-PNW. The pump energy density is 66.4µJ/cm2.
Fig. 5. (a) SEM images of the SCR cavity fold by a 500 nm diameter 80 µm length R6G-PNW. Inset: Dark-filed PL microscope image of the NW. (b) The acquired single mode emission spectrum for (a) at the pump energy density of 113 µJ/cm2.
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S0030-4026(16)30474-0 http://dx.doi.org/doi:10.1016/j.ijleo.2016.05.041 IJLEO 57667
To appear in: Received date: Revised date: Accepted date:
6-1-2016 17-2-2016 10-5-2016
Please cite this article as: Hanyang Li, Yan Wang, Jin Li, Shuangqiang Liu, Jun Yang, Waveguiding Performance of Rhodamine 6G Doped Polymer Nanowire, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.05.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Waveguiding Performance of Rhodamine 6G Doped Polymer Nanowire Hanyang Li1, Yan Wang1, Jin Li2, Shuangqiang Liu1, and Jun Yang1* 1
Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering
University, Harbin 150080, China 2
College of Information Science and Engineering, Northeastern University, Shenyang 110819,
China *Email address: [email protected]
1 / 12
Abstract We report waveguiding performance and guide loss both of straight and bent dye doped polymer nanowires. We also demonstrate Rhodamine 6G Doped Polymer nanowires with diameter of 500 nm, which were fabricated into self-coupled resonator. Single-mode lasing peak centered at 542.2 nm was obtained in the spectrum. The self-coupled resonators perform better mode selection compared with the linear structure nanowires, which can be utilized to realize tunable laser systems and enhance the coupling efficiency of emission in ultra-small resonators. Keywords: R6G-doped, nanowire, waveguiding, self coupled resonator.
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1 Introduction Recently, considerable progress was achieved in developing sub-wavelength sized optically pumped laser based on nanowires (NWs) [1-3]. The optical and optoelectronic properties of one-dimensional organic nanomaterials are fundamentally different from their inorganic counterpart [4], particularly the case for single dye-doped polymer nanowire (PNW), because their high photoluminescence efficiency and large stimulated emission cross-sections enhances the performance of corresponding devices [5]. Organic solid-state NWs [6-10] or semiconductor NWs [11]-[15] were controlled assembly and manipulated into arbitrary geometries, optical pumping of those NWs exhibit performance metrics such as narrow linewidths, high Q factors and low thresholds, which already show great promise applications for integrated, narrow band filters and chemical and biological sensing [16,17]. When optical path length of a ring-resonator is exactly a whole number of wavelengths, resonance occurs, such as micro-knot [18], micro-ring [19], and loop mirrors microcavity [20], which supports multiple resonances, the free spectral range (FSR) depends on the resonator length [21]. One method to increase mode selection ability of resonator is to use series-coupled method, because the series-coupled resonators can generate a mode selection mechanism by the Vernier effect [22]. The extinction ratio and the modulation can both be significantly enhanced. When both rings can be tuned individually, it becomes possible not only to move the resonance wavelength around but also switch the resonance on or off. However, complicated manipulation and careful balancing the coupling structure is needed if the double ring resonator constituted by a single NW, which is difficult to achieve in experiment. Additionally the coupling losses cause a decrease in Q factor. In our previous work [23], three Rhodamine6G PNWs with different diameter and were respectively folded into three laser cavities, which have similar geometry but different size, and laser output at different wavelengths. In this paper, we report the preparation methods of R6G-PNWs, both straight waveguide and bent waveguide were fabricated, the relationship between guide loss and propagation distance is described. Due to the coupling efficiency and the Vernier effect, the coupling microcavity constituted by dye-doped PNW could be used to realize tunable laser and single-mode lasing emission. 2 Preparation Methods R6G-PNWs were fabricated by directly drawing from fluorescent dyes doped molten PMMA. 3 / 12
Experimentally, 4 mg of R6G (Alfa Aesar) and 0.5 g methyl methacrylate (MMA) are dissolved into 20 ml of ethanol, the mixed solution is stirred at 60°C, 0.005 wt% of azodiisobutyronitrile (AIBN) is added into the mixed solution , heated at 75°C for pre -polymerization 2h, decreased to 60 °C for polymerization 2h, drying at 110 °C for 6h. The bulk R6G-PMMA with R6G is 0.79 wt% was obtained. We use a heating plate to melt R6G-PMMA (melt temperature 240°C-270°C) and keep temperature at around 270°C during the NWs drawing. A probe with the tip of several micrometers is immersed into the molten R6G-PMMA, and then draw the probe out of the melting quickly, the extended R6G-PMMA wire is quickly quenched in air, the process of the drawing is shown in Figure 1(a).
Fig. 1. (a) Schematic of R6G-PNWs fabrication by direct drawing process from molten R6G-PMMA. (b) Schematic of the instrumental setup for the R6G-PNWs lasing experiments.
Experiments were performed with the setup shown in Figure 1(b), the R6G-PNWs were deposited on a low-index MgF2 substrate and pumped with by frequency-doubled pulses of a Q-switched Nd:YAG laser (λ=532nm, pulse duration is 10ns). Pump laser was focused to a spot diameter of ~70 µm through a 40× objective (NA=0.65) which allows for lasing-emission from the NW and pump light power density measurements at the same sample spot, because there are different energies above and below the beam focus. The pulse energies were measured and averaged using an optical energy meter. Reflection of the pump light was removed by a 532 nm filter, lasing-emission of the R6G-PNWs was split by a beam splitter to a spectrometer (spectral resolution = 0.1 nm) and CCD. 3. Experimental Results and Analysis The waveguiding of the NW is a important parameter for the laser cavity, and the relationship between guide loss and propagation distance is of crucial importance in the paper. A single bent 4 / 12
R6G-PNW with diameter of 500 nm length of 40 µm is excited by the probe tip, and we keep the laser power at 0.2 mW and locate the excited spot along the bent R6G-PNW by adjusting the XYZ translation stage, as shown in Figure 2(a).
Fig. 2. (a) Dark-filed PL microscope images of a 500 nm diameter 40 µm length bent R6G-PNW excited by the probe tip at different spot along the R6G-PNW. The insets are the gray images of the excited and output spots. (b) Bent R6G-PNW, normalized intensity of the output PL dependent the guided distance, the red line is fitted using a first-order exponential function.
To quantitatively analyze the waveguide efficiency of the R6G-PNW, we calculate the PL intensity of the excited spot and the output spot by studying the image brightness, as used in previous [24]. The spot images are converted from RGB to gray styles in Adobe Photoshop, whose gray values are calculated by Matlab to characterize the corresponding intensity. The PL intensity of the output spot is normalized against the excited spot, then the decay of the guided normalized PL intensity dependent propagation distance (d) is obtained, as shown in Figure 2(b). Upon increasing d, the PL intensity decreases as ≈ exp (-αd), with a loss coefficient α = 605 cm-1. The exponential decay of light along the R6G-NW indicates the predominance of losses due to absorption in the organic dye R6G. The loss coefficient is comparable to those reported for electrospun polymer nanofibers. The loss can be decreased by doping polymer waveguides with donor/acceptor in a dual-dye system.
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Fig. 3. (a) Dark-filed PL microscope images of a 800 nm diameter 30 µm length straight R6G-PNW excited by the probe tip at different spot along the R6G-PNW. (b) Straight R6G-PNW, normalized intensity of the output PL dependent the guided distance.
We also studied the waveguide efficiency of the straight R6G-PNW, a single straight R6G-PNW with diameter of 800nm length of 30 µm is excited by the same probe tip, and the same laser power. Excited spot along the straight R6G-PNW is shown in Figure 3(a). PL intensity dependent propagation distance of this straight R6G-PNW is obtained, the loss coefficient of straight R6G-PNW is α = 525 cm-1, as shown in Figure 3(b). The evanescent field around the NW allows light guiding through sharp bends (micrometer level) with low optical loss, both for semiconductor NWs and polymer NWs. The transmission of a 300-nm-diameter PNW (n=1.49) guiding a broadband supercontinuum and monochromatic lasers. The range of the transmittance is from 60% to 80% in visible light. At the wavelength far from the absorption band of the dopants, measured optical loss of the PMMA NW is typically lower than 0.1 dB/mm [25]. The bending loss decreases with the increasing index due to stronger optical confinement. For example, with a refractive index of 1.38, the bending loss of the 450 nm diameter NW is 3.5 dB/90°; when the refractive index increases to 2.0, the bending loss decreases to a negligible value of 0.1 dB/90° [26]. Generally, the real refractive index of the gain material will be a little higher than that measured from the passive hybrid material since absorption of laser dye will add additional contribution to dispersion at the lasing wavelength. Due to the molecular relaxation process, the 6 / 12
refractive index change is on the order of 10-4.
Fig. 4. (a) SEM image of R6G-PNW with diameter of 900 nm and length of 36 µm, the R6G-PNW was cut into two segments, the length of the shorter one is 14 µm and the longer one is 22 µm. Inset: Dark-filed PL microscope image of the NW. (b) The spectra with the multiple peaks for the two segments straight R6G-PNW. The pump energy density is 66.4µJ/cm2.
To study the optical properties of the R6G-PNW, a R6G-PNW with 900 nm diameter and 50 µm length, which was cut into two segments then brought into together as shown in Figure 4(a). We had measured the emission of this straight geometry structure R6G-PNW. The pump energy density is 66.4µJ/cm2. The spectra with the multiple peaks were recorded and the result is shown in Figure 4(b). It is difficult to realize single-mode operation by solely increasing the pumping intensity in this straight geometry structure NW.
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Fig. 5. (a) SEM images of the SCR cavity fold by a 500 nm diameter 80 µm length R6G-PNW. Inset: Dark-filed PL microscope image of the NW. (b) The acquired single mode emission spectrum for (a) at the pump energy density of 113 µJ/cm2.
Another single R6G-PNW with 500 nm diameter 80 µm length was bent to a self-coupled resonator (SCR) geometry structure by two micromanipulators as shown in Figure 5(a). A single-mode lasing emission was obtained at the lasing peak centered at 545.2 nm as shown in Figure 5(b). The pump energy density is 113 µJ/cm2. Note that the small peak at wavelength of 532.5 nm is part of scattering pumping light, collected by the spectrograph through the filter. The SCR exhibited single emission peaks because of the particular geometry structure. The two inherent resonance paths provided by a single NW to realize the mode selection, this structure is a consequence of geometric optimization. Considering the NWs coupling with the evanescent field, we speculate that there are two propagation directions at the top junction — a forward direction and a backward direction. Therefore, the two ring resonators are formed two closed loops in the left and right, corresponding two resonant modes will also exit in the system, one circulating clockwise (CW mode) and the other circulating counterclockwise (CCW mode). When the left and right of the loop length are not equal, the left mode and right mode have different resonant 8 / 12
wavelength. According to the Vernier effect, only when the resonant wavelength satisfies Mλ0 = nL1 and Nλ0 = nL2 (where M and N are integers, L1 and L2 are the left and right loop length), the resonance can be amplified in the SCR. 4. Conclusion To summarize, we fabricated high light coupling micocavity with micromanipulation of a single R6G-PNW. The dye-doped NWs act as both gain media and laser cavity. The relationship between guide loss and propagation distance of straight R6G-PNW and bent R6G-PNW were measured, which indicated that R6G-PNW is a typical characteristic of active waveguides. The behavior of the single nanowire resonator was explained by comparison with the laser spectrum, as well as experimental evidence, the evanescent field around the NW allows light guiding through sharp bends (micrometer level) with low optical loss. Acknowledgements This work was funded by the National Natural Science Foundation of China (Grants No.61227013,61422505, 61307104,61405044), the Program for New Century Excellent Talents in University (NCET-12-0623), National Key Scientific Instrument and Equipment Development Project (No. 2013YQ040815), and Specialized Research Fund for the Doctoral Program of Higher Education (No.20122304110022).
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Figure Captions: Fig. 1. (a) Schematic of R6G-PNWs fabrication by direct drawing process from molten R6G-PMMA. (b) Schematic of the instrumental setup for the R6G-PNWs lasing experiments.
Fig. 2. (a) Dark-filed PL microscope images of a 500 nm diameter 40 µm length bent R6G-PNW excited by the probe tip at different spot along the R6G-PNW. The insets are the gray images of the excited and output spots. (b) Bent R6G-PNW, normalized intensity of the output PL dependent the guided distance, the red line is fitted using a first-order exponential function.
Fig. 3. (a) Dark-filed PL microscope images of a 800 nm diameter 30 µm length straight R6G-PNW excited by the probe tip at different spot along the R6G-PNW. (b) Straight R6G-PNW, normalized intensity of the output PL dependent the guided distance.
Fig. 4. (a) SEM image of R6G-PNW with diameter of 900 nm and length of 36 µm, the R6G-PNW was cut into two segments, the length of the shorter one is 14 µm and the longer one is 22 µm. Inset: Dark-filed PL microscope image of the NW. (b) The spectra with the multiple peaks for the two segments straight R6G-PNW. The pump energy density is 66.4µJ/cm2.
Fig. 5. (a) SEM images of the SCR cavity fold by a 500 nm diameter 80 µm length R6G-PNW. Inset: Dark-filed PL microscope image of the NW. (b) The acquired single mode emission spectrum for (a) at the pump energy density of 113 µJ/cm2.
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