January 1, 2014 / Vol. 39, No. 1 / OPTICS LETTERS
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Cascade-pumped random lasers with coherent emission formed by Ag–Au porous nanowires Yanrong Wang,1 Xiaoyu Shi,1 Yanyan Sun,1 Ruqiang Zheng,1 Sujun Wei,1 Jinwei Shi,1,* Zhaona Wang,1 and Dahe Liu1,2,3 1 2
Applied Optics Beijing Area Major Laboratory, Department of Physics, Beijing Normal University, Beijing 100875, China
Key Laboratory of Nondestructive Test (Ministry of Education), Nanchang Hangkong University, Nanchang 330063, China 3 e-mail: [email protected] *Corresponding auhtor: [email protected] Received October 28, 2013; accepted November 11, 2013; posted November 13, 2013 (Doc. ID 200273); published December 16, 2013 A series of sequentially cascade-pumped random lasers is reported. It consists of three random lasers in which the Ag–Au bimetallic porous nanowires play the role of scatterers, and the gain materials are coumarin 440 (C440), coumarin 153 (C153), and rhodamine 6G (R6G), respectively. The random laser with C440 is first pumped by a 355 nm pulsed laser. The emission of C440 pumps the C153, and the emission of C153 pumps the R6G sequentially. Low-threshold coherent emissions from the three random lasers are observed. The cascade-pumped random lasers can be achieved easily with low cost and can be used in applications conveniently. © 2013 Optical Society of America OCIS codes: (140.3460) Lasers; (140.5560) Pumping; (160.3918) Metamaterials. http://dx.doi.org/10.1364/OL.39.000005
A series of cascade-pumped lasers are important in integrated optics [1]. For example, a laser with certain power can be used for multichannel applications, a wideband laser is useful in the application of wavelength division multiplexing, and a narrowband laser can be employed for the use of modulation. Therefore the laser based on integrated photonics will play the key role in integrated optics [2]. Previously, Chiu et al. reported a cascadepumped picosecond dye laser system [3]. However, because a cavity is needed for the dye laser, and the emission is only in one direction, the optical layout for achieving cascade pumping is complicated. In contrast, the emission from the random laser has no fixed direction, so the emissions in different directions can be used as pumping beam and output beam as required. Since a random laser was suggested in principle [4,5] and verified in experiments [6], wide attention has been given to this topic. Many systems have shown the lasing behavior, such as neodymium-glass powders [7], dye-TiO2 solutions [8], nanoclusters of ZnO [9], conjugated polymer films [10], dye infiltrated opals [11], dye-TiO2 polymer films [12], laser dye within liquid crystals [13], and laser speckles with dye [14]. Generally, there are two kinds of mechanisms for random lasing: coherent feedback and incoherent feedback [15]. Up to now, coherent random lasers have been achieved in strong and weak scattering systems. The physical picture for lasing in a strong scattering systems is widely accepted: the photon localization is responsible for the emergence of coherent lasing in such a system [9]. On the other hand, the understanding of coherent random lasers in weak scattering regimes is still an open question. Many works have been reported to clarify this problem [16–24]. In applications, a random laser with multiwavelength emissions, even white light emission [25], is desired. However, for the case of multifrequency emissions, it is hard to obtain coherent feedback of different frequencies at the same time because (1) if a single pump beam is used, one of the gain materials will absorb 0146-9592/14/010005-04$15.00/0
most of the pumping energy, and coherent lasing cannot be built up for all gain materials; (2) high threshold is needed for coherent lasing. The cascade-pumped random lasers presented here have no such problem due to two reasons: (1) each random laser is pumped by an independent pumping source; (2) with Ag–Au bimetalic nanowires working as the scatterers, low-threshold value for coherent lasing of a random laser easily can be realized. Therefore, a cascade-pumped random laser will be an effective way to obtain a multiwavelength coherent emission. In contrast, [25] only achieved incoherent lasing with mixed wavelengths and ZnO nanoparticles cannot provide a low threshold for coherent lasing. In our system, Ag–Au bimetallic porous nanowires were used to provide scattering. Ag nanowires were synthesized by a polyvinylpyrrolidone-assisted reaction in ethylene glycol [26,27]. The Ag nanowires were then filtered by multicentrifugation, washed with acetone, and dispersed in ethanol to get the Ag nanowires ink with a concentration of 0.033 M for further experiments. After that, a chloroauric acid (HAuCl4) solution with a concentration of 4.8 mM was dropwise added into the Ag nanowire dispersions. Based on the galvanic replacement reaction between Ag and HAuCl4, Au–Ag bimetallic porous nanowires were synthesized. Figure 1 shows the SEM image of nanowires when the molar ratio γ M of the added HAuCl4 and Ag nanowires was γ M M Au 0.0072 and M Ag 0.288 (d), respectively. Figure 1(a) shows that the mean diameter of Ag nanowires is around 150 nm, and the mean length is about 10 μm. Compared with Figs. 1(a) and 1(b) shows the Ag nanowires were etched to form nanoscale pores and attached Au nanoparticles forming protuberances at the surfaces by adding little HAuCl4 into Ag nanowires dispersions. By measuring the transmission in the spectral range of 530–560 nm, the mean free path of the system is estimated to be l > 2 mm [28], which is the order of the system size; therefore, this is definitely a weak scattering system. In the experiments of the present work, γ M 0.0072, © 2014 Optical Society of America
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OPTICS LETTERS / Vol. 39, No. 1 / January 1, 2014 Table 1. Absorbance and Fluorescence of Three Laser Dyes Laser Dye C440 C153 R6G
Fig. 1. SEM images of (a) Ag nanowires and (b) Ag–Au bimetallic porous nanowires as scatterers.
the concentration for the Ag nanowires ink of ρ 1.01 × 108 mL−1 were chosen. The gain materials used were three laser dyes: coumarin 440 (C440), coumarin 153 (C153), and rhodamine 6G (R6G). Figure 2 shows the absorbance and fluorescence of the three laser dyes. Table 1 gives the related data. It can be found that the absorbance range of C440 is 323 to 400 nm, which matches with the tripling frequency at 355 nm of a Nd:YAG laser, and its emission is around 447 nm. The absorbance range of C153 is 344 to 464 nm, which matches with the emission wavelength 447 nm of C440, and its emission is around 548 nm. The absorbance range of R6G is 439 to 556 nm, which matches with the emission wavelength 548 nm of C153, and its emission is around 583 nm. Each dye solution in ethanol was put in a cuvette together with the prepared Ag–Au bimatellic nanowires to form a random laser. Respectively, the concentrations of the three dyes are 1.67 mg∕ml for C440, 0.417 mg∕ml for C153, and 0.15 mg∕ml for R6G. As used for cascadepumped lasers, the threshold value of each laser needs to be low. The thresholds of the three lasers were measured, and the results are shown in Fig. 3. It can be seen that the thresholds of the three random lasers are 0.0774 MW∕cm2 (C440), 0.769 MW∕cm2 (C153), and 0.7724 MW∕cm2 (R6G) respectively, which are all considerably low for a random laser. The three cuvettes are placed crosswise as shown in Fig. 4 to achieve optimum pumping. The third harmonic
Range of Absorbance (nm)
Range of Fluorescence (nm)
323–400 344–464 439–556
403–535 485–653 559–682
at 355 nm of a pulsed Nd:YAG laser with the specifications of 8 ns pulse duration, 10 Hz repetition rate, and 8 mm diameter of the output beam (Continuum Powerlite Precision 8000) was employed to pump the first random laser with C440. The emission of C440 excites the second random laser with C153 as the pumped source. Then the emission from C153 plays the role of pumped beam of the third random laser with R6G. In the above sequence, the cascade-pumped random lasers were achieved. From Fig. 4 the colors corresponding to the different single frequencies and their mixed color obviously can be seen. It should be addressed that our system also can be easily used to generate a single laser with various selected frequency intervals. For this purpose, one can improve the energy-conversion efficiency by many simple methods, e.g., depositing high reflective Al film on all the surfaces except the output window. Figure 5 shows the spectra of the coherent emissions from the three cascade-pumped random lasers. The measurements were taken at the direction perpendicular to the pump beam. It should be addressed that different
Fig. 3. Thresholds of three random lasers formed by laser dyes and Ag–Au bimetallic nanowires.
Fig. 2. Measured spectra of absorbance and fluorescence of the three laser dyes used. The dashed lines represent absorbance, and the solid lines represent fluorescence.
Fig. 4. Experimental setup geometry of cascade-pumped random lasers. The left shows the layout, and the right shows the pumping sequence and experimental result.
January 1, 2014 / Vol. 39, No. 1 / OPTICS LETTERS
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Fig. 6. Temporal profile of the emission of the random laser.
Fig. 5. Coherent emission spectra of cascade-pumped random lasers. (a) Three emission spectra measured by a spectrometer with a resolution of 0.4 nm. (b), (c), and (d) Sharp spikes of C440, C153, and R6G measured by a spectrometer with the resolution of 0.01 nm.
neutral-density filters were used for measuring the spectra, so the heights of every peak do not present real intensity of the emissions. The spectra at left were measured using an Ocean Optics fiber spectrometer MAYA 2000 with a resolution of 0.4 nm. It can be seen that three groups of narrow peaks around 447, 548, and 583 nm are obtained. More precise spectral measurements were taken further using a Princeton Instruments spectrometer Acton SP2750 with a resolution of 0.01 nm, and the results are shown at right. Sharp spikes correspond to the narrow peaks at left. The linewidth of each spike is less than 0.05 nm, which indicates typical coherent lasing. It should be pointed out that, when coherent lasing occurs, the wavelength of every sharp spike is fixed, although its intensity may change from shot to shot. It is just the characteristics of a random laser. It is known that the quantum efficiency of R6G is high, so R6G is chosen as the third random laser since the pumped energy for it is lowest in the three pumped sources. The output energy of the pulsed Nd:YAG laser should be adjusted to pump the first random laser with C440, and further, the output from the C440 will pump the second random laser with C153, so that the output energy from C153 is powerful enough to excite coherent emissions of the third random laser with R6G while remaining the coherent emission of C153 itself. In our experiments, when the output energy of the Nd:YAG laser is higher than 0.537 MW∕cm2 , three coherent lasing could be obtained simultaneously. To further prove the lasing, Fig. 6 gives the temporal profile of the emission of the first random laser in our cascade-pumped system. During the measurement, the actual pulse duration of the pumping Nd:YAG laser was 20 ns running at 14.17 MW∕cm2 , which is far beyond the threshold of the random lasers. It can be seen that the pulse duration of the random laser is 15.3 ns, which is
narrower than the pumping pulse. It is just the character of lasing. A low threshold is the guarantee for cascade-pumped random lasers. In our work, the Ag–Au porous nanowires provide the low threshold of the system. From Fig. 1(b), it can be seen that the scatters actually have the porous structure. Therefore, many nanogaps are formed on the surface of the nanowires [25,26]. The nanogaps play an important role to achieve coherent lasing with a low threshold. These nanogaps can lead to a distinguished advantage: the dye molecules can be readily concentrated to nanometer scales to increase the effective gain, and electrical field enhancement can help the reduction of the threshold. Future work will focus on obtaining more lasing modes, even a supercontinuum white laser, while keeping the low threshold, e.g., continuum working. The authors would like to thank the National Natural Science Foundation of China (Grant Nos. 11074024, 11104016, and 11374037), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20100003120009), and the Fundamental Research Funds of the Central Universities (No. 2013YB65) for financial support. References 1. R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer Verlag, 2002). 2. H. Rong, R. Jones, A. Liu, O. Cohen, D. Hak, A. Fang, and M. Paniccia, Nature 433, 725 (2005). 3. P. H. Chiu, S. Hsu, S. J. C. Box, and H.-S. Kowk, IEEE J. Quantum Electron. 20, 652 (1984). 4. R. V. Ambartsumyan, N. G. Basov, P. G. Kryukov, and V. S. Letokhov, IEEE J. Quantum Electron. 2, 442 (1966). 5. V. S. Letokhov, Sov. Phys. J. Exp. Theor. Phys. 26, 835 (1968). 6. N. Lawandy, R. Balachandran, A. Gomes, and E. Sauvain, Nature 368, 436 (1994). 7. C. Gouedard, D. Husson, C. Sauteret, F. Auzel, and A. Mingus, J. Opt. Soc. Am. B 10, 2358 (1993). 8. N. M. Lawandy, R. M. Balachandran, A. S. L. Gomes, and E. Sauvain, Nature 368, 436 (1994). 9. H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and P. R. H. Chang, Phys. Rev. Lett. 82, 2278 (1999). 10. S. V. Frolov, Z. V. Vardeny, K. Yoshino, A. A. Zakhidov, and R. H. Baughman, Phys. Rev. B 59, R5282 (1999). 11. S. V. Frolov, Z. V. Vardeny, A. A. Zakhidov, and R. H. Baughman, Opt. Commun. 162, 241 (1999). 12. G. Zacharakis, N. A. Papadogiannis, G. Pilippidis, and T. G. Papazoglou, Opt. Lett. 25, 923 (2000). 13. D. Wiersma and S. Cavalier, Nature 414, 708 (2001). 14. T. Zhai, Y. Zhou, S. Chen, Z. Wang, J. Shi, D. Liu, and X. Zhang, Phys. Rev. A 82, 023824 (2010).
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OPTICS LETTERS / Vol. 39, No. 1 / January 1, 2014
15. H. Cao, Waves in Random Media (2003), Vol. 13, p. R1. 16. V. M. Apalkov, M. E. Raikh, and B. Shapiro, Phys. Rev. Lett. 89, 016802 (2002). 17. S. Mujumdar, M. Ricci, R. Torre, and D. Wiersma, Phys. Rev. Lett. 93, 053903 (2004). 18. A. Yamilov, X. Wu, H. Cao, and A. L. Burin, Opt. Lett. 30, 2430 (2005). 19. X. Wu, W. Fang, A. Yamilov, A. A. Chabanov, A. A. Asatryan, L. C. Botten, and H. Cao, Phys. Rev. A 74, 053812 (2006). 20. C. Vanneste, P. Sebbah, and H. Cao, Phys. Rev. Lett. 98, 143902 (2007). 21. K. L. van der Molen, R. W. Tjerkstra, A. P. Mosk, and A. Lagendijk, Phys. Rev. Lett. 98, 143901 (2007).
22. H. E. Türeci, L. Ge, S. Rotter, and A. D. Stone, Science 320, 643 (2008). 23. J. Andreasen and H. Cao, Opt. Express 19, 3418 (2011). 24. S. Chen, J. Shi, X. Kong, Z. Wang, and D. Liu, Laser Phys. Lett. 10, 055006 (2013). 25. S. Chen, X. Zhao, Y. Wang, J. Shi, and D. Liu, Appl. Phys. Lett. 101, 123508 (2012). 26. N. L. Netzer, C. Qiu, Y. Zhang, C. Lin, L. Zhang, H. Fong, and C. Jiang, Chem. Commun. 47, 9606 (2011). 27. X. Xu, K. Kim, H. Li, and D. L. Fan, Adv. Mater. 24, 5457 (2012). 28. X. D. Zhang and Z. Q. Zhang, Phys. Rev. B 65, 155208 (2002).
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Cascade-pumped random lasers with coherent emission formed by Ag–Au porous nanowires Yanrong Wang,1 Xiaoyu Shi,1 Yanyan Sun,1 Ruqiang Zheng,1 Sujun Wei,1 Jinwei Shi,1,* Zhaona Wang,1 and Dahe Liu1,2,3 1 2
Applied Optics Beijing Area Major Laboratory, Department of Physics, Beijing Normal University, Beijing 100875, China
Key Laboratory of Nondestructive Test (Ministry of Education), Nanchang Hangkong University, Nanchang 330063, China 3 e-mail: [email protected] *Corresponding auhtor: [email protected] Received October 28, 2013; accepted November 11, 2013; posted November 13, 2013 (Doc. ID 200273); published December 16, 2013 A series of sequentially cascade-pumped random lasers is reported. It consists of three random lasers in which the Ag–Au bimetallic porous nanowires play the role of scatterers, and the gain materials are coumarin 440 (C440), coumarin 153 (C153), and rhodamine 6G (R6G), respectively. The random laser with C440 is first pumped by a 355 nm pulsed laser. The emission of C440 pumps the C153, and the emission of C153 pumps the R6G sequentially. Low-threshold coherent emissions from the three random lasers are observed. The cascade-pumped random lasers can be achieved easily with low cost and can be used in applications conveniently. © 2013 Optical Society of America OCIS codes: (140.3460) Lasers; (140.5560) Pumping; (160.3918) Metamaterials. http://dx.doi.org/10.1364/OL.39.000005
A series of cascade-pumped lasers are important in integrated optics [1]. For example, a laser with certain power can be used for multichannel applications, a wideband laser is useful in the application of wavelength division multiplexing, and a narrowband laser can be employed for the use of modulation. Therefore the laser based on integrated photonics will play the key role in integrated optics [2]. Previously, Chiu et al. reported a cascadepumped picosecond dye laser system [3]. However, because a cavity is needed for the dye laser, and the emission is only in one direction, the optical layout for achieving cascade pumping is complicated. In contrast, the emission from the random laser has no fixed direction, so the emissions in different directions can be used as pumping beam and output beam as required. Since a random laser was suggested in principle [4,5] and verified in experiments [6], wide attention has been given to this topic. Many systems have shown the lasing behavior, such as neodymium-glass powders [7], dye-TiO2 solutions [8], nanoclusters of ZnO [9], conjugated polymer films [10], dye infiltrated opals [11], dye-TiO2 polymer films [12], laser dye within liquid crystals [13], and laser speckles with dye [14]. Generally, there are two kinds of mechanisms for random lasing: coherent feedback and incoherent feedback [15]. Up to now, coherent random lasers have been achieved in strong and weak scattering systems. The physical picture for lasing in a strong scattering systems is widely accepted: the photon localization is responsible for the emergence of coherent lasing in such a system [9]. On the other hand, the understanding of coherent random lasers in weak scattering regimes is still an open question. Many works have been reported to clarify this problem [16–24]. In applications, a random laser with multiwavelength emissions, even white light emission [25], is desired. However, for the case of multifrequency emissions, it is hard to obtain coherent feedback of different frequencies at the same time because (1) if a single pump beam is used, one of the gain materials will absorb 0146-9592/14/010005-04$15.00/0
most of the pumping energy, and coherent lasing cannot be built up for all gain materials; (2) high threshold is needed for coherent lasing. The cascade-pumped random lasers presented here have no such problem due to two reasons: (1) each random laser is pumped by an independent pumping source; (2) with Ag–Au bimetalic nanowires working as the scatterers, low-threshold value for coherent lasing of a random laser easily can be realized. Therefore, a cascade-pumped random laser will be an effective way to obtain a multiwavelength coherent emission. In contrast, [25] only achieved incoherent lasing with mixed wavelengths and ZnO nanoparticles cannot provide a low threshold for coherent lasing. In our system, Ag–Au bimetallic porous nanowires were used to provide scattering. Ag nanowires were synthesized by a polyvinylpyrrolidone-assisted reaction in ethylene glycol [26,27]. The Ag nanowires were then filtered by multicentrifugation, washed with acetone, and dispersed in ethanol to get the Ag nanowires ink with a concentration of 0.033 M for further experiments. After that, a chloroauric acid (HAuCl4) solution with a concentration of 4.8 mM was dropwise added into the Ag nanowire dispersions. Based on the galvanic replacement reaction between Ag and HAuCl4, Au–Ag bimetallic porous nanowires were synthesized. Figure 1 shows the SEM image of nanowires when the molar ratio γ M of the added HAuCl4 and Ag nanowires was γ M M Au 0.0072 and M Ag 0.288 (d), respectively. Figure 1(a) shows that the mean diameter of Ag nanowires is around 150 nm, and the mean length is about 10 μm. Compared with Figs. 1(a) and 1(b) shows the Ag nanowires were etched to form nanoscale pores and attached Au nanoparticles forming protuberances at the surfaces by adding little HAuCl4 into Ag nanowires dispersions. By measuring the transmission in the spectral range of 530–560 nm, the mean free path of the system is estimated to be l > 2 mm [28], which is the order of the system size; therefore, this is definitely a weak scattering system. In the experiments of the present work, γ M 0.0072, © 2014 Optical Society of America
6
OPTICS LETTERS / Vol. 39, No. 1 / January 1, 2014 Table 1. Absorbance and Fluorescence of Three Laser Dyes Laser Dye C440 C153 R6G
Fig. 1. SEM images of (a) Ag nanowires and (b) Ag–Au bimetallic porous nanowires as scatterers.
the concentration for the Ag nanowires ink of ρ 1.01 × 108 mL−1 were chosen. The gain materials used were three laser dyes: coumarin 440 (C440), coumarin 153 (C153), and rhodamine 6G (R6G). Figure 2 shows the absorbance and fluorescence of the three laser dyes. Table 1 gives the related data. It can be found that the absorbance range of C440 is 323 to 400 nm, which matches with the tripling frequency at 355 nm of a Nd:YAG laser, and its emission is around 447 nm. The absorbance range of C153 is 344 to 464 nm, which matches with the emission wavelength 447 nm of C440, and its emission is around 548 nm. The absorbance range of R6G is 439 to 556 nm, which matches with the emission wavelength 548 nm of C153, and its emission is around 583 nm. Each dye solution in ethanol was put in a cuvette together with the prepared Ag–Au bimatellic nanowires to form a random laser. Respectively, the concentrations of the three dyes are 1.67 mg∕ml for C440, 0.417 mg∕ml for C153, and 0.15 mg∕ml for R6G. As used for cascadepumped lasers, the threshold value of each laser needs to be low. The thresholds of the three lasers were measured, and the results are shown in Fig. 3. It can be seen that the thresholds of the three random lasers are 0.0774 MW∕cm2 (C440), 0.769 MW∕cm2 (C153), and 0.7724 MW∕cm2 (R6G) respectively, which are all considerably low for a random laser. The three cuvettes are placed crosswise as shown in Fig. 4 to achieve optimum pumping. The third harmonic
Range of Absorbance (nm)
Range of Fluorescence (nm)
323–400 344–464 439–556
403–535 485–653 559–682
at 355 nm of a pulsed Nd:YAG laser with the specifications of 8 ns pulse duration, 10 Hz repetition rate, and 8 mm diameter of the output beam (Continuum Powerlite Precision 8000) was employed to pump the first random laser with C440. The emission of C440 excites the second random laser with C153 as the pumped source. Then the emission from C153 plays the role of pumped beam of the third random laser with R6G. In the above sequence, the cascade-pumped random lasers were achieved. From Fig. 4 the colors corresponding to the different single frequencies and their mixed color obviously can be seen. It should be addressed that our system also can be easily used to generate a single laser with various selected frequency intervals. For this purpose, one can improve the energy-conversion efficiency by many simple methods, e.g., depositing high reflective Al film on all the surfaces except the output window. Figure 5 shows the spectra of the coherent emissions from the three cascade-pumped random lasers. The measurements were taken at the direction perpendicular to the pump beam. It should be addressed that different
Fig. 3. Thresholds of three random lasers formed by laser dyes and Ag–Au bimetallic nanowires.
Fig. 2. Measured spectra of absorbance and fluorescence of the three laser dyes used. The dashed lines represent absorbance, and the solid lines represent fluorescence.
Fig. 4. Experimental setup geometry of cascade-pumped random lasers. The left shows the layout, and the right shows the pumping sequence and experimental result.
January 1, 2014 / Vol. 39, No. 1 / OPTICS LETTERS
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Fig. 6. Temporal profile of the emission of the random laser.
Fig. 5. Coherent emission spectra of cascade-pumped random lasers. (a) Three emission spectra measured by a spectrometer with a resolution of 0.4 nm. (b), (c), and (d) Sharp spikes of C440, C153, and R6G measured by a spectrometer with the resolution of 0.01 nm.
neutral-density filters were used for measuring the spectra, so the heights of every peak do not present real intensity of the emissions. The spectra at left were measured using an Ocean Optics fiber spectrometer MAYA 2000 with a resolution of 0.4 nm. It can be seen that three groups of narrow peaks around 447, 548, and 583 nm are obtained. More precise spectral measurements were taken further using a Princeton Instruments spectrometer Acton SP2750 with a resolution of 0.01 nm, and the results are shown at right. Sharp spikes correspond to the narrow peaks at left. The linewidth of each spike is less than 0.05 nm, which indicates typical coherent lasing. It should be pointed out that, when coherent lasing occurs, the wavelength of every sharp spike is fixed, although its intensity may change from shot to shot. It is just the characteristics of a random laser. It is known that the quantum efficiency of R6G is high, so R6G is chosen as the third random laser since the pumped energy for it is lowest in the three pumped sources. The output energy of the pulsed Nd:YAG laser should be adjusted to pump the first random laser with C440, and further, the output from the C440 will pump the second random laser with C153, so that the output energy from C153 is powerful enough to excite coherent emissions of the third random laser with R6G while remaining the coherent emission of C153 itself. In our experiments, when the output energy of the Nd:YAG laser is higher than 0.537 MW∕cm2 , three coherent lasing could be obtained simultaneously. To further prove the lasing, Fig. 6 gives the temporal profile of the emission of the first random laser in our cascade-pumped system. During the measurement, the actual pulse duration of the pumping Nd:YAG laser was 20 ns running at 14.17 MW∕cm2 , which is far beyond the threshold of the random lasers. It can be seen that the pulse duration of the random laser is 15.3 ns, which is
narrower than the pumping pulse. It is just the character of lasing. A low threshold is the guarantee for cascade-pumped random lasers. In our work, the Ag–Au porous nanowires provide the low threshold of the system. From Fig. 1(b), it can be seen that the scatters actually have the porous structure. Therefore, many nanogaps are formed on the surface of the nanowires [25,26]. The nanogaps play an important role to achieve coherent lasing with a low threshold. These nanogaps can lead to a distinguished advantage: the dye molecules can be readily concentrated to nanometer scales to increase the effective gain, and electrical field enhancement can help the reduction of the threshold. Future work will focus on obtaining more lasing modes, even a supercontinuum white laser, while keeping the low threshold, e.g., continuum working. The authors would like to thank the National Natural Science Foundation of China (Grant Nos. 11074024, 11104016, and 11374037), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20100003120009), and the Fundamental Research Funds of the Central Universities (No. 2013YB65) for financial support. References 1. R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer Verlag, 2002). 2. H. Rong, R. Jones, A. Liu, O. Cohen, D. Hak, A. Fang, and M. Paniccia, Nature 433, 725 (2005). 3. P. H. Chiu, S. Hsu, S. J. C. Box, and H.-S. Kowk, IEEE J. Quantum Electron. 20, 652 (1984). 4. R. V. Ambartsumyan, N. G. Basov, P. G. Kryukov, and V. S. Letokhov, IEEE J. Quantum Electron. 2, 442 (1966). 5. V. S. Letokhov, Sov. Phys. J. Exp. Theor. Phys. 26, 835 (1968). 6. N. Lawandy, R. Balachandran, A. Gomes, and E. Sauvain, Nature 368, 436 (1994). 7. C. Gouedard, D. Husson, C. Sauteret, F. Auzel, and A. Mingus, J. Opt. Soc. Am. B 10, 2358 (1993). 8. N. M. Lawandy, R. M. Balachandran, A. S. L. Gomes, and E. Sauvain, Nature 368, 436 (1994). 9. H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and P. R. H. Chang, Phys. Rev. Lett. 82, 2278 (1999). 10. S. V. Frolov, Z. V. Vardeny, K. Yoshino, A. A. Zakhidov, and R. H. Baughman, Phys. Rev. B 59, R5282 (1999). 11. S. V. Frolov, Z. V. Vardeny, A. A. Zakhidov, and R. H. Baughman, Opt. Commun. 162, 241 (1999). 12. G. Zacharakis, N. A. Papadogiannis, G. Pilippidis, and T. G. Papazoglou, Opt. Lett. 25, 923 (2000). 13. D. Wiersma and S. Cavalier, Nature 414, 708 (2001). 14. T. Zhai, Y. Zhou, S. Chen, Z. Wang, J. Shi, D. Liu, and X. Zhang, Phys. Rev. A 82, 023824 (2010).
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OPTICS LETTERS / Vol. 39, No. 1 / January 1, 2014
15. H. Cao, Waves in Random Media (2003), Vol. 13, p. R1. 16. V. M. Apalkov, M. E. Raikh, and B. Shapiro, Phys. Rev. Lett. 89, 016802 (2002). 17. S. Mujumdar, M. Ricci, R. Torre, and D. Wiersma, Phys. Rev. Lett. 93, 053903 (2004). 18. A. Yamilov, X. Wu, H. Cao, and A. L. Burin, Opt. Lett. 30, 2430 (2005). 19. X. Wu, W. Fang, A. Yamilov, A. A. Chabanov, A. A. Asatryan, L. C. Botten, and H. Cao, Phys. Rev. A 74, 053812 (2006). 20. C. Vanneste, P. Sebbah, and H. Cao, Phys. Rev. Lett. 98, 143902 (2007). 21. K. L. van der Molen, R. W. Tjerkstra, A. P. Mosk, and A. Lagendijk, Phys. Rev. Lett. 98, 143901 (2007).
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