Whispering-Gallery-Mode for Coherent Random Lasing in a Dye-Doped Polystyrene Encapsulated Silica-Glass Capillary

Round 1

Reviewer 1 Report

Dear Authors,

The paper entitled Whispering-Gallery-Mode for Coherent Random Lasing in a Dye-Doped Polystyrene Encapsulated Silica-Glass Capillary shows a study of the effect on the generation of coherent random lasing in a dye-doped polystyrene cylinder fabricated with silica-glass capillary. The influence of the presence of the capillary, of its diameter, and of the refractive index of the dye-doped polystyrene is also determined.

In particular, it is found that the capillary eases the generation of random lasing because of the presence of whispering gallery mode in the capillary. In addition, a 500 μm diameter capillary gives the best results and it is easier to obtain random lasing with strong WGM resonances.

The paper is very well written. It shows the purpose of this research, the procedures are explained clearly, and the results seem reasonable.

I would suggest only a few adjustments to make it suitable for publication:

• Line 56: “within and without” should be “with and without”
• Lines 86-87: “within and without” should be “with and without”
• Line182: ”simples” should be “samples”
• Line 198: “conformed” should be “confirmed”

Author Response

Point 1: Line 56: “within and without” should be “with and without”

Response 1: Thanks for the referee’s constructive suggestions here. It has been revised.

 

Point 2: Lines 86-87: “within and without” should be “with and without”

Response 2: Thanks for the referee’s constructive suggestions here. It has been revised.

 

Point 3: Line182: “simples” should be “samples”

Response 3: Thanks for the referee’s constructive suggestions here. It has been revised.

 

Point 4: Line 198: “conformed” should be “confirmed”

Response 4: Thanks for the referee’s constructive suggestions here. It has been revised.

Reviewer 2 Report

   The authors investigated random lasing from dye-doped polystyrene encapsulated in a silica-glass capillary. The effects of whispering gallery mode (WGM) resonance occurred due to the capillary on the emission spectrum and the threshold of random lasing were examined for capillaries with various diameters and random gain media with various refractive indices. The experimental results found that the random laser emission is enhanced when the effect of WGM is large.

   Although this type of random laser structures has already been studied, few detailed investigations have been conducted on the WGM assisted random lasing. In this respect, the manuscript can be accepted for publication in Processes if the comments listed below are dealt with appropriately.

 

1. The ring laser picture often used to describe coherent random lasing is not appropriate for the laser action presented in this manuscript, because the probability of forming closed loops among scatterers shown in Fig. 2 is very small in the diffusive regime.
   Reference:
   Andreasen, et al., “Modes of random lasers,” Advances in Optics and Photonics 3, 88–127 (2011) doi:10.1364/AOP.3.000088.
2. Figure 2(b) should appear as Figure 1.
3. The diameters of the scattering particles and the pumping spot should be given.
4. AIBN should be spelled out.
5. The term “Dye-doped mixed solution” should be presented where DDMS first appears (section 2.3).
6. Figure 1(c): the unit “micro J” is missing.
7. Figure 4: the colors of plots in Figs. 4(a) and 4(b) should be matched (for example, S1 is in red in Fig. 4(a) but in black in Fig. 4(b).).
8. Figures S1 and S2 in the supplementary material should be included in the manuscript.

Author Response

Point 1: The ring laser picture often used to describe coherent random lasing is not appropriate for the laser action presented in this manuscript, because the probability of forming closed loops among scatterers shown in Fig. 2 is very small in the diffusive regime.

Reference:

Andreasen, et al., “Modes of random lasers,” Advances in Optics and Photonics 3, 88–127 (2011) doi:10.1364/AOP.3.000088.

Response 1: Thanks for the referee’s critical comments on laser and nice suggestions here. Coherent random laser can be obtained by multiple light scattering in the random medium (RSC Adv., 2016, 6, 85538). Previous work shows that strong near-field scattering enhancement exists in DDPS aggregates in solution, which triggers coherent random laser (Chin. J. Chem. Phys., 2019, 32, 739-746). The coherent random laser can be obtained from a DDPS sample alone under the excitation power of 600 μJ. This result indicates that the close-loop is formed in the disorder medium. In the present study, under pumping of 532 nm light with energy of 70 μJ, coherent random laser does not produce from DDPS without capillary. However, under pumping of 532 nm light with energy of 50 μJ, coherent random laser produces from DDPS with capillary. Based on the experiment, the Whispering Gallery Mode is proposed to explain the experimental phenomenon. The random laser action suggests that the close-loop is formed in the disorder medium. Therefore, the ring laser picture is appropriate for the laser action presented in this manuscript.

 

Point 2：Figure 2(b) should appear as Figure 1.

Response 2：Thanks for the referee’s suggestions here. In this paper, at first, we compared the random lasing behaviors of the polymer solid sample with and without capillary. The result showed that random laser was produced when there was capillary. To analyze the mechanism responsible for the coherent RL from DDPS, the Whispering Gallery Mode is proposed to explain the experimental results as illustrated in Figure 2b. Therefore, the order of Figure 1 and Figure 2 is reasonable. There is no need to change it.

 

Point 3：The diameters of the scattering particles and the pumping spot should be given.

Response 3：Thanks for the referee’s constructive suggestions here. The diameter of the scattering group (POSS) is 0.54 nm, which has been already given in the Introduction Section in the original version. The diameter of the pumping spot is 100 nm, which has been given in the section of Materials and Methods in the revision.

 

Point 4：AIBN should be spelled out.

Response 4：Thanks for referee’s nice suggestions. AIBN has been spelled out in the revision。

 

Point 5：The term “Dye-doped mixed solution” should be presented where DDMS first appears (section 2.3).

Response 5：Thanks for the referee’s suggestion here. We have corrected it.

 

Point 6：Figure 1(c): the unit “micro J” is missing.

Response 6: Thanks for the referee’s constructive suggestions here. The unit “micro J” has been provided in Figure 1C in the revision.

 

Point 7：Figure 4: the colors of plots in Figs. 4(a) and 4(b) should be matched (for example, S1 is in red in Fig. 4(a) but in black in Fig. 4(b).).

Response 7：We thank the reviewer for your comments and valuable suggestions. We have corrected it.

 

Point 8：Figures S1 and S2 in the supplementary material should be included in the manuscript.

Response 8：Thanks for the referee’s constructive suggestions here. Supporting information S1 has been already inserted into Figure 2 in the original version. According to your suggestion, supporting information S2 has been inserted into the revision as Figure 4.

Reviewer 3 Report

The authors observed the differences in lasing threshold with/without capillary. While the experiment clearly shows the difference, the theoretic analysis is quite speculative. I would recommend the author conduct a thorough analysis before making the conclusions.

1. The authors think the formation of whispering gallery mode helps to lower the threshold. However, there is no numerical effects made to support the claim. There could be other mechanisms causing such effect. For example, the capillary may behaves like a cylindrical lens when pump light passing through it. Consequently, the focal point could have much larger intensity and trigger the RL emission much easier than without a capillary. The author really need to do a legitimate analysis to convince reader that the mechanism they described is the case.
2. The author use the ratio between lasing wavelength and linewidth as "Q" (line119, page 4). However, that's not Q is conventionally defined. Usually Q in WGM refers to the optical quality factor of a "passive" cavity, not an active one presented here. In fact, assuming the laser noise is quantum limited, then the emission linewidth is quadratically inverse to Q, not linearly inverse to it as stated here. I would recommend the authors to drop the "Q" descriptions throughout this paper. Just state the linewidth is enough. It will be good if the authors can measure the Q of liquid filled capillary (without the DDPS).
3. The emission spectra consistently show multiple peaks, could the authors explain why?
4. Fig. 1c, I recommend the author add a zoom-in plot of the data points for pump energy above 25uJ  as an inset. It is quite important to observe the slope of FWHM in this interval.
5. What is the pulse width of the pump laser?
6. It is also bothersome that I can't even see a formation of whispering gallery mode in Fig. 5. Maybe the authors should try to increase the microscope magnification to see if they can capture the laser (not pump) whispering gallery formation. At least that will confirm that whispering gallery is indeed formed.
7. In fact, by looking at Fig. 5 carefully, it actually seems all DDPSs that lases are attached to the inner wall of the capillary, not elsewhere in the hollow core as described in Fig. 2. That actually could be the reason for the enhancement. Nevertheless, the author should do due diligence to analyze their results.
8. The language should be improved. Some sentences are very difficult to understand. I recommend the authors to find some professionals for proofreading.

Author Response

Point 1: The authors think the formation of whispering gallery mode helps to lower the threshold. However, there is no numerical effects made to support the claim. There could be other mechanisms causing such effect. For example, the capillary may behaves like a cylindrical lens when pump light passing through it. Consequently, the focal point could have much larger intensity and trigger the RL emission much easier than without a capillary. The author really need to do a legitimate analysis to convince reader that the mechanism they described is the case.

Response 1: Thanks for the referee’s critical comments on the mechanisms and valuable suggestions. In the present study, by comparing the random lasing behaviors of the polymer solid sample with and without capillary, we found that under pumping of 532 nm light with energy of 70 μJ, coherent random laser did not produce from DDPS without capillary. However, under pumping of 532 nm light with energy of 50 μJ, coherent random laser produced from DDPS with capillary. Based on the experiment, the Whispering Gallery Mode is proposed to explain the experimental phenomenon. As shown in Fig. 6, all bright emission of random lasers comes from the capillary inner wall where the WGM exists. This result reveals that when the fluorescence reaches the boundary between DDPS and the capillary, due to the larger RI of DDPS than that of capillary glass, with the appropriate angle of incident light, most part of the light will be totally reflected in DDPS-Capillary interface to continue propagation, which can form the first WGM. The formation of whispering gallery mode helps to lower the threshold. As indicated by the referee, there are other mechanisms causing such effect, such as that the capillary may behaves like a cylindrical lens when pump light passing through it. It is valuable suggestion. We will investigate the effects of cylindrical lens on RL emission in detail in future.  

 

Point 2: The author use the ratio between lasing wavelength and linewidth as "Q" (line119, page 4). However, that's not Q is conventionally defined. Usually Q in WGM refers to the optical quality factor of a "passive" cavity, not an active one presented here. In fact, assuming the laser noise is quantum limited, then the emission linewidth is quadratically inverse to Q, not linearly inverse to it as stated here. I would recommend the authors to drop the "Q" descriptions throughout this paper. Just state the linewidth is enough. It will be good if the authors can measure the Q of liquid filled capillary (without the DDPS).

Response 2: Thanks for the referee’s constructive suggestions here. In this paper, the Q refers to the quality factor of the laser, which is defined as Q = λ/Δλ, where λ is the peak wavelength and Δλ is the line width of the peak (Advanced Optical Materials, 2014, 2, 88-93). However, Q in WGM refers to the optical quality factor of a "passive" cavity. According to your nice suggestion, the quality factor of the laser has been renamed as QF in the revision to avoid confusion with the Q of WGM.

 

Point 3: The emission spectra consistently show multiple peaks, could the authors explain why?

Response 3: Thanks for referee’s comments on the multiple peaks. The emission spectra consistently show multiple sharp peaks. The multiple peaks is the inherent nature of random lasing behavior, which is achieved when specific frequencies of light are multiply amplified by stimulated emission in randomly closed loop paths (ACS Photonics, 2014, 1, 1258-1263). Each sharp peak refers to one randomly closed loop path. We have explained the multiple peaks in the revision.

 

 

Point 4: Fig. 1c, I recommend the author add a zoom-in plot of the data points for pump energy above 25 uJ as an inset. It is quite important to observe the slope of FWHM in this interval.

Response 4: Thanks for the referee’s useful suggestion here. According to referee’s useful suggestion, we have checked the values of FWHM. FWHM are 0.432 nm，0.428 nm，0.446 nm，0.425 nm and 0.439 nm, respectively, in the range of pump energy from 25 mJ to 45 mJ. This result indicates that FWHM essentially does not change with increasing of pump energy from 25 mJ to 45 mJ. The slope of FWHM in this interval is close to zero. Because Figure 1C has already showed that the slope of FWHM in this interval is close to zero, we do not add a zoom-in plot of the data points for pump energy above 25 mJ as an inset.

 

Point 5: What is the pulse width of the pump laser?

Response 5: Thanks for the referee’s comment here. The pulse width of the pump laser is 10 ns, which has been already shown in the original version on line 62.

 

Point 6: It is also bothersome that I can't even see a formation of whispering gallery mode in Fig. 5. Maybe the authors should try to increase the microscope magnification to see if they can capture the laser (not pump) whispering gallery formation. At least that will confirm that whispering gallery is indeed formed.

Response 6: Thanks for the referee’s critical comments on the formation of whispering gallery mode. Coherent random laser can be obtained by multiple light scattering in the random medium (RSC Adv., 2016, 6, 85538). Previous work shows that strong near-field scattering enhancement exists in DDPS aggregates in solution, which triggers coherent random laser (Chin. J. Chem. Phys., 2019, 32, 739-746). The coherent random laser can be obtained from a DDPS sample alone under the excitation power of 600 μJ. This result indicates that the close-loop is formed in the disorder medium. In the present study, under pumping of 532 nm light with energy of 70 μJ, coherent random laser does not produce from DDPS without capillary. However, under pumping of 532 nm light with energy of 50 μJ, coherent random laser produces from DDPS with capillary. Based on the experiment, the Whispering Gallery Mode is proposed to explain the experimental phenomenon. As shown in Fig. 6, all bright emission of random lasers comes from the capillary inner wall where the WGM exists. This result reveals that when the fluorescence reaches the boundary between DDPS and the capillary, due to the larger RI of DDPS than that of capillary glass, with the appropriate angle of incident light, most part of the light will be totally reflected in DDPS-Capillary interface to continue propagation, which can form the first WGM. The formation of whispering gallery mode helps to lower the threshold. Our results confirm that whispering gallery is indeed formed.

 

 

Point 7: In fact, by looking at Fig. 5 carefully, it actually seems all DDPS that lases are attached to the inner wall of the capillary, not elsewhere in the hollow core as described in Fig. 2. That actually could be the reason for the enhancement. Nevertheless, the author should do due diligence to analyze their results.

Response 7: Thanks for referee’s positive comments and constructive suggestions. In fact, by looking at Fig. 6 in revision carefully, no laser spot come from the intracavity of the capillary. All bright emission of random lasers comes from the capillary inner wall where the WGM exists, which do confirm that WGM boosts random lasers. This result reveals that although the close-loop is formed in the disorder medium, coherent random laser does not produce from DDPS in the intracavity of the capillary owing to that the pumping energy is lower than the pumping threshold. When the fluorescence reaches the boundary between DDPS and the capillary, due to the larger RI of DDPS than that of capillary glass, with the appropriate angle of incident light, most part of the light will be totally reflected in DDPS-Capillary interface to continue propagation, which can form the first WGM. Because the formation of whispering gallery mode helps to lower the threshold, the coherent random laser produces from the capillary inner wall capillary. This issue has been discussed in the revision. 

 

Point 8: The language should be improved. Some sentences are very difficult to understand. I recommend the authors to find some professionals for proofreading.

Response 8: Thanks for referee’s valuable suggestion. We have checked and revised English carefully through the manuscript.