Photonic Hook Initiated Using an Air–Liquid Interface
Round 1
Reviewer 1 Report
In this paper, the authors demonstrate a novel photonic hook initiated by an air-liquid interface (ALI). This bent light focus is produced by a dielectric micro-cylinder partially immersed at the edge of a thin liquid film whose thickness is smaller than the diameter of the micro-cylinder. The photonic hook propagates horizontally in the liquid along the ALI at specific depths and can generate a force field that enables optical trapping in the region slightly beneath the ALI. The morphological analysis indicates that the contrast in refractive indexes of ALI causes this phenomenon at the shadow end of the micro-cylinder with a transverse dimension smaller than the diffraction limit. This study is interesting and helpful to the photonics field. The paper can be accepted by Photonics after the following issues are addressed.
(1) How will the refractive index of the liquid affect the properties of the novel photonic hook?
(2) For a liquid layer with defined thickness, how will the relative refractive index (n2/n0) of the liquid (n2) air (n0) affect the properties of the novel photonic hook? To ensure that the novel photonic hook can exist, what is the range of n2/n0?
Comments for author File: 
Comments.pdf
Author Response
Hello, thank you very much for the comments. All responses and a figure are included in the attached PDF file.
Author Response File: Author Response.pdf
Reviewer 2 Report
I am satisfied with the response from the authors. I will recommend acceptance.
Author Response
Reviewer 2:
Comments and Suggestions for Authors
I am satisfied with the response from the authors. I will recommend acceptance.
Response: Thank you very much for your support.
Reviewer 3 Report
In this work, the Authors demonstrate a novel photonic hook initiated by and propagating along the air-liquid interface (ALI), based on a setup of a long dielectric micro-cylinder or an optical fiber core partially immersed at the edge of a thin water film. The authors clearly and thoroughly provide the necessary background information by proposing brief but exhaustive state-of-the-art guides that allow the authors to justify specific technical choices. The manuscript is well written with good and clear English and is well organized in paragraphs. Overall, the work is publishable, subject to minor imperfections and minor shortcomings.
Indeed, the comments related to minor revisions are given below:
- The target application has to be clearly defined, according to the used power. The Authors discuss about trapping. A comparison with recent results have to be provided (see, e.g., https://doi.org/10.1103/PhysRevA.106.033511; https://doi.org/10.3390/photonics9060425; https://doi.org/10.1364/JOSAA.463732)
· In section 2, I suggest that the authors justify with more detail and information the choices regarding the photonic hook configuration describing any advantages and future challenges for fabrication.
· The results of this experimental work indicate that the proposed photonic hook has unique qualities due to its attributes of curved propagation along ALI. Since the results are also promising regarding possible fabrication, I suggest the authors include indications and information regarding fabrication steps in a future perspective.
· Line 137: Delete the letter “ α ”from the paragraph "Results and discussion".
· Line 299: The proposed photonic hook's future developments are unclear. I suggest going into more detail about the applicability to generate more interest in the reader.
· line 388: Reference 42 is underlined.
Author Response
Reviewer 3:
Comments and Suggestions for Authors
In this work, the Authors demonstrate a novel photonic hook initiated by and propagating along the air-liquid interface (ALI), based on a setup of a long dielectric micro-cylinder or an optical fiber core partially immersed at the edge of a thin water film. The authors clearly and thoroughly provide the necessary background information by proposing brief but exhaustive state-of-the-art guides that allow the authors to justify specific technical choices. The manuscript is well written with good and clear English and is well organized in paragraphs. Overall, the work is publishable, subject to minor imperfections and minor shortcomings.
Indeed, the comments related to minor revisions are given below:
- The target application has to be clearly defined, according to the used power. The Authors discuss about trapping. A comparison with recent results have to be provided (see, e.g., https://doi.org/10.1103/PhysRevA.106.033511; https://doi.org/10.3390/photonics9060425; https://doi.org/10.1364/JOSAA.463732)
Response: Thank you very much. Compared to the reference listed above, the photonic hook and its trapping potential reported in this paper are highly related to the ALI or the interface between the two media. This is unique compared to the normal optical trapping approaches. The results in the references above were all achieved in a fully immersive environment and did not involve the interfaces. The new references and their comparisons to the current result are added on Page 2 of the manuscript and marked in red as
‘Besides, Yu et al. used a light field with a nonuniform spatial deformation to create multiple focal points for realising trapping in the longitudinal direction [34]. Brunetti et al. designed a dielectric nanobowtie dimer made up of two tip-to-tip silicon triangles to trap nanoparticles between the nanocavity [35]. Liang et al. investigated the optical trapping of multiple Rayleigh particles at different locations by four-petal Gaussian vortex beams [36]. However, all these recent results about optical trapping were achieved in a fully immersive environment and did not involve the effect on ALI or the interface between the two media. Wei et al. studied the optical force on metal nanorod and Rayleigh particle by a photonic jet [37, 38].’
- In section 2, I suggest that the authors justify with more detail and information the choices regarding the photonic hook configuration describing any advantages and future challenges for fabrication.
Response: Thank you very much. The main optical properties and physical design principles of the photonic hook were summarized and discussed in [7]. It is noted that the advantages of the proposed photonic hook are mainly based on its property propagating in the small area beneath and along the ALI. The fabrication challenges are precise controls of the optical fibre core (microcylinder)’s position and the thickness of the water film. The advantages and future fabrication challenges are added on page 8 and marked in red as
‘In fact, because of the property that the proposed photonic hook only exists in the small area beneath ALI, it could be used for future optical liquid-level sensors [54] and optical trapping in bio-medical applications [55]. The fabrication challenges of the proposed setup would focus on the placement of the optical fibre core (microcylinder) and precise control of the thickness of the water film.’
- The results of this experimental work indicate that the proposed photonic hook has unique qualities due to its attributes of curved propagation along ALI. Since the results are also promising regarding possible fabrication, I suggest the authors include indications and information regarding fabrication steps in a future perspective.
- Line 137: Delete the letter “ α ”from the paragraph "Results and discussion".
Response: Thank you very much. The additional α caused by a typo was deleted in the manuscript.
- Line 299: The proposed photonic hook's future developments are unclear. I suggest going into more detail about the applicability to generate more interest in the reader.
Response: Thank you very much. More future developments and potential are added at the end of the article.
- line 388: Reference 42 is underlined.
Response: Thank you very much. The additional underline was deleted.
Reviewer 4 Report
General Comments: The manuscript titled " Photonic hook initiated by air-liquid interface " presents a comprehensive study on the behavior of photonic hooks in various scenarios, particularly focusing on the role of refractive index gradients. The authors combine modeling and experimental data to support their findings. While the study is promising and contributes to the understanding of photonic hooks, there are several major revisions needed to enhance its clarity, comprehensibility, and overall impact.
Major Revisions:
Inadequate Introduction: The introduction section does not provide sufficient context for readers unfamiliar with the topic. It would be helpful to include a brief overview of the field of photonic hooks and the specific problem the article aims to address. This will make the manuscript more accessible to a broader audience.
Data Interpretation: The article presents a significant amount of data related to optical forces and Poynting vector flows. However, it lacks a thorough interpretation of these findings. The authors should provide insights into the implications of the data and how it relates to the overall study. A more detailed discussion of the data's significance is needed.
Incomplete Conclusion: The conclusion section is relatively brief and does not adequately summarize the key findings and their significance. A more comprehensive conclusion that ties together the results and their implications for the field is needed. This will help readers understand the broader implications of the study.
Experimental Methods: The provided text primarily focuses on presenting and discussing results rather than detailing experimental methods. To assess the clarity and comprehensibility of the experimental methods, the authors should look for the following elements:
Equipment and Materials: The methods section should list all the equipment and materials used in the experiments. This includes details about the experimental setup, and any specialized instruments.
Procedures: Provide a step-by-step account of the experimental procedures. This should be detailed enough for another researcher to replicate the experiments.
Data Collection: Explain how data was collected, including the parameters measured, the frequency of data collection, and any sensors or instruments used for this purpose.
Data Analysis: Explain the methods used to analyze the data, including any statistical techniques or software packages employed.
Reproducibility: Discuss any efforts made to ensure the reproducibility of the experiments.
References: Cite any relevant references to established methods or procedures that were followed.
Role of Refractive Index Gradient: The manuscript discusses the role of the gradient of the refractive index in driving the deflection of photonic hooks. Providing a more intuitive explanation of how this gradient affects the behavior of electromagnetic fields could help readers grasp the concept better. Enhancing the clarity of this concept is crucial for understanding the study's core findings.
Practical Implications: While the manuscript mentions potential applications such as optical trapping and super-resolution imaging, it could delve into more detail about why these findings are significant for these applications. How could the ability to achieve FWHMs smaller than DLs impact these fields, and what are the practical benefits? Elaborating on the practical implications would strengthen the manuscript's impact.
Table 1: The table comparing FWHMs of photonic hooks and DLs is informative, but it might be helpful to include a brief explanation or interpretation of the data. For example, why is it important that FWHMs are shorter than DLs, and what does the percentage decrease signify in practical terms? Providing such context will enhance the reader's understanding of the table's significance.
The manuscript has the potential to make a valuable contribution to the field of photonic hooks, but it requires significant revisions to address the issues mentioned above.
Author Response
Reviewer 4:
Comments and Suggestions for Authors
General Comments: The manuscript titled " Photonic hook initiated by air-liquid interface " presents a comprehensive study on the behavior of photonic hooks in various scenarios, particularly focusing on the role of refractive index gradients. The authors combine modeling and experimental data to support their findings. While the study is promising and contributes to the understanding of photonic hooks, there are several major revisions needed to enhance its clarity, comprehensibility, and overall impact.
Major Revisions:
Inadequate Introduction: The introduction section does not provide sufficient context for readers unfamiliar with the topic. It would be helpful to include a brief overview of the field of photonic hooks and the specific problem the article aims to address. This will make the manuscript more accessible to a broader audience.
Response: Thank you very much. More introductions about the photonic hook and its developments are added to and marked in red in paragraph 1 on page 1 of the manuscript with the references. It is shown as
‘Moreover, the radius of the curvature of a photonic hook is smaller than the wavelength of the illuminating light. This property is not presented in the family of Airy-like beams and is considered the smallest curvature radius of electromagnetic waves to date [7]. Serving as an example of an asymmetric optical phase shifting, a photonic hook is theoretically caused by the difference in phase velocity and interference of the wave produced by the contrast of the local refractive indexes inside or around the dielectric particles [9-11]. Initially, it was realised in the mean of a geometrical asymmetry, e.g. a wavelength-scaled dielectric cuboid combined with a wedge prism, then developing to the methods breaking the symmetries of refractive indexes of materials and illuminating light. This adds a newfound degree of simplicity.’
Data Interpretation: The article presents a significant amount of data related to optical forces and Poynting vector flows. However, it lacks a thorough interpretation of these findings. The authors should provide insights into the implications of the data and how it relates to the overall study. A more detailed discussion of the data's significance is needed.
Response: Thank you very much. As a rule, experimental measurement of the Poynting vector field often turns out to be impossible. In such a situation, a theoretical description of this field is the only tool for its research. The data of Poynting vector flow and optical forces is used to analyse the forming mechanism of the proposed photonic hook and explore the potential application in optical trapping, respectively. The Poynting vector and optical forces as vectors are derived from the equations (2)-(5), and then computed and visualised by CST and MATLAB to generate the results presented in the paper.
Incomplete Conclusion: The conclusion section is relatively brief and does not adequately summarize the key findings and their significance. A more comprehensive conclusion that ties together the results and their implications for the field is needed. This will help readers understand the broader implications of the study.
Response: Thank you very much. The conclusion of this paper has been rewritten and marked in red on page 10 as
‘This study investigates a novel photonic hook initiated by and propagating along the ALI. It exists at specific depths of a liquid film that is used to partially immerse a micro-cylinder and shows a unique bending and horizontal development at the ALI. Its formation mechanism is thought to be related to the refractive index contrasts around ALI in the shadow direction of the micro-cylinder, based on an analysis of the distributions of the electric field intensities and Poynting vector flows at various water film thicknesses. This is different from the previously reported photonic hooks, which are all caused by particle asymmetries regardless of shape or refractive indexes at the irradiation end. The results indicate that the proposed photonic hook has unique qualities due to its attributes of the curved propagation along ALI with a transverse dimension smaller than the diffraction limit, as well as the simulated optical forces. Optical trapping, interface cell culture, and liquid-level sensing could be the potential applications of this newfound photonic hook.’
Experimental Methods: The provided text primarily focuses on presenting and discussing results rather than detailing experimental methods. To assess the clarity and comprehensibility of the experimental methods, the authors should look for the following elements:
Equipment and Materials: The methods section should list all the equipment and materials used in the experiments. This includes details about the experimental setup, and any specialized instruments.
Procedures: Provide a step-by-step account of the experimental procedures. This should be detailed enough for another researcher to replicate the experiments.
Response: Thank you very much. The current results reported in the paper are based on the numerical simulations. The experimental verification is carried out in our labs at present, and the experimental details will be included in the following publications.
Data Collection: Explain how data was collected, including the parameters measured, the frequency of data collection, and any sensors or instruments used for this purpose.
Data Analysis: Explain the methods used to analyze the data, including any statistical techniques or software packages employed.
Response: Thank you very much. The current results reported in the paper are based on the numerical simulations. CST studio as the numerical simulation software is able to export all modelling data according to the meshing plan. A MATLAB programme collects all data and generates the force fields using the equations (4) and (5). This process is marked in red on page 3.
Reproducibility: Discuss any efforts made to ensure the reproducibility of the experiments.
References: Cite any relevant references to established methods or procedures that were followed.
Response: Thank you very much. The current results reported in the paper are based on the numerical simulations. The experimental verification is carried out in our labs at present, and the experimental details will be included in the following publications. The references related to the simulation have been cited in the manuscript.
Role of Refractive Index Gradient: The manuscript discusses the role of the gradient of the refractive index in driving the deflection of photonic hooks. Providing a more intuitive explanation of how this gradient affects the behavior of electromagnetic fields could help readers grasp the concept better. Enhancing the clarity of this concept is crucial for understanding the study's core findings.
Response: Thank you very much. As the detailed introduction marked in red in paragraph 1, a photonic hook is caused by the difference in phase velocity and interference of the wave inside the particle. The contrast and gradient of refractive indexes at the interface between two media can lead to these phenomena. When the incident light passes the microcylinder and propagates along the ALI, the upper and bottom of the wavefront will be at two different velocities, and that causes the bending. This is marked in red and added on page 4 as
‘In this case, the light encounters corresponding contrasts of refractive indexes at the boundaries separated by the ALI (from glass to air in the upper half; from glass to water in the lower half) at the shadow end after propagation through the entire dielectric micro-cylinder. When the incident light propagates along the ALI, its wavefront travels in two media – the upper part in the air and the bottom part in the water, which results in the difference of phase velocity to bend the focus for forming a photonic hook. It is indicated that the gradient of the refractive index can drive the deflection of the photonic hooks at the appropriate depths to advance the ALI and further elongate them, which results in the morphological differences shown in Fig. 3.’
Practical Implications: While the manuscript mentions potential applications such as optical trapping and super-resolution imaging, it could delve into more detail about why these findings are significant for these applications. How could the ability to achieve FWHMs smaller than DLs impact these fields, and what are the practical benefits? Elaborating on the practical implications would strengthen the manuscript's impact.
Table 1: The table comparing FWHMs of photonic hooks and DLs is informative, but it might be helpful to include a brief explanation or interpretation of the data. For example, why is it important that FWHMs are shorter than DLs, and what does the percentage decrease signify in practical terms? Providing such context will enhance the reader's understanding of the table's significance.
The manuscript has the potential to make a valuable contribution to the field of photonic hooks, but it requires significant revisions to address the issues mentioned above.
Response: Thank you very much. The proposed photonic hook is able to provide the trapping capability to manipulate the particles in a non-straight trajectory relying on its property of light bending. This can be used for manipulation of the objects behind or around the obstacles, which is normally impossible to trap by the normal optical trapping system.
Besides, a curved light beam, such as a photonic hook, can yield a larger field of view for near-field imaging with a high contrast and resolution because of its asymmetric excitation pattern. Meanwhile, the FWHM of the photonic jet refers to the width of the intensity distribution at half of its maximum intensity. In the context of imaging resolution and the diffraction limit, this parameter is crucial as it defines the spatial extent of the focused light. The diffraction limit is a fundamental constraint on the resolution of optical systems. According to the Rayleigh criterion, the minimum resolvable distance (or minimum resolvable feature size) in an optical system is determined by the wavelength of light and the numerical aperture of the optical system. If FWHM is smaller than the diffraction limit, the features that is smaller than the diffraction limit can be resolved as separate entities. Therefore, the proposed photonic hook can play a role in breaking the diffraction limit by concentrating light into a highly localized region. This would be meaningful for building trapping and imaging 2 in 1 optical setup.
This content is marked in red and added on page 5.
Round 2
Reviewer 3 Report
The Authors have modified the manuscript according to the Reviewer suggestions.
Reviewer 4 Report
After a thorough review of the manuscript submitted by the author, I am pleased to inform you that the suggested changes have been satisfactorily implemented. The manuscript now meets the required standards of quality for publication.
The author's responses to my suggestions demonstrated a commendable commitment to improving the work. The revisions made have substantially enhanced the clarity, cohesion, and strength of the argument presented in the manuscript.
I would like to commend the author's effort and dedication to enhancing the quality of their work. I believe that this manuscript is now ready to proceed to the publication process.
