In order to timely and comprehensively reflect the latest development trends of optical information encryption and storage technology, "China Laser" will publish a special topic on "Multidimensional Optical Information Encryption and Storage Technology" in the 18th issue of 2023.

The back cover article of the special topic is sourced from the research group of Professor Xiao Shumin and Professor Ruan Qifeng at Harbin Institute of Technology (Shenzhen). The article summarizes the principle of two-photon polymerization lithography (TPL) 3D printing technology, several common structural color 3D structures, and printing schemes. It also reviews the research progress of TPL technology in the application of structural color stereo information and dynamic information, and finally looks forward to the development prospects of this field.

Su Sihua, Wang Kaiyang, Huang Can, Jin Limin, Xiao Shumin, Ruan Qifeng Research progress on structural color based on two-photon polymerization 3D printing technology [J] China Laser, 2023, 50 (18): 1813007

Cover Interpretation

   

The Cover Borrows The Face Mask Of The Chinese Treasure Sichuan Opera, Presenting The Principle Of Using Micro Nano 3D Printing Technology To Achieve Multiple High-precision Structural Color Patterns. Analogous To Sichuan Opera Artists Who Agilely Complete Facial Changes With Superb Skills In Every Gesture, The Angle Multiplexed Grating Prepared By Two-photon Aggregation 3D Printing Technology Can Achieve Micro Nano Optical "facial Changes": Under Oblique Incident White Light Illumination, Gently Rotating The Sample Can Quickly Switch Multiple Sets Of Color Micro Patterns In The Microscope. Multi/dynamic Structural Colors Based On Two-photon Aggregation 3D Printing Technology Have Important Applications In Optical Anti-counterfeiting, Information Storage, And Optical Sensing Fields.

1、 Introduction

The Graceful And Magnificent Display Of Peacocks On Their Screens, The Dazzling Light Of Butterfly Wings, And The Existence Of Many "color Masters" In Nature. Similarly, Researchers Can Use 3D Printing Technology And Optical Principles To Fine Tune Special 3D Printed Samples Under White Light Illumination At Different Angles, Presenting A "flowing" Light Luxury Color And Continuously Presenting A Chameleon Like Dreamlike Color.

The Process Of 3D Printing Can Be Likened To The Production Of Sugar Paintings. A Sugar Painter Will First Imagine A Sugar Painting Model In Their Mind, And Then Solidify The Hot Syrup Layer By Layer On A Flat Surface To Form A Three-dimensional Object With Ups And Downs. The Current 3D Printing Technologies Mainly Include Fused Deposition Modeling (FDM), Direct Ink Writing (DIW), Resonant Laser Printing (RLP), Photopolymerization Stereolithography (SLA), And Digital Light Processing (DLP). However, There Are Certain Difficulties In The Manufacturing Of Submicron Scale 3D Structures Using These Technologies. The Two-photon Polymerization Lithography (TPL) Technology Can Easily Print Polymer Structures With Sub Micron Precision, Making It One Of The Highest Precision 3D Printing Processes Currently Available.

This Article Provides A Brief Overview Of The Principles Of TPL 3D Printing Technology, Several Common Structural Color 3D Structures, And Printing Schemes. It Also Reviews The Research Progress Of TPL Technology In The Application Of Structural Color 3D Information And Dynamic Information, And Finally Looks Forward To The Development Prospects Of This Field.

2、 Principle Of Two Photon Aggregation 3D Printing Technology

Two Photon Absorption (TPA) Is A Third-order Nonlinear Optical Effect That Refers To The Process In Which A Molecule Absorbs Two Photons Simultaneously When Transitioning From The Ground State To The Excited State. TPL Technology Utilizes Femtosecond Laser To Excite The Initiator In The Photosensitive Adhesive, Which Induces Molecular Polymerization After Absorbing Two Photons. Based On The Threshold Characteristics Of Two-photon Polymerization, Photosensitive Adhesive Polymerization Is Limited To The Focal Point Of Femtosecond Laser, Thus Possessing Extremely High Processing Accuracy. The Minimum Line Width For TPL Printed Structures Can Be Less Than 100 Nm.
In The TPL Printing System, A Galvanometer And A Light Shutter Are Used To Scan The Focused Beam Point By Point And Layer By Layer To Achieve Printing Of Complex Structures. Printing Systems Can Generally Be Divided Into Immersion (left In Figure 1) Or Oil Immersed Configurations (right In Figure 1). The Advantage Of Immersion Configuration Is That It Can Prepare Three-dimensional Structures With Larger Heights (several Millimeters). To Optimize Printing Accuracy, The Refractive Index Of The Photoresist Used Should Match The Numerical Aperture Of The Objective Lens. Oil Immersed Configuration Can Prevent The Objective Lens From Being Contaminated By Photoresist, But The Printing Height Is Limited By The Working Distance Of The Objective Lens.

Figure 1 Immersion (left) and oil immersed (right) configurations of two-photon polymerization 3D printing process

      Compared To Other Planar Micro Nano Fabrication Techniques, TPL's Key Advantage Lies In Its Ability To Fabricate High-precision True Three-dimensional Structures. In Addition, By Combining Photoresist Materials That Can Respond To External Stimuli, TPL Can Also Manufacture 4D Micro Nano Structures With Shape And Performance Changing Over Time (time Being The Fourth Dimension).

3、 Structural color

     

In Daily Life, The Colors We Observe Come From Glowing Or Non Glowing Objects. For Non Luminous Objects, Their Color Sources Can Be Mainly Divided Into Two Types: One Is That The Object Absorbs Part Of The Incident Light While Reflecting Or Transmitting The Remaining Light, Which Can Be Called Pigment Color [Figure 2 (a)]. The Facial Makeup Of Sichuan Opera Artists Is Carefully Drawn With Various Colored Pigments, Which Is A Vivid Example Of Pigment Colors. Another Type Is The Color Generated Through The Interaction Between Light And Micro Nano Scale Structures, Known As Structural Color.
Structural Color Originates From The Interference, Diffraction, And Scattering Interactions Between Light And Micro/nanostructures. The Colorful Soap Bubbles Commonly Seen Are The Colorful Structural Colors Generated Based On The Interference Effect Of Thin Films [Figure 2 (b)]. By Adjusting The Geometric Parameters Of The Structure (such As Period, Size, Shape, Surrounding Environment, Etc.), Its Optical Response In The Visible Spectrum Can Be Changed, Resulting In A Colorful Appearance.

The Team Led By Xiao Shumin From Harbin Institute Of Technology (Shenzhen) Utilized Silicon Metasurfaces Combined With Refractive Index Matching Layers To Present Highly Saturated Structural Colors. By Preparing Silicon Nanostructures Of Different Sizes Pixel By Pixel, The Vibrant Color Micro Images Of "Peacock And Orchid" Were Successfully Displayed [Figure 2 (c)]. Compared To Traditional Dyes And Pigments, Structural Colors Have Become A Hot Topic In Micro Nano Optics Research Due To Their Advantages Of Cleanliness, Environmental Friendliness, Anti Fading, And Ultra-high Printing Resolution.

Figure 2: Colors of non luminous objects: (a) Pigments of flowers; (b) The structural color of soap bubbles (derived from Pixabay); (c) High precision and high saturation structural color images prepared by micro nano methods (from Nature Communications, 2020, 11:1864)

4、 Structural Color Multiple/three-dimensional information

     After implementing the method of storing two-dimensional image information such as photos, people began to try to use angle information to achieve stereoscopic imaging. Researchers have begun exploring the preparation of three-dimensional structural color samples and the presentation methods of color stereoscopic information. Assembling nanospheres into opal structures is a relatively simple micro nano manufacturing process, often used to prepare large-area uniform structural colors.
On this basis, researchers used the opal structure assembled by nanospheres as a template, and selectively solidified the photoresist between some spheres using TPL. After development, the template was removed to produce complex three-dimensional structural color samples [Figure 3 (a)]. By changing the size of the nanospheres, various anti opal structures with bright colors, high reflectivity, and narrow stopbands can be generated. Editing information on different planes can achieve the effect of reading different colored characters from multiple focal planes separately [Figure 3 (b)].

 

Figure 3: Implementation of inverse opal structure color: (a) Three dimensional inverse opal structure model (from Nano Letters, 2021, 21:8602); (b) The effect of reading different colored characters on multiple focal planes (from Nano Letters, 2021, 21:8602)

   

Joel Yang From Singapore University Of Science And Technology And Ruan Qifeng From Harbin Institute Of Technology (Shenzhen) Collaborated To Present Multiple/three-dimensional Structural Color Information Using Special Optical Design. The Team Decoupled The Effects Of Grating Height And Period On Structural Color, Resulting In TPL Printed Samples Presenting Different Color Images Under Vertical Or Oblique Illumination Conditions. Furthermore, The Directional Effect Of Angle Multiplexed Gratings Was Utilized To Achieve The Effect Of Sequentially Displaying Multiple Cartoon Color Images When Rotating The Sample [Figure 4 (a)].

In Addition, The Team Used TPL To Print Castle Structures Combining Micro Lenses And Nano Columns At Once, Creating High-resolution Light Field Prints. In This System, The Incident Light Passes Through Structured Color Nanocolumn Pixels And Is Collected By Microlenses And Projected Onto The Observer, Who Can See The Color Of The Selected Pixel With The Naked Eye. By Utilizing The Principle Of Light Field Display To Design Structured Color Nanowires At Different Positions Under Each Lens, 3D Color Cartoon Faces With 25 Or Even 625 Viewing Angles Can Be Achieved [Figure 4 (b)]. These Nano Columns That Store Color Information Are Hidden Within The Castle Structure And Cannot Be Directly Replicated By The Template Method, Thus Possessing High Anti-counterfeiting Performance.

Figure 4 (a) presents multiple/three-dimensional structural color information in combination with lighting conditions (from Nano Letters, 2022, 22:8189) or (b) utilizes the principle of light field display (from Nature Communications, 2021, 12:3728)

5、 Structural color dynamic information

       

The Reversible Tuning Of 3D Printed Structural Colors Is Mainly Achieved Through External Stimuli, Including But Not Limited To Changes In Humidity, Temperature, PH, Mechanical Strain, Light, Magnetic Field, And Voltage Bias.
For Example, Researchers Have Successfully Prepared Surface Relief Elastomers Using TPL Printing And Reverse Replication Techniques, And Applied Groove Structures Capable Of Carrying Dynamic Structural Colors To Encrypt Multiple Sets Of Optical Information (Figure 5). By Applying Stress In Different Directions, The Color Of The Groove Structure Can Undergo Reversible And Repeatable Changes. Unlike Traditional Bragg Diffraction Gratings That Require Large-area Periodic Arrays, This Work Only Requires A Single Groove To Produce Rich Structural Colors.
Based On The Unique Color Change Pattern Of Flexible Grooves, Multiple Dynamic Adjustment Schemes For Structural Colors Can Be Designed. By Synergistically Controlling Directional Strain And Capillary Force, An Integrated Elastomer With Six Different Patterns Can Be Achieved [Figure 5 (a)].

In Addition, They Also Designed And Prepared Relief Samples With Staggered Nano Groove Structures. When The Elastic Material Was Stretched Along Orthogonal Directions, The Originally Transparent Areas Could Present Images Of Trees And Flowers In Sequence [Figure 5 (b)]. These Micro Nano Structures With Adjustable Shape/color Cycles Have Important Application Value In Fields Such As Anti-counterfeiting, Information Storage, And Strain Sensing.

Figure 5 Surface relief elastomer is used to present dynamic structural color information. (a) Schematic diagram of integrated elastomer (derived from Advanced Materials, 2022, 34: 2108128); (b) Schematic diagram of flowers and trees designed (from Advanced Materials, 2022, 34: 2108128)

6、 Summary and Prospect

   The Two-photon Polymerization Lithography Technology Provides Rich Design Freedom For Micro Nano Fabrication And Brings New Possibilities For Structural Color Applications. Although Preliminary Progress Has Been Made In TPL Based 3D Structural Color Printing Technology, Its Potential Has Not Been Fully Explored.
At Present, There Is Still A Problem Of Low Processing Efficiency When Applying This Technology To Industrial Production, And Further Improvement And Optimization Are Needed. Future Research Work Can Also Explore The Use Of TPL To Manipulate More Degrees Of Optical Freedom. In Summary, TPL Micro Nano 3D Printing Technology Has Broad Application Prospects In The Field Of Optics. Through Continuous Research And Exploration, We Can Further Explore And Optimize This Technology, Promote Its Application In Scientific Research And Industrial Production, And Achieve More Profound Development In Fields Such As Optical Communication, Optical Encryption, And Data Storage.

Introduction To The Research Group

   

Ruan Qifeng's research group at Harbin Institute of Technology (Shenzhen)（ http://faculty.hitsz.edu.cn/ruanqifeng ）Mainly engaged in research in the fields of micro nano optics and micro nano manufacturing. The Micro Nano Photonics Laboratory, where the research group is located, is an important component of the Micro Nano Optoelectronic Information System Theory and Technology Laboratory of the Ministry of Industry and Information Technology and the Guangdong Provincial Key Laboratory of Semiconductor Materials and Intelligent Photon Systems; Long term commitment to interdisciplinary basic and applied research in the field of micro nano optics, including physics, materials science, and micro nano processing; We have a comprehensive platform for designing, preparing, and testing micro nano structures. Welcome students/scholars interested in related research fields to join.

Journal Introduction:

Founded in 1974 by the director of the Chinese Academy of Sciences, sponsored by the Shanghai Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences and the Chinese Optical Society, and published by the China Laser Journal, China Laser is a flagship Chinese academic journal that comprehensively reports the latest research achievements in the field of laser technology. In 2021, it will be changed to a bimonthly publication and "English long abstracts" will be published to enhance the international dissemination of journal articles. In 2021 and 2022, we will respectively create a special issue on "Frontier Laser Manufacturing" and a special issue on "Biomedical Photonics".

China Laser is currently indexed by search systems such as EI, ESCI, AJ, CA, INSPEC, Scopus, CSCD, etc. Has won titles such as "Top 100 Science and Technology Journals", "Top 100 Outstanding Academic Journals in China", "China's Excellent Science and Technology Journals", and "China's Most Influential Academic Journals" multiple times. Selected for the "China Science and Technology Journal Excellence Action Plan" in 2019. In 2021, it was nominated for the Journal Award at the 5th China Publishing Government Awards. Selected as "T1 level" in the "Classification Catalogue of High quality Scientific and Technological Journals in Optical Engineering and Optics" in 2022.

Science Editor | Professor Ruan Qifeng and Su Sihua from Harbin Institute of Technology (Shenzhen)

Editor | Shen Lingling