Femtosecond-pulse Laser Interactions with Materials: Fundamentals and Applications for Non-thermal Ultra-precise Machining

박정규 과학기술연합대학원대학교 2012 국내박사

The intense femtosecond pulsed laser interaction with materials is a subject of practical interest as well as material science interest. Significant advancements in solid state lasers and the chirped pulse amplification technique using Ti:sapphire laser media have led to the challenging phase of this research works. The application fields include high-intensity physics, femtosecond pulsed laser machining, high order harmonics generation, and so on. The femtosecond laser machining is one of the most promising technologies among the femtosecond laser applications because machining physics is drastically different from those of conventional nanosecond and longer pulsed laser machining. In this dissertation, the first, the motivation, background and previous studies at the introduction of this dissertation were presented. The second, femtosecond laser absorption and ablation were theoretically described. The ablation of metals by femtosecond laser is theoretically described using two-temperature model. Femtosecond laser ablation is expressed by physics theory that takes account of electron contribution calculated by a two-temperature model. The ablation rate and the thickness of heat-affected zone are estimated theoretically. The third, machining in various materials was studied experimentally. In the experimental first works, the internal modification in transparent materials including glass and flexible polymer (PDMS) using femtosecond laser pulses irradiation has been studied. The first, optical diffraction gratings embedded in BK-7 glass by low-density plasma formation using a femtosecond laser was studied. In this work, the optical embedded diffraction gratings with the internal refractive index modification in BK-7 glass plates using low-density plasma formation excited by a high-intensity femtosecond (130 fs) Ti: sapphire laser (λp=790 nm) was demonstrated and studied. The refractive index modifications with diameters ranging from 400 nm to 4 μm were photoinduced after plasma formation occurred upon irradiation with peak intensities of more than 1 × 10^13 W/cm^2. The graded refractive index profile was fabricated to be a symmetric around from the center of the point at which low-density plasma occurred. The maximum refractive index change (∆n) was estimated to be 1.5 × 10^-2. The several optical embedded gratings in BK-7 glass plate were demonstrated with refractive index modification induced by the scanning of low-density plasma formation. The second, internal modification in transparent materials using plasma formation induced by a femtosecond laser was studied. In this research, the fabrication of internal diffraction gratings with photoinduced refractive index modification in transparent materials was demonstrated using low-density plasma formation excited by a femtosecond (130 fs) Ti: sapphire laser (λp = 800 nm). The refractive index modifications with diameters ranging from 1 μm to 3 μm were photoinduced after plasma formation occurred upon irradiation with peak intensities of more than 2.0 × 10^13 W/cm^2. The graded refractive index profile was fabricated to be a symmetric around from the center of the point at which low-density plasma occurred. The third, flexible gratings fabricated in polymeric plate using femtosecond laser irradiation were studied. In this research, flexible gratings embedded in PDMS (poly-dimethlysiloxane) were fabricated using femtosecond laser pulses. Photo-induced gratings in flexible PDMS plate were directly written by a high-intensity femtosecond (130 fs) Ti: Sapphire laser (λp = 800 nm). Refractive index modifications with 4 μm diameters were photo-induced after irradiation of the femtosecond pulses with peak intensities of more than 1 × 10^11 W/cm^2. The graded refractive index profile was fabricated to be symmetric around the center of the focal point. The diffraction efficiency of the grating samples is measured by a He-Ne laser. The maximum value of refractive index change (∆n) in the laser-modified regions was estimated to be approximately 3.17 × 10^-3. In the experimental second works, vibration assisted femtosecond laser machining effect on metal was studied. In this work, I demonstrate a novel approach to improve laser machining quality on metals by vibrating the optical objective lens with a frequency (of 500Hz) and various displacements (0~16.5 μm) during a femtosecond laser machining process. The laser used in this experiment is an amplified Ti:sapphire fs laser system that generates 100 fs pulses having an energy of 3.5 mJ/pulse with a 5 kHz repetition rate at a central wavelength of 790 nm. It is found that both the wall surface finish of the machined structures and the aspect ratio obtained using the frequency vibration assisted laser machining are improved, compared to those derived via laser machining without vibration assistance. This is the first report of low frequency vibration of an optical objective lens in the femtosecond laser machining process being exploited to obtain significantly improved surface roughness of machined side walls and increased aspect ratios. In the experimental final works on this dissertation, laser cleaning effect on the wafer using plasma shockwave and plasma filament excited by femtosecond laser irradiation. The first, removal of nanoparticles from a silicon wafer using plasma shockwaves excited with a femtosecond laser was studied. In this work, experiments on the cleaning effect of 100nm-sized polystyrene latex (PSL) particles on silicon wafers using plasma shockwaves excited via a femtosecond (130 fs) Ti:Sapphire laser (λp=790 nm) are reported. The removal efficiency depended on the gap distance between the focused laser beam point and the silicon wafer surface; however some cases exhibited damaged surfaces due to the excessive laser intensity. The cleaning efficiency was strongly dependent on the gap distance between the plasma formation point and the surface. The rem

Comparison of a femtosecond laser and a mechanical microkeratome for LASIK : 라식수술시 Femtosecond Laser와 미세각막절개도를 사용한 각막절편의 비교

김재용 울산대학교 대학원 2004 국내박사

목적: 가토에서 Femtosecond laser (IntraLase�)를 사용하여 각막절편을 만들었을 때의 각막기질의 염증세포 침윤 및 apoptosis 정도와 유착강도를 미세각막절개도(ACS™)를 사용하여 각막절편을 만든 경우와 비교해 보고자 하였다. 연구대상 및 방법: (실험 1) 가토안을 대조군과 4개 실험군으로 나누었고 실험군은 femtosecond laser로 각막절편만 만든 군(FS군), femtosecond laser로 절편을 만들고 excimer laser ablation을 시행한 군(FS+라식군), 미세각막절개도로 각막절편만 만든 군(MM군), 미세각막절개도로 각막절편을 만들고 excimer laser ablation을 시행한 군(MM+라식군)이 포함되었다. 총 60마리 가토안에서 수술 후 4시간과 24시간에 안구적출 후 관련된 조작을 가하고 H&E 및 TUNEL assay를 시행하였다. (실험 2) 가토 8마리 8안에서는 femtosecond laser를 이용하여 각막절편을 만들었고, 다른 가토 8마리 8안에서는 미세각막절개도를 사용하여 각막절편을 만들었다. 1개월 및 3개월 후 tension meter를 사용하여 각막절편 유착 강도를 측정, 비교하였다. 결과: (실험 1) 술후 24시간에는 중심각막 및 주변절편에서의 염증세포 침윤이 FS군에서 MM 군보다 의미있게 많았고 FS+라식군에서 MM+라식군보다 의미있게 많았다. 술후 4시간 및 24시간에 모든 실험군의 TUNEL 양성세포수가 대조군과 비교시 차이가 있었으나 (P<0.05) 실험군 간의 의미 있는 차이는 보이지 않았다. (실험 2) 1개월째 각막절편의 평균 유착강도는 femtosecond laser를 사용한 경우 126.7 gf이었고 미세각막절개도를 사용한 경우 65 gf였다. 3개월째 각막절편의 평균유착강도는 femtosecond laser를 사용한 경우 평균 191.3gf 이었고 미세각막절개도를 사용한 경우 127.5gf이었다. 술후 3개월째 양군간에 통계학적인 차이가 있었다(P=0.038). 결론: 라식수술시 각막절편 형성에 있어서 femtosecond laser를 사용한 경우 기존의 미세각막절개도를 사용한 경우보다 술후 1일째 apoptosis에는 차이가 없었으나 각막기질 및 주변부 절편의 염증세포 침윤이 많이 발생하였고 술후 3개월에 강한 각막절편유착이 발생하였다. Femtoseond laser는 수술후 강한 항염증 치료가 필요하지만 강한 절편유착을 만들 수 있었다. Purpose: Our goal was to compare the corneal stromal inflammation and apoptosis around the flaps at the early postoperative period and adhesion strength of corneal flaps at the late postoperative period between those made with a femtosecond (FS) laser, an IntraLase FS™ and those made with a mechanical microkeratome (MM), an ACS™ microkeratome in rabbit. Materials and Methods: (Study 1) involved five reference groups: (1) FS group with only flaps made with a femtosecond laser, (2) FS+LASIK group with flaps made with a femtosecond laser followed by excimer laser ablation, (3) MM group with flaps made with a mechanical microkeratome, (4) MM+LASIK group with flaps made with a mechanical microkeratome followed by excimer laser ablation, and (5) control group in which no procedure was performed. Twelve eyes of each group were enucleated and processed with H & E staining and a TUNEL assay at 4 and 24 hours after procedures. (Study 2) involved two reference groups: (1) FS group with flaps made with a femtosecond laser (eight rabbits’ eyes) and (2) MM group with flaps made with a mechanical microkeratome (eight eyes). At 1 and 3 months after the procedures, the adhesion strength was measured by a tension meter. Results: In Study 1, at 24 hours after the procedures the inflammatory cell infiltration at the central cornea and at the peripheral interface was significantly greater in the FS group than in the MM group, and significantly greater in the FS+LASIK group than in the MM+LASIK group. The TUNEL assay revealed a significantly higher apoptotic cells in each experimental group than in the control group, but no significant differences between the four experimental groups at 4 and 24 hours after procedure. In the FS group of Study 2, 126.67 and 191.33 gram force (gf) were needed at 1 month and 3 months to detach the flaps: in the MM group, the corresponding values were 65 and 127.5 gf. The adhesion strength was significantly higher in the FS group than in the MM group at 3 months (P=0.038). Conclusions: A femtosecond laser produces greater corneal stromal inflammation at 1 day after surgery without increase in apopotosis and stronger flap adhesion at 3 months after surgery than a mechanical microkeratome, although a femtosecond laser may require stronger antiinflammatory drugs to be administered.

Study on nonlinear multi-dimensional direct laser writing by using ultrashort high power laser

박창현 Graduate School, Yonsei University 2020 국내박사

Au cours des 30 dernières années, alors que la largeur d'impulsion des lasers a été réduite et que les lasers haute puissance ont été développés, des recherches sur l'interaction entre le photon et les matériaux utilisant des lasers femtosecondes ont été activement menées. La haute densité d'énergie des lasers pulsés femtosecondes permet des processus de photoionisation non linéaire de plusieurs manières selon. Cet article rapporte une étude d'une écriture laser directe de type Argentum dans des verres de phosphate de zinc contenant de l'argent en induisant une déformation d'absorption non linéaire d'impulsions laser femtoseconde. Lorsque des verres de phosphate de zinc contenant de l'argent sont irradiés avec des impulsions laser femtoseconde, des amas en forme d'anneau se forment en raison d'une absorption non linéaire. Les propriétés de fluorescence et l'indice de réfraction de l'amas d'argent induit par cette déformation sont différents de ceux du verre d'origine. Des comparaisons simultanées de microsondes chimiques, de NSOM et de modélisation numérique ont été utilisées pour analyser la distribution des espèces d'argent induite par laser. Les résultats renforcent considérablement la compréhension des modifications matérielles de ces verres dans un régime d'interaction non thermique. En particulier, il a été constaté que la distribution spatiale des espèces dans des verres contenant de l'argent produits par irradiation laser femtoseconde a un effet significatif sur la sélectivité de la gravure chimique. Les séparateurs de faisceau en forme de Y, des structures 2D fabriquées à l'aide de DLW de type A, ont été fabriqués et leurs performances ont été mesurées. Il a été confirmé qu'un guide d'onde de forme générale dans laquelle l'indice de réfraction du noyau est plus grand que celui du revêtement peut être produit par le biais d'un DLW de type A. Étant donné que le DLW de type A induit toujours des changements d'indice de réfraction positifs de 2,7 x 10-3 à 5,1 x 10-3, il est très approprié pour fabriquer un guide d'onde. Une jonction Y symétrique et diverses jonctions Y asymétriques ont été conçues et fabriquées à l'aide de guides d'ondes à double ligne, et les rapports de sortie ont été mesurés en fonction de la transition de la position d'injection. Il a été confirmé que le ratio de sortie pourrait être de 96% -4% à 57% -43% en raison de l'irradiance différente dans le processus d'écriture entre la branche supérieure et la branche inférieure. Ainsi, les verres DLW en argent contenant du phosphate de zinc peuvent être utilisés facilement et rapidement pour fabriquer le type de dispositif optique souhaité avec uniquement le processus d'écriture. Enfin, en recherchant le stockage optique de données 5D (ODS) à l'aide de DLW de type A, le champ d'application a été élargi. Nous avons démontré le stockage optique de données 5D encodé dans des modifications orientées DLW de type A en utilisant un rayonnement laser relativement faible par rapport au DLW conventionnel. Cinq dimensions ont été obtenues en ajoutant l'orientation du motif d'ellipse et l'intensité de fluorescence à la position 3D. Le motif d'ellipse a été créé par mise au point anamorphique, et l'orientation a été ajustée à 16 niveaux en utilisant SLM. De plus, un dispositif AOM a été utilisé pour régler l'irradiance laser femtoseconde de 16 niveaux. Pour confirmer la possibilité du 5D ODS proposé, deux images différentes ont été simultanément intégrées dans une image par DLW de type A. Et les deux images originales différentes au format bitmap 4 bits ont été restaurées avec succès. Les fidélité de lecture correspondantes de 60,5% et 25,1% ont été obtenues pour la direction d'orientation et les niveaux d'intensité de fluorescence, respectivement. De plus, il est démontré que la précision de lecture peut être considérablement améliorée à 85,0% et 47,1% lorsque le format bitmap 3 bits a été appliqué. En utilisant la technologie proposée, nous avons atteint une densité de données maximale de 14,9 Gb / cm3, et nous pensons que des densités de stockage de données allant jusqu'à 119,2 Gb / cm3 (en utilisant NA = 1,3 cible pétrolière) peuvent être atteintes. En conclusion, les caractéristiques de fluorescence du DLW de type A dans des verres de phosphate de zinc contenant de l'argent ont été étudiées et son utilité en tant qu'application multidimensionnelle a été confirmée. Nous pensons que cette technologie a un grand potentiel pour la structuration à l'échelle nanométrique dans les semi-conducteurs et la fabrication de dispositifs optiques à micro-échelle. In the past 30 years as the pulse width of lasers has been narrowed and high-power lasers have been developed, researches on the interaction between photon and materials using femtosecond lasers have been actively conducted. The high energy density of femtosecond pulsed lasers enables nonlinear photoionization processes in several ways. This paper reports a study of a type Argentum direct laser writing in silver containing zinc phosphate glasses by inducing a nonlinear absorption deformation of femtosecond laser pulses. When silver-containing zinc phosphate glasses are irradiated with femtosecond laser pulses, ring-shaped clusters are formed due to non-linear absorption. The fluorescence properties and the refractive index of the silver cluster induced by this deformation are different from those of the original zinc phosphate glass. Simultaneous comparisons of chemical micro probes, near-field scanning optical microscopy (NSOM), and numerical modeling were used to analyze the laser-induced silver species distribution. The results significantly strengthen the understanding of material modifications in such glasses in a non-thermal interaction regime. In particular, it has been found that the spatial distribution of species in silver-containing glasses produced by femtosecond laser irradiation has a significant effect on chemical etching selectivity. The Y-shaped beam splitters, 2D structures made by using type A DLW, was fabricated and its performance was measured. It was confirmed that a waveguide of a general shape in which the refractive index of the core is larger than that of cladding can be produced by using a type A DLW. Since the type A DLW always induces positive refractive index changes from 2.7x10-3 to 5.1x10-3, it is very suitable for making waveguide. A symmetric Y-junction and various asymmetric Y-junctions were also designed and fabricated using double line waveguides. Their output ratios were measured according to the transition of the inject position. It was confirmed that the output ratio could be varied from 96%-4% to 57%-43% due to the different irradiance in the process of writing between the upper branch and lower branch. So DLW in silver containing zinc phosphate glasses can be utilized easily and quickly to fabricate the desired type of optical device with only the writing process. Finally, the scope of application was further expanded by investigating 5D optical data storage (ODS) using type A DLW. We have demonstrated 5D optical data storage encoded in orientated type A DLW modifications by using a relatively low laser irradiance compared to conventional DLW. Five dimensions were achieved by adding the orientation of ellipse pattern and fluorescence intensity to 3D position. The ellipse pattern was created by anamorphic focusing, and the orientation was adjusted to 16 levels by employing SLM. In addition, AOM device was used to adjust the femtosecond laser intensity of 16 levels. To confirm the possibility of the proposed 5D ODS, two different images were simultaneously embedded simultaneously in one image by type A DLW. And it was demonstrated that the two different original images of 4-bit bitmap format were successfully restored. The corresponding reading fidelities of 60.5% and 25.1% were obtained for the orientation direction and fluorescence intensity levels, respectively. In addition, it is shown that the reading accuracy can be greatly improved to 85.0% and 47.1% when 3-bit bitmap format was applied. Using the proposed this technology, we showed a maximum data density of 14.9 Gb/cm3 can be achieved, and it can reach up to 119.2 Gb/cm3 (using NA = 1.3 oil target). In conclusion, the fluorescence characteristics of type A DLW in silver containing zinc phosphate glasses were studied, and its utility as a multi-dimension application was confirmed. We believe this technology has great potential for nano-scale patterning in semiconductor and fabrication of micro-scale optical devices. 지난 30년간 극초단 고출력 레이저가 개발되고 연구됨에 따라, femtosecond 펄스 레이저를 이용한 photon과 물질간의 상호작용에 대한 연구가 활발히 이루어지고 있다. 높은 에너지 밀도를 가지는 femtosecond 레이저 펄스는 물질의 특성에 따라 다양한 메커니즘으로 비선형 광이온화를 가능케한다. 이 논문은 femtosecond laser pulse를 은 이온을 함유한 황화 아연 유리에 조사함으로써, 비선형 흡수 변형을 유도하는 type A Direct Laser Writing (DLW)에 관한 특성 및 응용에 관한 연구를 보고한다. 이 샘플에 femtosecond laser pulse train을 조사하면, 비선형 흡수로 인한 은 이온의 재배치과정을 통해 ring의 형태를 가지는 cluster를 형성한다. 이렇게 형성된 cluster는 기존의 유리 샘플과는 다른 fluorescence 특성과 refractive index를 가지게 된다. Chemical micro probe, near-field scanning optical microscopy (NSOM), 수치 계산을 동시에 비교하여 은 이온들의 분포를 분석하였다. 이를 통해 유리 샘플과 같은 non-thermal interaction regime에서 물질의 변형에 대한 이해를 높일 수 있다. 특히 femtosecond laser로 유도된 은 이온 종의 공간적 분포가 Chemical etching selectivity에 미치는 영향 분석하였다. Type A DLW을 통해 만들어진 2차원 구조물인 Y형태의 beam splitter를 제작하고, 그 성능을 측정하였다. 이러한 방법으로 만들어진 광도파관은 일반적인 형태의 광도파관이 가지는 core 물질의 refractive index가 cladding의 refractive index 보다 높은 특징을 가진다. 조사되는 레이저 세기와 쓰기 속도에 따라 다르지만, 2.7x10-3 에서 5.1x10-3 의 positive refractive index 변화를 가지고, 이는 광도파관으로 활용하기에 적당하다. 대칭적 Y-junction, 비대칭적 Y-junction을 설계, 제작하고 그 성능을 비교하였다. Type A DLW은 레이저 beam이 focusing되는 voxel의 외곽부분에만 silver cluster를 남기기 때문에, 2차원 구조에서는 두 줄의 writing patter을 남긴다. 이렇게 만들어진 광도파관은 입사 coupling이 굉장히 크기 때문에 입사 위치에 따른 beam splitter의 output을 측정하였다. 각기 다른 레이저 세기로 upper branch와 lower branch를 만든 비대칭적 Y-junction은 57%-43%에서 96%-4%까지의 beam splitting 성능을 가진다. 이렇게 은 이온을 함유한 황화 아연 유리에 Direct Laser Writing방식으로 2차원 구조물을 제작하면 높은 구조적 자유도를 가지고 빠르게 제작할 수 있기 때문에, micro-size 광학부품을 제작하는데 유용하다. 2차원에서 확장하여, 5차원 광학적 정보 저장(5D Optical Data Storage; 5D ODS)에 대한 연구를 하여 type A DLW의 활용성을 더욱 높일 수 있음을 확인하였다. Type A DLW은 기존 다른 그룹의 5D ODS 연구에서 사용된 레이저 세기보다 낮은 레이저 세기를 활용할 수 있다는 강점을 가진다. 5차원은 기존의 공간적 3차원에 타원 패턴의 방향 그리고 형광 세기를 더해주어 구현하였습니다. 타원 패턴은 symmetrical focusing이 아닌 anamorphic focusing을 하여 유도하였고, 방향은 Spatial Light Modulator (SLM)의 holographic mask를 회전시켜 16단계로 변화시켰다. 형광 세기는 Writing pattern을 제작할 때, Acousto-Optic Modulator (AOM)를 사용하여 레이저의 세기를 조절하여 16단계로 변화시켰다. 이렇게 제안된 5D ODS의 가능성을 확인하기 위해, 서로 다른 2개의 사진을 타원의 방향, 형광 세기로 나타낼 수 있도록 변환하고, 이를 하나의 writing 이미지가 동시에 내포하도록 제작하였다. 이렇게 제작된 이미지를 4-bit 형식으로 디코딩하여 원래의 정보를 다시 얻었다. 타원의 방향에 포함된 정보는 60.5%의 정확도, 형광 세기에 포함된 정보는 25.1%의 정확도로 복원되었다. 그리고 정확도를 높이기 위해 3-bit 형식으로 디코딩하였을 때는 각각 85.0%, 47.1%의 정확도로 복원되었다. Type A DLW을 이용한 5D ODS는 14.9 Gb/cm3의 최대 저장 밀도를 가지고, 좀 더 높은 Numerical Aperture를 가지는 Objective lens를 사용하면 최대 119.2 Gb/cm3까지 저장 밀도를 증가시킬 수 있다. 종합적으로 이 논문은 은 이온이 함유된 황화 아연 유리에 femtosecond laser pulse를 이용한 Direct Laser Writing의 특성을 연구하고, 다차원 응용에 대한 연구 결과를 보여준다. 우리는 이 기술이 반도체 분야에서 nano-size의 pattering, micro-size의 광학 부품 분야 등에 폭 넓게 활용될 것이라 전망한다.

Femtosecond Laser-induced Nano and Microscale Structures on Metal Surfaces

박태훈 인하대학교 대학원 2024 국내박사

This thesis presents the use of femtosecond lasers to generate nano- and micro- scale laser-induced structures on the surfaces of metals with significant industrial relevance, exploring the principles of their formation, applications, and the resulting physical and optical characteristics of metal surfaces. Chapter 1 presents the background of this study. Chapter 2 describes the introduction of a novel femtosecond laser-induced columnar structure (fs-CS) on pure aluminum surfaces, which has rarely been reported. This chapter discusses the initial growth mechanism and evolution of these structures, proposing that the redeposition of nano- and microscale particles and clusters from the laser plume is critical in the initial growth. Furthermore, the evolution of the structure and surface morphology were found to be substantially influenced by four hydrodynamic phenomena induced by laser impact. Chapter 3 presents the investigation of the optical properties of CSs on aluminum surfaces. These structures exhibited reflectance characteristics that differed from those reported in previous studies in the visible spectrum. The unique microstructure naturally formed during the growth of CSs is suggested to contribute significantly to these optical characteristics. Chapter 4 discusses the functionalization of metal surfaces for structural coloration and optical authentication using femtosecond laser- induced structures. By controlling the polarization, the tunability of the structural coloration of the laser-induced periodic surface structure and its potential application in deep learning-based optical authentication systems are demonstrated. Finally, Chapter 5 focuses on improving the surface characteristics of metals, including ferrous, alloy steel, and galvanized steel, by creating femtosecond laser-induced structures. This study proposes the optimization of the expressed characteristics of metal surfaces by controlling the laser parameters to regulate the shape and density of nano- and micro-scale structures, thereby enhancing surface functionalities.

극초단 레이저를 이용한 나노구조 금속 및 반도체 박막의 광학적 전기적 특성 향상

지석영 과학기술연합대학원대학교 2022 국내박사

As lasers are applied to various industrial fields, many studies are being conducted to investigate the interaction between materials and lasers. Various materials such as metals, semiconductors, and polymers used in industrial fields can be processed with a laser method suitable for each. Currently, there is a lot of laser research going on to improve the properties of materials using laser annealing process. We conducted a study on the change in properties of gold, which is a metallic material, and amorphous silicon, which is a semiconductor material, using a femtosecond laser that minimizes the thermal influence of the surrounding while selectively applying high laser peak energy. In addition, it was confirmed that the absorptivity according to the wavelength can vary depending on the structure of the material, and it was hypothesized that the difference in the absorptivity causes the difference in the heat source generated by the laser, and that even the same material can change the thermal distribution depending on the structure. Therefore, to show that the nanostructure and bulk heat distribution are different during laser irradiation, theoretical analysis through simulation and experimental comparative analysis through actual experiments were performed. In the case of gold, the electrical and optical properties of the nano-Au pattern were improved through a laser annealing process using a femtosecond laser of the wire grid polarizer (WGP) nano-pattern. The laser annealing process with a femtosecond laser was performed by selecting the laser parameters that could increase the grain size of the metal without damaging the underlying silicon substrate. The heat generated by the femtosecond laser is rapidly generated, increasing the internal grain size of the nanopattern. As the temperature raised by the laser decreases rapidly, the nanostructured metal pattern does not change its shape. In the case of WGP subjected to femtosecond laser annealing process, it was possible to manufacture WGP with more than 300% efficiency compared to WGP without thermal annealing process. While studying the recrystallization of amorphous silicon using a femtosecond laser, a study on the difference between the nanostructure and the bulk structure was conducted. It was confirmed through experiments how recrystallization occurs when the same laser energy is applied to amorphous silicon having a bulk and a nanostructure. It was found that recrystallization of nanostructured poly silion can be realized with high efficiency even with low laser energy. In addition, we have succeeded in manufacturing high-efficiency polysilicon using a complex laser system that combines the advantages of a CW laser and a femtosecond laser. It can be seen that the Raman peak of poly silicon generated by our complex laser system is 518 cm-1, which is close to the Raman peak of 520 cm-1 of single crystal silicon. Through the study of changes in the physical properties of gold, a metal material, and amorphous silicon, a semiconductor material, using a femtosecond laser, it was confirmed that the physical properties of a variety of other materials could be improved by laser annealing process using a femtosecond laser. In addition, it was confirmed that the thermal distribution of the laser is different in the bulk and nanostructures due to the difference in the absorption rate according to the structure, which will suggest the direction of research on the laser annealing process. I think the results of this paper will be a key technology for many researchers to improve the properties of materials with a simple process in the future.

The impact of mechanical treatments on titanium implant surfaces : a comparison of sandblasted, acid- etched, and femtosecond laser-treated surfaces = 다양한 티타늄 임플란트 표면에서 기계적 처치 방법에 따른 효과

은승모 경북대학교 대학원 2024 국내박사

본 연구는 기계적 처치 방법이 임플란트 표면의 거칠기에 미치는 영향을 조사하였으며, machined, femtosecond laser-treated, 그리고 sandblasted with large-grit sand and acid-etched (SLA) 처리된 표면을 비교하였다. 본 연구는 일반적으로 사용되는 임플란트 표면에 대한 비수술적 처치 방법들의 영향을 평가하며, 여기에는 특히 femtosecond laser-treated implant surface에 중점을 두어 machined 및 SLA surface와 비교하고, 기계적 처치가 다양한 임플란트 표면에 미치는 영향을 이해하는 것을 목표로 한다. Titanium (Ti) disk 제작 및 SLA 공정과 femtosecond laser 표면처리를 수행하여 세 종류의 티타늄 디스크를 준비하였다. 각 표면 처리 그룹의 디스크 시편은 티타늄 큐렛, 초음파 티타늄 스케일러, 그리고 티타늄 브러시를 이용한 기계적 처리방법으로 후 처리를 수행했다. 이 후 scanning electron microscopy, confocal laser scanning, contact angle analyzer를 이용하여 surface roughness, weight percentage를 평가하고, wettability를 비교했다. 통계분석은 군간 비교를 위해 Kruskal-Wallis H test를 이용하여 수행하였다(α = 0.05). Machined surface group에서는 티타늄 스케일러로 인한 상당한 표면 변형이 관찰되었다. SLA surface group에서는 티타늄 큐렛과 비교하여 티타늄 스케일러로 인한 변형이 더 크게 나타났다. Femtosecond laser-treated surface group은 다른 그룹과 유사한 결과를 보였다. 기계적 처치를 하지 않은 control group에서, machined surface group 및 SLA group 간 Ra 값에서 유의한 차이는 나타나지 않았다. 그러나 femtosecond laser-treated surface는 SLA 표면보다 더 높은 Ra 값을 나타냈다. 티타늄 큐렛이나 티타늄 브러시의 적용은 femtosecond laser-treated surface의 거칠기를 더 증가시켰으며, 티타늄 스케일러 처리는 SLA 표면의 Ra 값을 감소시켰다. Femtosecond laser-treated implant surface는 conventional하게 처리된 임플란트 표면과 비교 가능한 결과를 나타내었다. 다양한 처리 방법 중에서는 티타늄 스케일러만이 표면 거칠기를 유의한 수준으로 감소시켰으며, 다른 처리 방법들은 거칠기 수준에 미미한 변화를 일으켰다. Femtosecond laser-treated group과 machined 및 SLA surface group 간에는 조성적인 차이가 있지만, 이러한 차이의 정도는 처리 방법에 따라 현저하게 다르지 않았다. 특히, femtosecond laser-treated group은 접촉각을 통해 관찰한 결과대로 SLA 그룹에 비해 향상된 친수성을 나타냈다. Femtosecond laser-treated implant의 잠재적인 생물학적 이점을 고려할 때, 이들은 미래에 통상적인 임플란트 표면 처리에 대한 실질적인 대안으로 부상할 수 있다.

Parametric Study of Femtosecond Laser-Assisted Selective Removal of Neovascular Structures in Animal Eye : 초고속레이저를 이용한 적출된 돼지안구망막상 신생혈관의 선택적 제거연구 :

MEHRA SINGH SIDHU Korea Research Institute of Standard and Science 2014 국내박사

Rapid advances in the generation and amplification of ultrashort laser pulses have opened up many avenues to understand the interaction between the laser pulse and biological materials. The ultrashort pulse makes it easy to achieve extremely high peak intensity with low pulse energies, which induces plasma-based ablations with multiphoton nonlinear absorptions for transparent media. In this work, femtosecond laser-assisted parametric studies were made for the selective treatment of retinal blood vessels in porcine eyes in vitro and reduction of neovascular structures in cornea tissue of rat eyes in vivo. The near infrared (NIR) femtosecond laser (fs-laser) was employed to develop an efficient method for non-invasive vitreoretinal surgery in eyes. Blood vessels laid on/in retinal or corneal tissues were very precisely exposed to a titanium-sapphire (Ti-Sapphire) fs-laser beam (wavelength = 800 nm, pulse width = 150 fs, pulse repetition rate = 1 kHz). From in vitro studies on retina tissues taken from enucleated porcine eyes, we systematically determined the perforation threshold for primary retinal blood vessels (4.7 ± 1.9 J/cm2) and the ablation threshold for an inner limiting membrane (ILM) overlaid on the blood vessels (2.2 ± 1.1 J/cm2). With increasing laser fluence, a selective ablation of concerted retina layers was possible without any apparent damage to the underlying retinal layers. We measured the ablation depth of retinal tissues as a function of laser fluence which was divided into two regimes, namely, optical penetration depth ( 8.2 ±2.2 μm : Fth( ) = 2.2 ± 0.9 J/cm2) at a laser fluence lower than 25.3 J/cm2 and electronic heating depth (l = 69.7 ± 8.7 μm : Fth(l) = 25.3 ± 13.9 J/cm2) at a fluence higher than 25.3 J/cm2, where Fth( ) and Fth(l) were defined as respective ablation thresholds. Thus, gradually varying the laser fluence allowed us to systematically control the ablation depths within the retinal tissues. On the basis of the lumen diameter and the direction of blood flow from the optic nerve to the periphery of the retina, the retinal blood vessels were classified into four different types of blood vessels: primary, secondary, tertiary and quaternary. From the analysis of vessel perforation probability as a function of laser fluence, we found that the perforation threshold (Fth) for blood vessels decreases with a decreasing vessel diameter (D) and vessel wall thickness (L), for the primary vessel with D = 146.1 ± 10.6 μm and L = 10.0 ± 0.3 μm, Fth = 4.7 ± 1.9 J/cm2, for the secondary with D = 118.8 ± 7.6 μm and L = 7.4 ± 0.1 μm, Fth = 2.8 ± 0.4 J/cm2, for the tertiary with D = 93.1 ± 4.9 μm and L = 6.6 ± 0.7 μm, Fth = 1.5 ± 0.4 J/cm2, and for the quaternary with D = 51.4 ± 7.3 μm and L = 5.8 ± 0.9 μm, Fth = 0.6 ± 0.2 J/cm2. The perforation threshold fluence for quaternary and tertiary blood vessels was 4 and 1.5 times lower than the retina ablation threshold (2.2 ± 0.9 J/cm2), respectively. In order to elucidate the dependence of the perforation threshold on the vessel wall thickness, we measured the perforation threshold fluences for an artificial blood vessel mimicked-channel, which was made of poly-di-methyl-siloxane (PDMS) film with two different thicknesses (10 and 20 μm). While the ablation threshold was found to be independent of film thickness, the perforation threshold was dependent on the film thickness. The perforation threshold for 10 μm PDMS film thickness was 2.5 J/cm2 and for 20 μm it was 4.3 J/cm2. This observation was quite consistent with the fact that retinal blood vessel perforation thresholds decreased with a decreasing vessel wall thickness. Thus, a careful radiant exposure to an fs-laser might allow us to selectively perforate the neovascular structures without any apparent change in normal blood vessels because the diameter and wall thickness of several tens of micrometers for the neovascular vessels was much smaller than that of normal blood vessels. Further, we applied fs-laser treatment of blood vessels to intact porcine eyes (ex situ). For this, we have indigenously designed an fs-laser microsurgery prototype equipped with an ophthalmoscope-assisted real-time imaging system to obtain images while irradiating the target tissues. By using the galvano scanner integrated with this high quality retina surface image of the intact porcine eye ball, we could irradiate with the fs-laser beam a specific part of interest and treat the retinal blood vessels without any apparent damage outside the laser spots. After fs-laser exposure, using optical coherence tomography (OCT) and a histological characterization technique, the threshold fluences for ILM ablation (2.5 ± 0.5 J/cm2) and the perforation threshold for primary blood vessels (5.0 ± 0.1 J/cm2) were determined. Using fs-laser microsurgery system, in vivo studies were conducted for the selective treatment of artificially induced corneal neovascularizations in rat eyes. An fs-laser pulse was scanned in the region of neovascular structures grown with a varying laser fluence range from 2.2 J/cm2 to 8.6 J/cm2. The minimal visible laser lesion (MVL) threshold for corneal neovascular structures was 4.3 J/cm2. Scanning electron microscopy (SEM) and OCT characterization of the anterior segment after laser irradiations revealed a localized degeneration of neovascular structures without any observable changes in adjacent tissues. Furthermore, there was a 30% reduction in corneal neovascularizations after five days of fs-laser exposure. Nevertheless, more in vivo studies on long-term reduction of corneal neovascularization are necessary to confirm the current study. In conclusion, this in vitro, ex situ, and in vivo study unequivocally suggests that an fs-laser beam can be utilized for reliable and safe treatment for neovascular structures with high-spatial resolution. Fs-laser assisted ablation thresholds for vascularized transparent materials also remain unaf

Micropatterning on implantable nanofibers by femtosecond laser ablation process

정용우 Korea University 2017 국내석사

Mimicking native extracellular matrix(ECM) has benefits to acquire reliable results from experiments or induce positive feedback to regenerate artificial tissues. Developing an artificial ECM that closely mimics the native tissue environments is important for controlling cell fate. Electrospinning, one of the methods for fabricating native ECM that mimic the structural features. Our approach is to create hierarchical micro/nano structures to electrospun fibers by a combination of electrospinning and femtosecond laser process. Femtosecond laser has a very short pulse duration that allows to fabricate delicate small ablation areas without giving thermal damage comparing with other lasers such as nanosecond lasers. In this study we used femtosecond laser to create microgrooves on poly-L-lactide(PLLA) nanofiber sheets. Observing the ablation on PLLA nanofibers with femtosecond laser, parameters were varied with pulse energy and beam spot size. Controlling pulse energy created more heat affect zone than controlling spot size. Data was merged to fabricate minimal heat damage from fs laser ablation to pattern microgroove on nanofiber sheets. Groove of 5μm width and 5μm depth was applied in 10μm, 25μm, 80μm interval on PLLA nanofiber sheets. Original PLLA nanofiber and laser patterned PLLA nanofiber showed similar characteristics of chemical and mechanical properties. These PLLA nanofiber sheets were used to regulate myoblast to differentiate myotube assembly. Femtosecond laser ablation for patterning microgrooves on nanofiber sheets can be utilized potentially as an in vitro model for understanding the role of micro-nano topography to study cell-material interactions.

Study on Design and Characterization of High Energy Femtosecond Laser Oscillators

Seong-Hoon Kwon 광주과학기술원 대학원 2021 국내박사

With a repetition rate of about 100 MHz, the pulse energy that can be obtained directly from a conventional femtosecond laser is only a few nJ, so laser amplifier systems are generally used in experiments that require high energy pulses. However, the amplifier systems cause increase in cost and size. Extended-cavity femtosecond lasers can produce the pulse energy of 100 nJ or more, which results from reducing the laser repetition rate and hence increasing the pulse energy. Thus, the extended-cavity femtosecond lasers can replace the laser amplifier systems in many experiments such as laser processing and frequency conversion, requiring sub-μJ pulses. In this thesis, we developed and analyzed an extended-cavity femtosecond laser for laser ablation. Since the extended-cavity femtosecond laser had to be installed in a limited space, the Herriott multipass cavity (HMPC) configuration and the telescope cavity configuration using 4 mirrors had been utilized for designing the extended cavity. The extended cavity allowed the laser repetition rate to be reduced to about 9.7 MHz, resulting in 145 nJ, 115 fs pulses. The extended-cavity femtosecond laser was installed in a laser machining system for processing silicon wafers. In the laser machining system, the laser beam focused through an optical microscope objective lens was irradiated to the silicon wafers with a fluence of 0.33 J/cm2. As a result, it was confirmed that laser processing is possible using this laser oscillator without any amplifier systems. The details are covered in Chapter 3. On one hand, the pulse energy scalability in femtosecond laser oscillators is limited by the instabilities such as cw generation and multiple pulsating, resulting from excessive nonlinear effects in a gain medium. One of well-known methods of suppressing the instabilities is to reduce the peak power below the instability threshold by stretching the pulse. In positive dispersion regime, the chirped-pulse oscillators generate heavily-chirped pulses requiring external compression and thereby offer the potential of scaling the pulse energy. On the other hand, due to the low peak power of the chirped-pulse, it may be difficult to operate the Kerr-lens mode-locking of the chirped pulse oscillators without a saturable absorber. Among saturable absorbers, the semiconductor saturable absorber mirrors (SESAMs) have been frequently used because it is easy to use as an end mirror. In such the chirped-pulse oscillators with a SESAM, the intracavity dispersion can be controlled through a pair of prisms placed near an output coupler. However, this placement results in a spatially chirped output beam because of the angular dispersion occurring in the prism pair. To eliminate the spatial chirp, additional prisms are required out of the cavity. However, this increases a system size and introduces unwanted dispersion compensation. In order to solve these problems, we designed the chirped-pulse oscillator with the SESAM located at the middle of the cavity. In the cavity of the oscillator, the prism pair and output coupler are placed in different cavity-arms. As a result, the spatial-chirp-free output beam is generated even when the intracavity GDD is compensated by the prism pair. However, the output characteristics of the laser oscillator with the SESAM inside the cavity have not been reported, so the effects of the SESAM position on pulse evolution were firstly analyzed. In Chapter 4, the analysis was done by comparing two lasers that have the different SESAM positions (The cavity end or the middle of the cavity). The two compared lasers generated about 50 nJ pulses in positive dispersion regime, and this result was analyzed via the effective loss induced at a SESAM. In negative dispersion regime, it was found that the upper energy limit of a stable single pulse in the newly designed-cavity laser is lower than that of the conventional laser. This result was analyzed via soliton dynamics. Numerical simulations confirmed that pulse contrast and the stability against cw generation is better when SESAM is inside the cavity than at the cavity end. As mentioned above, in order to demonstrate that the newly designed-cavity laser can produce the output beam with no spatial chirp, we developed the chirped-pulse oscillator with the SESAM in the middle of the cavity that has a prism pair. The developed oscillator produced 172 nJ chirped-pulses that were compressed to 98 fs after pulse compression. An additional prism pair out of the cavity is not required to compensate for the spatial chirp so the developed oscillator can be said to be a more compact laser oscillator than the conventional laser oscillators.

Improvement of corrosion property of biodegradable metal via femtosecond laser surface modification

박재호 高麗大學校 大學院 2016 국내석사

Magnesium alloy has been significantly recognized as orthopedic materials. It has similar mechanical properties of bones than any other biomaterials, and it resolves inside of body with bone generation promotion. However, the corrosion rate of magnesium alloy is accelerated when it is existed around body fluid with lots of chloride compounds. Moreover, the rapid change of hydrogen gas and pH that occurs at the beginning of corrosion can cause excessive inflammation and fibrosis surroundings tissue which limit the alloy to be used as human body implant. Herein, we introduced femtosecond laser surface modification for decreasing the corrosion rate of magnesium and alloy. Laser surface engineering is emerging as an useful tool for surface modification of materials due to its simple and effective process. Surface modification engineering by laser is possible for non-contact machining and selective process depending on the conditions of the laser such as laser beam size, wavelength, power and so on. It is possible to expect different effects, therefore it is used in variety of fields. In this study, we applied femtosecond laser surface modification processing to Mg and Mg alloy in two different ways to improve bio-corrosion properties. First, by controlling the laser power and repetition rate, we found that the corrosion rate can be significantly decreased with laser conditions of energy per pulse of 40uJ, repetition rate of 500Hz, and scan speed of 2mm/s. Initial corrosion rate was decreased as 65% compared to non-treated samples by Mg ion elution and amount of hydrogen gas analysis under HBSS(Hank’s solution). Therefore, we observed the surface and phase change with SEM(Scanning Electron Microscope) and XRD(X-ray diffraction). By femtosecond laser surface modification, we can control corrosion rate more precisely, which will allow biomaterials to be used in various fields through optimization of laser treatment on diversity of metal. Second, using femtosecond(fs) laser surface modification, we gave Mg surface super hydrophilic wettability so that we expected improvement of adhesion ability of biodegradable coating layer for controlling corrosion rate. Laser treatment group and non laser treatment group was spin-coated with biodegradable polymer PLGA(poly lactic-co-glycolic acid) in the same condition. Immersion test was conducted in cell media for 7 days. In immersion test, Mg ion assay showed that coatings on laser treated Mg has less Mg ion than the control group. And we could observe that cross-section of laser treated group shows coating layers infiltrate deeper onto the surface. From these data, we expected improvement of adhesion strength due to imbedded coating layer between pits and grooves. Henceforth, through fs laser surface modification, it can be expected to be utilizing in variety of coating technique engineering.