Presented By Under supervision 1 Department of Physics, Nagoya - PowerPoint PPT Presentation

Presented By Under supervision 1 Department of Physics, Nagoya University, Nagoya, 464-8602, Japan 2 National Institute for Fusion Science, Toki, Gifu 509-5292, Japan

Content ➢ Introduction ➢ Aim ➢ Hypothesis ➢ Method ➢ Results and Discussion ▪ Formation Mechanism of Periodic Nanograting Structure By Weibel Instability ▪ Formation Mechanism of Periodic Nanograting Structure By Surface Plasma Wave ➢ Conclusion

Introduction What Is the Periodic Nanograting Structure? ❑ Nanomaterial: Is define as a material with structure size(in at least one dimension) in nanoscale(1 nm = 10 − 9 m). Nanoscale is refer to structure with length scale of 1 to 100 nm. ❑ Nanograting structure: It is kind of periodic structure has been observed on the surfaces of the materials after irradiating it with laser beam of different intensity. The first observation for the periodic nanograting structure was 1999. Periodic nanograting structures are one of the contemporary science issues that gained much attention in the last 20 years due to their vast usage in industrial applications. ❑ This structures is called LIPSS ( Laser Induced Periodic Surface Structures).

➔ LIPSS ( periodic nanograting structure) has following characteristics: ◆ The periodicity is usually less than laser wavelength. ◆ The groove direction is perpendicular and/or parallel to laser polarization direction. ➔ Periodic nanograting structure is produced by repeating irradiation of femt femtose osecond cond laser pulses with the intensity about 10 13 W/cm 2 or higher.

Interspaces of LIPSS were in the range of 0.5 λ L – 0.85 λ L Copper Dt = 100 fs d = 40 m m l = 800 nm Intensity ~ 10 13 W/cm 2 Courtesy to Prof. M. Hashida ? cm

Application of the Periodic Nanograting Structure SCM415(alloy steel) ❑ Friction reduction: Φ =30 [mm] T. Kato and N. Abe, 25% Laser Research 37,510 (2009) reduction ❑ Metal coloring: A.Vorobyev, et al., Laser Photonics B. Rev. 1-23 (2012)

Aim ❑ Producing and controlling periodic nanograting structure is my major duty. ❑ Investing and explaining formation mechanism of periodic nanograting structure under the effect of relativistic and nonrelativistic laser beam is the second major duty. ❑ Study the accompanied physics related to formation mechanism of periodic nanograting structure is the keywords of the thesis work.

Hypothesis ❑ The maximum laser intensity can be used to produce the periodic nanograting structure in 15 W/cm 2 . Our simulati experimental work is about I = = 10 10 13 13 – 10 10 15 ation on study dyin ing g case contain ined ed 18 W/cm 2 - μ m 2 and 10 18 two cases es with h relativ ivist istic ic and nonrelat ativi ivistic stic laser r beam of intensity ity I = = 10 16 W/cm 2 - μ m 2 . 10 16 I = 10 ❑ Many researches assume that surface plasma wave has a role in forming periodic nanograting structure but without any evidences. Within in this s simulati ation on study dy we can get a c clear eviden ence ce to existen ence ce of surface ce plasm sma wave and explain ain its role in formation ion mechan anism. ism. ❑ Getting periodic nanograting structure by using relativistic laser beam can be considered as the first trial in this field. We could d expla lain in this s case by using relativi ivist stic ic laser r intensi sity.

Method The particle article-in in-cel cell ( PIC) PIC) code code is used to study plasma physics by researchers. PIC code compute both of Maxwell ’ s equations and equations of motion for electrons and ions in 2D space and 3D velocity space. ❖ Computer Physics Communications 204 (2016) 141 – 151

Systems Comparison ❑ The experimental system Pre-formed plasma Metal Laser Sample ❑ The simulation system Mimic plasma Target Plasma Laser Sample

Result and Discussion Part I The Formation Mechanism of Periodic Nanograting Structure by Weibel Instability

The Simulation Parameters ❖ Plasma Parameter: - M i ／ m e = 1836 - A = 1, Z = 1 - T e = 1.0 , T i = 0.1 keV ✿ Target plasma – X = 2.0 〜 12.0 µm – Y = -3.6 〜 3.6 µm ❖ Laser Parameter: λ L = 800 nm – n target = 10 n cr incidence angle = 0° ✿ Mimic plasma τ rise = 15 fs, p-polarized, plane wave I L = 10 18 W/cm 2 -µm 2 – X = 0 〜 2.0 µm continuously irradiated beam – Y = -3.6 〜 3.6 µm The Max experim rimen ental tal 13 ~ 10 15 W/cm 2 – n thin = 0.7 n cr 10 13 10 15 I= 10 ✿ simulation Time : 0 ~ 500 fs

Simulation Results ● Electron density profile in the x-y plane at snapshot t = 250 fs. The periodic nanograting structure observed as self-organized structure along the boundary between mimic plasma and target plasma at t = 250 fs. 13 tips are formed along y-axis from -3.0 to 3.0 µm, and the average interspace size of 0.46 µm is shorter than the laser wavelength of 0.8 µm.

Electron Density Profile x-component of electron current density z-component of magnetic field t =150 0 fs fs t =200 0 fs fs

Current density (J/en cr c) Current density ( J xe ) and magnetic field ( B z ) in a relation with time development. : The maximum amplitude of J xe and B z : The minimum amplitude of J xe and B z

Weibel Instability can be understood simply as the result of the superposition of two counter-streaming beams

Result and Discussion Part II The Formation Mechanism of Periodic Nanograting Structure by Surface Plasma Wave

The Simulation Parameters Y ❖ Plasma Parameter : 10 - M i ／ m e = 1836/16 = 114.8 Target Plasma - A = 1, Z = 1 - T e = 1.0 , T i = 0.1 keV X 0 12 2 ✿ Target plasma – X = 2.0 〜 12.0 µm - 10 – Y = - 10.0 〜 10.0 µm – n target = 10 n cr ✿ Vacuum is surrounded target plasma everywhere. flattop shape ( f = 10 µm) I L = 10 16 W/cm 2 - μ m 2 Laser Parameter: ❖ λ L = 800 nm continuously irradiated beam The Max experimental incidence angle = 0° I= 10 13 ~ 10 15 W/cm 2 τ rise = 5 fs, p-polarized ✿ simulation Time: 0~500 fs

The electron density profile in the x-y plane in snapshots taken at (a) t =150 fs, (b) t =200 fs, (c) t = 250 fs, (d) t = 300 fs,(e) t = 350 fs,(f) t = 400 fs, (g) t = 450 fs, and (h) t = 500 fs. t = 250 0 fs fs t = 200 0 fs fs t = 300 0 fs fs t = 150 0 fs fs t = 400 0 fs t = 450 50 fs t = 500 0 fs fs t = 350 0 fs fs

t = 300 fs Intermediate area Sparse area Y [micron]

Average electron density (n e /n cr ) versus distance x (µm) with different density at (a) t = 200, (b) t = 300, (c) t = 400, and (d) t = 500 fs. n sparse ~ 0.1 n cr cr n inter ~ 5 5 n cr cr

Surface Plasma Wave ( SPW) Wavelength of SPW with different density values. The used relation to calculate wavelength of SPW is The graph is drawn for the range n sparse = 0 ~ 0.9 n cr depend on n sparse and n inter and n inter = 1 ~ 5 n cr 𝜕 𝑡𝑞 = 𝜕 𝑀 2 𝜕 𝑞𝑓 𝑜 𝑓 𝜁 = 1 − 2 = 1 − 𝑜 𝑑𝑠 , 𝜕 𝑀 𝜁 : is the permittivity of medium 𝜁 𝑗𝑜𝑢𝑓𝑠 . 𝜁 𝑡𝑞𝑏𝑠𝑡𝑓 𝑙 𝑡𝑞 = 𝑙 𝑀 𝜁 𝑗𝑜𝑢𝑓𝑠 + 𝜁 𝑡𝑞𝑏𝑠𝑡𝑓 The used relation to calculate wavelength of SPW is depend on n sparse and n inter 2 − (𝑜 𝑡𝑞𝑏𝑠𝑡𝑓 + 𝑜 𝑗𝑜𝑢𝑓𝑠 ) 𝜇 𝑡𝑞 𝑜 𝑑𝑠 = (1 − 𝑜 𝑡𝑞𝑏𝑠𝑡𝑓 )(1 − 𝑜 𝑗𝑜𝑢𝑓𝑠 𝜇 𝑀 𝑜 𝑑𝑠 ) 𝑜 𝑑𝑠

Standing wave and Ponderomotive force ● The mechanism of surface plasma wave (SPW) y generation is based on the collective behavior of the excited electrons in the y-direction due to the electric field component of the laser beam. 𝜕 𝑡𝑞 = 𝜕 𝑀 e ● The bidirectional collective behavior of the E L electrons in both positive and negative y-axis, leads to SPW propagation near to the interface, x producing the so- called standing wave. The ponderomotive force of standing wave plays a major e role in forming seeds or tips for the development of periodic nanograting structures at early stages since it is repeated each 𝛍 sp /2 .

Define the position of pressure balance P L =P p. 𝑄 𝑀 = 2𝐽 𝑀 𝑑 = 6.67𝐽 𝑀 𝑋/𝑑𝑛 2 𝑁𝑐𝑏𝑠 , 2 [𝜈𝑛][𝑁𝑐𝑏𝑠]. 𝑄 𝑞 = 1.79 𝑜 𝑈𝑓 𝑙𝑓𝑊 /𝜇 𝑀 The position at which the pressure got balanced is at density n = 2.384 n cr is the interface position.

➢ Specify n sparse and n inter as the first local minima on the left hand side and x 1,2 (µm) as the first local maxima on the right hand side of x 1,2 (µm). ➢ We can calculate wavelength of SPW from the previous relation. time n inter /n cr n sparse /n cr 𝛍 sp / 𝛍 L (fs) 200 4.58 0.10 0.91 300 4.83 0.14 0.95 400 5.26 0.17 0.98 500 5.51 0.19 1.00

Comparison graph between the simulation and theoretical interspace. Time (fs) Theoretical Simulated Interspace Interspace 𝛍 sp /2 ( µm ) ( µm ) 200 0.36 0.26 300 0.38 0.33 400 0.39 0.40 500 0.40 0.50