Hybrid periodic microstructures on chromium films were prepared by SLM-assisted nanosecond laser technique

2024-09-26 13:30:06

Spatial light modulator is a dynamic component which can change the amplitude, phase and polarization state of incident light in real time under the control of external signal. The application of spatial light modulator in laser processing can realize dynamic beam shaping, and has the advantages of programmable, easy to control, easy to integrate, low loss and high refresh frequency. And with the improvement of the damage threshold of spatial light modulators, the application fields of laser processing are also expanding, such as metamaterial structure manufacturing, microfluidic, 3D printing, optical storage, material surface modification, quantum dots and other fields.

Thesis information:

In this paper, an effective technique for the preparation of different hybrid period structures on chromium (Cr) films with different thickness using a spatial light modulator (SLM) assisted 1064nm nanosecond laser is presented. For 1000nm Cr films, regular two-scale MG-LIPSs can be prepared by combining the periodic modulation grating (MG) generated by SLM with the laser induced periodic surface structure (LIPSS), whose morphological characteristics are controlled by the laser flux, the number of effective pulses and the MG period. Due to the diffraction effect of MG and LIPSS, the surface of MG-LIPSS pattern exhibits vivid anisotropic structural colors. Due to the more significant thermal stress of the thinner films compared to MG-LIPss, a complex periodic structure consisting of MG and cracks (MGC) is formed on the 200nm Cr films. Although the cracks of MGC are randomly distributed, MGC has a long order characteristic of certain transmittance and can be used as a transmission grating with diffraction effect. These results show that the SLM-based light field modulated laser processing provides an efficient, economical and controllable method for preparing large area periodic structures on Cr films. In addition, changes in film thickness can be used to explore hybrid microstructure with specific properties for different applications, such as optical components and anti-counterfeiting measures.

The following are part of the experimental process and results:

The light source uses a commercial nanosecond laser, which provides a 1064nm, 50ns linear polarization pulse laser. The repetition frequency is set to 3kHz during the experiment, and the corresponding maximum output power of the laser is 0.45W. The light emitted from the laser passes through a 4× beam expanding mirror, so that the light spot is filled with the liquid crystal light valve target surface. The phase spatial light modulator (FSLM-2K70-VIS) is used in the experiment, the pixel size is 8um, and the resolution is 1920×1080. After being modulated by a spatial light modulator, it is acted on the sample through a lens, and the processing process is monitored in real time by CCD to ensure that the sample surface is always in the processing plane. The hologram was generated by Gerchberg Saxton algorithm.

FIG. 1 (a) Experimental device (Phase spatial light modulator, model: FSLM-2K70-VIS); (b) Original and modulated beams.

FIG. 2 SEM morphology of 1000nmCr thin films formed by MG-LIPSS under 4 different modulation periods Γ as laser flux increases. Scale: 5μm.

FIG. 3 SEM morphology of MG-LIPSS formed by (a)-(c) 1000nmCr films under different effective pulse numbers. Scale: 5μm.

FIG. 4 (a)0.27J/cm² and (e) 0.32J /cm² correspond to AFM measurements of MG-LIPSS structures under different laser irradiation, respectively. (b) and (f) correspond to two-dimensional fast Fourier transforms of SEM images (a) and (e), respectively. (c) and (d) Two-dimensional diagrams of LIPSS and MG cross sections corresponding to (a) MG-LIPss. (g) and (h) are two-dimensional diagrams of the LIPSS and MG cross sections corresponding to (e) MG-LIPss. Scale: 5μm.

Figure 5 (a-b) MicroRaman spectra of MG-LIPSS prepared at two different laser fluxes F at different locations. (c-f) EDS results of MG-LIPSS prepared at different laser fluxes F (collection points are marked in red in the figure). Scale: 5μm.

FIG. 6 SEM morphology of MGC formed under different processing conditions for Cr film at 200nm. (a) Γ2 = 8 μm, F = 0.16 J/cm². (b) Γ3 = 9 μm, F = 0.16 J/ cm². (c) Γ4 = 13 μm, F = 0.16 J/cm². (d) Γ4 = 13 μm, F = 0.30 J/cm². Scale: 5μm.

Figure 7 Rainbow structure color of MG-LIPSS. (a) White light diffraction diagram of MG-LIPSS mixed periodic structure formed on a 1000nmCr film, with LIPSS and MG producing rainbow structure colors in two orthogonal directions, respectively. (b) The Chinese character pattern of "Sun Yat-sen University" is coated with 1000nm Cr on a glass wafer with a diameter of 100 mm. (c) Processed samples. (d) and (e) color the "Sun Yat-sen University" pattern and the dragon pattern respectively. (f) and (g)MG-LIPSS "3" are different representations of the colors of the iridescent structure at different viewing angles. Scale: 5mm.

The parameters of the spatial light modulator used in this experiment are as follows:

Model number	FSLM-2K70-P03	Modulation type	Phase pattern
Liquid crystal type	Reflecting type	Gray level	8 bits, 256 levels
Liquid crystal mode	PAN	Driving mode	figure
Resolution	1920×1080	Pixel size	8.0μm
Effective region	0.69" 15.36mm×8.64mm	Filling factor	87%
flatness（PV）	Before calibration：5λ After calibration：1λ	flatness（RMS）	Before calibration：1/3λ After calibration：1/10λ
Refresh frequency	60Hz	Response time	≤30ms
Optical efficiency	75%@1064nm	Angle of alignment	0°
Phase range	2π@1064nm Max：2.1π@1064nm	Spectral range	450nm-1100nm
Gamma adjust	support	Phase correction	support(808nm/1064nm)
linearity	≥99%	Phase stability（RMS）	≤0.13π
Damage threshold	Continuous: ≤20W/cm2(no water cooling) ≤100W/cm2(water-cooled)	Diffraction efficiency	1064nm 60%@ L8 66%@ L16 75%@ L32

In order to further extend the application of spatial light modulator in industry, this paper is developed High damage, square large target surface spatial light modulator:

Model number	FSLM-2K73-P03HP	Modulation type	Phase pattern
Liquid crystal type	Reflecting type	Gray level	8 or 10 bits optional
Liquid crystal mode	PAN	Driving mode	figure
Resolution	2048×2048	Pixel size	6.4μm
Effective region	0.73" 13.1mm×13.1mm	Filling factor	93%
Refresh frequency	60 Hz（8bit）*	Input power supply	12V 3A
Angle of alignment	0°	Data interface	HDMI
Phase range	2π@1064nm Max：3.5π@1064nm	Spectral range	1000nm-1100nm
Optical efficiency	95%±5%@1064nm	Response time	≤30ms
Gamma correction	support	Phase correction	支持(1064nm)
linearity	≥99%	Phase stability (RMS)	＜0.03π
Damage threshold	Continuous: ≤1000W/cm2(no water cooling) Pulse: Peak power density (10GW/cm2) Average power density (100W/cm2) @1064nm/290fs/200KHz (water cooled)	Diffraction efficiency	1064nm 56%@ L8 72%@ L16 85%@ L32

Write at the end:

With the further development of laser processing technology and the increasing demand for high-precision and high-efficiency processing indicators, spatial light modulator, as a key optical component, will play an important role. The application of spatial light modulator in laser processing is not limited to a single technical field, its wide application prospects cover a number of fields, such as industrial manufacturing, scientific research, optoelectronics, etc., for the advancement and innovation of laser processing technology provides a strong support and driving force, is expected to promote laser processing technology to a more advanced, more complex direction.