CN111474616A - A method for fabricating subwavelength metal gratings with broad-beam femtosecond laser double pulses - Google Patents

Abstract

The invention discloses a method for preparing a sub-wavelength metal grating by a wide-beam femtosecond laser double pulse. The method can realize the high-efficiency controllable preparation of the large-area sub-wavelength metal grating, does not need precise mechanical parts, does not have a mask, does not have a chemical corrosive agent, and has loose processing environment; the whole preparation process is simple and easy to operate, and secondary damage of stress to the surface of the metal sample can be effectively avoided due to non-contact processing; the surface structure prepared by the method is more regular and uniform in spatial arrangement, and has a large laser spot irradiation range and high preparation efficiency.

Description

A method for fabricating subwavelength metal gratings with broad-beam femtosecond laser double pulses

technical field

The invention relates to the technical field of laser processing, in particular to a method for preparing subwavelength metal gratings with wide-beam femtosecond laser double pulses.

Background technique

As an important tool for analyzing spectral information, regulating light transmission performance, and detecting the properties of optical and electromagnetic fields, gratings play a pivotal role in all aspects of national defense and life. Compared with traditional gratings, subwavelength metal gratings have many unique features in optical transmission performance, such as wide response spectrum, high transmittance and extinction ratio, and good polarization correlation, and have become the most potential optical components in the future. Usually, metal gratings of sub-wavelength scale are the core components in spectrometers, monochromators, lasers and other equipment. In addition, because the all-metal grating element has the characteristics of good voltage resistance, high temperature resistance, and resistance to damage, it can be used normally in a variety of extreme environments, which greatly improves the stability of instruments and equipment. Sensing and other fields have broad application prospects.

Considering that subwavelength metal gratings are important components constituting advanced instruments, extensive and in-depth research on their fabrication methods has been carried out. At present, the more mature preparation methods include: mechanical scribing, electron beam etching, extreme ultraviolet lithography and nanoimprinting, etc. However, these preparation methods all have their own limitations, which seriously limit the further development of subwavelength metal gratings. In addition, mechanical scribing machines, electron beam etching systems, etc. are expensive, high maintenance costs, and demanding processing materials and environment, resulting in high processing costs and time-consuming problems; The operation process of the wavelength metal grating is complex, requiring mask processing and the use of various chemical etchants, resists, etc., a large amount of environmental pollutants are easily generated during the preparation process, and there are problems such as low preparation efficiency. Compared with the above-mentioned traditional subwavelength metal grating fabrication methods, femtosecond lasers have become a new tool for fabricating subwavelength structures due to their high peak power and small heat-affected zone. Among them, the method and mechanism of (quasi) periodic surface structure induced by femtosecond laser have been widely studied by many scholars. In conclusion, by controlling parameters such as the spatial intensity distribution, temporal characteristics, energy density, pulse number, and polarization direction of the incident femtosecond laser, the period, shape, and depth of the subwavelength structures on the material surface can be changed. However, the formation of sub-wavelength (quasi) periodic structures on metal surfaces by femtosecond lasers is mostly concentrated in the case where a single incident beam is spatially tightly focused by an objective lens or other spherical optical elements, in which there is an obvious focused spot radiation. However, the problems of small illumination area, low preparation efficiency in large area, uneven distribution of surface structure and poor regularity, especially on the surface of metal tungsten with high melting point and high hardness, the rapid preparation of large area subwavelength grating has not been successfully realized. In summary, there are still many difficulties in the field of efficient fabrication of large-area high-quality subwavelength metal gratings using femtosecond lasers.

SUMMARY OF THE INVENTION

The invention aims to overcome the problems of the existing femtosecond laser on the metal surface to form subwavelength gratings, such as small focus spot irradiation area, low preparation efficiency in large area, uneven distribution of surface structure and poor regularity, etc. The example of rapid fabrication of large-area subwavelength gratings on the surface of high-melting-point, high-hardness materials has not been successfully realized.

To achieve the above object, the present invention adopts the following technical solutions:

A method for preparing subwavelength metal gratings with broad-beam femtosecond laser double pulses, comprising the following steps:

S1. The femtosecond laser light source outputs the processing light source and enters the optical beam expander;

S2. The optical beam expanding device expands the processing light source, and the beam-expanded light source enters the energy adjustment device;

S3, the energy adjustment device performs energy adjustment on the beam-expanded light source, and the energy-adjusted light source is incident into the double-pulse generating device;

S4. The double-pulse generating device outputs co-linear transmission of femtosecond laser pulses in space, delayed in time, and mutually perpendicular polarization directions, and the femtosecond laser double-pulses are incident into the beamline focusing device;

S5. The beamline focusing device spatially focuses the femtosecond laser double pulses and then irradiates the surface of the metal to be processed on the sample moving scanning device to prepare a subwavelength metal grating. The sample moving scanning device controls the to-be-processed metal grating by controlling the The translation speed of the processed metal changes the number of overlapping pulses irradiated on the metal surface to be processed, thereby controlling the morphology of the microstructure of the metal surface to be processed.

Further, the purity of the metal to be processed is greater than 99%, and the roughness of the surface of the metal to be processed is 5-10 nanometers. Preferably, the metal to be processed is metal tungsten with a purity of 99.95% and a surface roughness of 7.71 nanometers.

Further, the femtosecond laser light source is a Ti:sapphire chirped pulse amplification laser, the repetition frequency of the output light source is 1 kHz, the pulse width is 40 femtoseconds, the center wavelength is 800 nanometers, and the spot diameter is 5 mm.

Further, the optical beam expander is composed of plano-concave plano-convex lenses, the focal lengths of the plano-concave plano-convex lenses are 38.1 mm and 125 mm, respectively, and the light spot diameter of the light source output by the optical beam expander is 15 mm.

Further, the energy adjustment device is composed of a half-wave plate and a Glan-Taylor prism, and the direction of the crystal axis of the half-wave plate is rotated to control the continuous adjustment of the energy of the output light source of the energy adjustment device.

Further, the double-pulse generating device is a yttrium vanadate birefringent crystal, and the angle between the polarization direction of the light source entering the double-pulse generating device and the optical axis of the yttrium vanadate birefringent crystal is 30°. Or 60°, the energy ratio of the femtosecond laser double pulse output by the double pulse generating device is √3:1.

Further, the thickness of the yttrium vanadate birefringent crystal is 1.684 mm, and the delay time of the femtosecond laser double pulse output by the double pulse generating device is 1.2 picoseconds.

Further, the beamline focusing device is a plano-convex cylindrical lens, the diameter of the plano-convex cylindrical lens is 25.4 mm, the focal length is 50 mm, and it is made of fused silica material. The femtosecond laser double pulses output by the double pulse generating device are spatially focused in the direction perpendicular to the lens generatrix.

Further, the sample moving scanning device controls the translation speed of the metal to be processed, so that the scanning speed of the light source irradiating the surface of the metal to be processed is 0.01-0.03 mm/sec.

The method for preparing subwavelength metal gratings with wide-beam femtosecond laser double pulses of the present invention can realize efficient preparation of subwavelength metal gratings on the surface of metal tungsten, a material with high melting point and high hardness. Compared with the existing methods such as mechanical scribing, electron beam etching and photolithography, it can not only realize the efficient and controllable preparation of large-area sub-wavelength metal gratings, but also requires no precision mechanical parts, no masks, no chemical etchants, and no processing environment. Loose; the whole preparation process is simple and easy to operate, and because it is non-contact processing, the secondary damage to the surface of the metal tungsten sample can be effectively avoided; The surface structure of the laser is more regular and uniform in spatial arrangement, the laser spot irradiation range is large, and the preparation efficiency is high. In conclusion, the preparation of subwavelength gratings on metal surfaces based on time-delayed broad-beam femtosecond laser double pulses with polarization crossing proposed in the present invention is a non-contact processing method with low processing cost, high efficiency and controllability.

Description of drawings

1 is a schematic diagram of the optical path of the method for efficiently preparing a subwavelength metal grating with a wide-beam femtosecond laser double pulse according to the present invention;

Fig. 2 is the optical picture and the laser intensity distribution of the light spot focused on the metal tungsten surface in one embodiment of the present invention;

3 is an optical photograph and a scanning electron microscope (SEM) image of a subwavelength tungsten grating prepared in an embodiment of the present invention;

4 is an atomic force scanning microscope (AFM) image of a subwavelength tungsten grating prepared in an embodiment of the present invention and its structural depth measurement curve;

5 is a SEM image of the prepared subwavelength tungsten grating structure morphology when the polarization direction of the light source incident on the yttrium vanadate birefringent crystal and the optical axis direction of the yttrium vanadate birefringent crystal are 30° and 60°;

Fig. 6 is the corresponding relation diagram of the laser energy after passing through the energy adjusting device and the prepared subwavelength tungsten grating structure depth;

Description of reference numbers:

110-femtosecond laser light source; 120-optical beam expanding device; 130-energy conditioning device; 140-double pulse generating device; 150-beamline focusing device; 160-sample moving and scanning device.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

As shown in FIG. 1, it is a schematic diagram of the optical path of the method for efficiently preparing a subwavelength metal grating with a wide-beam femtosecond laser double pulse according to the present invention, wherein along the direction of the optical path, a femtosecond laser light source 110, an optical beam expander 120, and an energy adjustment device 130 are sequentially placed , a double pulse generating device 140 , a beamline focusing device 150 , a sample moving and scanning device 160 .

The femtosecond laser light source 110 is used to generate a femtosecond laser beam, the optical beam expander 120 is used to expand the femtosecond laser beam, the energy adjustment device 130 is used to continuously adjust the energy of the femtosecond laser beam, and the double pulse generating device 140 For generating femtosecond laser double pulses with a fixed time delay, the beamline focusing device 150 is used to focus the femtosecond laser double pulses on the surface of the metal to be processed, and the sample moving and scanning device 160 is used to fix and move the metal to be processed.

The method for preparing subwavelength metal gratings with wide-beam femtosecond laser double pulses of the present invention comprises the following steps:

S1. The femtosecond laser light source 110 outputs the processing light source, and enters the optical beam expander 120;

S2. The optical beam expander 120 expands the processing light source, and the beam-expanded light source enters the energy adjustment device 130;

S3. The energy adjustment device 130 performs energy adjustment on the beam-expanded light source, and the energy-adjusted light source enters the double-pulse generating device 140;

S4. The double-pulse generating device 140 outputs the femtosecond laser double pulses with colinear transmission in space, time delay, and mutually perpendicular polarization directions, and the femtosecond laser double pulses are incident into the beamline focusing device 150;

S5. The beamline focusing device 150 spatially focuses the femtosecond laser double pulses and then irradiates the surface of the metal to be processed on the sample moving scanning device 160 to prepare a subwavelength metal grating. The sample moving scanning device 160 controls the translation speed of the metal to be processed by controlling the To change the number of overlapping pulses irradiated to the metal surface to be processed, and to control the morphology of the microstructure of the metal surface to be processed.

The purity of the metal to be processed is generally required to be greater than 99%, and the surface must be mechanically polished with a roughness of 5-10 nanometers. In this embodiment, the metal to be processed is a metal tungsten disk with a diameter of 15 mm, the purity is 99.95%, and the surface roughness is 7.71 nanometers.

In this embodiment, the femtosecond laser light source 110 is a Ti:Sapphire chirped pulse amplification laser, the repetition frequency of the output light source is 1 kHz, the pulse width is 40 femtoseconds, the center wavelength is 800 nanometers, and the spot diameter is 5 mm.

In this embodiment, the optical beam expander 120 is composed of plano-concave plano-convex lenses, the focal lengths of the plano-concave plano-convex lenses are 38.1 mm and 125 mm, respectively, and the light spot diameter of the light source output by the optical beam expander 120 is 15 mm.

In other embodiments, the optical beam expander 120 can be composed of plano-concave and plano-convex lenses of other focal lengths to form beam expanders with different beam expansion magnifications, and the diameter of the beam spot after beam expansion is controlled and adjusted to match the diameter of the metal to be processed.

In this embodiment, the energy adjustment device 130 is composed of a half-wave plate and a Glan-Taylor prism, and rotating the crystal axis direction of the half-wave plate controls the continuous adjustment of the output light source energy of the energy adjustment device 130 .

In this embodiment, the double-pulse generating device 140 is a yttrium vanadate birefringent crystal with a thickness of 1.684 mm. When the angle is 30° or 60°, the femtosecond laser double pulses output by the double pulse generating device 140 are colinear in space and 1.2 picoseconds in time, and the polarization directions are perpendicular to each other, and the energy ratio is √3:1.

In this embodiment, the beamline focusing device 150 is a plano-convex cylindrical lens, the diameter of the plano-convex cylindrical lens is 25.4 mm, the focal length is 50 mm, and it is made of fused silica material. The femtosecond laser double pulse output by the pulse generator is spatially focused in the direction perpendicular to the lens bus, and the femtosecond laser double pulse maintains the original size in the parallel direction of the lens bus, so that the focused femtosecond laser double pulse irradiation range presents An elongated oval shape with a spot length of about 15 mm in the direction of the major axis of the ellipse.

As shown in FIG. 2 , the optical image and the laser intensity distribution of the light spot focused on the metal tungsten surface in this embodiment, the processing light source output by the femtosecond laser light source 110 is generated by the optical beam expander 120 , the energy adjustment device 130 , and the double pulse. After the device 140 and the beamline focusing device 150, the femtosecond laser double pulses are focused on the metal tungsten surface to present an elongated ellipse shape, wherein the long axis of the ellipse is equal to the beam diameter of the femtosecond laser double pulse incident on the beamline focusing device 150. is 15 mm, and the short axis of the ellipse is the focusing direction.

In addition, the normalized laser intensity distribution at the position of the dotted line of the spot in Fig. 2 shows that the laser intensity changes sharply along the edge to the center position in the short axis direction, while there is no beam spatial focusing effect along the long axis direction, which makes the illumination Femtosecond laser double pulses to the metallic tungsten surface have a slowly varying intensity distribution in this direction. Based on this, the femtosecond laser double pulse outputted by the beamline focusing device 150 scans the metal tungsten along the short axis direction, which can not only obtain a larger spot illumination and a single scanning area, but also achieve subwavelength tungsten within the spot range. Uniform generation of gratings.

The sample moving scanning device 160 is generally composed of a three-dimensional precision moving platform and a computer. The computer program controls the translation speed of the metal to be processed on the three-dimensional precision moving platform to change the number of overlapping pulses irradiated on the metal surface to be processed, thereby controlling the metal surface microstructure. By controlling the moving direction of the metal to be processed, the prepared area on the metal surface can be changed. Generally, the scanning speed of the light source irradiating the metal surface to be processed is controlled to be 0.01-0.03 mm/sec. In this embodiment, the scanning speed of the light source irradiating the surface of the metal tungsten to be processed is 0.02 mm/sec.

As shown in FIG. 3 , the optical photo and scanning electron microscope (SEM) image of the subwavelength tungsten grating prepared in this embodiment show that the subwavelength tungsten grating prepared according to this embodiment has a high degree of regularity, and The entire surface of the sample is uniformly distributed, with a grating period of 510±20 nanometers, which can show iridescent colors under sunlight, indicating that it has a certain light-splitting ability and spatial dispersion effect. In addition, the subwavelength tungsten grating prepared in this embodiment has no surface damage caused by mechanical stress, the surface morphology of the grating is relatively consistent, and only undergoes ultrasonic cleaning with simple chemical solvents such as acetone, deionized water, and ethanol during the processing. Therefore, There are no problems such as residual impurities and environmental pollution.

As shown in FIG. 4 , the atomic force scanning microscope (AFM) image of the subwavelength tungsten grating prepared in this embodiment and its structural depth measurement curve, it can be seen that the structural depth of the subwavelength tungsten grating prepared in this embodiment is about 200 nanometers, and the groove surface is relatively smooth, which achieves a better grating preparation effect.

As shown in FIG. 5 , when the polarization direction of the light source incident on the yttrium vanadate birefringent crystal 140 and the optical axis direction of the yttrium vanadate birefringent crystal 140 are 30° and 60°, the prepared subwavelength tungsten grating structure morphology SEM image. It can be seen that under a given incident laser energy irradiation and other parameter control conditions unchanged, when the yttrium vanadate birefringent crystal 140 rotates to some specific directions (for example: the angle with the incident laser polarization direction is 30 ° and 60°), the subwavelength tungsten grating structure regularly arranged in the entire spot area can be obtained. At this time, due to the crystal birefringence effect, the processing beam is a time-delayed double pulse with collinear transmission, vertical polarization, and energy ratio of √3:1, and the direction of the subwavelength tungsten grating structure is always perpendicular to the polarization direction of the high-energy laser pulse ( (shown by the black double arrow in the figure). In addition, when the angle between the optical axis of the crystal and the polarization direction of the incident light is not equal to 30° or 60°, that is, when the ratio of the delayed double pulse energy is not equal to √3:1, the subwavelength fringe structure obtained on the tungsten surface has Regularity deteriorates. Based on this, using a dual-pulse femtosecond laser with an energy ratio of √3:1 and a vertical polarization with a fixed time delay is an important processing parameter to realize the fabrication of regular subwavelength tungsten gratings.

As shown in FIG. 6 , it is a graph of the corresponding relationship between the laser energy after passing through the energy adjustment device and the depth of the prepared subwavelength tungsten grating structure. When the control conditions remain unchanged, the structure depth of the fabricated subwavelength tungsten grating also changes. The measurement results show that with the gradual increase of the femtosecond laser energy output by the energy adjustment device 130, the structural depth of the prepared subwavelength tungsten grating exhibits a characteristic of first increasing and then decreasing. When the laser energy is 194 mW, the maximum structure depth is about 210 nanometers; if the femtosecond laser energy output by the energy adjusting device 130 continues to increase to 254 mW, the structure depth is reduced to about 85 nanometers. The above results show that the femtosecond laser energy output by the energy adjusting device 130 is an important processing parameter to effectively control the depth of the grating grooves when using the method for efficiently fabricating subwavelength metal gratings with wide-beam femtosecond laser double pulses of the present invention.

The method for preparing subwavelength metal gratings with wide beam femtosecond laser double pulses provided by the invention can realize the efficient preparation of subwavelength metal gratings on the surface of metal tungsten, a high melting point and high hardness material, and has the advantages of short time consumption, simple and easy operation, Low cost, non-contact processing and other characteristics are conducive to the commercialization of large-area subwavelength metal gratings.

Compared with the existing methods such as mechanical scribing, electron beam etching, photolithography, etc., the method for preparing subwavelength metal gratings by wide-beam femtosecond laser double pulses provided by the present invention can not only realize the high-efficiency and controllability of large-area subwavelength metal gratings. The whole preparation process is simple and easy to operate, and because it is non-contact processing, it can effectively avoid the secondary damage to the surface of the metal tungsten sample by stress; another On the one hand, compared with the case of single-beam femtosecond laser irradiation, the surface structure prepared by this method is more regular and uniform in spatial arrangement, the laser spot irradiation range is large, and the preparation efficiency is high. To sum up, the preparation of subwavelength gratings on metal surfaces based on time-delayed broad-beam femtosecond laser double pulses with crossed polarizations proposed by the present invention is a non-contact processing method with low processing cost, high efficiency and controllability.

The specific embodiments of the present invention do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made according to the technical concept of the present invention shall be included in the protection scope of the claims of the present invention.

Claims (10)

1. a method for preparing subwavelength metal gratings with wide-beam femtosecond laser double pulses, is characterized in that, comprises the following steps: S1. The femtosecond laser light source outputs the processing light source and enters the optical beam expander; S2. The optical beam expanding device expands the processing light source, and the beam-expanded light source enters the energy adjustment device; S3, the energy adjustment device performs energy adjustment on the beam-expanded light source, and the energy-adjusted light source is incident into the double-pulse generating device; S4. The double-pulse generating device outputs co-linear transmission of femtosecond laser pulses in space, delayed in time, and mutually perpendicular polarization directions, and the femtosecond laser double-pulses are incident into the beamline focusing device; S5. The beamline focusing device spatially focuses the femtosecond laser double pulses and then irradiates the surface of the metal to be processed on the sample moving scanning device to prepare a subwavelength metal grating. The sample moving scanning device controls the to-be-processed metal grating by controlling the The translation speed of the processed metal changes the number of overlapping pulses irradiated on the metal surface to be processed, thereby controlling the morphology of the microstructure of the metal surface to be processed. 2 . The method for preparing subwavelength metal gratings with wide-beam femtosecond laser double pulses according to claim 1 , wherein the purity of the metal to be processed is greater than 99%, and the roughness of the surface of the metal to be processed is 5. 3 . -10 nm. 3 . The method for preparing subwavelength metal gratings by wide-beam femtosecond laser double pulses according to claim 2 , wherein the metal to be processed is metal tungsten with a purity of 99.95% and a surface roughness of 7.71 nanometers. 4 . 4. the method for preparing subwavelength metal grating with wide-beam femtosecond laser double pulse according to claim 1, is characterized in that, described femtosecond laser light source is Ti:sapphire chirped pulse amplifying laser, and the repetition frequency of output light source is 1 kHz, pulse width of 40 femtoseconds, center wavelength of 800 nm, and spot diameter of 5 mm. 5. The method for preparing subwavelength metal gratings by wide-beam femtosecond laser double pulses according to claim 1, wherein the optical beam expander is composed of plano-concave plano-convex lenses, and the focal lengths of the plano-concave plano-convex lenses are respectively 38.1 mm and 125 mm, the light spot diameter of the light source output by the optical beam expander device is 15 mm. 6. The method for preparing subwavelength metal gratings by wide-beam femtosecond laser double pulses according to claim 1, wherein the energy adjustment device is composed of a half-wave plate and a Glan-Taylor prism, and the rotating The direction of the crystal axis of the half-wave plate controls the continuous adjustment of the energy of the output light source of the energy adjustment device. 7. The method for preparing subwavelength metal gratings with wide-beam femtosecond laser double pulses according to claim 1, wherein the double-pulse generating device is a birefringent crystal of yttrium vanadate, and the incident enters the double-pulse generating device The angle between the polarization direction of the light source and the optical axis of the yttrium vanadate birefringent crystal is 30° or 60°, and the energy ratio of the femtosecond laser double pulse output by the double pulse generator is √3:1 . 8 . The method for preparing subwavelength metal gratings by wide-beam femtosecond laser double pulses according to claim 7 , wherein the thickness of the birefringent crystal of yttrium vanadate is 1.684 mm, and the output of the double pulse generator is 1.684 mm. 9 . The delay time of the femtosecond laser double pulse is 1.2 picoseconds. 9 . The method for preparing subwavelength metal gratings by wide-beam femtosecond laser double pulses according to claim 1 , wherein the beamline focusing device is a plano-convex cylindrical lens, and the diameter of the plano-convex cylindrical lens It is 25.4 mm and has a focal length of 50 mm. It is made of fused silica material. The beamline focusing device spatially focuses the femtosecond laser double pulses output by the double pulse generating device in a direction perpendicular to the lens generatrix. 10 . The method for preparing subwavelength metal gratings by wide-beam femtosecond laser double pulses according to claim 1 , wherein the sample moving scanning device controls the translation speed of the metal to be processed, so as to realize irradiation to the metal to be processed. 11 . The scanning speed of the light source for processing metal surfaces is 0.01-0.03 mm/sec.

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