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Laser micro machining beyond the diffraction limit using a photonic nanojet
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CIRP-1626; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Laser micro machining beyond the diffraction limit using a photonic nanojet Tsutomu Uenohara, Yasuhiro Takaya (1)*, Yasuhiro Mizutani Department of Mechanical Engineering, Osaka University, Suita, Osaka, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Conventional laser machining with a focused laser beam is difficult to use for sub-micrometre machining because of the optical diffraction limit. We therefore propose a laser machining method that uses a photonic nanojet with a fine-beam profile generated from a dielectric microsphere illuminated by laser light. The beam diameter is several hundred nanometres, which exceeds the diffraction limit. By controlling the intensity distribution and position, the processed feature size can be controlled at the submicrometre scale. A hole with a diameter of approximately 200 nm was machined using a photonic nanojet with a wavelength of 800 nm. ß 2017 Published by Elsevier Ltd on behalf of CIRP.
Submitted by Yasuhiro Takaya (1), Osaka, Japan Keywords: Laser micro machining Nano structure Photonic nanojet
1. Introduction Laser machining is used for various applications, such as cutting, welding, drilling, cleaning, additive manufacturing, surface modifying, and micromachining [1]. The feature size of laser micromachining with a focused laser beam is determined by the beam diameter of the focused laser irradiating a work piece. The beam diameter of a focused laser is limited by the diffraction limit, which is given by dbeam ¼
l 2nsina
(1)
where dbeam is the minimum beam spot diameter, l is the wavelength of the incident laser, n is the refractive index of the medium where the beam propagates, and a is the beam divergence angle. The term n sin a in Eq. (1) is the numerical aperture (NA). To obtain a small beam diameter at a particular wavelength, an objective lens with a high NA is needed. The feature size of laser micromachining with a focused laser beam is limited by the diffraction limit of the laser beam. We therefore propose a laser micromachining approach that employs a photonic nanojet (PNJ) to obtain a feature size below the diffraction limit [2]. A PNJ is a fine high-intensity light beam that is generated at the backside of a dielectric microsphere illuminating a laser light. A PNJ has a small beam diameter of several hundred nanometres, which is less than the diffraction limit of the wavelength of the incident laser light. Moreover, a PNJ propagates a long distance of several micrometres while maintaining a high intensity without divergence. Lecler et al. argued that the beam diameter of a PNJ is below the diffraction limit because the peak
* Corresponding author. E-mail address: [email protected] (Y. Takaya).
intensity appears near the surface of the microsphere [3]. Because a PNJ has a small beam diameter, the intensity is enhanced compared with the intensity of the incident laser light. Based on these features, a PNJ enables laser micromachining with a feature size below the diffraction limit. Many studies have investigated the intensity distribution control of a PNJ. The intensity distribution of a PNJ can be controlled by the wavelength [4,5] and polarisation [6] of the incident laser light, the diameter [4,7] and refractive index [4,8] of the microsphere, and the refractive index of the medium surrounding the microsphere [9–12]. The important parameters of PNJ for laser micromachining are the beam diameter, which is defined by the full width at half maximum (FWHM), the distance from the microsphere surface to the position of peak intensity (i.e. focal length), and the intensity enhancement ratio. A PNJ is used for laser micromachining because of the high intensity and small diameter of its beam. However, some groups have reported performing laser micromachining using a PNJ under the condition that the microspheres are arrayed on the sample surface [13–15]. In these studies, the variation in the machining diameter and depth was investigated by fluence control of the incident laser. Controlling the width and depth of a structure with high resolution at the sub-micrometre scale is difficult with only fluence control. Moreover, the peak intensity of the PNJ is not effectively used because it appears some distance away from the microsphere surface [16]. Thus, the relationship between the intensity distribution of a PNJ in the propagation direction and the characteristics of laser machining with a PNJ has not been clarified. This paper presents a laser micromachining approach in which the peak intensity of a PNJ is effectively utilised by position control of the PNJ in the propagation direction. An investigation of laser micromachining with a feature size below the diffraction limit through fluence control, as well as position control of the PNJ in the propagation direction, is also presented.
http://dx.doi.org/10.1016/j.cirp.2017.04.068 0007-8506/ß 2017 Published by Elsevier Ltd on behalf of CIRP.
Please cite this article in press as: Uenohara T, et al. Laser micro machining beyond the diffraction limit using a photonic nanojet. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.068
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CIRP-1626; No. of Pages 4 2
T. Uenohara et al. / CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
2. Finite-difference time-domain simulation of a PNJ Fig. 1 shows the intensity distribution of a PNJ calculated by the two-dimensional finite-difference time-domain (2D-FDTD) method. A microsphere with a diameter of 8.0 mm and a refractive index of 1.44 was illuminated by a plane wave with a wavelength of 800 nm. The medium surrounding the microsphere was air with a refractive index of 1.00. Fig. 1(b) and (c) shows the intensity distribution of the PNJ in the x direction at the position of peak intensity and in the z direction (i.e. propagation direction of the PNJ). The statement z = 0 represents the position of the microsphere surface. Under these conditions, the beam diameter defined by the FWHM at the position of peak intensity is 437 nm, which is less than the beam diameter of 444 nm at the focal point with an objective lens having an NA of 0.9. The peak intensity appeared at a 1.1 mm distance from the microsphere surface. The PNJ was confirmed to propagate a long distance while maintaining a high intensity without divergence. To efficiently perform laser micromachining with a PNJ, the peak intensity of the PNJ based on the intensity distribution in the propagation direction must be utilised.
Fig. 2. Schematic diagram of the experimental setup. The red line shows the machining laser. The blue line shows the reflected light for the confocal optical system to detect the positions of the Si substrate and microsphere. The yellow line represents the light for observation.
the position. Stage 1 controlled the position of the Si substrate as a work piece in the z direction. Stage 2 controlled the threedimensional position of the microsphere held by the micropipette. The position control of the PNJ in the propagation direction was performed by controlling the distance h, between the microsphere bottom surface and Si substrate surface. To control distance h, the positions of the microsphere and Si substrate had to be detected. The confocal optical system was introduced to detect the surface positions of the microsphere and Si substrate. The surface position was detected by scanning the sample in the z direction and detecting the reflected light intensity with a photodetector set behind a pinhole. Fig. 3(a) depicts the procedure for controlling distance h. First, the Si substrate is scanned in the z direction with stage 1, and its surface is detected. Second, the Si substrate is moved downward by a specific distance, D. Then, the microsphere is inserted between the focused laser beam spot and Si substrate, and its top surface is detected by scanning of the microsphere held by the micropipette with stage 2. In this procedure phase, it is necessary to scan the microsphere while correlating the centre of the microsphere with the optical axis of the focused laser beam. By scanning the microsphere in x and y directions, the centre of the microsphere can be detected in the same way as the microsphere top. Thus, h is given by Dd
(2)
Fig. 1. Intensity distribution of a PNJ: (a) on the x–z plane, (b) in the x direction, and (c) in the z direction. z = 0 represents the microsphere surface. The beam diameter (FWHM) at the position of peak intensity is 437 nm. The peak intensity appears at a 1.1 mm distance from the microsphere surface.
3. Experimental setup and procedure Hole machining was performed by position control of the PNJ in the propagation direction as well as by fluence control. Fig. 2 shows a schematic diagram of the experimental setup. A femtosecond laser was used for machining with a wavelength of 800 nm, pulse duration of 100 fs, and repetition rate of 80 MHz. The incident laser power was adjusted with an attenuator. The number of irradiation pulses was adjusted with a mechanical shutter. The incident laser beam was focused with an objective lens having an NA of 0.5. It was irradiated to a silica particle with a diameter of 8.00 mm (standard deviation of 0.15 mm) and refractive index of 1.44 as a dielectric microsphere. In this experimental setup, the minimum beam diameter was 0.8 mm and calculated by Eq. (1). To control the position of the PNJ in the propagation direction, the position of the microsphere over the work-piece surface had to be controlled. A micropipette was introduced to hold the microsphere and control
Fig. 3. (a) Schematic diagram of the control procedure for distance h. Confocal signals when the following are scanned: (b) Si substrate and (c) microsphere held by a micropipette.
Please cite this article in press as: Uenohara T, et al. Laser micro machining beyond the diffraction limit using a photonic nanojet. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.068
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CIRP-1626; No. of Pages 4 T. Uenohara et al. / CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
3
Fig. 4. Schematic diagram of the laser irradiation condition for the microsphere. A focused laser beam with an NA of 0.5 is irradiated to a microsphere with 16 mm defocusing.
where d denotes the diameter of the microsphere (8.0 mm). In this experiment, D was set to 30 mm. Fig. 3(b) shows the signal of the confocal optical system while the Si substrate is scanned in the z direction. Fig. 3(c) shows the signal of the confocal optical system while the microsphere is scanned in the z direction. The experiment scanning pitch was 0.1 mm. The position where the peak intensity was detected by the photodetector was determined by the sample surface position. Thus, the positions of the Si substrate surface and microsphere top surface could be detected by scanning in the z direction. In this experimental setup, the position of the sample surface was detected with a resolution of 0.1 mm. Fig. 4 shows the laser irradiation condition for the microsphere held by a micropipette. The focused laser beam with an NA of 0.5 was irradiated to the microsphere to increase the fluence of the incident laser light. The relative positions of the focused laser beam and microsphere were controlled by moving the microsphere and Si substrate in the z direction while maintaining h, as shown on the right side of Fig. 3(a). The focused laser beam was defocused by 16 mm so that the microsphere could be regarded as being irradiated with a plane wave when the radius of curvature of the wavefront of the focused beam was increased. Thus, the intensity distribution of the PNJ in this experiment was almost the same as that shown in Fig. 1. The fluence [J/cm2] was determined by the incident laser energy [J] irradiated to a microsphere and the laser irradiation area [cm2]. The fluence was controlled by the incident laser energy with a constant laser irradiation area. Hole machining was performed with fluences of 7, 5.3, 3.5, and 1.8 mJ/cm2 and distances h of 1.0, 1.5, 2.0, and 2.5 mm. As shown in Fig. 1(c), h was set to include the peak intensity of the PNJ based on the intensity distribution in the propagation direction. The number of irradiation laser pulses was 160,000. Experiments of hole machining were performed five times under each condition. Machined holes were measured with an atomic force microscope (AFM). The hole diameter and depth were evaluated from the cross-sectional profile of the machined holes. 4. Results and discussion Hole machining could be performed when h was 1.5 and 2.0 mm and the fluence was 7 mJ/cm2. At h of 1.0 and 2.5 mm, hole machining could not be performed. Fig. 5 shows the measurement results of the machined holes when h is 1.5 and 2.0 mm and the fluence is 7 mJ/cm2. Fig. 5(a’) and (b’) show the cross-sectional profiles at the deepest position. The machined structure has a conical shape. In addition, although the bumpy structure appears surrounding the machined hole, this shape appears when irradiated with a fluence lower than the ablation threshold [17]. This shape reflects the beam profile in the x direction of the PNJ. Based on the average height (i.e. reference height z = 0) of the region around the machined hole, the hole diameter and depth were defined. The distance between the intersections of the reference height and the cross-sectional profile is defined as the hole diameter. The distance from the reference height to the deepest
Fig. 5. Measurement results for a machined hole with a fluence of 7 mJ/cm2 and distances h of (a, a0 ) 1.5 mm and (b, b0 ) 2.0 mm. (a) and (b) show AFM images. (a0 ) and (b0 ) show cross-sectional profiles of the machined hole.
position in the z direction is defined as the hole depth. The bumpy parts around the machined hole were not evaluated. Hole machining could be performed when h was only 1.5 mm with fluences of 5.3, 3.5, and 1.8 mJ/cm2. Fig. 6 shows the measurement results of the machined holes when h was 1.5 mm and the fluence was 5.3, 3.5, and 1.8 mJ/cm2. Table 1 presents the evaluation results for the hole diameter and depth of the machined hole at an h of 1.5 mm. The hole diameter is 0.48 mm and the hole depth is 188 nm when h is 2.0 mm and the fluence is 7 mJ/cm2. By controlling the PNJ position in the propagation direction, submicrometre hole diameters could be realised at both distances h of 1.5 and 2.0 mm. Hole machining could not be performed when h was 1.0 and 2.5 mm because the fluence of the PNJ at that position was below the ablation threshold of Si substrate. These results are reflected by the PNJ intensity distribution in the propagation direction, as shown in Fig. 1(c). The hole diameter and depth were confirmed to change according to the intensity distribution of the PNJ by position control of the PNJ in the propagation direction. Hole machining below the diffraction limit could not be performed with only position control of the PNJ in the propagation direction. Hole machining could be performed with all fluences only at the distance h of 1.5 mm. Thus, the fluence of the PNJ at the distance h of 1.5 mm was greater than the ablation threshold of the Si substrate. Thus, the peak intensity of the PNJ in the propagation direction appeared approximately 1.5 mm from the microsphere surface in this experiment. The hole diameter changed according to the fluence of the incident laser. Especially, the hole diameter of 0.18 mm, which was below the diffraction limit of the wavelength of 800 nm, could be realised when distance h was 1.5 mm and the fluence was 1.8 mJ/cm2. Considering that the ablation threshold of the Si substrate was 200 mJ/cm2 [18], the peak fluence of the PNJ was over 100 times higher than that of the incident laser light. The relationship between the diameter and depth of the structure was determined by the PNJ intensity distribution and the sample position. The laser irradiated region where the intensity of PNJ was greater than the ablation threshold increases in x and z
Please cite this article in press as: Uenohara T, et al. Laser micro machining beyond the diffraction limit using a photonic nanojet. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.068
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In an experiment, when the fluence was 7 mJ/cm2, the hole diameter and depth were changed by position control of the PNJ in the propagation direction. The experimental results reflected the intensity distribution of the PNJ in the propagation direction. By position control of the PNJ in the propagation direction, the peak intensity, which appeared a distance from the microsphere surface, could be effectively used. A hole diameter of 0.18 mm, which was below the diffraction limit for a wavelength of 800 nm, was realised by fluence control and position control of the PNJ in the propagation direction. Thus, laser micromachining with a PNJ can realise a feature size below the diffraction limit by combining position control of the PNJ in the propagation direction and fluence control. It remains a challenge for future research to control the machining width and depth with a high resolution of several tens of nanometres in scale by position control of the PNJ in the propagation direction with a high resolution and fluence control. Acknowledgement This work was supported by JSPS KAKENHI Grant Number JP24246028.
References
Fig. 6. Measurement results of machined holes with fluences of (a, a0 ) 5.3, (b, b0 ) 3.5, and (c, c0 ) 1.8 mJ/cm2 when distance h is 1.5 mm. (a–c) show AFM images, and (a0 –c0 ) show cross-sectional profiles of the machined holes.
Table 1 Evaluation results of machined holes with a distance h of 1.5 mm. Fluence [mJ/cm2]
Diameter [mm]
Depth [nm]
7.0 5.3 3.5 1.8
0.73 0.43 0.58 0.18
429 67 34 57
directions when the fluence is high. Thus, the hole diameter and hole depth increased with the fluence increase. By combining the position control of the PNJ in the propagation direction and the fluence control, machining below the diffraction limit could be performed because the peak intensity of the PNJ and threshold effect were effectively used. 5. Conclusion This paper presented a laser micromachining approach that uses a PNJ to realise machining with a feature size below the diffraction limit through position control of the PNJ in the propagation direction and fluence control. Position control of the PNJ in the propagation direction is realised by controlling the relative positions of the microsphere held by a micropipette and the Si substrate. Distance h between the microsphere bottom surface and Si substrate surface is measured with a confocal optical system.
[1] Li L, Hong M, Schmidt M, Zhong M, Malshe A, In’t Veld BH, Kovalenko V (2011) Laser Nano-Manufacturing – State of the Art and Challenges. CIRP Annals – Manufacturing Technology 60(2):735–755. [2] Chen Z, Taflove A (2004) Photonic Nanojet Enhancement of Backscattering of Light by Nanoparticles: A Potential Novel Visible-Light Ultramicroscopy Technique. Optics Express 12(7):1214–1220. [3] Lecler S, Takakura Y, Meyrueis P (2005) Properties of a Three-Dimensional Photonic Jet. Optics Letters 30(19):2641–2643. [4] Leitz K-H, Quentin U, Alexeev I, Schmidt M (2012) Process Investigations of Optical Trap Assisted Direct-Write Microsphere Near-Field Nanostructuring. CIRP Annals – Manufacturing Technology 61:207–210. [5] Lee S, Li L, Wang Z (2014) Optical Resonances in Microsphere Photonic Nanojets. Journal of Optics 16:015704. [6] Liu Y, Wang B, Ding Z (2011) Influence of Incident Light Polarization on Photonic Nanojet. Chinese Optics Letters 9(7):072901. [7] Liu C-Y, Wang Y-H (2014) Real-Space Observation of Photonic Nanojet in Dielectric Microspheres. Physica E 61:141–147. [8] Ku Y-L, Kuang C-F, Hao X, Li H-F, Liu X (2013) Parameter Optimization for Photonic Nanojet of Dielectric Microsphere. Optoelectronics Letters 9(2): 153–156. [9] Wang ZB, Guo W, Pena A, Whitehead DJ, Luk’yanchuk BS, Li L, Liu Z, Zhou Y, Hong MH (2008) Laser Micro/Nano Fabrication in Glass with Tunable-Focus Particle Lens Array. Optics Express 16(24):19706–19711. [10] Pikulin A, Afanasiev A, Agareva N, Alexandrov AP, Bredikhin V, Bityurin N (2012) Effects of Spherical Mode Coupling on Near-Field Focusing by Clusters of Dielectric Microspheres. Optics Express 20(8):9052–9057. [11] Chen A, Taflove A, Backman V (2006) Highly Efficient Optical Coupling and Transport Phenomena in Chains of Dielectric Microspheres. Optics Letters 31(3):389–391. [12] Ding H, Dai L, Yan C (2010) Properties of the 3D Photonic Nanojet Based on the Refractive Index of Surroundings. Chinese Optics Letters 8(7):706–708. [13] Kallepalli LND, Grojo D, Charmasson L, Delaporte P, Ute´za O, Merlen A, Sangar A, Torchio P (2013) Long Range Nanostructuring of Silicon Surfaces by Photonic Nanojets from Microsphere Langmuir Films. Journal of Physics D: Applied Physics 46:145102. [14] Khan A, Wang Z, Sheikh MA, Whitehead DJ (2010) Parallel Near-Field Optical Micro/Nanopatterning on Curved Surfaces by Transported Micro-Particle Lens Arrays. Journal of Physics D 43:305302. [15] Pena A, Wang Z, Whitehead D, Li L (2010) Direct Writing of Micro/Nano-Scale Patterns by Means of Particle Lens Arrays Scanned by a Focused Diode Pumped Nd:YVO4 Laser. Applied Physics A 101:287–295. [16] Ferrand P, Wenger J, Devilz A, Pianta M, Stout B, Bonod N, Popov E, Rigneault H (2008) Direct Imaging of Photonic Nanojets. Optics Express 16(10): 6930–6940. [17] Rueda JH, Gotte N, Siegel J, Soccio M, Zielinski B, Sarpe C, Wollenhaupt M, Ezquerra TA, Baumert T, Solis J (2015) Nanofabrication of Tailored Surface Structures in Dielectrics Using Temporally Shaped Femtosecond-Laser Pulses. ACS Applied Material Interfaces 7(12):6613–6619. [18] Jeschke HO, Garcia ME, Lenzner M, Bonse J, Kruger J, Kautek W (2002) Laser Ablation Thresholds of Silicon for Different Pulse Durations: Theory and Experiment. Applied Surface Science 197–198:839–844.
Please cite this article in press as: Uenohara T, et al. Laser micro machining beyond the diffraction limit using a photonic nanojet. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.068
CIRP-1626; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Laser micro machining beyond the diffraction limit using a photonic nanojet Tsutomu Uenohara, Yasuhiro Takaya (1)*, Yasuhiro Mizutani Department of Mechanical Engineering, Osaka University, Suita, Osaka, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Conventional laser machining with a focused laser beam is difficult to use for sub-micrometre machining because of the optical diffraction limit. We therefore propose a laser machining method that uses a photonic nanojet with a fine-beam profile generated from a dielectric microsphere illuminated by laser light. The beam diameter is several hundred nanometres, which exceeds the diffraction limit. By controlling the intensity distribution and position, the processed feature size can be controlled at the submicrometre scale. A hole with a diameter of approximately 200 nm was machined using a photonic nanojet with a wavelength of 800 nm. ß 2017 Published by Elsevier Ltd on behalf of CIRP.
Submitted by Yasuhiro Takaya (1), Osaka, Japan Keywords: Laser micro machining Nano structure Photonic nanojet
1. Introduction Laser machining is used for various applications, such as cutting, welding, drilling, cleaning, additive manufacturing, surface modifying, and micromachining [1]. The feature size of laser micromachining with a focused laser beam is determined by the beam diameter of the focused laser irradiating a work piece. The beam diameter of a focused laser is limited by the diffraction limit, which is given by dbeam ¼
l 2nsina
(1)
where dbeam is the minimum beam spot diameter, l is the wavelength of the incident laser, n is the refractive index of the medium where the beam propagates, and a is the beam divergence angle. The term n sin a in Eq. (1) is the numerical aperture (NA). To obtain a small beam diameter at a particular wavelength, an objective lens with a high NA is needed. The feature size of laser micromachining with a focused laser beam is limited by the diffraction limit of the laser beam. We therefore propose a laser micromachining approach that employs a photonic nanojet (PNJ) to obtain a feature size below the diffraction limit [2]. A PNJ is a fine high-intensity light beam that is generated at the backside of a dielectric microsphere illuminating a laser light. A PNJ has a small beam diameter of several hundred nanometres, which is less than the diffraction limit of the wavelength of the incident laser light. Moreover, a PNJ propagates a long distance of several micrometres while maintaining a high intensity without divergence. Lecler et al. argued that the beam diameter of a PNJ is below the diffraction limit because the peak
* Corresponding author. E-mail address: [email protected] (Y. Takaya).
intensity appears near the surface of the microsphere [3]. Because a PNJ has a small beam diameter, the intensity is enhanced compared with the intensity of the incident laser light. Based on these features, a PNJ enables laser micromachining with a feature size below the diffraction limit. Many studies have investigated the intensity distribution control of a PNJ. The intensity distribution of a PNJ can be controlled by the wavelength [4,5] and polarisation [6] of the incident laser light, the diameter [4,7] and refractive index [4,8] of the microsphere, and the refractive index of the medium surrounding the microsphere [9–12]. The important parameters of PNJ for laser micromachining are the beam diameter, which is defined by the full width at half maximum (FWHM), the distance from the microsphere surface to the position of peak intensity (i.e. focal length), and the intensity enhancement ratio. A PNJ is used for laser micromachining because of the high intensity and small diameter of its beam. However, some groups have reported performing laser micromachining using a PNJ under the condition that the microspheres are arrayed on the sample surface [13–15]. In these studies, the variation in the machining diameter and depth was investigated by fluence control of the incident laser. Controlling the width and depth of a structure with high resolution at the sub-micrometre scale is difficult with only fluence control. Moreover, the peak intensity of the PNJ is not effectively used because it appears some distance away from the microsphere surface [16]. Thus, the relationship between the intensity distribution of a PNJ in the propagation direction and the characteristics of laser machining with a PNJ has not been clarified. This paper presents a laser micromachining approach in which the peak intensity of a PNJ is effectively utilised by position control of the PNJ in the propagation direction. An investigation of laser micromachining with a feature size below the diffraction limit through fluence control, as well as position control of the PNJ in the propagation direction, is also presented.
http://dx.doi.org/10.1016/j.cirp.2017.04.068 0007-8506/ß 2017 Published by Elsevier Ltd on behalf of CIRP.
Please cite this article in press as: Uenohara T, et al. Laser micro machining beyond the diffraction limit using a photonic nanojet. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.068
G Model
CIRP-1626; No. of Pages 4 2
T. Uenohara et al. / CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
2. Finite-difference time-domain simulation of a PNJ Fig. 1 shows the intensity distribution of a PNJ calculated by the two-dimensional finite-difference time-domain (2D-FDTD) method. A microsphere with a diameter of 8.0 mm and a refractive index of 1.44 was illuminated by a plane wave with a wavelength of 800 nm. The medium surrounding the microsphere was air with a refractive index of 1.00. Fig. 1(b) and (c) shows the intensity distribution of the PNJ in the x direction at the position of peak intensity and in the z direction (i.e. propagation direction of the PNJ). The statement z = 0 represents the position of the microsphere surface. Under these conditions, the beam diameter defined by the FWHM at the position of peak intensity is 437 nm, which is less than the beam diameter of 444 nm at the focal point with an objective lens having an NA of 0.9. The peak intensity appeared at a 1.1 mm distance from the microsphere surface. The PNJ was confirmed to propagate a long distance while maintaining a high intensity without divergence. To efficiently perform laser micromachining with a PNJ, the peak intensity of the PNJ based on the intensity distribution in the propagation direction must be utilised.
Fig. 2. Schematic diagram of the experimental setup. The red line shows the machining laser. The blue line shows the reflected light for the confocal optical system to detect the positions of the Si substrate and microsphere. The yellow line represents the light for observation.
the position. Stage 1 controlled the position of the Si substrate as a work piece in the z direction. Stage 2 controlled the threedimensional position of the microsphere held by the micropipette. The position control of the PNJ in the propagation direction was performed by controlling the distance h, between the microsphere bottom surface and Si substrate surface. To control distance h, the positions of the microsphere and Si substrate had to be detected. The confocal optical system was introduced to detect the surface positions of the microsphere and Si substrate. The surface position was detected by scanning the sample in the z direction and detecting the reflected light intensity with a photodetector set behind a pinhole. Fig. 3(a) depicts the procedure for controlling distance h. First, the Si substrate is scanned in the z direction with stage 1, and its surface is detected. Second, the Si substrate is moved downward by a specific distance, D. Then, the microsphere is inserted between the focused laser beam spot and Si substrate, and its top surface is detected by scanning of the microsphere held by the micropipette with stage 2. In this procedure phase, it is necessary to scan the microsphere while correlating the centre of the microsphere with the optical axis of the focused laser beam. By scanning the microsphere in x and y directions, the centre of the microsphere can be detected in the same way as the microsphere top. Thus, h is given by Dd
(2)
Fig. 1. Intensity distribution of a PNJ: (a) on the x–z plane, (b) in the x direction, and (c) in the z direction. z = 0 represents the microsphere surface. The beam diameter (FWHM) at the position of peak intensity is 437 nm. The peak intensity appears at a 1.1 mm distance from the microsphere surface.
3. Experimental setup and procedure Hole machining was performed by position control of the PNJ in the propagation direction as well as by fluence control. Fig. 2 shows a schematic diagram of the experimental setup. A femtosecond laser was used for machining with a wavelength of 800 nm, pulse duration of 100 fs, and repetition rate of 80 MHz. The incident laser power was adjusted with an attenuator. The number of irradiation pulses was adjusted with a mechanical shutter. The incident laser beam was focused with an objective lens having an NA of 0.5. It was irradiated to a silica particle with a diameter of 8.00 mm (standard deviation of 0.15 mm) and refractive index of 1.44 as a dielectric microsphere. In this experimental setup, the minimum beam diameter was 0.8 mm and calculated by Eq. (1). To control the position of the PNJ in the propagation direction, the position of the microsphere over the work-piece surface had to be controlled. A micropipette was introduced to hold the microsphere and control
Fig. 3. (a) Schematic diagram of the control procedure for distance h. Confocal signals when the following are scanned: (b) Si substrate and (c) microsphere held by a micropipette.
Please cite this article in press as: Uenohara T, et al. Laser micro machining beyond the diffraction limit using a photonic nanojet. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.068
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CIRP-1626; No. of Pages 4 T. Uenohara et al. / CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
3
Fig. 4. Schematic diagram of the laser irradiation condition for the microsphere. A focused laser beam with an NA of 0.5 is irradiated to a microsphere with 16 mm defocusing.
where d denotes the diameter of the microsphere (8.0 mm). In this experiment, D was set to 30 mm. Fig. 3(b) shows the signal of the confocal optical system while the Si substrate is scanned in the z direction. Fig. 3(c) shows the signal of the confocal optical system while the microsphere is scanned in the z direction. The experiment scanning pitch was 0.1 mm. The position where the peak intensity was detected by the photodetector was determined by the sample surface position. Thus, the positions of the Si substrate surface and microsphere top surface could be detected by scanning in the z direction. In this experimental setup, the position of the sample surface was detected with a resolution of 0.1 mm. Fig. 4 shows the laser irradiation condition for the microsphere held by a micropipette. The focused laser beam with an NA of 0.5 was irradiated to the microsphere to increase the fluence of the incident laser light. The relative positions of the focused laser beam and microsphere were controlled by moving the microsphere and Si substrate in the z direction while maintaining h, as shown on the right side of Fig. 3(a). The focused laser beam was defocused by 16 mm so that the microsphere could be regarded as being irradiated with a plane wave when the radius of curvature of the wavefront of the focused beam was increased. Thus, the intensity distribution of the PNJ in this experiment was almost the same as that shown in Fig. 1. The fluence [J/cm2] was determined by the incident laser energy [J] irradiated to a microsphere and the laser irradiation area [cm2]. The fluence was controlled by the incident laser energy with a constant laser irradiation area. Hole machining was performed with fluences of 7, 5.3, 3.5, and 1.8 mJ/cm2 and distances h of 1.0, 1.5, 2.0, and 2.5 mm. As shown in Fig. 1(c), h was set to include the peak intensity of the PNJ based on the intensity distribution in the propagation direction. The number of irradiation laser pulses was 160,000. Experiments of hole machining were performed five times under each condition. Machined holes were measured with an atomic force microscope (AFM). The hole diameter and depth were evaluated from the cross-sectional profile of the machined holes. 4. Results and discussion Hole machining could be performed when h was 1.5 and 2.0 mm and the fluence was 7 mJ/cm2. At h of 1.0 and 2.5 mm, hole machining could not be performed. Fig. 5 shows the measurement results of the machined holes when h is 1.5 and 2.0 mm and the fluence is 7 mJ/cm2. Fig. 5(a’) and (b’) show the cross-sectional profiles at the deepest position. The machined structure has a conical shape. In addition, although the bumpy structure appears surrounding the machined hole, this shape appears when irradiated with a fluence lower than the ablation threshold [17]. This shape reflects the beam profile in the x direction of the PNJ. Based on the average height (i.e. reference height z = 0) of the region around the machined hole, the hole diameter and depth were defined. The distance between the intersections of the reference height and the cross-sectional profile is defined as the hole diameter. The distance from the reference height to the deepest
Fig. 5. Measurement results for a machined hole with a fluence of 7 mJ/cm2 and distances h of (a, a0 ) 1.5 mm and (b, b0 ) 2.0 mm. (a) and (b) show AFM images. (a0 ) and (b0 ) show cross-sectional profiles of the machined hole.
position in the z direction is defined as the hole depth. The bumpy parts around the machined hole were not evaluated. Hole machining could be performed when h was only 1.5 mm with fluences of 5.3, 3.5, and 1.8 mJ/cm2. Fig. 6 shows the measurement results of the machined holes when h was 1.5 mm and the fluence was 5.3, 3.5, and 1.8 mJ/cm2. Table 1 presents the evaluation results for the hole diameter and depth of the machined hole at an h of 1.5 mm. The hole diameter is 0.48 mm and the hole depth is 188 nm when h is 2.0 mm and the fluence is 7 mJ/cm2. By controlling the PNJ position in the propagation direction, submicrometre hole diameters could be realised at both distances h of 1.5 and 2.0 mm. Hole machining could not be performed when h was 1.0 and 2.5 mm because the fluence of the PNJ at that position was below the ablation threshold of Si substrate. These results are reflected by the PNJ intensity distribution in the propagation direction, as shown in Fig. 1(c). The hole diameter and depth were confirmed to change according to the intensity distribution of the PNJ by position control of the PNJ in the propagation direction. Hole machining below the diffraction limit could not be performed with only position control of the PNJ in the propagation direction. Hole machining could be performed with all fluences only at the distance h of 1.5 mm. Thus, the fluence of the PNJ at the distance h of 1.5 mm was greater than the ablation threshold of the Si substrate. Thus, the peak intensity of the PNJ in the propagation direction appeared approximately 1.5 mm from the microsphere surface in this experiment. The hole diameter changed according to the fluence of the incident laser. Especially, the hole diameter of 0.18 mm, which was below the diffraction limit of the wavelength of 800 nm, could be realised when distance h was 1.5 mm and the fluence was 1.8 mJ/cm2. Considering that the ablation threshold of the Si substrate was 200 mJ/cm2 [18], the peak fluence of the PNJ was over 100 times higher than that of the incident laser light. The relationship between the diameter and depth of the structure was determined by the PNJ intensity distribution and the sample position. The laser irradiated region where the intensity of PNJ was greater than the ablation threshold increases in x and z
Please cite this article in press as: Uenohara T, et al. Laser micro machining beyond the diffraction limit using a photonic nanojet. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.068
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CIRP-1626; No. of Pages 4 T. Uenohara et al. / CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
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In an experiment, when the fluence was 7 mJ/cm2, the hole diameter and depth were changed by position control of the PNJ in the propagation direction. The experimental results reflected the intensity distribution of the PNJ in the propagation direction. By position control of the PNJ in the propagation direction, the peak intensity, which appeared a distance from the microsphere surface, could be effectively used. A hole diameter of 0.18 mm, which was below the diffraction limit for a wavelength of 800 nm, was realised by fluence control and position control of the PNJ in the propagation direction. Thus, laser micromachining with a PNJ can realise a feature size below the diffraction limit by combining position control of the PNJ in the propagation direction and fluence control. It remains a challenge for future research to control the machining width and depth with a high resolution of several tens of nanometres in scale by position control of the PNJ in the propagation direction with a high resolution and fluence control. Acknowledgement This work was supported by JSPS KAKENHI Grant Number JP24246028.
References
Fig. 6. Measurement results of machined holes with fluences of (a, a0 ) 5.3, (b, b0 ) 3.5, and (c, c0 ) 1.8 mJ/cm2 when distance h is 1.5 mm. (a–c) show AFM images, and (a0 –c0 ) show cross-sectional profiles of the machined holes.
Table 1 Evaluation results of machined holes with a distance h of 1.5 mm. Fluence [mJ/cm2]
Diameter [mm]
Depth [nm]
7.0 5.3 3.5 1.8
0.73 0.43 0.58 0.18
429 67 34 57
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Please cite this article in press as: Uenohara T, et al. Laser micro machining beyond the diffraction limit using a photonic nanojet. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.068