fabrication of high effective silicon diffractive optics for the terahertz range by femtosecond laser ablation

doi: 10.1016/j.phpro.2016.11.030 ScienceDirect International Conference "Synchrotron and Free electron laser Radiation: generation and application", SFR-2016, 4-8 July 2016, Novosibirs

Physics Procedia 84 ( 2016 ) 170 – 174

1875-3892 © 2016 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of SFR-2016.

doi: 10.1016/j.phpro.2016.11.030

ScienceDirect

International Conference "Synchrotron and Free electron laser Radiation: generation and

application", SFR-2016, 4-8 July 2016, Novosibirsk, Russia

Fabrication of high-effective silicon diffractive optics for the terahertz range by femtosecond laser ablation

V.S Pavelyeva,b*, M.S Komlenokc,d, B.O Volodkina, B.A Knyazeve,f, T.V

Kononenkoc,d, V.I Konovc,d, V.A Soifera,b, Yu.Yu Choporovae,f

a Samara University, 34, Moskovskoe shosse, Samara 443086, Russian Federation

b Image Processing Systems Institute of the Russian Academy of Sciences, 151, Molodogvardejskaya St, Samara 443001, Russian Federation

c A.M Prokhorov General Physics Institute RAS, 38 Vavilov St., Moscow 119991, Russian Federation

d National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 31 Kashirskoye shosse, Moscow 115409, Russian

Federation

e Budker Institute of Nuclear Physics SB RAS, 11, akademika Lavrentieva Ave, Novosibirsk 630090, Russian Federation

f Novosibirsk State University, 2, Pirogova St,, Novosibirsk 630090, Russian Federation

Abstract

Comparison of the two laser sources (UV nanosecond and IR femtosecond) used for the formation of micro-relief

at the silicon surface showed the advantage of the second one A four-level silicon diffractive THz Fresnel lens has

been fabricated by laser ablation at high repetition rate (f = 200 kHz) of femtosecond Yb:YAG laser Features of the

lens were investigated in the beam of the Novosibirsk free electron laser at the wavelength of 141 μm Detailed results of investigation of fabricated lens micro-relief are presented The measured diffractive efficiency of the lens

is in good agreement with the theoretical prediction

© 2016 The Authors Published by Elsevier B.V

Peer-review under responsibility of the organizing committee of SFR-2016

Keywords: multilevel diffractive optics; femtosecond laser ablation; diffractive microrelief; diffraction efficiency

* Corresponding author Tel.(Fax):+7-846-267-48-43

E-mail address:pavelyev10@mail.ru

© 2016 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of SFR-2016.

1 Introduction

The development of coherent, high power sources of THz radiation (see Kulipanov et al (2015)) has formed a need for optical elements to control this radiation It is known that high-power THz beams can be controlled by silicon diffractive optical elements (DOEs) The widespread lithographic etching of a silicon substrate , used in fabrication of binary elements (Agafonov (2013, 2015), Knyazev et al (2015)), has disadvantages in the case of multilevel ones Formation of multilevel microrelief by lithographic etching requires an expensive and complicated procedure of photomask alignment, and binary (two-level) elements, in turn, have limited energy efficiency A principal possibility of the microfabrication of high-effective power silicon diffractive opticsfor terahertz range by laser ablation was shown by Komlenok et al (2015) We present here results of investigation of the fabricated lens in the beam of a free electron laser at the wavelength of 141 μm Comparison of application of nanosecond UV and femtosecond IR laser ablation for forming THz micro-relief at the silicon substrate is also reported, and the advantage of the application of femtosecond laser source is demonstrated

2 Experiment

A high-resistance silicon sample was irradiated with an excimer KrF laser (Optosystems Ltd., CL 7100, τ = 20

ns, λ = 248 nm, f = 50 Hz) or a disk Yb : YAG laser (Dausinger and Giessen, λ = 1030 nm, τ =400 fs, f = 200 kHz)

in normal ambient conditions For irradiation with the excimer laser, the projection optical setup (fig.1a) was used to illuminate the surface of the sample uniformly (demagnification factor of 20×) Square mask with the size of 4 mm was used for irradiation, so size of laser spot on a surface was 200 μm Laser fluence on the surface was controlled with filters mounted before the mask The sample was moved by the translation stage In the experiments with the

Yb : YAG laser, the beam was focused onto the silicon surface in a spot with a diameter of 10 μm (1/e intensity level) The sample was mounted on a rotating platform on the translation stage (fig 1b) The rotating platform was used to obtain radial symmetry for lens formation The depth of a surface structure was controlled by moving the axis of rotation with different velocity of the translation stage

Fig 1 Optical systems for sample irradiation with the excimer laser (a) and the Yb:YAG laser (b)

3 Results and discussion

Since wavelengths and pulse durations of the lasers used were different, the optical absorption lengths were also drastically different For the IR radiation, the optical absorption coefficient α is 125 cm-1 (optical absorption length

l a= 80 μm), and for UV – α = 1.8 106 cm-1 (l a≈ 6 nm) (see Green and Keevers (1995)) Tested structure consisted of three different depth levels of 14.5, 29.1 and 43.6 μm was chosen to compare two laser sources In the case of the

UV irradiation, the laser fluence was 4 J/cm2 and slightly exceeded the threshold of silicon ablation, which was about 1 J/cm2 The sample was mounted on the translation stage and was shifted with different steps: 20 and 200

μm, corresponded to the 0.1 and 1 of the size of laser spot on the surface To reach required depth of structure, the

number of pulses was varied from 3000 to 10000 The result of laser ablation was analyzed by means of the scanning electron microscopy (SEM) and is presented in the figure 2

Fig 2 SEM images of silicon surface after laser irradiation of the excimer laser (λ = 248 nm, 4 J/cm 2 ) with different level of overlap 20 (a)

and 200 (b) μm

SEM images demonstrate that the surface after laser irradiation is not smooth and consists of melted material In figure 2a, periodic structures had appeared due to the small shift between two spots and the redeposition of ablated material Also significant quantity of the redeposited material was observed on the boundary and formed breastworks For their removal, the sample was dipped into the hydrofluoric acid, which is widely used for silicon oxide etching The result of the sample etching is presented in figure 3

Fig 3 SEM images of the silicon surface after etching in the hydrofluoric acid (5 min.) with different level of overlap of laser spots 20 (a)

and 200 (b) μm

It is seen that the chemical etching helps to remove breastworks and material redepositon, however significant surface roughness still observed The surface morphology in the case of UV nanosecond irradiation is determined by

significant material overheating because of small value of l a ≈ 6 nm and relatively small heat diffusion length

σ = ( χ ∙ τ)1/2

≈ 130 nm (where heat diffusivity χ = 0.9 cm2/s, pulse duration τ = 20 ns) that results in conical tips formation (fig 2b and 3b), observed also by Fizenkop et al (2008), Georgiev et al (2004), and Sánchez et al (1999)

A result of IR femtosecond laser irradiation is presented in figure 4 In this case material redeposition wasn’t observed and the surface roughness was less than for the UV nanosecond laser action This phenomenon may be

explained by significant larger optical penetration depth l a = 80 μm and, correspondingly, less material overheating

The depth of the ablated structures was controlled with white-light interferometry (WLI) microscope (Zygo, NewView 5000) Figure 4c demonstrates good agreement of obtained depth of the structures with calculated values

Fig 4 SEM images of the silicon surface after IR fs irradiation with different magnification (a, b) Surface profile obtained with

a WLI microscope (c)

Fig 5 SEM image of the part of four-level Fresnel lens with maximum depth of 43.6 μm, corresponding to the phase function of π/2

In view of the above results, we chose ablation with the femtosecond laser as a method for Fresnel lens manufacture The following parameters were selected for fabrication of the lens focusing 141-μm THz: the focal length of 120 mm , 250-μm radial step of phase-function quantization, and 4 quantization levels of the microrelief The laser fluence on the surface was 15 J/cm2 that considerably exceeded the threshold of silicon ablation, which is about 0.5 J/cm2 for the laser source with similar parameters (Bonse et al (2002)) Such significant energy excess was needed to ablate a large amount of material Diameter of the lens was 30 mm and surface structure was consisted of concentric rings of different width and three different depths: 14.5, 29.1 and 43.6 μm, corresponding to the phase function of the lens 3π/2, π and π /2 The SEM image of the part of four-level Fresnel lens with maximum

depth of 43.6 μm, is presented in figure 5 The analysis of the surface micro-relief by means of electron microscopy has shown that laser ablation technique provides fabrication of the diffractive structures with high accuracy

The fabricated Fresnel lens has been tested at the Novosibirsk free-electron laser facility described by Kulipanov

et al (2015) at a wavelength of 141 μm The diffraction efficiency of the lens was measured to be 35.9%, and the intensity distribution along the optical axis was in a good agreement with numerical calculations performed by Komlenok et al (2015) The diffraction efficiency obtained was limited by the Fresnel reflection and can be increased about twice by application of an antireflection coating

4 Conclusion

Comparison of the results of the silicon ablation by excimer and disk Yb : YAG lasers showed the advantage of the IR femtosecond irradiation, which is manifested in smother surface, the absence of breastworks and higher productivity So four-level silicon diffractive THz Fresnel lens has been fabricated by femtosecond laser ablation at high repetition rate (f = 200 kHz) and tested in the beam of the free electron laser at the wavelength of 141 μm The results of the lens investigation have shown the diffraction efficiency of 35.9 %, which is in good agreement with numerical calculations

The experiments performed have opened new possibilities for the use of a high repetition rate (f = 200 kHz) IR femtosecond laser ablation to fabricate effective multilevel THz DOEs

Acknowledgements

Study of laser ablation performed at the General Physics Institute, Russian Academy of Sciences, were supported

by a grant from the Russian Science Foundation (Project No 14-22-00243) Testing of the silicon lens was carried out at the Novosibirsk free electron laser facility belonging to the Siberian Synchrotron and Terahertz Radiation Center using terahertz focusing system (beamline) developed and assembled under support of the Russian Science Foundation grant 14-50-00080 Computer design and simulation of DOE were supported by the Ministry of Education and Science of the Russian Federation (Project No 1879) The authors are grateful to the NovoFEL team for support of the experiment

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