Chirped Multilayer Mirror Based on Silicon Nitride ... - OSA Publishing

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Chirped Multilayer Mirror Based on Silicon Nitride (Si3N4) With Air‐Gap Interlayers I. A. Sukhoivanov , O.V. Shulika , S. O. Yakushev1, S. I. Petrov1, V.V. Lysak2,1 3,1

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1Kharkov National University of Radio Electronics, Kharkov, Ukraine Gwangju Institute of Science and Technology, Gwangju, Republic of Korea 3University Guanajuato, Salamanca, Mexico [email protected]; [email protected];

Abstract: Chirped multilayer mirror based on silicon nitride with air‐gap interlayers is proposed and designed. The mirror provides high reflectivity and good dispersion properties in the range λ = 400–1200 nm supporting the few‐cycle pulses processing. ©2008 Optical Society of America OCIS codes: (230.1480) Bragg reflectors; (320.5520) Pulse compression; (320.5540) Pulse shaping

1. Introduction and aim Chirped mirrors (CMs) are widely used for dispersion control and compression of ultrashort pulses in femtosecond lasers. The main trend of innovations in CM’s designing is increasing of mirrors bandwidth and retaining of small phase ripples within this bandwidth at the same time. Recent applications of CMs such as operating on few‐cycle pulses (pulse duration involves only few periods of the electrical field) require broader bandwidth and precise dispersion control. Titanium dioxide ( TiO2 ) and silicon dioxide ( SiO2 ) are commonly used for the fabrication of CMs in the wavelength range around λ = 800 nm, where Ti:Sapphire lasers oscillate. However, TiO2 does not support a broader bandwidth due to sizeable absorption below λ = 500 nm; hence, new materials and design approaches are desirable. In this paper, we present design of chirped mirror based on silicon nitride ( Si3 N 4 ) instead of titanium dioxide. Silicon nitride has low absorption and continuous refractive index in the wide spectral range from the ultraviolet to the near infrared. However, as well as other proposed materials ( Nb2 O5 , Ta2O5 , Hf 2O5 ) Si3 N 4 has a smaller refractive index than TiO2 (2.0 and 2.5 at 800 nm, respectively) and therefore total refractive index contrast is smaller. Owing to that the high reflectivity bandwidth of CM is restricted. In order to avoid this problem we propose substituting of SiO2 with air‐gap interlayers. Due to the smaller refractive index of air as compared to SiO2 , the total refractive index contrast of Si3 N 4 / Air is kept high.


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a)

Fig. 1. Si3 N 4 ‐air‐gap CM design consisting of 54 layers. a) Layer thicknesses of the designed CM; b) Reflectance of the designed CM (green line). Group delay dispersion of the designed CM (blue lines). Red line demonstrates target GDD‐line (undistorted curve).

2. Design of chirped mirror We have designed Si3 N 4 / Air CM which consists of 54 layers, Fig. 1. a). Note that the first few layers close to the surface of the structure are rather thin; special care is required in the fabrication of these layers. Fig. 1. b) shows spectral characteristics of the designed CM. The mirror’s bandwidth covers the wavelength range from 400 to 1200 nm (1.5 octave), supporting a reflectivity of over 98 % in this range. We have used a double‐chirped technique and numerical optimization (conjugate gradient method) in order to suppress phase ripples. The amplitude of GDD oscillations around the undistorted curve is about 80 fs 2 , with a GDD value of about -17 fs2 at 800 nm. 3. Analysis of pulse compression with designed CM In order to estimate the capability of the developed CM design, we have performed an analysis of the pulse compression in the time domain. A sapphire crystal of 2.0 mm in length was selected as the dispersion source. The crystal produces a GDD value of 115 fs 2 at 800 nm. In order to compensate for such dispersion, eight reflections from the designed CM were used. The duration of the initial transform‐ limited pulse was set at 5 fs (two periods of electrical field at 800 nm). This pulse was launched into the sapphire crystal which results in broadening. Subsequent pulse compression after eight reflections was calculated, resulting in the formation of the compressed pulse, Fig. 2. Comparing the pulse shapes shows that compression with CM is rather strong but incomplete due to residual GD oscillations in the designed CM. This problem can be eliminated using more complex optimization algorithms or performing sophisticated CM designs.


Fig. 2 Waveforms of initial transform‐limited, broadened

Fig. 3 Dependence of pulse amplitude on the angle of incidence.

incident, and compressed reflected pulses, respectively (Left‐

Red line – P‐polarization, green line – S‐polarization.

Right). Insertion shows calculated polarization‐gate FROG trace of the reflected pulse.

Next we have examined an oblique incidence of laser pulse on the surface of CM. Dependence of the pulse amplitude on the angle of incidence shows clearly a stability of developed CM’s design. Fig. 3 0

shows that amplitude of the reflected pulse doesn’t decrease sufficiently until 10 in case of P‐ polarization state of incident radiation. If incident radiation has S‐polarization state amplitude of reflected pulse keeps until

200 . Further increasing of incident angle resulted in strong decreasing of

reflected pulse amplitude as well as general pulse degradation. The reason is strong rising of phase oscillation amplitude owing to oblique incidence. 4. Conclusion Chirped multilayer mirror based on silicon nitride with air‐gap interlayers is proposed and designed. The mirror consists of 54 layers and provides high reflectivity and good dispersion properties in the wavelength range λ = 400–1200 nm supporting the few‐cycle pulses processing. The subsequent time‐ domain analysis of the pulse compression using the designed CM shows that our mirror provides good reconstruction for initial transform‐limited pulse of 5 fs, giving a pulse of 6.2 fs FWHM after eight 0

bounces. Numerical calculations show that designed CM provides stable pulse compression until 10 0

and 20 angle of incidence in case of P‐ and S‐polarization respectively.