OPTOELECTRONICS AND ADVANCED MATERIALS – RAPID COMMUNICATIONS Vol. 2, No. 11, November 2008, p. 686 - 688 Air-gap silicon nitride chirped mirror for few-cycle pulse compression S. O. IAKUSHEV * , O. V. SHULIKA, V. V. LYSAK a , I. A. SUKHOIVANOV b Lab. Photonics, Kharkov National University of Radio Electronics, 14 Lenin Avenue, Kharkov 61166, Ukraine a Facultad de Ingeneria Mecanica Electrica y Electronica (FIMEE), Universidad de Guanajuato, Comunidad de Palo Blanco, C.P. 36730, Salamanca, GTO, Mexico b Department of Information and Communication Gwangju Institute of Science and Technology, 1, Oryong-dong, Buk-gu, 500-712, Gwangju, Republic of Korea We propose new design solution for broadband chirped mirrors. It consists in application of silicon nitride with air-gap interlayers. This proposal is the first application of silicon nitride to a chirped mirror, to the best of our knowledge. The proposed material combination allows to realize CMs supplying bandwidth from ultraviolet to near infrared. Using this idea we have made design of silicon nitride - air-gap chirped mirror for compression of few-cycles laser pulses in the near- infrared. Our CM provides high reflectivity and good dispersion properties in the wavelength range 400-1200 nm. Time domain analysis shows that designed CM allows to operate on few-cycles laser pulses. (Received September 14, 2008; accepted September 26, 2008) Keywords: Chirped mirror, Few-cycle pulse, Pulse compression 1. Introduction Chirped mirrors (CMs) have been widely used for dispersion control in ultrafast optics; CMs are typically intended for use in intracavity dispersion compensation in femtosecond solid state lasers. Therefore, they are designed in the near-infrared wavelength range, where the Ti:Sapphire laser crystal has fluorescence. The typical bandwidth of CMs bounded at 400 nm of the spectral bandwidth supports the generation of sub-10-fs pulses [1]. However, recent applications of CMs, such as those operating on few-cycle pulses, require a broader bandwidth and more precise dispersion control [2; 3]. Titanium dioxide 2 TiO and silicon dioxide 2 SiO are commonly used for the fabrication of CMs in the wavelength range around 800 nm, owing to high refractive index contrast. However, 2 TiO does not support a broader bandwidth due to sizeable absorption below 500 nm; hence, new materials and design approaches are proposed [2, 3]. We have discussed the application of silicon nitride 4 3 N Si in the design of ultrabroadband CMs [4]. In this letter we propose new design solution consisting of the application of 4 3 N Si as an alternative high-index material to 2 TiO , which combines with air-gap interlayers to form low-index layers. Note that 4 3 N Si has low absorption and a continuous refractive index in a wide spectral range [5], though 4 3 N Si has a smaller refractive index than 2 TiO (2.0 and 2.5 at 800 nm, respectively). Other proposed materials have the same problem. Since the contrast of refractive indices is important in achieving broadband reflection, air is a favorable solution for increasing the contrast due to its refractive index smaller than that of 2 SiO . CMs based on the combination of 4 3 N Si and air-gap interlayers are capable of providing a bandwidth covering the range over the ultraviolet-visible- near infrared frequencies. Dispersion control inside this bandwidth is only limited by two factors pertaining to CM performance. First, the phase ripples rise with bandwidth expansion, requiring more complex and efficient methods for their suppression. Second, is increasing the number of layers in the CM design required to provide as wide bandwidth of high reflection as possible. However a progress in fabrication technologies allows to rise the complexity of CM design. 2. Proposed structure of the chirped mirror In this letter, we present the design of an 4 3 N Si -air- gap chirped mirror with 54 layers. This design has shown good reflective and dispersion properties in the wavelength range from 400 to 1200 nm. Moreover, subsequent analysis of the pulse compression shows that designed CM provides required dispersion compensation and pulse reconstruction. The simplest way to produce a CM is based on the linear modulation of a local Bragg wavelength (a Bragg mirror has a constant Bragg wavelength over the stack). However, such a simple design has a serious drawback; ripples of the reflection phase have the potential to distort the pulse shape and can even destroy pulse formation in the laser cavity. There have been proposed numerous methods to avoid this imperfection. Here, to eliminate the impedance mismatch we applied a double-chirped technique [1] to the structural