Influence of boron concentration on nonlinear absorption and ultrafast dynamics in GaSe crystals Ahmet Karatay a, * , Mustafa Yuksek b , Hüseyin Ertap c , Ali Kemal Mak c , Mevlüt Karabulut c , Ayhan Elmali a a Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Ankara, Turkey b Department of Electrical and Electronical Engineering, Faculty of Engineering, Kafkas University, 36100 Kars, Turkey c Department of Physics, Faculty of Science and Letters, Kafkas University, 36100 Kars, Turkey article info Article history: Received 1 June 2016 Received in revised form 1 July 2016 Accepted 5 July 2016 Keywords: Semiconductor Nonlinear absorption Z-scan Ultrafast pump-probe Doping effect abstract The nonlinear absorption properties and ultrafast dynamics of pure and boron doped GaSe crystals have been studied by open aperture Z-scan and ultrafast pump probe spectroscopy techniques. All of the studied crystals showed nonlinear absorption under 100 fs pulse duration and 1200 nm wavelength excitations. Nonlinear absorption coefficients increase with increasing the doping ratio of boron atoms in crystals. These findings indicate that free carrier density increase with boron doping and this behavior leads to excited state absorption. Second harmonic generation signals of crystals were detected with the help of fiber optic spectrometer. The blue shift in the energy of the second harmonic generation signals was observed in boron doped crystals. Ultrafast pump probe experiments indicate that the excited state absorption signal with long lifetime observed for undoped GaSe crystal switches to bleach signal for boron doped GaSe crystals at 625 nm probe wavelength. The effects of increasing doping ratio were observed on ultrafast dynamics as a switching time changes. Our experimental results indicate that it is possible to control nonlinear absorption properties, frequency conversion and ultrafast dynamics of GaSe crystal by changing boron doping ratio. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Gallium selenide (GaSe) crystal is one of the attractive and promising crystals, because of its extreme physical properties: such as high optical damage threshold [1,2], broadband transparency [3,4], enormous birefringence [5] and high second order suscepti- bility [6]. Especially, as bulk GaSe is a well known nonlinear crystal with second order optical nonlinear coefficient which is larger than that of the widely used many crystals [7,8]. Further, GaSe is sig- nificant nonlinear crystal for middle infrared frequency conversion since GaSe crystal have low absorption properties in wavelength from near to far infrared region (0.65e18 mm) [9,10]. In previous studies some dopants like In, Te, Ag, Ge, Sn and Er were used to improve nonlinear optical applications of GaSe crys- tals [10e13]. For instance, Suhre D.R. et al. [11], Freng Z.S. et al. [12] and Singh N.B. et al. [10,13] had studied the nonlinear optical properties of In, Er and Ag doped GaSe crystals and they had reported that the second harmonic generation varied with dopant concentrations. Here, we studied the second harmonic generation of GaSe crystals as a function of boron concentration under 1200 nm wavelength with 100 fs pulse duration. Nonlinear absorption (NA) is defined as an increase in absorp- tion coefficient with increasing optical intensity. NA become more popular research field due to numerous applications in science and technology such as Q-switching, mode-locking [14,15], upconver- sion lasing [16], and optical limiting [17]. There has been an increasing interest in the nonlinear optical properties of atomic layered materials. The layered semiconductors such as GaS [18,19], GaSe [7,20e22], and Ge [23] and Sn [24] doped GaSe, InSe and Ho doped InSe [25] have been widely investigated due to their nonlinear optical properties and potential applications for semi- conductor devices [26]. In our previous studies with nanosecond and picoseconds pulsed lasers at 1064 nm wavelength we showed that the saturable absorption and nonlinear absorption of GaSe crystal can be controlled with Ge and Sn dopings [23,24]. Time-resolved reflective and transmission pump-probe spec- troscopy is a useful tool to identify the carrier dynamics and * Corresponding author. E-mail address: akaratay@eng.ankara.edu.tr (A. Karatay). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat http://dx.doi.org/10.1016/j.optmat.2016.07.005 0925-3467/© 2016 Elsevier B.V. All rights reserved. Optical Materials 60 (2016) 74e80