Fluorescence fluctuation of Rhodamine 6G dye for high repetition rate laser excitation Nageshwar Singh a,n , Hemant K. Patel b , S.K. Dixit c , H.S. Vora d a Magnetic and Superconducting Materials Section, Materials & Advanced Accelerator Sciences Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, MP, India b Cryo-engineering and Cryo-module Development Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, MP, India c Laser Systems Engineering Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, MP, India d Laser Engineering Support Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, MP, India article info Article history: Received 3 April 2012 Received in revised form 12 July 2012 Accepted 25 July 2012 Available online 3 August 2012 Keywords: Fluorescence Rhodamine 6G Stationary and flowing medium Fluctuations High repetition rate excitation abstract In this paper, fluorescence from Rhodamine 6G dye for stationary and flowing liquid medium, excited by copper vapor laser, operating at 6 kHz pulse repetition frequency, was investigated. Large fluctuations in spectral width (about 5 nm) and spectral intensity in the fluorescence from stationary dye solution were observed, while fluctuations in the spectral width diminish in a flowing dye medium. However, this increases spectral intensity and slightly red shifts the fluorescence peak emission wavelength. Theoretical analysis was carried out to explain the observed results by incorporating the temperature induced refractive index, beam deflection and spectral variation in stationary dye solution. Numerical analysis of thermal load and contour of temperature in the optical pumped region inside the dye cell in stationary, 0.2 and 1.5 m/s flow velocity was also investigated to support our analysis. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Rhodamine 6G (Rh6G) is a very efficient and one of the most widely used dye as an active media for tunable lasers [1–3]. In addition, Rh6G had also been used as an efficient energy transfer partner in a binary mixture with other dyes to improve the efficiency and broaden the spectral tuning range of dye lasers [4–7]. The emission spectral characteristics of energy transfer dye laser (ETDL) had been extensively investigated theoretically and experimentally [8–10], using Rh6G as donor with numerous other dyes as an acceptor pair. Fluorescence spectrum of Rh6G is closely resembled the mirror image of the long-wavelength absorption band [1]. However, its intrinsic emission properties are affected by kinetics of nonradiative processes [1,3] and environmental factors [3] such as the solvent (solute–solvent interaction, polar- ity and pH), the temperature, the concentration of the dye, etc. Fluorescence feature of Rh6G is also influenced by excited-state absorption and self-absorption of molecular fluorescence [11,12]. The spectral and luminescence properties of dye molecules (such as the fluorescence quantum efficiency, width, and profile of the electronic spectra) and also the lasing characteristics are affected by nature of the solvent(s) [13,14]. The concentrations of lasing dye in an active medium alter the efficiency of dye laser [3,15]. The kinetics of intersystem crossing rate and triplet state life time of Rh6G, which deteriorates the performance of dye laser, had been studied [16]. The thermo-optical properties of the dye laser solvent(s), characterized in terms of refractive-index changes with temperature, and their influences on lasing characteristics has been studied [17,18] extensively. Ahmed et al. [19] have reported fluorescence from Rh6G dye dissolved in liquid solvent with added TiO 2 particles (in a thin 0.016  1 cm 2 and 1  1 cm 2 cells) to understand random scatter- ing under lamp excitation (514 nm). The fluorescence, in the presence of 1 cm thick cell, was red shifted with respect to the intrinsic fluorescence in presence of 0.016 thin cell. They observed that the presence of random scattering in the lumines- cent bodies affects the spectrum of the fluorescent radiation observed at the surface of these bodies. The primary effects observed were red shifts in the emission spectra with respect to the intrinsic fluorescence emission. Hunga et al. [20] reported the fluorescence changes of Rhoda- mine 6G in ethanol solutions using a wide range of pumping field fluence at 532 nm at room temperature. Blue shift of the fluores- cence spectra and fluorescence quenching of the dye molecule in solution were observed at high excitation fluence values. The effects are interpreted as the result of population redistribution in the solute–solvent molecular system induced by the high fluence field and the fluence dependence of the radiationless decay mechanism. Fischer and Georges [21] had measured the fluores- cence quantum yield of Rh6G in ethanol. Hammond [11,12] had Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.07.022 n Corresponding author. Tel.: þ91 731 2488372; fax: þ91 731 2488300. E-mail address: nageshwar@rrcat.gov.in (N. Singh). Journal of Luminescence 134 (2013) 607–613