Spectroscopic and Laser Properties of the Near-Infrared Tunable Laser Material Yb3+-Doped CaF2 Crystal A. Jouini,*,†,‡ A. Brenier,‡ Y. Guyot,‡ G. Boulon,‡ H. Sato,† A. Yoshikawa,† K. Fukuda,† and T. Fukuda†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 808–811
Institute of Multidisciplinary Research for AdVanced Materials,Tohoku UniVersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan, and Physico-Chimie des Matériaux Luminescents, UMR CNRS 5620, UniVersité Claude Bernard-lyon 1, 10 Rue Ampère, Villeurbanne 69622, France ReceiVed December 5, 2006; ReVised Manuscript ReceiVed February 9, 2007
ABSTRACT: High-quality oxygen-free 5 mol %Yb3+-doped CaF2 single crystal grown by the Czochralski method has been studied for its broadband absorption and emission properties. We present its spectroscopy at room and low temperatures and laser operation will be demonstrated around 1.05 µm with a slope efficiency of 57% with various transmission output couplers. The laser wavelength could be tuned between 1030 and 1070 nm. 1. Introduction The trivalent ytterbium ion, Yb3+, is an adequate candidate for high-power (>50 W cm-1), high-efficiency (>50%), and ultrafast laser emissions. It has several advantages compared with Nd3+. Its very simple electronic-level scheme (only two multiplets, 2F5/2 and 2F7/2) leads to a quantum defects smaller than Nd3+-doped crystals (10% instead of 25%). The radiative lifetime of the Yb3+(2F5/2) excited electronic manifold (upper laser manifold) of Yb:YAG was found to be around 1 ms, four times longer than the lifetime of the Nd3+(4F3/2) upper manifold of Nd:YAG laser crystal and without detrimental processes such as excited state absorption, concentration quenching, or upconversion energy transfer. What was needed to exploit all these favorable properties was an efficient high-power laser-diode emission matching the absorption band of the Yb3+ ion. This need was fullfilled by diode lasers based on InGaAs quantum wells emitting in the 900–980 nm region, developed in the 1990s to pump Er-doped fiber amplifiers for commercial use in the telecommunications industry.1 On the other hand, CaF2 (fluorite structure) is a well-known fluoride crystal that has the advantages of a wide transparent wavelength range (0.15–9 µm), low phonon energy (∼320 cm-1), and high thermal conductivity (10 W m-1 K-1) at the same time: in fact, to the best of our knowledge, the first continuous-wave (CW), diode-pumped, and ceramic lasers were based on a CaF2 matrix activated by Tm2+, U3+, or Dy2+.2–4 The present work is a continuation of previous efforts whithin the framework of collaboration between the Japanese (IMRAM) and the French (LPCML) laboratories on Yb-doped fluoride and oxide single crystals for laser applications.5–9 High-purity Yb3+-doped CaF2 single crystal was grown using the Czochralski (Cz) method. Broadband room temperature absorption was suitable for laser diode pumping. Fluorescent emissions from the 2F5/2 multiplet were recorded at 300 and 12 K under pulsed infrared laser excitation of 5 mol % Yb3+-doped CaF2 at 940 nm, and the decay times of these emissions were measured. CW laser output has been also demonstrated for 5
* Corresponding author. E-mail:
[email protected]. Fax: 81-22-2175102 † Tohoku University. ‡ Université Claude Bernard-lyon 1.
mol % Yb3+-doped CaF2 under high-power InGaAs laser-diode pumping, and the laser parameters values will be given. 2. Experimental Section Crystal growth was performed in a Cz system with a resistive heater made of high-purity graphite. The starting material was prepared from commercially available CaF2 and YbF3 powders of high purity (>99.99%). The dopant concentration of YbF3 were 5 mol%. The starting materials were placed in a high-purity graphite crucible with 0.5 wt% ZnF2 powder as an oxygen scavenger. Vacuum treatment was performed prior to growth. The system was heated from room temperature to 1000 °C for a period of 12 h under a vacuum (∼1 × 10-3 Pa). Both rotary and diffusion pumps were used to achieve ∼1 × 10-3 Pa and effectively eliminate water and oxygen from the growth chamber and the starting material. Subsequently, high-purity Ar gas (99.9999%) was slowly introduced into the furnace to avoid contamination of the crystal and to prevent reduction of the Yb3+ ions to Yb2+ ions. Thereafter, the starting materials were melted at approximately 1460 °C. The pulling rate was 3.0 mm/h, and the rotation rate was 15 rpm. Growth orientations were controlled using [111]-oriented CaF2 seed crystal. After the growth, the crystal was cooled to room temperature at a rate of 50 °C/h. To identify the obtained phase, we carried out powder X-ray diffraction (XRD) analysis in air at room temperature using a RINT Ultima (RIGAKU) diffractometer with a CuKR X-ray source (40 kV, 40 mA). The diffraction pattern was scanned over the 2θ range from 3 to 80° with steps of 0.02°. The crystallinity of the crystals was measured by X-ray rocking curve (XRC) analysis using a RIGAKU advanced thin film X-ray system (ATX-E). CuKR1 radiation was used with a multilayer X-ray mirror. The XRC profiles were taken with a 4-bounce Ge (220) channel-cut monochromator. The beam divergence was 12 arcsec. Five millimeter square samples were prepared with a thickness of 1.9 and 3 mm, respectively, for all measurements. Both faces of the sample were polished and antireflection-coated for laser experiments. Room-temperature absorption spectra were recorded with a UV– visible (vis)–near IR (NIR) lambda 9000 Perkin-Elmer dual-beam spectrometer. Excitation of the Yb3+ fluorescence was performed with a frequency-doubled Nd:YAG laser (10 ns, 10 Hz) pumping a Quantel three-amplifier-stage dye laser containing a mixture of DCM and LD700 and followed by a hydrogen Raman cell shifter to generate a beam in the 950–980 nm range. The specific infrared fluorescence is selected by using a Jobin Yvon HR250 monochromator fit with a 600 grooves/ mm grating blazed at 1 µm and detected by a fast North Coast germanium cell cooled by liquid nitrogen. The signal was analyzed using a Lecroy 9410 digital oscilloscope coupled to a computer. The lifetimes have been directly measured on one side of the sample, which is directly excited by the laser beam. Because the size of sample has been chosen to be small, the excited crystal part is small, and radiative trapping due to a geometrical effect can be assumed to be weak.
10.1021/cg060883r CCC: $40.75 2008 American Chemical Society Published on Web 01/30/2008
Yb3+-Doped CaF2 Crystal
Figure 1. [111] oriented 5 mol % Yb-doped CaF2 grown by the Czochralski method.
Crystal Growth & Design, Vol. 8, No. 3, 2008 809
Figure 3. X-ray rocking curve of (a) first grown and (b) laser Ybdoped CaF2 crystal.
Figure 2. X-ray topography of (a) first grown and (b) laser Yb-doped CaF2 crystal. Experimental lifetime values have been fitted to a single exponential profile with excellent agreement.
3. Results and Discussion 3.1. Growth and X-ray Characterization. Yb-doped CaF2 single crystals were grown by two different methods: the simple melting under CF4 atmosphere and the laser-heated pedestal growth (LHPG) under Ar atmosphere.5 Their laser performances, low power (
