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Double stimulated emission of mixed color centers in AN LiF crystal
Volume 55, number 4
OPTICS COMMUNICATIONS
DOUBLE STIMULATED IN AN LiF CRYSTAL
15 September 1985
EMISSION OF MIXED COLOR CENTERS
Li-xing ZHENG and Liang-feng WAN Department of Physics, Tianjin University, Tianjin, People’s Republic of China
Received 2 May 1985; revised manuscript received 12 June 1985
Generation of double stimulated emission utilizing mixed F;-F, reported for the first time.
1. Optical properties of F$ centers
Laser emission of F, centers and Fi centers in LiF crystals has been obtained by several authors [l-3] and in our laboratory [4], respectively. This work reports generation of the stimulated emission with a double broad band utilizing mixed Fi-F, centers in an LiF crystal at RT. In LiF, an Fi center consists of three neighbouring halide ion vacancies in a regular triangle in a (111) plane with two electrons trapped. In the lattice the F; center presents C,, symmetry. Optical properties of such centers can be described emp1oying.a model based on the analogy with the Hi moleculeion in a dielectric medium. The scheme of the lowest energy states is shown in fig. 1. The fundamental absorption band at 460 nm is due to the transition from the ground state lA1 to the fust excited state 1E. The emission band at 530 nm is due to transition between relaxed states. normal singlet
triplet -2
3A
relaxed
1E -_-3E,
Fig. 1. Energy levels diagram of Fl center.
0 0304018/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
centers in an LiF crystal at room temperature (RT) is
The Fl center is well localized. The luminescence light has a small Stokes shift (0.36 ev). The data measured in our laboratory exhibit that the quantum efficiency of the F; center =O.S, the fluorescence decay time = 18 ns at RT. Fz centers have very long stored lifetime at RT, high opto- and them-to-stability. The absorption band of Ff centers overlaps considerably that of F, centers at 450 nm so that an absorption region with a single peak, called M region, is formed [5]. Monochromatic excitation in the M region gives a broad gr,een emission band of Fz centers and a red one of F, centers, which is due to double excitation. By the use of this phenomenon, one can develop a tunable laser in a broader region or with double wavelength output. The latter is the aim of the present investigation. Estimating the emission cross section of the F2 center as 5.5 X 10-l 7 cm2 and that of the F; center as 2.5 X 10-l 7 cm2, we knew that the laser threshold value of F$ centers was higher than that of F2 centers. In order to obtain stimulated emission of Fi and F2 centers simultaneously, therefore, it is necessary that the concentration of F; centers should be sufficiently higher than that of F2 centers while the concentration of F2 centers exceeds the threshold value. Analysis of the spectrum showed, however, that the concentration of F2 centers was higher than that of F; centers in LiF colored by y-ray or electron beam at RT. Moreover, the existence of N centers at 540 nm in LiF would self-absorb luminescence light 277
15 September 1985
OPTICS COMMUNICATIONS
Volume 55, number 4
77
520
500
700
600 wavelength
(nm)
Fig. 2. Emission spectrum due to excitation at 450 nm
from F; centers. So the essential key is how to produce a color center crystal. Having made a lot of experiments, we found a proper procedure of coloration, i.e. irradiation by electron beam at liquid-nitrogen temperature and then aggregation and transformation in the dark at RT. The sample produced under these conditions has following characteristics: (a) The sample has chiefly green color, little inclination toward orange. (b) Excitation at 450 nm gives a strong green emission band of F; centers and a weak red one of F2 centers (see fig. 2). It is noteworthy that the relative intensity in the fluorescence spectrum observed is almost independent of excitation wavelength from 410 nm to 490 nm (M region). (c) There are no N centers. Absorption loss in LiF is very small. In addition, it was also found that using oxygen-doped LiF crystals yielded expected result more easily than using pure ones. The mechanism of formation of F; centers will be discussed in another paper.
2. Experimental results The 3 5 cm long colored LiF used in our experiment was cut from a single crystal grown in our laboratory. It was transversely pumped with a nitrogenlaser pumped dye laser (Coumarin-I) operating at 460 nm with 1 Hz repetition rate. The absorption coefficient of the sample at the pump wavelength is 278
.
670 640 wavelength (nm)
690
710
Fig. 3. Relative output intensity as a function of emission ‘wavelength.
about 35 cm-l. The pumping light from the dye laser was focused by a cylindrical lens to form a line on the front surface of the sample. The optical resonator was a two-plane mirror cavity separated by 6 cm. The transmission of the output mirror was 90%. Its weak reflectivity is sufficient, for both Fi and F2 centers have high optical gain. We did not try to obtain optimum coupling. Strong emission of highly directional light was observed clearly. On the screen the near field of stimulated emission presented a bright green-yellow spot, the far field a green spot surrounded by red because of different divergence of F; and F2 center emission. The divergence of the light beam was about 15 mrad. Setting a prism in front of the output mirror, one can see simultaneously both a green and a red spot separated on the screen. There was no yellow light between the two spots. Fig. 3 depicts the spectrum of stimulated emission obtained. Since the resonator was nonselective, the wavelength and bandwidth of stimulated emission are determined by the wavelength dependence of F; and F2 center gain coefficient. The stimulated emission of F$ centers extends from 520 nm to 545 nm with a peak at 530 nm, that of F2 centers from 670 nm to 7 15 nm with a peak at 695 nm. The stimulated emission with overall bandwidth about 70 nm is generated simultaneously, which shows that a mixed color center laser can match with a mixed dye laser even in the visible region. It is possible to obtain individually tuning output with double wavelength lines by the use of dispersive elements in the cavity. Average output energy per pulse reached 60 /.IJ
Volume 55, number 4
OPTICS COMMUNICATIONS
when the pump energy per pulse was 640 fl. Opticoptic conversion efficiency was about 9.4%. Green emission of Fi centers first vanished and then the red one of F, centers in the course of decreasing the pump energy gradually. The treshold value of pump energy for F2 centers was 90 /JJ and that for Fz centers 15OcLJ. It was observed that F, centers were bleached resulting in red stimulated emission drop during the experiment. No output drop and no bleaching effects of I$ centers occurred after lo4 pulses had been shot, which shows thata stability of F; centers under laser action at RT is much better than that of F2 centers. The relative intensity between double stimulated emission can be altered by controlling irradiation condition and dose. Only F2 center laser action was to be observed if LiF crystal was y-rayed at RT. Using
15 September 1985
Coumarin-120 at 440 nm or Coumarin-152 at 485 nm dye laser as a pumping light, we also obtained double stimulated emission of mixed color centers. Yellow stimulated emission can be probably obtained if the pump energy is increased.
References [ 1] Yu.L. Gusev, S.N. Konoplin and S.I. Marennikov, Sov. J. Quantum Electron. 7 (1977) 1157. PI R.W. Boyd, J.F. Owen and K J. Teegarden, IEEE J. Quantum Electron. QE-14 (1978) 697.
I31 T. Kulinski, F. Kaczmarek, M. Ludwiczak and Z. Blaszczak, Optics Comm. 35 (1980) 120.
14 L.X. Zheng, S.Z. Guo and L.F. Wan, Chinese Physics Letters, to be published.
[5 J. Nahum and D.A. Wiegand, Phys. Rev. 154 (1967) 817.
279
OPTICS COMMUNICATIONS
DOUBLE STIMULATED IN AN LiF CRYSTAL
15 September 1985
EMISSION OF MIXED COLOR CENTERS
Li-xing ZHENG and Liang-feng WAN Department of Physics, Tianjin University, Tianjin, People’s Republic of China
Received 2 May 1985; revised manuscript received 12 June 1985
Generation of double stimulated emission utilizing mixed F;-F, reported for the first time.
1. Optical properties of F$ centers
Laser emission of F, centers and Fi centers in LiF crystals has been obtained by several authors [l-3] and in our laboratory [4], respectively. This work reports generation of the stimulated emission with a double broad band utilizing mixed Fi-F, centers in an LiF crystal at RT. In LiF, an Fi center consists of three neighbouring halide ion vacancies in a regular triangle in a (111) plane with two electrons trapped. In the lattice the F; center presents C,, symmetry. Optical properties of such centers can be described emp1oying.a model based on the analogy with the Hi moleculeion in a dielectric medium. The scheme of the lowest energy states is shown in fig. 1. The fundamental absorption band at 460 nm is due to the transition from the ground state lA1 to the fust excited state 1E. The emission band at 530 nm is due to transition between relaxed states. normal singlet
triplet -2
3A
relaxed
1E -_-3E,
Fig. 1. Energy levels diagram of Fl center.
0 0304018/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
centers in an LiF crystal at room temperature (RT) is
The Fl center is well localized. The luminescence light has a small Stokes shift (0.36 ev). The data measured in our laboratory exhibit that the quantum efficiency of the F; center =O.S, the fluorescence decay time = 18 ns at RT. Fz centers have very long stored lifetime at RT, high opto- and them-to-stability. The absorption band of Ff centers overlaps considerably that of F, centers at 450 nm so that an absorption region with a single peak, called M region, is formed [5]. Monochromatic excitation in the M region gives a broad gr,een emission band of Fz centers and a red one of F, centers, which is due to double excitation. By the use of this phenomenon, one can develop a tunable laser in a broader region or with double wavelength output. The latter is the aim of the present investigation. Estimating the emission cross section of the F2 center as 5.5 X 10-l 7 cm2 and that of the F; center as 2.5 X 10-l 7 cm2, we knew that the laser threshold value of F$ centers was higher than that of F2 centers. In order to obtain stimulated emission of Fi and F2 centers simultaneously, therefore, it is necessary that the concentration of F; centers should be sufficiently higher than that of F2 centers while the concentration of F2 centers exceeds the threshold value. Analysis of the spectrum showed, however, that the concentration of F2 centers was higher than that of F; centers in LiF colored by y-ray or electron beam at RT. Moreover, the existence of N centers at 540 nm in LiF would self-absorb luminescence light 277
15 September 1985
OPTICS COMMUNICATIONS
Volume 55, number 4
77
520
500
700
600 wavelength
(nm)
Fig. 2. Emission spectrum due to excitation at 450 nm
from F; centers. So the essential key is how to produce a color center crystal. Having made a lot of experiments, we found a proper procedure of coloration, i.e. irradiation by electron beam at liquid-nitrogen temperature and then aggregation and transformation in the dark at RT. The sample produced under these conditions has following characteristics: (a) The sample has chiefly green color, little inclination toward orange. (b) Excitation at 450 nm gives a strong green emission band of F; centers and a weak red one of F2 centers (see fig. 2). It is noteworthy that the relative intensity in the fluorescence spectrum observed is almost independent of excitation wavelength from 410 nm to 490 nm (M region). (c) There are no N centers. Absorption loss in LiF is very small. In addition, it was also found that using oxygen-doped LiF crystals yielded expected result more easily than using pure ones. The mechanism of formation of F; centers will be discussed in another paper.
2. Experimental results The 3 5 cm long colored LiF used in our experiment was cut from a single crystal grown in our laboratory. It was transversely pumped with a nitrogenlaser pumped dye laser (Coumarin-I) operating at 460 nm with 1 Hz repetition rate. The absorption coefficient of the sample at the pump wavelength is 278
.
670 640 wavelength (nm)
690
710
Fig. 3. Relative output intensity as a function of emission ‘wavelength.
about 35 cm-l. The pumping light from the dye laser was focused by a cylindrical lens to form a line on the front surface of the sample. The optical resonator was a two-plane mirror cavity separated by 6 cm. The transmission of the output mirror was 90%. Its weak reflectivity is sufficient, for both Fi and F2 centers have high optical gain. We did not try to obtain optimum coupling. Strong emission of highly directional light was observed clearly. On the screen the near field of stimulated emission presented a bright green-yellow spot, the far field a green spot surrounded by red because of different divergence of F; and F2 center emission. The divergence of the light beam was about 15 mrad. Setting a prism in front of the output mirror, one can see simultaneously both a green and a red spot separated on the screen. There was no yellow light between the two spots. Fig. 3 depicts the spectrum of stimulated emission obtained. Since the resonator was nonselective, the wavelength and bandwidth of stimulated emission are determined by the wavelength dependence of F; and F2 center gain coefficient. The stimulated emission of F$ centers extends from 520 nm to 545 nm with a peak at 530 nm, that of F2 centers from 670 nm to 7 15 nm with a peak at 695 nm. The stimulated emission with overall bandwidth about 70 nm is generated simultaneously, which shows that a mixed color center laser can match with a mixed dye laser even in the visible region. It is possible to obtain individually tuning output with double wavelength lines by the use of dispersive elements in the cavity. Average output energy per pulse reached 60 /.IJ
Volume 55, number 4
OPTICS COMMUNICATIONS
when the pump energy per pulse was 640 fl. Opticoptic conversion efficiency was about 9.4%. Green emission of Fi centers first vanished and then the red one of F, centers in the course of decreasing the pump energy gradually. The treshold value of pump energy for F2 centers was 90 /JJ and that for Fz centers 15OcLJ. It was observed that F, centers were bleached resulting in red stimulated emission drop during the experiment. No output drop and no bleaching effects of I$ centers occurred after lo4 pulses had been shot, which shows thata stability of F; centers under laser action at RT is much better than that of F2 centers. The relative intensity between double stimulated emission can be altered by controlling irradiation condition and dose. Only F2 center laser action was to be observed if LiF crystal was y-rayed at RT. Using
15 September 1985
Coumarin-120 at 440 nm or Coumarin-152 at 485 nm dye laser as a pumping light, we also obtained double stimulated emission of mixed color centers. Yellow stimulated emission can be probably obtained if the pump energy is increased.
References [ 1] Yu.L. Gusev, S.N. Konoplin and S.I. Marennikov, Sov. J. Quantum Electron. 7 (1977) 1157. PI R.W. Boyd, J.F. Owen and K J. Teegarden, IEEE J. Quantum Electron. QE-14 (1978) 697.
I31 T. Kulinski, F. Kaczmarek, M. Ludwiczak and Z. Blaszczak, Optics Comm. 35 (1980) 120.
14 L.X. Zheng, S.Z. Guo and L.F. Wan, Chinese Physics Letters, to be published.
[5 J. Nahum and D.A. Wiegand, Phys. Rev. 154 (1967) 817.
279