Laser induced two-photon decay in Pr3+:LaAlO3

Laser induced two-photon decay in Pr3+:LaAlO3

Volume 7, number 1 OPTICS LASER INDUCED COMMUNICATIONS TWO-PHOTON Claude DELALANDE January DECAY 1973 IN Pr3+: LaAlO, and Andre MYSYROWIC...

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Volume

7, number

1

OPTICS

LASER

INDUCED

COMMUNICATIONS

TWO-PHOTON

Claude DELALANDE

January

DECAY

1973

IN Pr3+: LaAlO,

and Andre MYSYROWICZ

Laboratoire d’optique Quantique du CNRS a?zd UuiversitC Paris Sud, 91405 Orsay, France

Received

16 November

1972

A laser induced two-photon decay has been observed between the 3Po and 3Fz levels of trivalent in LaAlOa. The differential transition cross section was found to be da/da = 1 .l X 1O-27 cm2.

Laser induced two-photon decay processes have been observed in several instances [ l-5] . Of particular interest is the knowledge of the corresponding transition cross section u, which provides information upon the feasibility of the stimulated two-photon emission process [6] . In this respect, rare-earth ions in a crystalline host have attracted the attention of several authors [6,7] . As is well known, the inner unfilled electronic 4f shell, which is responsible for the transitions in the visible, is well shielded against crystal field effects, thus the corresponding absorption and emission lines are narrow. Also, because of strongly allowed 4f-5d transitions in the UV, one expects transition cross sections to be large. We present here the experimental observation of two-photon decay in Pr 3+:LaA103 and a measurement of the corresponding cross section. The levels of interest are shown in fig. I. The level 3Po is metastable, with a radiative lifetime of 32 psec. We studied the transition between 3Po and 3F2, for which all two-photon selection rules are satisfied. In this experiment, we used the intense emission (250 mJ, lpsec) of a flash-pumped coumarine dye laser, the wavelength of which was adjusted to the peak of the 3H4-3P1 absorption line, for optimum pumping. Subsequently, non-radiative transfer occurred to the 3Po level. The two-photon transition was induced by the presence of an intense radiation at fixed frequency wl, provided by a prism Qswitched ruby laser (0.2 J, 30 nsec) (see fig. 2). The ruby laser pulse was arranged to be coincident in 10

praseodymium

, 4f5d

cm-’

I/ --50000

--40000

--30000

21202 20594

--2ocm

(b)

(a)

4f2 --10000 11

35

t

1

5285

-0

3% Fig. 1. Levels of interest

of Pr3+:LaA103.

time with the maximum of the one-photon (3Po -3H4) luminescence intensity. We observed the process “a” in fig. 1, corresponding to a simultaneous absorption and emission of two photons, with fiw, -AU, = AE, rather than process “b”, the simultaneous emission of

Volume

7, number

OPTICS

1

January

COMMUNICATIONS

1973

30-

Ruby

Laser

I

25.

Monochromator

20

Fig. 2. Experimental set-up: (1) ruby laser intensity (2) two-photon decay signal output; (3) one-photon nescence intensity output (see text).

output; lumi.

photonshw1 ++rw2 = AE, because the corresponding cross section is larger and the detected UV photons hw2 are more easily discriminated against spurious noise. In the absence of an intermediate resonance, both processes are related in a simple manner [9]. The emitted photons were detected in a direction perpendicular to the incident beams through a monochromator and associated lenses. Diaphragms ensured that only the volume of the sample which was irradiated by the two lasers was seen by the entrance slit of the monochromator. Detection was performed with a fast photomultiplier (56 UVP Radiotechnique). The maximum signal corresponded to an average of 0.3 photoelectrons per pulse, so that single photoelectron counting techniques were used. The weakness of the emission is explained by the severe filtering conditions and the difficulty of achieving a large population in the 3P, level, because of Laporte’s rule. The 3P0 population was monitored via a monochromator and a photomultiplier. The ruby laser intensity was observed with a fast rise-time photocell. In order to minimize signal errors, the peak laser intensity was kept well below threshold for sample surface burning. Also, a ruby rod of good quality was used and the beam was approximately free of hot spots. The results are shown in fig. 3. The observed emission line falls exactly at the calculated position. Its width is instrument-limited. The value of the differential scattering cross section was measured, using the method described in ref. [2] , which avoids much of the uncertainty associated with absolute measurements. Taking a value of 0.4 for the total radiative quantum efficiency from 3P,

15

10

5

-I i)

C 3300

3330

3360

3390

3420

Fig. 3. Spectrum of the detected signal. The horizontal dashed line corresponds to the average noise level.

[lo], do/da

we find the value = 1.1 X 1O-27 cm2

for the differential cross section for emission with all polarizations. This value is larger by three orders of magnitude than typical vibrational Raman cross sections. This quantity can be evaluated in the free ion case. Following Axe [7], the main contribution to the transition tensor is assumed to arise from the 5d orbitals lying at 50 000 cm-l. We calculated the angular part of the tensor for levels 3P, and 3F,; the radial part is given by Rajnak [8] . The theoretical value for do/dR is found to be smaller by one order of magnitude than the experimental value. This is not surprising, in view of the crude approximations used in the calculation. Also, from the experimental value of da/da, we estimate the value 6 = 1O-49 cm4 set-l , with 6 defined by the relation dp/dt = 6 F2, where p represents the probability of emitting simultaneously two photons of equal energy, F the incident photon flux per unit surface. 11

Volume

7, number

1

OPTICS

COMMUNICATIONS

We are grateful to Professor J. Ducuing for his constant interest in this work. We acknowledge very fruitful discussions with Dr. S. Feneuille and C. Delsart .

References [I]

S. Yatsiv, M. Rokni and S. Barak, Phys. Rev. Letters (1968) 1282. [ 21 J. Ducuing, G. Hauchecorne, A. Mysyrowicz and F. Pradere, Phys. Letters 28A (1969) 746.

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1973

[ 31 P. Brlunlich, R. Hall and P. Lambropoulos, Phys. Rev. 5 (1972) 1013. [4] A.P. Veduta, M.D. Galanin, B.P. Kirsanov and Z.A. Chizhikova, JETP Letters 11 (1970) 96. (51 P. Platz, Appl. Phys. Letters 17 (1970) 537. [6] P.P. Sorokin and N. Braslau, IBM J. Res. Develop. 8 (1964) 177. [ 71 G.D. Axe, Phys. Rev. 136 (1964) A42. [ 81 K. Rajnak, J. Chem. Phys. 37 (1962) 2440. [9] W. Zernik, Phys. Rev. 132 (1963) 320. [lo] C. Delsart, private communication.