Excited state absorption and laser potential of Mn5+-doped Li3PO4

31 January 1997

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical Physics Letters 265 (1997) 264-270

Excited state absorption and laser potential of Mn 5+-doped Li3PO 4 M.F. Hazenkamp a, H.U. Giidel a, S. Kiick b, G. Huber b, W. Rauw c, D. Reinen c a Departementfiir Chemie. Unwersitiit Bern, Freiestrasse 3, CH-3000 Bern 9. Switzerland b lnstitutffir Laser-Physik, Universitfit Hamburg, Jungiusstrasse 9a, D-20355 Hamburg. Germany c Fachbereich Chemie und Zentrum fiir Materialwissenschaften, Philipps Universitfit Marburg, Hans-Meerwein-strasse, D-35032 Marburg, Germany

Received 28 October 1996

Abstract

Excited state absorption (ESA) spectra at room temperature of Mn 5+-doped Li3PO4 crystals are reported. A broad and strong (O'EsA = 1 × 10 18 cm 2) ESA band due to the ~E ~ 1T~ and 1E ---, IT2 transitions is observed between 10000 and 15000 cm ] which prevents efficient near-infrared pumping for laser operation. For MnS+-doped Li3PO 4 there is no ESA at the stimulated-emission wavelength, in contrast to the laser material Srs(VOa)3F:Mn 5+. This is due to the higher crystal field strength A and the weaker distortion of the host tetrahedron in Li3PO4. Finally, the laser potential of MnS+-doped materials, in general, is evaluated.

1. I n t r o d u c t i o n

Since the demonstration of near-infrared (NIR) laser oscillations of Cr4+-doped Mg2SiO 4 [1,2] the spectroscopic and laser properties of many materials doped with the 3d 2 ions Cr 4+, Mn 5+ and Fe 6+ in tetrahedral oxo coordination have been investigated [3-8]. At room temperature, cra+-doped crystals show broadband luminescence in the NIR due to a spin-allowed transition and are therefore the most interesting with respect to tunable laser applications. For MnS+-doped crystals the luminescence consists of a sharp line in the NIR due to a spin-forbidden transition [5]. Laser oscillation was demonstrated in MnS+-doped Ba3(VO4) 2, Sr3(VO4) 2 [7] and Sr5(VOa)3F [8], however, with high thresholds and rather low slope efficiencies. It was shown for Srs(VO4)3F:Mn 5+ that excited state absorption

(ESA) at both the pump and laser wavelengths has a detrimental effect on the lasing properties [8-10]. In order to find materials with more favourable ESA properties we measured the ESA spectra of Mn 5+doped Li3PO 4. In a systematic study of the luminescence properties of Mn 5+ in a variety of host lattices it was found that the MnO 3- tetrahedron was least distorted in this lattice and non-radiative quenching processes were also the smallest [5].

2. E x p e r i m e n t a l The high-temperature modification of Li3PO 4 crystallises in the orthorhombic space group Pmnb [1 1]. Small blue single crystals with dimensions of about I m m 3 were grown from a L i O H / C s C I flux as described in Ref. [12]. The Mn concentration in

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M.F. Hazenkamp et a l . / Chemical Physics Letters 265 (1997) 264-270

the flux was 1% with respect to P. The resulting Mn concentration in the crystal was determined using electron microprobe analysis to be 0.25% (3.15 × 1019 ions/cm3). The ground state absorption spectrum at room temperature was measured on a Cary 5e double beam spectrophotometer. The excited state absorption spectra were obtained with the pump and probe technique by measuring the transmission spectrum and the difference in transmission of the pumped and the unpumped crystal as described in detail in Ref. [13]. Pumping was performed with a Ti:A120 3 laser operating at 720 nm with an output power of 400 mW. The transmitted probe beam of a tungsten lamp (250 W) was dispersed by a 0.5 m single monochromator and detected with either a Si diode or a cooled InSb detector. It can be shown that [13] lp -- I u - = C ( OGS A -[- O'SE -- O'ES A ) ,

( 1)

lu

where Ip(.) is the transmitted intensity in the pumped (unpumped) case, C is a constant which depends on the details of the experimental apparatus and O-~sA, O-SE and trESA are the cross sections for ground state absorption, stimulated emission and excited state

265

absorption, respectively. Positive (Ip - l u ) / l . values correspond to situations in which either ground state absorption bleaching or stimulated emission are stronger than excited state absorption, whereas negative values correspond to a situation in which excited state absorption dominates. The angular overlap model (AOM) calculations were performed with the program LIGFIELD [ 14].

3. Results Fig. 1 shows the unpolarised ground state absorption (GSA) spectrum at room temperature of Mn 5÷doped Li3PO 4. The spectrum is similar to previously reported spectra of this material [5,15]. No polarised spectra were measured since it was reported that the absorption of MnS+-doped Li3PO 4 is practically unpolarised [15]. A strong broadband centered at 15380 cm -j dominates the spectrum in the visible and near-infrared region. Strong absorption is observed above 28000 cm -j. Some weak sharp structure appears at about 13840 and 8940 cm-~. The luminescence spectrum of this material consists of a single sharp line at 8940 cm-~ accompanied by a weak sideband structure [5].

140 LMCT 120

3T1(F)

100

3

80

~

.-. 'E o 'E

2

6O

40

20 3TI(P)

10000

15000

i . . . . . . . . 2 0 0 0 0 -1, 25000 energy ( c m )

0 30000

Fig. I. Unpolarised ground state absorption spectrum at room temperature of Mn 5+ -doped Li3PO 4.

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M.F. Hazenkamp et al. / Chemical Physics Letters 265 (1997) 264-270

t.O- a 3 0.5"%

0.0

- ,~,:~7,~

•

-0.55000

10000

15000

20000

energy(em"1) 2.0 b 1.5. '~ 1 . o ,?

~0.50.0 ~ ,t

.........

5000

i .........

i .........

10000 energy

15000

tion from pure Td symmetry is small [1 l]. We assume the same geometry for the MnO 3- tetrahedron so that we can interpret our spectra to a good approximation using Td symmetry. From the relevant Tanabe-Sugano diagram which is shown in Fig. 3 an assignment of bands to the various d - d transitions is straightforward. The result is included in Fig. 1. The GSA spectra of various MnS+-doped crystals have been reported and discussed in the literature [5,8,10,15-17]. These spectra are all quite similar. The three barely resolved maxima at 14750, 15380 and 16010 cm -I on the strong absorption band centered at 15380 cm-~ are due to the splitting into three components of the 3TI(F) state in C s symmetry. The steep rise to the first ligand-to-metal charge-transfer transition (LMCT) centered at 32000 cm-1 is observed above 28000 cm -1. The observed sharp line spontaneous emission at 8940 cm -l is due to the I E ~ 3 A 2 transition. The intensity of the zero-phonon line relative to the total emission intensity is 88%. Due to the C s site symme-

i,

20000

~

(era-1)

1 T2

3T1

50

Fig. 2. (a) Unpolarised (Ip - l u ) / l u spectrum at room temperature of MnS+-doped Li3PO4. Between 14100 and 13500 cm -1 the pump beam at 13900 cm-i was blocked in order to protect the detector. (b) O'EsA - O'SE spectrum obtained by subtraction of the O'GSA spectra from the appropriately scaled ( l p - l u ) / l u spectrum in (a).

The unpolarised ( l p - l u ) / l u spectrum at room temperature is shown in Fig. 2a. A strong ground state bleaching band is centered at 15380 cm -l which corresponds to the band in the GSA absorption spectrum in Fig. 1. Excited state absorption (ESA) bands are centered at about 5000 and 12000 c m - i and above 18000 c m - 1. A weak sharp positive signal is observed at 8940 cm -1.

~

',

40

~

3T2

30

et2

1A1

n~

tG

/

1D 3p

-----r

1E

4. Discussion 4.1. Spectroscopic assignments

In Li3PO 4 the Mn 5÷ ion (3d 2) substitutes for the pS+ ion and is tetrahedrally coordinated by oxygen ions. The site symmetry of pS+ is C s but the devia-

SF

00

~/

3A2

~

1

2

Dq/B

3

4

Fig. 3. Relevant part of the Tanabe-Sugano diagram for d 2 (Td). C = 4.25B. The dotted line indicates the appropriate D q / / B ratio of about 2.2 for MnS+-doped Li3PO 4.

M.F. Hazenkamp et aL / Chemical Physics Letters 265 (1997) 264-270

try the I E state is split into two components which are only 17 c m - l apart [5]. At room temperature both lines are broadened and merge into one emission line. The decay time at 10 K of 1.22 ms is essentially radiative [5]. At room temperature it is 1.11 ms, only slightly shorter than at 10 K, indicating that non-radiative processes from the ~E state are not dominant at room temperature. By subtracting the G S A spectrum (Fig. 1) from the appropriately scaled ( l p - lu)/l u spectrum (Fig. 2a) and inverting the y-axis one obtains the ESA-SE spectrum in Fig. 2b with OrESA > 0 and trSE < 0. The scaling factor C in Eq. (1) was estimated by using a similar method as in Ref. [9]: the lower limit for C is that for which O'ESA --O'SE becomes negative at energies where there cannot be stimulated emission. A rough upper limit is given by such a value of C for which the ESA-SE spectrum still shows structures or bands which are evidently due to G S A bleaching. The general appearance of the ESA-SE spectrum in Fig. 2b is similar to that of Srs(VOa)3F:Mn 5+ as reported in Refs. [9,10], which suggests that our estimate for C was reasonable. The reliability of our estimate for C can be checked by using a second method. The small negative peak in Fig. 2b at 8940 c m - ~ is due to SE from the ~E metastable state. O'SE at 8940 c m - l is 0.65 × 10 -19 c m 2. We can compare this value with that calculated from the radiative lifetime using Eq. (2)

[8]: 1

8"n"n2

~'rad

A2

O"SE (/"peak)

AV(lo-o/Itot)

(2)

where ~'rad is the radiative lifetime (1.22 × 10 -3 s), n the refractive index (1.56 [18]), A v the effective linewidth of the sharp emission peak at 8940 c m - ] ( 1 . 5 × 1012 H z ) and lo_o/lto t is the intensity of the zero-phonon emission peak relative to the total spontaneous emission intensity (88% [5]). We obtain O'SE(8940 c m - t ) = 1 x 10 -19 c m 2, in good agreement with the above value. We assign the weak ESA with the three maxima around 5000 c m - i in Fig. 2b to the IE ~ ~A i transition, which is spin allowed, but symmetry forbidden in pure T d symmetry. The peak at 4890 c m - ] is assigned to the I E ~ IA] zero-phonon transition and the peaks at 5190 and 5670 c m -1 to vibrational

267

sidebands involving an M n - O bending vibration of 300 c m - ~ and an M n - O stretching vibration of 780 cm -~, respectively. The strong broad ESA centered at about 12000 cm-1 is assigned to the combined I E ~ I T 2 a n d I E ~ IT 1 transitions, which are both spin and symmetry allowed. The onset of the strong ESA transition above 18000 cm -1 is ascribed to a I E ~ L M C T transition. Ligand field ESA transitions from the I E state to the higher lying I E(G), I T2(G) and IA 1(S) states (not shown in the Tanabe-Sugano diagram in Fig. 3) are weak because they correspond to two-electron transitions. They are overlapped by strong one-electron I E ~ L M C T bands. To check the assignments of the GSA and ESA spectra we have performed a ligand field calculation, assuming cubic symmetry. We used the Racah B and zl as fitting parameters to reproduce the energies of the excited states with respect to the 3A 2 ground state. We set C / B = 4.25, the free ion value [17]. The experimental energies can be fitted with A = 11300 cm -1 and B = 510 cm - l . These parameters are similar to those found for other MnS+-dope d crystals [10,12,17,19]. Table 1 shows a comparison of the calculated and observed energies of the ligand field transitions. The overall agreement confirms our assignment. However, the absolute agreement is only fair. In Refs. [17,19] this was explained with the 'symmetry-restricted covalency' argument, meaning that Racah B in MnO 3- is less reduced by covalency in the e 2 configuration than in the et 2 and t 2 configurations. In addition it was found that there is strong mixing of d - d states with low lying chargetransfer states which contributes to the partial breakdown of the ligand field model.

Table 1 Comparison of calculated and observed transition energies in c m - t for MnS+-doped Li3PO 4. For the calculation we assumed pure Td symmetry, C / B = 4.25 (free ion ratio) and the fitted parameters B = 510 c m - ] and A = 11300 c m - l Transition 3A2 ~

Calculated

Observed

IE 3T2 IA~ 3Tt(F) ~T2 IT I 3TI(P)

8350 11300 14730 16040 19320 21640 24980

8940 12200 13840 15380 = 20500 = 21600 22200

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4.2. Laser potential Excited state absorptions at the pump a n d / o r laser wavelength are important loss mechanisms for laser oscillation. ESA at the pump wavelength results in a low pumping efficiency and, as a consequence, in a high laser threshold power. ESA at the laser wavelength affects both the threshold power and the slope efficiency [10]. From Fig. 2a it appears that between 9000 and 13500 cm-~ ESA dominates over GSA so that optical pumping in this spectral range is inefficient. Note that this spectral region includes the wavelengths of readily available laser diodes. This situation is similar to that of the laser material Srs(VOa)3F:Mn 5+ in which strong broad ESA bands between 15400 and 8700 cm -~ prevent efficient pumping [8-10]. Around 16000 cm -1 (666 nm) OESA is not very large for MnS+-doped Li3P Q so that efficient pumping is possible at this wavelength. However, in that case nearly half of the pump energy is converted into heat in the crystal due to the large quantum defect. In Fig. 4a the section of the (Ip - l u ) / l u spectrum around the spectral region of the emission is enlarged. The stimulated emission peak at 8940 c m - t lies just below the tail of the strong and broad t E ~ ~T2 ESA band. Hence, no losses due to ESA at the laser wavelength are expected. This is different for the laser material Srs(VO4)3F:Mn 5+, in which the tail of the broad ESA band overlaps with the SE peak, see Fig. 4b. This induces important losses in the case of Srs(VO4)3F:Mn 5+ [10]. This overlap between the broad ~E ~ ~T2 ESA transition and ~E--*3A2 emission transition is a function of the ligand field strength A and the distortion of the MnO 3- tetrahedron. The influence of the distortion of the MnO 3 tetrahedron on the overlap is illustrated in Fig. 5. In Fig. 5a the calculated energies of the ~E ~ 3A 2 luminescence transition and the ~E ~ IT 2 ESA transition are plotted as a function of a trigonal angular distortion of the MnO 3- tetrahedron. The distortion is expressed by an angle 0 which corresponds to the angle between the M n - O bond along the C 3 axis to either one of the three other M n - O bonds. Thus, 0 > 109.47 ° (0 < 109.47 °) corresponds to an angular elongation (compression) of the tetrahedron. The energies were calculated using the angular overlap model taking

a

"~=0 -v=

I 8600

b 7600

I

1E_>IES1 T2 '

9000 9400 energy(cm -1)

r 8000

i'~ 9800

1E-~IT2 ' ESA" ~'.,~ i t 8400 8800 energy(cm -1)

Fig. 4. Comparison of the (lp - l u ) / l u spectra of MnS+-dope d Li3PO 4 (a) and MnS+-doped Srs(VO4)3F (b) in the spectral region of the ~E ---, 3A 2 stimulated emission. The ordinate scales are arbitrary.

B = 520 cm -~, C = 4.25B, e,, = 12000 cm - t , e~ = 0.25e,~ and the effective spin-orbit coupling constant ~'~ff= 120 cm-~. Upon trigonal distortion the IT 2 state splits into two components, and one of the components shifts to lower energies. The energy of the ~E state is only weakly dependent on the distortion (see Fig. 5a). Keeping in mind that the ESA transition has a typical width of some 2000 c m - ~ it is clear that the overlap between ESA and the sharp line luminescence may become significant for strongly distorted MnO 3- complexes. In Fig. 5b the effect of an angular tetragonal distortion is shown. This lowers the symmetry from T d to D2d. In this case the distortion angle 0 is defined as the tetrahedron angle which is bisected by the S 4 axis. Hence, 0 > 109.47 ° (0 < 109.47 °) corresponds to an angular compression (elongation) along the S 4 axis. The A O M parameters are the same as in

M.F. Hazenkamp et al. / Chemical Physics Letters 265 (1997) 264-270 12000-

IE~1T2~

11000'E 10000-

9000-

1

3

.

•

E ~ A 2 emtsslon

8000i

i

106

i

108 110 e(degrees)

i

i

112

114

12000-

11000E ~10000-

90001 E__ 3A2

emission

8000 i

106

i

i

108 110 e (degrees)

i

i

112

114

Fig. 5. Calculated energies of the ] E ~ 3A 2 emission and the ~ E ~ JT 2 ESA transition for a trigonal (a) and tetragonal (b) distortion of the M n O 3- tetrahedron. The angular overlap model parameters are e,,=12000 c m - ~ , e . ~ = 3 0 0 0 c m - ~ , B = 5 2 0 c m - J, C = 4.25B and ~reff = 120 c m - i for both calculations. The angular distortion angle 0 is defined in the text.

Fig. 5a. The effect on the t T 2 excited state and thus on the ~E -~ IT 2 ESA is seen to be much stronger for small distortions than in Fig. 5a. Thus, ESA is expected to become significant at smaller tetragonal distortions than in the trigonal case. Fig. 4 shows that for Srs(VO4)3F the lowest ESA transition has shifted several hundred wavenumbers to lower energy compared with Li3PO 4. The PO 3host tetrahedron of Li3PO 4 is an almost perfect tetrahedron [11], whereas the VO 3- tetrahedron in the apatite Srs(VO4)3F is trigonally elongated with typical distortion angles of 111-112 ° [12]. From Fig. 5a it follows that this distortion can account for only 60 c m - ~ to the observed shift of the ESA. Therefore ,:1 must also significantly contribute to this shift. The energy of the ~T 2 state is much more strongly dependent on ,6 than the energy of ~E. Hence, for lower A

269

the ~E ~ ~T2 transition shifts to lower energy with respect to the emission transition. The ligand field calculations reported here and those for Srs(VO4) 3F:Mn 5+ in Refs. [8,10] do not allow an accurate determination of A. However, if we take the energy of the 3A 2 ~ 3TI(F) transition in the GSA spectrum as a relative measure for A it appears that zl for Srs(VO4)3F is some 300 cm -~ smaller than for Li3PO 4 !10]. This is due to the larger V - O distance of 1.71 A in VO43- than the P - O distance of 1.56 ,~ in PO43-. The MnO 3- guest thus encounters a larger hole in Srs(VOa)3F. We will now evaluate the laser potential of MnS+-doped materials in general. From the extensive spectroscopic study in Ref. [5] it was found that MnS+-doped crystals essentially show sharp line emission from the I E state. Even for systems with small A and strong tetragonally distorted host tetrahedra (such as some spodiosites) it was found that the onset of the 3T2 absorption band lies some 800 cm-~ above the JE emission, so that this triplet state cannot be significantly populated at room temperature and no spin-allowed broadband luminescence was observed. This means that MnS+-based lasers will be three-level lasers with high thresholds and without the possibility to broadly tune the laser wavelength. In the present study and related ones [8-10] we find that for MnS+-doped crystals there is a strong and broad ESA band in the NIR due to the tE---~ ~T2, ~T l transitions which prevents efficient pumping in this spectral range. This ESA band may even overlap with the emission, which decreases the slope efficiency. Thus, MnS+-doped crystals have several properties, which are unfavourable for laser materials. On the other hand, high quantum efficiencies at room temperature are often found for MnS+-doped crystals due to the small H u a n g - R h y s factor of the emission transition. In Ref. [5] it was found that the highest quantum efficiencies are found for materials with less distorted host tetrahedra. In the present study we find that the overlap of the t E ~ ~T2 ESA band with the emission wavelength can be reduced or eliminated by choosing a host with an undistorted host tetrahedron and a large A. In conclusion, it is probable that only MnS+-doped host lattices with large A and undistorted host tetrahedra, such as Li3PO 4, have potential as laser materials, despite

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M.F. Hazenkamp et a l . / Chemical Physics Letters 265 (1997) 264-270

their three-level character. It is worthwhile optimising the crystal quality of Li3PO 4 or to find other host materials which meet the above requirements.

Acknowledgements The authors thank Professor Dr. R. Giovanoli of the Universit~tt Bern for the microprobe analysis. This work was financially supported by the Human Capital and Mobility program of the European Union and the Swiss National Science Foundation.

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