RF double resonances in the C4v centre CaF2: Ho3+

JOURNAL OF

LUMINESCENCE ELSEVIER

Journal of Luminescence 58 (1994) 332 334

Zeeman dependence of optical/RF double resonances in the C4,~,centre CaF2 : Ho3 + J.P.D. Martin*, T. Boonyarith, N.B. Manson Laser Physics Centre, Research School 0/Physical Sciences and Engineering, Australian National Unitersity, C’anherra, ACT, 0200, Australia

Abstract The low-field Zeeman splitting of the superhyperfine resonances associated with the two lowest electronic states of the C 3~are measured using RF bleaching ofspectral holes. The magnitude of the splitting is consistent 4~centre of CaF2: Ho with an earlier determination of the enhanced magnetic moments of the ground state levels. In addition, the number of observed superhyperfine Zeeman components indicates that there is fast electron—electron spin flip relaxation between the two ground electronic states.

1. Introduction This paper reports on optically detected nuclear resonance (ODNR) measurements in the C 4,, centre 3 + in low magnetic fields. This Ho3 + CaF2 : Ho centre exhibits strong mixing between the two lowest electronic states, both singlets, and the mixing results in anomalously large psuedoquadrupole splittings, hyperfine levels with appreciable but differing magnetic moments, and large superhyperfine splittings which depend on the hyperfine level involved [1,2]. The splittings of the superhyperfine resonances in a field can be measured using ODNR techniques and the magnitudes shown to be consistent with above mixed states. 2. Previous work 51 51 8(A1) and 8(A2) are the lowest energy states separated by 1.7 cm 1 of the C4,, centre *

Corresponding author,

CaF 3 ~ The states are strongly admixed by 2 : Ho the large hyperfine interaction between the Ho3 + 4f electrons and its nucleus. The strong mixing leads to theenough psuedoquadrupole splitting (Fig. is large to give rise to structure in 1) thewhich optical transitions to the excited states 5F 5F 5(E) and 5(A1) [1,2]. RF bleaching of spectral hole burning in any of the optical hyperfine lines in zero magnetic field has been previously reported to exhibit superhyperfine resonances associated with the interstitial ion and the nearest-neighbour ions as well as the second and third shell of ions [1]. Different sets of superhyperfine resonances were obtained for each hyperfine level. The ratio of the frequencies of these resonances in zero magnetic field scaled with the size of the enhanced holmium nuclear moment.

3. Zeeman splitting The Zeeman splitting of the superhyperfine levels can be calculated by combining the followingterms

0022-2313/94 $07.00 © 1994 Elsevier Science B.V. Al! rights reserved SSDI 0022 2313(93)E0174-V

J.P.D. Martin ci al.

Journal of Luminescence 58 (1994) 332 334

F

I1 I 7/2 +5/2 +3/2

A2

+1/2

+7/2

1/2 .

7/2

+

.

333

singlet states; (ii) the axial hyperfine interaction, mA812, the electronic Zeeman term, gJJ1B12H2,and axial superhyperfine interaction, nAi11,2,couple the two singlets; and (iii) mgJ~LNH2and ng~~1~H~ are the bare Zeeman terms. The superhyperfine splittings of the interstitial fluorine ion when m + + ±~ are obtained from (2) by taking the difference in energy between levels with n = + 4 and n 4. An approximate —

~,

~,

—

SI

~

8

expression for these energy differences is

—

A1

I

+3/2 ~5/2

I

EA2A1(m,4)

+

+7/2

+

1/2

‘

+7/2

1/2

~ zero field ~Fig. 1. Energy levels of the 51

in field 51 9(A1), 9(A2) states illustrating the optical hole burning through a superhyperfine structure associated with the ±~ hyperfine levels.

with the Hamiltonian used by [1] gj/..LNIH~pp gFJLNF. Happ,

gJf1BJHapp

~°Zeeman

(1)

where

IIB(IIN) are the Bohr (nuclear) magneton, the electronic (nuclear) spectroscopic splitting factors of the Ho3 + ion within a particular multiplet, and q~is the nuclear spectroscopic split-

gj(gj)

ting of thethe fluorine Byfactor inspecting groupion. symmetry properties of the C4,, centre, it can be shown that for the m = ± ± ± hyperfine levels, only terms containing the z-component of the orbital angular momentum operator [1], J1, couple the PA1>1 ~,m>l i,n> and the PA2>1 ~,m>l ~,n> states resulting in the modified energy levels neglecting the small direct quadrupole terms

4,

EA2A1(m,n)

4, 4

E0 + ~(P1 + P2)(m2 mgI/AN H2

+ ([A + (mA

4(P1

ngF/ANHZ

2

—

1)

~

=

+

{

A 2 —

g~/1 N + g8~~A1 {1,2} A

x H1 +

(3)

...

In a magnetic field, therefore, the Zeeman splitting 5PA associated with the 1 >1 ~m>14, n> state is differ4,m>14,n> state and, hence, there is aofseparate superhyperfine resonance ent from that the IPA2>1 frequency with each hyperfine state. Owing to the strong mixing, accurate estimates of the magnitude of the Zeeman splittings are obtained only by substituting the previously fitted values [1] of A 25.61 GHz, A 81 GHz, A8165 = 0.840 2F 2 =+4.72 2.6273/IN /1Ho = GHz,g8 = 1.2427, 1 + 4.’25j~N, and A 11 = 7.71 MHz into the exact eigenvalues of Eq. (2). The previously determined value for A11 [1] was modified to fit the zero field data. The estimated splittings for the A1 state are: 13.82 kHz/G (m = ±~) and 16.39 kHz/G (m = ± and for the A2 state: 5.81 kHz/G (m = ± and 8.38 kHz/G (m = ± Note that the resultant Zeeman splittings are much greater than the bare moment of 4.0 kHz/G for the fluorine ion. —

fl, 4)

4).

4 Experimental results

21}]2

P2){m 2

8 + g8/ABHZ 2)~2, where (i) E

21’

—

EA2A1(m,

+

I +

—

1/2 1/2

+ nA11)

(2)

x {1,2}

0 and A describe the effect of the free ion and the crystal field terms, respectively, on the two

A single crystal of Ho3~(0.0005%) doped CaF was oriented, mounted on a 5

turn

2 RF coil in

a glass cryostat and cooled to pumped helium ternperatures. The laser, resonant with a hyperfine line,

334

J.P.D. Martin et al.

/ Journal of Luminescence 58

depopulates the resonant hyperfine and superhyperfine levels and this results in a decrease in the laser absorption. When RF coincident with the ground state hyperfine or superhyperfine resonances is applied, population is restored to the resonant ground state and the absorption, and the consequent emission, increases. The increase in the emission level is termed the ODNR signal. The RF signal was swept from 0 to 30 MHz and the ODMR averaged for repeated sweeps.

(a)

/\

/ I

‘~

I

I ~.

/

“....

I

~ 2 .

I

- -

-~

‘~

~

I

(b)

I

I .E

f~

/ \

~

ç

(1994) 332 334

The ODNR spectrum obtained in this way for hole burning in the extreme lines corresponding to transitions from the ± and + hyperfine levels are shown in Figs. 2(a) and (b), respectively. The central line is the superhyperfine resonance at zero field and the dashed curves with peaks displaced to the high and low energy gives the superhyperfine components for a field of 78.5 G obtained by burning in the separate Zeeman components. From Fig. I, it can be seen that only one superhyperfine double resonance signal is expected for each case. However, two signals are present, and the frequencies are close to those predicted for the coupled pair of ground state singlets shown by the vertical lines in the figure. There is, therefore, a strong signal for

4

4

the ground electronic state resonant with the laser together with a slightly weaker signal for the nonresonant electronic state. The depth of the optical hole is determined by the dynamic equilibrium between optical pumping, optical decay, electron electron spin flips and nuclear nuclear spin flips, and the ODNR signal is a measure of the change in the dynamical situation caused by the RF. The second signal occurs due to the combined effect of the RF and fast electron spin flips between the two

electro:i::tates.

12

~

It has been shown that there are separate superhyperfine resonance with each hyperfine level in the

frequency offset (MHz)

ground state and the Zeeman shifts are consistent with that calculated from the enhanced magnetic

Fig. 2. ODNR spectrum associated with hole burning in the 5I 5F 8(A1)=~. 5(E) optical transition: (a) for the + ~ hyperfine levels and (b) for the + ~ hyperfine levels. The central line in each case is for zero field with frequencies of 23.49 MHz ( + ~) and 18.35 MHz ( + ~), respectively. The dashed curves with peaks to lower energy are for a field of 78.5 G and hole burning in the lower energy optical Zeeman component. The dashed curves to higher energy are for the hole burning in the higher Zeeman component. The solid lines indicate the predicted Zeeman shifts of the superhyperfine resonances.

moments. References [1] J.P.D. Martin, T. Boonyarith, N.B Manson, M. Mujaji and GD. Jones, J. Phys.: Condens. Matter 5 (1993) 1333. [2] J.P.D. Martin, T. Boonyarith, NB. Manson and Z. Hasan, J. Phys.: Condens. Matter 4 (1992) L411.