Observation of the hyperfine structure and anticrossings of hyperfine levels in the luminescence spectra of LiYF4:Ho3

Abstract

Resolved hyperfine structure and narrow inhomogeneously broadened lines in the optical spectra of a rare-earth-doped crystal are favorable for the implementation of various sensors. Here, a well-resolved hyperfine structure in the photoluminescence spectra of LiYF₄:Ho single crystals and the anticrossings of hyperfine levels in a magnetic field are demonstrated using a self-made setup based on a Bruker 125HR high-resolution Fourier spectrometer. This is the first observation of the resolved hyperfine structure and anticrossing hyperfine levels in the luminescence spectra of a crystal. The narrowest spectral linewidth is only 0.0022 cm^-1. This fact together with a large value of the magnetic g factor of several crystal-field states creates prerequisites for developing magnetic field sensors, which can be in demand in modern quantum information technology devices operating at low temperatures. Very small random lattice strains characterizing the quality of a crystal can be detected using anticrossing points.

Fig. 1. PL spectrum of LiYF₄:Ho³⁺ (0.1 at. %): T = 6 K, B = 0; λ_ex = 638.3 nm.

a Setup: 1—cryostat with magnetic coils (gray hatched), 1a—temperature controller, 1b—current source for magnetic coils; 2—temperature-stabilized diode laser, 2a—temperature controller and current source for laser; 3—PL module; 4—Fourier spectrometer; 5—PL registration module, 5a—preamplifier and an analog to digital converter; 6—workstation for spectra calculation and automatic control of magnetic field and temperature. b The whole spectral region studied. Identified intermultiplet transitions of Ho³⁺ as well as the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ trace impurity are indicated. Inset presents the scheme of energy levels of Ho³⁺; the observed transitions are shown by arrows. c ⁵I₆ → ⁵I₇ and d ⁵I₇ → ⁵I₈ luminescent transitions. Insets show the spectral lines highlighted in blue on an enlarged scale. The numbers n_i → n_f indicate the initial and final levels of the corresponding transition, numbered from the lowest level in the CF multiplet (Table 1).

Fig. 2. ⁵I₅ CF levels with resolved HFS.

The π- (k⊥c, E||c; black trace) and σ- (k⊥c, E⊥c; red trace) polarized absorption spectra of a LiYF₄:Ho³⁺ (0.1 at. %) single crystal at the temperature 3.5 K in zero magnetic field. δσ = 1/2 L = 0.01 cm⁻¹ (L is the maximal displacement of a moving mirror in the Fourier spectrometer).

Fig. 3. Isotopic structure of hyperfine components in the spectra of crystals with different ⁶Li and ⁷Li isotope compositions.

Isotopic structure in the a absorption and b, c luminescence spectra of ⁷Li_1–x⁶Li_xYF₄:Ho³⁺ (0.1 at %) crystals with x = 0.07 (natural abundance of Li isotopes; green traces), x = 0.9 (red traces), and x = 0 (blue traces). a ⁵I₈ Г₂ (6.85) → ⁵I₇ Г₃₄ (5155.75), b ⁵I₇ Г₃₄ (5155.75) → ⁵I₈ Г₂ (6.85), and c ⁵I₆ Г₁ (8673.4) → ⁵I₇ Г₁ (5162.8) optical transitions. T = 6 K. δσ = 1/2 L = 0.001 cm⁻¹ (L is the maximal displacement of a moving mirror in the Fourier spectrometer).

Fig. 4. Hyperfine structure of several photoluminescence lines in the ⁵I₆ → ⁵I₇ transition of Ho³⁺:⁷LiYF₄ at T = 6 K.

Lines correspond to the transitions a 2 → 2 [⁵I₆ Г₁ (8673.4) → ⁵I₇ Г₃₄ (5155.75)] and 3 → 3 [⁵I₆ Г₃₄ (8680.3) → ⁵I₇ Г₁ (5162.8)]; b 1 → 2 [⁵I₆ Г₂ (8670.9) → ⁵I₇ Г₃₄ (5155.75)]; c 2 → 3 [⁵I₆ Г₁ (8673.4) → ⁵I₇ Г₁ (5162.8)]. Red traces are experimental spectra registered with δσ = 0.001 cm⁻¹, blue curves represent the results of fit with sets of Loretzians. Note the stretched wavenumber scale at c.

Fig. 5. Evolution of the hyperfine structure with increasing magnetic field B||c.

Photoluminescence intensity map in the magnetic-field – wavenumber scale for two transitions from the ⁵I₅ Г₃₄ (11,241.6) CF level to the levels of the ⁵I₇ CF multiplet: ⁵I₇ Г₂ (5152.3) (top line) and ⁵I₇ Г₃₄ (5155.75) (bottom line). ⁷LiYF₄:Ho³⁺ (0.1 at. %), T = 10 K. B||c.

Fig. 6. Anticrossings of the hyperfine levels in the luminescence spectra of ⁷LiYF₄:Ho³⁺ in a magnetic field B||c.

a, b Luminescence intensity maps in the magnetic-field – wavenumber scale for the a ⁵I₇ Г₃₄ (5155.75) → ⁵I₈ Г₂ (6.85) and b ⁵I₆ Г₂ (8670.9) → ⁵I₇ Г₃₄ (5155.75) transitions in ⁷LiYF₄:Ho³⁺ (0.1 at. %) at 6 K. Anticrossings of the hyperfine levels are observed. c Experimental and d simulated fragment of the spectrum corresponding to the ⁵I₆ Г₂ (8670.9) → ⁵I₇ Г₃₄ (5155.75) transition of ⁷LiYF₄:Ho³⁺ (0.1 at. %) in a zero magnetic field (in black) and in the field of 140 mT (in red), indicated by an arrow in b.

Fig. 7. The ⁵F₅ Г₃₄ (15,495.4) level demonstrates the largest deformation splitting.

a Luminescence intensity map in the magnetic-field – wavenumber scale and b calculated frequency vs magnetic field dependences for the ⁵F₅ Г₃₄ (15495.4) → ⁵I₆ Г₂ (8670.9) transition in ⁷LiYF₄:Ho³⁺ (0.1 at. %) at 10 K. c Experimental and d calculated spectra of this transitions at several values of the magnetic field. A strong horizontal line in a corresponds to the singlet–singlet transition ⁵F₅ Г₁ (15,512.7) → ⁵I₆ Г₂ (8687.75).