Kaizo Nakamura

Photoluminescence Spectra of Eu2+ Centers in Ca(S, Se):Eu and Sr(S, Se):Eu

Photoluminescence and excitation spectra are observed for Ca(S, Se):Eu2+ and Sr(S, Se):Eu2+ powder phosphors at various temperatures between 4.2 and 300 K. The emission band of Ca(S1-x Sex ):Eu2+ at 300 K shifts almost linearly with an increase in x, from 652 nm (x=0) to 597 nm (x=1). In Sr(S1-x Sex ):Eu2+, the emission band shifts also linearly from 620 nm (x=0) to 571 nm (x=1). At low temperatures, vibronic structures on emission bands are observed for CaS:Eu2+, CaSe:Eu2+, SrS:Eu2+ and SrSe:Eu2+. The excitation processes are attributed to two transitions; the intraionic 4f7→4f65d transition within Eu2+ and the fundamental absorption by host crystal, followed by the transfer of the excitation energy to an unexcited Eu2+ ion.

Polariton Luminescence in Monoclinic ZnP2 Crystal

Exciton polariton luminescence and its temperature dependence have been investigated in monoclinic ZnP 2 crystals. The energies of transverse and longitudinal excitons ( E T and E L ) are determined by a fit of the reflection spectrum to a simple polariton model. In the vicinity of the 1 s exciton, two emission bands are observed at energies E T and E L . They are attributed to the luminescence from lower and upper branch polaritons, respectively. In particular, the lower polariton luminescence band exhibits a well-defined doublet structure at 2 K. Both components are separated from each other by about 1 meV with comparable intensities and the same polarization dependence. With increasing temperature, the higher-energy component decreases rapidly as compared to the lower-energy component. The origin of these emission components is discussed.

Luminescence from the aggregated Cu+ centers in SrS:Cu+

The photoluminescence (PL) and excitation spectra, PL decay curves and time-resolved PL spectra are measured at 80 K for the SrS:Cu+ powder phosphors activated with a large amount of Cu2S. The shape and peak position of the PL spectra depend on both the activator concentration and the excitation wavelength. The PL spectra consist of four kinds of Cu+ bands, α, β, γ and δ. The α band peaking at 513 nm at 80 K is attributed to the isolated Cu+ center, the β band peaking at 543 nm at 80 K to the paired Cu+ center, and the γ (558 nm at 80 K) and δ (about 600 nm at 80 K) bands to the highly aggregated Cu+ centers.

Intersystem Conversion between Singlet and Triplet Exciton States in ZnP2

Detailed excitation spectra for singlet and triplet luminescence in ZnP 2 are observed at 2 K by using a Ti:sapphire laser. When a singlet exciton is excited, conversion to a triplet exciton occurs over a wide excitation energy range. The spin memory of the exciton is easily lost under the band-to-band excitation condition. When the triplet exciton is excited, the excitation spectrum for the triplet luminescence shows a clear hydrogen-like series up to n =9, which reflects the absorption spectrum of a spin triplet exciton series with s - d doublet structure for n ≥3. The excitation spectrum for the singlet luminescence under the excitation of the triplet exciton is observed for the first time as a new exciton series appearing only for n ≥3. The new exciton series results from the contribution of the d -envelope function of the exciton, and the s -envelope function alone does not give rise to the singlet luminescence.

Temperature Dependence of Exciton Reflection Spectra in Monoclinic Zinc Diphosphide

Reflection spectra near the fundamental edge of monoclinic ZnP 2 crystal have been measured at various temperatures from 2 K to 200 K. As the temperature is raised from 2 to 80 K, minimum value of the reflectivity R min due to the lowest 1s exciton is found to decrease. The R min turns to increase above 80 K. The model calculations of reflection spectra based on the dead layer model are performed for different values of the exciton dissipative damping constants. The analysis of the reflection spectra obtained at various temperatures reveals that this change of the spectra results from monotonical increase of the exciton damping with increasing temperature in the existence of exciton-free “dead layer”. It is suggested that the main contribution to the exciton damping is the scattering by phonons.

Exciton Transitions in Indium Halides

Reflection, absorption and emission spectra of InBr, InI and InBr 1- x I x mixed crystals have been measured at 2 K by using polarized light. In InBr, a prominent exciton peak is found at 2.48 eV for the polarization E // b , which is on the high energy side of the lowest direct exciton peak at 2.33 eV allowed for E // c . Three square root steps due to indirect excitons have been observed for E // b in contrast to four steps for E // c . Six momentum conserving phonons are confirmed. Luminescence from free and shallow bound indirect excitons is identified in InBr. The first exciton peak in InBr 1- x I x mixed crystals shifts continuously from InI to InBr. Indirect-direct crossover at x =0.5 leads to the conclusion that the lowest energy band gap of InI is of direct type. The indirect transition in InBr is assigned to the transition from S to T point in the Brillouin zone.

Photoluminescence of the SrS:Mn2+ Phosphor and Pb2+-Sensitized Luminescence of the SrS:Pb2+, Mn2+ Phosphor

Emission and excitation spectra of the Mn 2+ ion in the SrS:Mn 2+ and SrS:Pb 2+ , Mn 2+ phosphor are investigated at various temperatures. The analysis of the excitation spectra of the Mn 2+ emission band leads to the Racah parameters B =712 cm -1 , C =3352 cm -1 and the crystal field parameter D q =576 cm -1 with the covalency parameter e =0.046. By the transient spectroscopy technique, it is found that the emission band due to the single Mn 2+ ion center has a peak at 553 nm and a decay time of ∼5.5 ms, while that due to the paired Mn 2+ center has a peak at 565 nm and a decay time of 0.6 ms. Energy transfer from Pb 2+ ion to Mn 2+ ion is observed in SrS:Pb 2+ , Mn 2+ . By observing the change of band shapes, the intensities and decay characteristics of both the Pb 2+ and Mn +2 lumimescence, it is concluded that the transfer is of resonance type. It is found that the energy of Pb 2+ ion is transferred preferentially to the isolated Mn 2+ ion.

Influence of Paired Mn2+ Centers on the Luminescence Spectra of CaS:Mn2+

The photoluminescence decay of the CaS:Mn2+ phosphor activated with a fairly large amount of Mn2+ (1-2mol%) consists of slow (8.5 ms at 80 K) and fast (550 µs at 80 K) exponential decay components due to the single and paired Mn2+ centers, respectively. From the measurements of time-resolved emission spectra, the emission bands due to the single and paired Mn2+ centers were found to be located at 590 and 599 nm, respectively, at 80 K. Five bands in the time-resolved excitation spectrum of the paired Mn2+ center are observed on the slightly low-energy side relative to those of the single Mn2+ center. The time-resolved excitation spectra were also investigated for SrS:Mn2+ phosphor.

Scattering spectra in orthorhombic indium bromide under resonant excitation

Abstract Emission spectra in indirect gap InBr are measured at liquid helium temperature under Ar+ laser excitation. They consist of many weak lines up to 20 with LO phonon energy (16.4 meV) spacing. They shift with incident laser lines. Even-numbered lines are stronger than odd-numbered lines. They show resonant enhancements when photon energy of scattered light is near the direct exciton energy, 2.33 eV.

Reflection spectra of orthorhombic indium iodide

Abstract The reflection spectra of InI single crystal have been measured using linearly polarized light. Pronounced anisotropy is observed between two directions of polarization E ∥ c and E ∥ a . The first peak at 2.018 eV for E ∥ c is direct allowed exciton transition with binding energy 4.3 meV. The E ∥ a spectrum suggests that the transition is forbidden.