The secondary emission spectra Of beta -ZnP2 under the resonant excitation ill the Is exciton region were investigated in detail. Several scattering lines with the Stokes shifts of less than 6 meV were found. By analyzing the experimental results, using a model based on the exciton-polariton picture, we, attributed the observed scattering lines to the Brillouin lines. The sound velocities,, of tho LA and TA phonons were derived to be v(LA) = 4.8(+/-0.2) x 10(5) cm/s and v(TA) = 2.8(+/-0.2) x 10(5) cm/s, respectively. The resonant enhancement of the Brillouin lines was observed around tho transverse exciton energy E-T, which reflects high transmissivity and a small group velocity of the exciton-polaritons at E-T. We also found that the Rayleigh scattering line was resonantly enhanced between the longitudinal exciton. energy E-L and E-T due to the surface roughness. which explains the drastic reduction of the luminescence from the lower brunch polaritons in intensity reported earlier. The excitation energy, dependence of the Brillouin line width is also discussed.

Magnetic field effects on the excitation spectra of the 1s triplet exciton luminescence in monoclinic β-ZnP2 were observed at 2K. Under the magnetic field of 1T, the luminescence band splits into two components with the separation of about 0.2meV. The excitation spectrum for each of the Zeeman-split components was measured for the first time. The spectrum for the one component is definitely different to the other.

Various aspects of luminescence in beta -ZnP2 have been investigated at low temperatures. Formation of exciton molecules is confirmed under the CW resonance excitation into the lowest Is triplet exciton absorption band. The exciton molecule emission band M appears at a moderately weak laser power. Low energy component C-L1 Of the singlet luminescence comes from the anti-Stokes acoustic phonon scattering of the triplet excitons, From the outgoing resonance of the LO Raman line, it is confirmed that the high energy component C-L2 has many strong phonon sidebands, while low energy component C-L1 has no sideband. Furthermore, a novel luminescence C-T resulted from the anti-Stokes scattering of the triplet excitons has been found. From detailed excitation spectra, we can deduce some of the relaxation paths of the exciton polaritons.

Nonradiative relaxation processes of excitons have been investigated by means of photocalorimetric (PC) spectroscopy in the exciton resonance region of beta -ZnP2 by detecting temperature increase of the crystal under excitation with a Ti:sapphire laser at 4.5 K. For singlet excitons, power of the light absorbed by the crystal is converted to that of the thermal heating of the crystal almost at a constant rate as high as about 80% in the energy range from the Is longitudinal exciton energy E-L to the band gap energy E-g. The heat generation, however, is largely suppressed in the Is reflection band; where the exciton luminescence is also decreased in intensity, which suggests the presence of other optical process in this region such as resonant Rayleigh scattering. From comparison of the PC spectrum with excitation spectrum for the singlet exciton luminescence, it is found that a ratio of the PC signal to the luminescence intensity increases at the exciton resonances n = 1, 2, 3 and 4. This fact indicates that the rate of the nonradiative decay of excitons relative to the emission of the luminescence is high at the exciton resonances. The PC spectrum for triplet excitons is also presented.

Photocalorimetric (PC) spectra have been measured in the exciton resonance region of β-ZnP2 by detecting the temperature increase of the crystal under excitation with a Ti:sapphire laser at 4.5K. For singlet excitons, the power of the light absorbed by the crystal is converted to that of the thermal heating of the crystal at almost an equal rate in the energy range from the 1s longitudinal exciton energy EL to the band gap energy Eg. The amounts of both singlet exciton luminescence and heat generation decrease in the 1s reflection band, which suggests the presence of other optical process such as resonant Rayleigh scattering in this region. The excitation spectrum for the singlet exciton luminescence is also observed. The ratio of the PC signal to the luminescence intensity is enhanced at the exciton resonances n=1–4. This fact indicates that the rate of the nonradiative decay of excitons relative to the emission of the luminescence is large at the exciton resonances. The PC spectrum for triplet excitons is also presented.

We observe the luminescence spectrum of the singlet exciton polariton (E‖c) in β-ZnP2 at 2K under excitation into the lowest triplet exciton absorption (E‖b) by measuring the excitation spectra. A sharp excitation peak is observed at the triplet exciton energy Et for each excitation spectrum, which appears through the anti-Stokes acoustic phonon scattering of the triplet excitons into the singlet exciton polariton states lying above 2meV. The luminescence spectrum is obtained by plotting the intensities of the excitation peaks against the monitoring energy. The spectrum has similar shape to the singlet polariton luminescence. The excitation efficiency increases steeply again below the low-energy tail of the ordinary singlet luminescence. Since the singlet exciton polariton has the character of light in the low-energy region, the scattered polariton goes out easily from the crystal surface. As the monitoring energy is lowered, the excitation spectra have almost equidistant (∼0.14meV) satellite peaks on both sides of the main peak at Et. The energy distance corresponds to the LA phonon energy connecting the light branch with the bottleneck in the triplet exciton polariton.

Temporal behaviors of the radiative recombination of excitons have been studied in beta-ZnP2 by using picosecond laser pulses at 2K. Under interband excitation with E//c, the singlet exciton luminescence decays with two exponential components, 250ps and 1.25 ns. The latter value is reasonable for the allowed exciton transition. The triplet exciton luminescence decays also with two exponential components, 150ps and about 2.05ns. These decay times of the triplet luminescence are surprisingly short, because the triplet exciton is only weakly allowed and expected to have a long lifetime. The relative intensity of the fast component of the triplet luminescence decreases, as the excitation density decreases. This component is attributed to the formation of the excitonic molecule via collision of two triplet excitons. The decay time of the slow component does not change largely with the excitation power, suggesting effective nonradiative decay processes. When the exciton is created in a bulk of the crystal under excitation into the triplet exciton band with E//b, the triplet luminescence shows a build-up component, for which exciton diffusion mechanism is suggested. The de-excitation processes of the triplet exciton are briefly discussed.

A new type of experiment was performed to investigate the effective nonradiative process causing the fast decay of the forbidden Is triplet exciton luminescence with polarization E//b in beta-ZnP2. Excitation spectra of anti-Stokes Is singlet exciton luminescence with E//c have been measured by varying the excitation energy with E//b around the is triplet exciton absorption peak which is located about 2 meV below the singlet exciton luminescence. A sharp peak of excitation efficiency has been confirmed at the triplet exciton resonance for the first time. Temperature dependence of the excitation peak is in accordance with the thermal behavior of the singlet exciton polariton luminescence both from lower- and upper-branches. This anti-Stokes acoustic phonon scattering of triplet exciton into singlet states is one of the main causes of the short lifetime of triplet exciton in beta-ZnP2. Spin flip mechanism is briefly discussed.

Luminescence spectra of tetragonal ZnP2 crystal are measured at various temperatures. Below the indirect exciton threshold, about 30 intrinsic indirect exciton luminescence bands are confirmed. Amidst the broad bound exciton luminescence band, several emission bands appear even at 2 K. Intensities of these bands increase with increasing temperature, especially in the high energy part of the band. They are also intrinsic exciton luminescence caused through the indirect exciton transition processes accompanied by two phonons at k(1) and k(2) conserving the exciton wavevector; k(ex) = k(1) + k(2). Energy difference between the lowest edges of one- and two-phonon luminescence bands 59.4 meV is equal to the largest Raman shift energy. Phonons participating in the luminescence are mainly momentum conserving phonons appearing in the one-phonon luminescence bands and the ones at the Gamma point of the Brillouin zone.

Magnetic field effects on the triplet exciton are investigated in β-ZnP2. The sharp luminescence line of the 1s triplet exciton weakly allowed for the polarization E ∥ b splits into two or three lines with g-value of 4.0 according to the direction of the applied magnetic field H ∥ b or H ∥ c and also to the polarization E ∥ b or E ∥ c for which the luminescence is observed. These results are explained simply by a group theory.

AbstractElectronic transitions in biaxial β‐ZnP2 are studied by two‐photon absorption. The anisotropy of the crystal leads to a large energy splitting of the P excitons. The energy shift of the P excitons and Landau transitions are observed in a magnetic field up to 6T. We present a theory which describes the anisotropic P excitons in a magnetic field. The analysis of the experimental data allows the determination of the anisotropic dielectric constants and effective masses of the valence and conduction bands.

Detailed excitation spectra for singlet and triplet luminescence bands in ZnP2 are observed by using a Ti:sapphire laser. When the triplet exciton is excited, the excitation spectrum for the triplet luminescence shows clear hydrogenic series up to n=9, which reflects the absorption spectrum of the spin triplet exciton. 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. Magnetic field effects of the excitation spectra under the triplet exciton excitation are also shown.

Secondary emission spectra in the higher region of exciton band of ZnP2 single crystal are measured at 6 K with a tunable Ti:sapphire laser as an exciting light source. The luminescence bands due to the radiative recombination of the 2s, 3s and 4s excitons are clearly observed separately. It is also found that the 1LO Raman line with energy of 32.2 meV shows resonance enhancement near the 2s exciton band. On the contrary, the intensity of the 1LO Raman line becomes anomalously weak when it is just superimposed on the 3s luminescence peak. The shapes of these luminescence bands change considerably when the 1LO Raman line approaches them. The changes in intensity of the 2s and 3s exciton bands with the excitation energy are briefly discussed in regard to the relaxation processes of polariton.

Relaxation processes of exciton polaritons in monoclinic zinc diphosphide (beta) -ZnP2 have been investigated in detail by measuring resonant secondary emission spectra under excitation into the energy range from the interband to the exciton-bands region. In addition to the 1LO line of 32.2 meV already reported, we have found more than 30 resonant Raman lines ranging from 9.4 meV to 58.9 meV by varying the excitation energy. These lines are observed distinctly only when their scattered energies fall into the vicinity of the 1s exciton energy. It is confirmed that among phonons of 72 modes the LO phonon of 32.2 meV preferentially governs the relaxation processes of exciton polaritons in (beta) -ZnP2. We have also observed a significant intensity decrease of the exciton polariton luminescence when the excitation energy is lowered across the 1s longitudinal exciton energy EL. A brief discussion is made in this connection.

Secondary emission spectra from the excited states of exciton series in black monoclinic ZnP 2 single crystal are measured at 6K under the band-to-band excitation condition. The luminescence bands due to the radiative recombination of the 2 s , 3 s and 4 s excitons are clearly observed separately. The changes in intensities of the 1LO Raman line with the energy of 32.2 meV and the luminescence bands with the excitation energy are studied in detail. The 1LO Raman line shows resonance enhancement due to the out-going resonant effect when it comes near the 2 s exciton band. On the contrary, when it is just superimposed on the 3 s luminescence peak the intensities of the 2 s and 3 s luminescence bands and the 1LO Raman line have the minimum values. This weakness is explained with the relaxation of the exciton polariton through the 2LO Raman process.

Resonant secondary emission of monoclinic zinc diphosphide (beta-ZnP2) has been investigated in detail under excitation into the energy range from the interband to the exciton-bands region. In addition to the 1LO line of 32.2 meV already reported, we have found more than 30 resonant Raman lines ranging from 9.4 meV to 58.9 meV by varying the excitation energy. These lines are observed distinctly when their scattered energies fall into the vicinity of the Is exciton energy. It is confirmed that among phonons of 72 modes the LO phonon of 32.2 meV preferentially governs the relaxation processes of exciton polaritons in beta-ZnP2. We have also observed a significant intensity decrease of the exciton polariton luminescence when the excitation energy is lowered across the 1s longitudinal exciton energy E(L). A brief discussion is made in this connection.