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Physical review. B, Solid state

Optical pumping in nitrogen-doped GaP

Robert F. Leheny, Jagdeep Shah

Published Oct 15, 1975• 12 citations
Publisher

Abstract

We report on the results of optical-pumping experiments in GaP doped with the isoelectronic impurity nitrogen. In the first experiment we have measured the saturation of absorption at the $A$ bound exciton as a function of pump intensity, using as the pump a pulsed dye laser tuned to the $A$ line. The observed saturation behavior is in quantitative accord with a simple three-level system involving the $A$ and $B$ bound-exciton level and the N ground state. The second experiment involves measuring the absorption at the $A$ line with a weak probe (a tunable dye laser) while the crystal is being pumped with an intense pump dye laser tuned above the indirect gap. In this case also the absorption saturates with increasing pump power and we can infer approximately 10% efficiency for the capture of excitons at N impurities. In this second experiment, a simple model predicts an amplification by a factor of 8.5 but none is observed. We conclude from these measurements that, when pumped above the indirect gap, the dominant excited state of N impurities in our crystal is a single electron trapped at N. We believe that this is caused by the presence of other impurities in the crystal and is not an intrinsic limitation of the system.

DOI
10.1103/physrevb.12.3268

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We report on the results of optical-pumping experiments in GaP doped with the isoelectronic impurity nitrogen. In the first experiment we have measured the saturation of absorption at the AAA bound exciton as a function of pump intensity, using as the pump a pulsed dye laser tuned to the AAA line. The observed saturation behavior is in quantitative accord with a simple three-level system involving the AAA and BBB bound-exciton level and the N ground state. The second experiment involves measuring the absorption at the AAA line with a weak probe (a tunable dye laser) while the crystal is being pumped with an intense pump dye laser tuned above the indirect gap. In this case also the absorption saturates with increasing pump power and we can infer approximately 10% efficiency for the capture of excitons at N impurities. In this second experiment, a simple model predicts an amplification by a factor of 8.5 but none is observed. We conclude from these measurements that, when pumped above the indirect gap, the dominant excited state of N impurities in our crystal is a single electron trapped at N. We believe that this is caused by the presence of other impurities in the crystal and is not an intrinsic limitation of the system.
Excitons are sometimes trapped at impurities in crystals and at defects on interfaces in the confined systems, such as quantum wells and microcrystallites embedded in a host matrix. We discuss these trapping effects on the third-order optical polarization under nearly resonant pumping of the exciton. First we predict an enhancement of ${\mathrm{\ensuremath{\chi}}}^{(3)}$(\ensuremath{\Omega};\ensuremath{\Omega},-\ensuremath{\Omega},\ensuremath{\Omega}) by a factor ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{n}\ensuremath{\rightarrow}\mathit{T}}$/${\mathrm{\ensuremath{\Gamma}}}_{\mathit{T}\ensuremath{\rightarrow}\mathit{g}}$ multiplicatively on that enhanced by the large excitonic dipole moments. Here ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{n}\ensuremath{\rightarrow}\mathit{T}}$ and ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{T}\ensuremath{\rightarrow}\mathit{g}}$ are, respectively, the rate at which the exciton is trapped and the decay rate of the trapped electron into the ground state. In many cases, ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{n}\ensuremath{\rightarrow}\mathit{T}}$ is much larger than ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{T}\ensuremath{\rightarrow}\mathit{g}}$ so that a large enhancement of ${\mathrm{\ensuremath{\chi}}}^{(3)}$ is expected. Second, the four-wave mixing spectrum \ensuremath{\Vert}${\mathrm{\ensuremath{\chi}}}^{(3)}$(2${\mathrm{\ensuremath{\Omega}}}_{1}$-${\mathrm{\ensuremath{\Omega}}}_{2}$;${\mathrm{\ensuremath{\Omega}}}_{1}$,-${\mathrm{\ensuremath{\Omega}}}_{2}$,${\mathrm{\ensuremath{\Omega}}}_{1}$)\ensuremath{\Vert} under two beams ${\mathrm{\ensuremath{\Omega}}}_{1}$ and ${\mathrm{\ensuremath{\Omega}}}_{2}$ is shown to consist of three hierarchical structures as a function of ${\mathrm{\ensuremath{\Omega}}}_{1}$-${\mathrm{\ensuremath{\Omega}}}_{2}$, i.e., the sharpest spike around ${\mathrm{\ensuremath{\Omega}}}_{1}$-${\mathrm{\ensuremath{\Omega}}}_{2}$=0 with the linewidth \ensuremath{\Vert}${\mathrm{\ensuremath{\Omega}}}_{1}$-${\mathrm{\ensuremath{\Omega}}}_{2}$\ensuremath{\Vert}=${\mathrm{\ensuremath{\Gamma}}}_{\mathit{T}\ensuremath{\rightarrow}\mathit{g}}$, the steep shoulder with the center also at ${\mathrm{\ensuremath{\Omega}}}_{1}$-${\mathrm{\ensuremath{\Omega}}}_{2}$=0 and the width ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{n}}$ (the exciton decay rate), and the wide structure with the width ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{n}\mathit{g}}$ around ${\mathrm{\ensuremath{\Omega}}}_{1}$=${\mathrm{\ensuremath{\omega}}}_{1\mathit{s}}$ and ${\mathrm{\ensuremath{\Omega}}}_{2}$=${\mathrm{\ensuremath{\omega}}}_{1\mathit{s}}$. Here ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{n}\mathit{g}}$ is the relaxation rate of the exciton polarization and \ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{1\mathit{s}}$ the lowest exciton energy. Third, the differential transmission spectrum is found to show the strong absorption saturation peak or the sharp induced absorption dip as a function of pump-probe detuning ${\mathrm{\ensuremath{\Omega}}}_{2}$-${\mathrm{\ensuremath{\Omega}}}_{1}$, depending upon the pump frequency ${\mathrm{\ensuremath{\Omega}}}_{1}$ relative to the exciton peak ${\mathrm{\ensuremath{\omega}}}_{1\mathit{s}}$.
We report low-temperature results of accumulated and two-pulse picosecond photon-echo experiments on the optical transition of the nitrogen-bound exciton in GaP:N. The observed temperature dependence of the dephasing of the so-called A line at 534.95 nm can be explained in terms of (stimulated) phonon processes involving the lower B state and, at higher temperatures, the free-exciton band. The low-temperature-echo decay time of 25.5\ifmmode\pm\else\textpm\fi{}0.5 ps is shown to be determined by the lifetime of a A state. Analysis of the factors governing the intensity of the accumulated photon echo as a function of temperature leads to the conclusion that in the B state temperature-independent excitation transfer occurs. The rate constant of this phenomenon, which we ascribe to excitation tunneling, is 107{10}^{7}107 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.
Nonlinear optical absorption in commercially available diamond crystals containing nitrogen-vacancy (NV) centers is investigated using femtosecond open-aperture Z-scan measurements at a wavelength of 1032 nm. Experiments were performed on a high-purity electronic-grade diamond and on samples with moderate and high NV concentrations. While the undoped crystal exhibits third-order nonlinear absorption, the NV-doped samples show a pronounced saturable absorption response. To elucidate the microscopic origin of this effect, the Z-scan results are analysed in conjunction with linear transmission spectra, which reveal the presence of additional nitrogen-related defect complexes, including H2 ( NVN−\hbox {NVN}^{-}NVN− ) centers. Although the excitation wavelength is detuned from the principal zero-phonon lines of both NV and H2 defects, it overlaps with the broader near-infrared absorption band associated with vibronic transitions. A quantitative analysis based on an effective two-level model indicates that the observed saturable absorption is most consistently explained by a dominant contribution from H2 defects, while a weaker off-resonant contribution from NV centers cannot be excluded. The extracted parameters, including the linear absorption coefficient and saturation intensity, are supported by a reconstruction of the defect-related absorbance spectrum and by consistency with previously reported nonlinear refractive index data. These results highlight the importance of complex defect environments in determining the nonlinear optical response of NV-doped diamond and provide insight into defect-specific contributions relevant for diamond-based nonlinear and quantum photonic applications.
The behaviour of an optically thick column of barium vapour resonantly 10 -2 illuminated at 553.5m by a 50 ns, 10 Wm pulse from a flashlamp pumped dye 19 21 -3 laser system was investigated for barium number densities in the range 10 -10 m in the presence of He buffer gas at 100 Torr.A variety of optical saturation effects associated with the principal resonance transition were studied.In marked contrast to earlier work, saturation of the intensity of the scattered light near 553.5 nm with increasing laser power was found for a suitable geometry of the collection optics but at a laser intensity typically two orders of magnitude higher than that predicted by simple theory, despite the fact that this theory was found to describe correctly transmission measurements along the length of the vapour column.The apparent contradiction between the fluorescence and transmission results, which indicated that the scattered radiation intensity continued to rise with incident laser power, despite the population of the upper resonance level having reached a limiting saturation value, could be understood in terms of a simple model which took into consideration the spectral distribution of the scattered light and its transfer through the unexcited vapour.However, in common with other similar experiments, saturation of the scattered radiation intensity with laser power could not be totally achieved when a more usual collection optics geometry was employed.This was interpreted as a consequence of laser intensity variation across the face of the laser beam.A number of subsidiary experiments were performed.These included spatially resolved intensity measurements of the scattered radiation and time resolved metastable level population measurements.In addition, evidence for laser induced ionization was sought using two different methods and this enabled an upper limit of 10% ionization to be placed on the experiments in the present work.
A theory of nonlinear phenomena has been developed for a photonic crystal in the presence of a pump and a coupling laser field. The crystal is doped with an ensemble of four-level nanoparticle impurities. It is considered that the impurity particles are not only interacting with the photonic crystal but also with each other via dipole-dipole interaction. An expression for the susceptibility has been obtained using the density matrix method. The nonlinear effects due to the coupling and the pump fields have been included in the formulation. The absorption spectrum has been calculated in the presence of the strong coupling and pump fields for an isotropic photonic crystal made from dielectric spheres. The photonic crystal has a gap to midgap ratio of about 21%. It is predicted that the absorption spectrum in the photonic crystal can have zero, one, two or three absorptionless states by tuning one of the transition energies within the bands. This is an interesting phenomenon which can be used to make photonic switching devices. We have also calculated the absorption spectrum in the presence of the dipole-dipole interaction. It is found that a symmetric absorption spectrum changes to an asymmetric one due to this interaction. It is also found that there is a large enhancement in the absorption and the dispersion simultaneously for certain values of the detuning and concentration.
We show that a laser beam, tuned in the transparency region of a direct-gap semiconductor, not only shifts but also splits the exciton levels. This splitting originates from the degeneracy of the valence band as the (${\ensuremath{\sigma}}_{+},{\ensuremath{\sigma}}_{\ensuremath{-}}$) parts of the light are not similarly coupled to the various exciton components. In the case of a fourfold-degenerate valence band, we find that, at large detuning, the shifts of the (2\ifmmode\times\else\texttimes\fi{}4) eightfold exciton are exactly the shifts of a (2+4) six-level atom. This is just the generalization of the simple band-structure case with one electron and one hole band in which the large detuning excitonic shift has the same value as that of the two-level (dressed) atom. We give the selection rules to observe the various shifted lines for typical pump-probe experiments. We also discuss the experimental conditions necessary to see an exciton red shift at the two-photon absorption threshold, when the molecular biexciton is stable.