Intrinsic Point Defects (Vacancies and Antisites) in CdGeP\u3csub\u3e2\u3c/sub\u3e Crystals

Cadmium germanium diphosphide (CdGeP2) crystals, with versatile terahertz-generating properties, belong to the chalcopyrite family of nonlinear optical materials. Other widely investigated members of this family are ZnGeP2 and CdSiP2. The room-temperature absorption edge of CdGeP2 is near 1.72 eV (720 nm). Cadmium vacancies, phosphorous vacancies, and germanium-on-cadmium antisites are present in as-grown CdGeP2 crystals. These unintentional intrinsic point defects are best studied below room temperature with electron paramagnetic resonance (EPR) and optical absorption. Prior to exposure to light, the defects are in charge states that have no unpaired spins. Illuminating a CdGeP2 crystal with 700 or 850 nm light while being held below 120 K produces singly ionized acceptors (VCd−) and singly ionized donors (GeCd+), as electrons move from VCd2− vacancies to GeCd2+ antisites. These defects become thermally unstable and return to their doubly ionized charge states in the 150–190 K range. In contrast, neutral phosphorous vacancies (VP0) are only produced with near-band-edge light when the crystal is held near or below 18 K. The VP0 donors are unstable at these lower temperatures and return to the singly ionized VP+ charge state when the light is removed. Spin-Hamiltonian parameters for the VCd− acceptors and VP0 donors are extracted from the angular dependence of their EPR spectra. Exposure at low-temperature to near-band-edge light also introduces broad optical absorption bands peaking near 756 and 1050 nm. A consistent picture of intrinsic defects in II-IV-P2 chalcopyrites emerges when the present CdGeP2 results are combined with earlier results from ZnGeP2, ZnSiP2, and CdSiP2

Degenerate Frequency Two Beam Coupling in Organic Media Via Phase Modulation

The following research was born out of the observation of a counter-propagating beam (CPB) originating from the interaction of a pump beam and a Fresnel reflection inside an organic liquid held inside a fused silica cuvette. The strong influence on overlap with the Fresnel reflection is evidence for degenerate frequency two-beam coupling (TBC) via a transient phase grating. It is well known that TBC requires a phase shift between the fields and the induced grating which is supplied by the finite temporal response of the nonlinearity. TBC also requires a frequency shift between the interacting fields. For degenerate frequencies to couple, this shift must arise from self or cross phase modulation in the medium. In this work, we present a strong, degenerate frequency TBC in two organic systems using a single nanosecond beam where the probe is generated from the Fresnel reflection of the cuvette and the necessary phase and frequency shifts are the result of the thermo-optic effect and population redistribution. Its effect on the analysis of nonlinear transmission experiments and its relationship with Stimulated Rayleigh Bragg Scattering (SRBS) is also presented

Dynamic Holography in Resonant Nonlinear Media: Theory and Application

Two beam coupling (TBC) is a coherent interaction in which energy is transferred from one laser beam to another and has promising applications in real-time holography and coherent beam combing. We have recently shown efficient degenerate frequency TBC for counter-propagation geometries in isotropic two-photon absorbing media pumped with a nanosecond pulsed laser. When an interference pattern is generated in this media, single and two photon absorption initiates a population redistribution resulting in a holographic grating with the same modulation period and phase initially. However, due to temporal convolution of self- and cross-phase modulation, the grating will begin to shift in time relative to the interference pattern thus allowing coherent energy transfer to evolve. A comprehensive theoretical and numerical model is presented consistent with empirical results and historical observations of both energy and phase coupling. Numerical simulations indicate the presence self-oscillation due to nonlinear phase wrapping and strong excited state absorption inhibit energy transfer in a co-propagating geometry. However with proper temporal phase conditioning and choice of medium thickness, significant energy transfer can be achieved in the co-propagating case

Impact of the Liquid Crystal Director Twisting on Two-Beam Energy Exchange in a Hybrid Photorefractive Inorganic-Liquid Crystal Cell

We studied the energy transfer between light beams on the director grating in a hybrid photorefractive liquid crystal (LC) cell assuming the propagation of light waves in the cell to be in the Mauguin regime. This approach makes it possible to trace the change of the gain coefficient dependence on the director grating spacing with the change of the LC director twist. Conditions for the LC flexoelectric parameters and the director helix pitch necessary for transformation the gain coefficient dependence from the nematic to cholesteric type are obtained. The influence of the director splay and bend deformations on the gain coefficient is also studied

Degenerate Frequency Two-Beam Coupling in Organic Media via Phase Modulation with Nanosecond Pulses

This work presents a theoretical treatment using population redistribution and the thermo-optic effect to mediate degenerate frequency two-beam coupling (TBC) in the nanosecond regime in third-order nonlinear organic solutions. We show experimentally that the energy transfer is indeed a result of TBC and can be modeled using self- and cross-phase modulation to produce the required frequency shift. As a result of the relatively long lifetimes and large phase shifts induced by population redistribution and thermo-optic effects, the coupling efficiency can be significant. For the special case when a single input beam is aligned to overlap with the Fresnel reflection of the sample/air interface, coupling efficiencies can easily exceed 50% of the incident pump energy, which can account for a severe deleterious effect in nonlinear transmission experimentation

Charge Trapping by Iodine Ions in Photorefractive Sn\u3csub\u3e2\u3c/sub\u3eP\u3csub\u3e2\u3c/sub\u3eS\u3csub\u3e6\u3c/sub\u3e Crystals

Electron paramagnetic resonance (EPR) is used to establish the role of iodine as an electron trap in tin hypothiodiphosphate (Sn2P2S6) crystals. Iodine ions are unintentionally incorporated when the crystals are grown by the chemical-vapor-transport method with SnI4 as the transport agent. The Sn2P2S6 crystals consist of Sn2+ ions and (P2S6)4− anionic groups. During growth, an iodine ion replaces a phosphorus in a few of the anionic groups, thus forming (IPS6)4− molecular ions. Following an exposure at low temperature to 633 nm laser light, these (IPS6)4− ions trap an electron and convert to EPR-active (IPS6)5− groups with S = 1/2. A concentration near 1.1 × 1017 cm−3 is produced. The EPR spectrum from the (IPS6)5− ions has well-resolved structure resulting from large hyperfine interactions with the 127I and 31P nuclei. Analysis of the angular dependence of the spectrum gives principal values of 1.9795, 2.0123, and 2.0581 for the g matrix, 232 MHz, 263 MHz, and 663 MHz for the 127I hyperfine matrix, and 1507 MHz, 1803 MHz, and 1997 MHz for the 31P hyperfine matrix. Results from quantum-chemistry modeling (unrestricted Hartree–Fock/second-order Møller–Plesset perturbation theory) support the (IPS6)5− assignment for the EPR spectrum. The transient two-beam coupling gain can be improved in these photorefractive Sn2P2S6 crystals by better controlling the point defects that trap charge

Photoinduced Trapping of Charge at Sulfur Vacancies and Copper Ions in Photorefractive Sn\u3csub\u3e2\u3c/sub\u3eP\u3csub\u3e2\u3c/sub\u3eS\u3csub\u3e6\u3c/sub\u3e Crystals

Electron paramagnetic resonance (EPR) is used to monitor photoinduced changes in the charge states of sulfur vacancies and Cu ions in tin hypothiodiphosphate. A Sn2P2S6 crystal containing Cu+ (3d10) ions at Sn2+ sites was grown by the chemical vapor transport method. Doubly ionized sulfur vacancies (V2+S) are also present in the as-grown crystal (where they serve as charge compensators for the Cu+ ions). For temperatures below 70 K, exposure to 532 or 633 nm laser light produces stable Cu2+ (3d9) ions, as electrons move from Cu+ ions to sulfur vacancies. A g matrix and a 63,65Cu hyperfine matrix are obtained from the angular dependence of the Cu2+ EPR spectrum. Paramagnetic singly ionized (V+S) and nonparamagnetic neutral (V0S) charge states of the sulfur vacancies, with one and two trapped electrons, respectively, are formed during the illumination. Above 70 K, the neutral vacancies (V0S) are thermally unstable and convert to V+S vacancies by releasing an electron to the conduction band. These released electrons move back to Cu2+ ions and restore Cu+ ions. Analysis of isothermal decay curves acquired by monitoring the intensity of the Cu2+ EPR spectrum between 74 and 82 K, after removing the light, gives an activation energy of 194 meV for the release of an electron from a V0S vacancy. Warming above 120 K destroys the V+S vacancies and the remaining Cu2+ ions. The photoinduced EPR spectrum from a small concentration of unintentionally present Ni+ ions at Sn2+ sites is observed near 40 K in the Sn2P2S6 crystal

Spectroscopic Structure–Property Relationships of a Series of Polyaromatic Platinum Acetylides

To develop a structure–spectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon) ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings (nFAR) and ligand topology (polyacene (<b>L</b>), polyphenanthrene (<b>Z</b>), or compact(<b>C</b>)). We measured ground-state absorption, fluorescence, and phosphorescence spectra. We also performed nanosecond and femtosecond transient absorption experiments. To extend the range of compounds in the structure–property relationship, we did DFT calculations on an expanded series of chromophores. Both the DFT results and experiments show that the S₁ and T₁ state energies are a function of both nFAR and the ligand topology. In the <b>L</b> chromophores, the S₁ and T₁ state energies decrease linearly with nFAR. In contrast, the S₁ and T₁ state energies of the <b>Z</b> chromophores oscillate around a fixed value with increasing nFAR. The <b>C</b> chromophores have behavior intermediate between the <b>L</b> and <b>Z</b> chromophores. A parallel series of calculations on the ligands shows the same behavior. The S₁–S_n energy obtained from ultrafast time-resolved spectra has a linear variation in nFAR. The rate constant for nonradiative decay, <i>k</i>_nr, was calculated from the S₁ state lifetime and decreases with an increasing number of π electrons in the aromatic ring. The result is consistent with the spin–orbit coupling caused by the central platinum heavy atom decreasing with larger nFAR. The present work shows that the framework developed for the analysis of poly(aromatic hydrocarbon) properties is useful for the understanding of the corresponding platinum acetylide complexes