Year: 1993

Spectroscopy and analysis of radiative and nonradiative processes in Ti 3+:Al22O3 crystals
Grinberg M, Mandelis A, Fjeldsted K, Othonos A

Phys. Rev. B, DOI: 10.1103/PhysRevB.48.5922Download
Experimental spectroscopic and time-resolved luminescence measurements of the Ti3+:Al2O3 crystalline system have been performed. The absorption and fluorescence spectra obtained have led to a reconsideration of prevailing theoretical models for the radiative relaxation process in Ti:sapphire crystals. A realistic, two-dimensional configuration-coordinate space model has been constructed to describe the Jahn-Teller distortion of the system. Consideration of the nonlinear contributions to the electron-lattice coupling potential was found to contribute little to the second luminescence emission maximum, thus indicating that the electron-phonon interaction is essentially linear in the ground state. The main effect of the nonlinearity was found to be in the value of the nonradiative activation energy. The spin-orbit coupling was also incorporated in the model and was shown to be responsible for the nonradiative transitions in Ti3+:Al2O3, while the phonon structure of the ground (2T2) state of the system results from both spin-orbit and nonlinear coupling with the lattice.

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Single-ended infrared photothermal radiometric measurement of quantum efficiency and metastable lifetime in solid-state laser materials: The case of ruby (Cr3+:Al2O3)
Mandelis A, Munidasa M, Othonos A

IEEE Journal of Quantum Electronics, , DOI: Download
A simple, self-consistent frequency-scanned photothermal radiometric (PTR) detection scheme was applied to the measurement of the metastable state de-excitation parameters of a ruby laser rod. The radiative quantum efficiency and lifetime in this material were calculated from the extrema of the photothermal phase and the amplitude slope versus the excitation laser beam modulation frequency curves, without recourse to the conventional second measurement to eliminate effects of photothermal saturation. This technique simplifies significantly the experimental methodology, guarantees uniqueness of the measured quantities, and increases the measurement range of lifetimes as compared to other photothermal-based methods. Therefore, it may prove valuable for fast industrial quality control, as well as for fundamental studies of laser materials

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Absolute nonradiative energy-conversion-efficiency spectra in Ti3+Al2O3 crystals measured by noncontact quadrature photopyroelectric spectroscopy
Mandelis A, Vanniasinkam J, Budhudu S, Othonos A, Kokta M

Phys. Rev. B, DOI: 10.1103/PhysRevB.48.6808Download
Photopyroelectric spectroscopy (PPES) was used in a noncontact experimental configuration to obtain high-resolution absolute spectra of the optical-to-thermal energy-conversion efficiency (nonradiative coefficient), ηNR(λ), of Ti:sapphire laser crystals with widely different figures of merit. Optical-absorption-coefficient β(λ) spectra were obtained from the PPES signal in the purely optical transmission mode. These were subsequently used self-consistently in a theoretical model along with PPES-signal quadrature spectra (purely thermal transmission mode) to extract ηNR(λ) spectra. These spectra were found to depend on the optical quality of the bulk and of the polished surfaces of the Ti:sapphire material. Therefore, they can be used for fundamental energy-conversion studies of the excited-state deexcitation manifold, as well as for establishing a practical monitoring criterion of crystal growth and processing quality in Ti3+:Al2O3.

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Fiber Bragg grating laser sensor
Othonos A, Alavie AT, Melle S, Karr SE, Measures RM

Optical Engineering, DOI: 10.1117/12.147704Download
An erbium-doped fiber laser utilizing a broadband mirror as the end reflector and an intracore Bragg grating as the output coupler is designed and developed. This arrangement is used as a laser sensor to improve interrogation efficiency of intracore Bragg gratings over broadband sensor interrogation methods. Wavelength tuning of the fiber laser has been achieved by varying the temperature and strain on the Bragg grating, demonstrating an improved SNR with respect to the previous techniques that use broadband interrogation of the Bragg grating sensor.

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Year: 1992

Free carrier and lattice-heating-induced changes to the reflectivity of epitaxial GeSi alloys following picosecond pulse excitation
Othonos A, Van Driel HM, Young JF, Baribeau J-

Solid State Communications, DOI: DOI: 10.1016/0038-1098(92)90361-CDownload
Transient reflectivity characteristics for epitaxial Ge1-xSix (x=0, 0.05, 0.1, 0.25) alloys have been measured at [lambda] = 0.575[mu]m, using 2ps pump and probe pulses with photogenerated carrier densities up to 1019 cm-3. For all x we are able to separately determine negative (free carrier) and positive (lattice heating) contributions to the reflectivity change; the relative contributions of the latter are found to increase with increasing x. The evolution of the free carrier part is dominated by carrier-temperature-dependent diffusion.

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Year: 1991

Correlation of hot-phonon and hot-carrier kinetics in Ge on a picosecond time scale
Othonos A, Van Driel HM, Young JF, Kelly PJ

Phys. Rev. B, DOI: 10.1103/PhysRevB.43.6682Download
We report room-temperature measurements of time-resolved reflectivity from intrinsic crystalline Ge induced and probed by λ=0.575 μm, 2-ps pulses and for photogenerated carrier densities up to ∼1019 cm-3. These data are used together with our earlier picosecond-time-resolved Raman-scattering data at 77 and 300 K to self-consistently determine transient nonequilibrium carrier and phonon effects. The combined data sets are modeled with a Boltzmann transport description of coupled electron and phonon kinetics to understand the temporal and spatial evolution of carrier density and temperature and the generation and decay of Raman-active phonons. Using known material parameters in the model, we can account for the magnitude and temporal evolution of the electron, hole, and phonon thermodynamic parameters. In particular, the model accounts for the relative delay between the peak of the pulse and the nonequilibrium phonon population, the number of nonequilibrium phonons generated, and the initially rapid diffusion of hot carriers away from the sample surface on a 5-ps time scale.

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Non-Equilibrium Phonon Dynamics in Ge and GeSi Alloys
Young JG, Lockwood DJ, Baribeau JM, Kelly PJ, Othonos A, Driel HM

Light Scattering in Semiconductor Structures and Superlattices, DOI: Download
The ability of the Raman scattering technique to coherently probe specific excitation modes in a semiconductor,providing both spectral and thermodynamics information, makes it a powerful tool for time-resolving relaxation processes.

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Year: 1989

Fluorescence studies of multiple-photon ionization processes: Four- and five-photon ionization of Sr at wavelengths of 558--590 nm
Haugen HK, Othonos AS

Phys. Rev. A, DOI: 10.1103/PhysRevA.39.3392Download
An ‘‘all-optical’’ technique is presented for the study of multiphoton-ionization processes, based on fluorescence detection of the ions which are produced. A vapor is utilized in a moderate-density regime (∼1011–1015 cm-3) such that collisional effects are negligible. The technique is applied to multiple-photon ionization of Sr in the wavelength range of 558–590 nm at laser intensities ∼1–50 GW/cm2. Fine-structure-resolved emission of the Sr+ (5p2P3/2/,1/d0) ions which are formed is observed at λ=407.8 and 421.5 nm, following either four- or five-photon absorption. No emission is observed from higher ionic states, with a detection sensitivity ∼1:103 with respect to the ionic-resonance emission

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Hot-carrier dynamics in Ge on single picosecond timescales: Comparing Raman and reflectivity experiments with a self-consistent kinetic model
Othonos A, Van Driel H, Young JF, Kelly PJ

Solid-State Electronics, DOI: DOI: 10.1016/0038-1101(89)90276-1Download
Through comparisons of picosecond Raman and transient reflection experiments with a comprehensive kinetic model of photo-excited carrier and lattice dynamics in Ge, we demonstrate the ability of these techniques to probe subtle aspects of non-equilibrium carrier dynamics in group IV materials at moderate injected carrier densities. Using material parameters taken from the literature, the temporal evolution of the non-equilibrium optical phonon population generated by the relaxation of photo-excited electrons and holes is obtained by solving a coupled set of Boltzmann equations for the electron and hole particle and thermal currents. The results of the calculation agree, in absolute terms, with the experimentally observed evolution of the non-equilibrium optical phonon population. The calculation also predicts that the hot plasma initially diffuses rapidly away from the sample surface, on a 5 picosecond timescale, and subsequently diffuses much slower as the carrier temperature decays to the lattice temperature, and the density gradient diminishes due to the hot carriers which have already migrated into the material. This prediction is verified by comparison of the calculated change in reflectivity due to the plasma, and picosecond reflectivity measurements performed at room temperature with 575 nm pulses.

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Year: 1988

Fluorescence studies of multiphoton ionization of Sr: Production of excited ionic states
Haugen HK, Othonos A

Phys. Rev. A, DOI: 10.1103/PhysRevA.38.6448Download
fine-structure sublevels in the singly charged ion are fully resolved and are found to be nonstatistically populated. Four- and five- (or more) photon processes are observed to populate Sr+(5p2P) in the intensity regime of I∼1-50 GW/cm2, but no emission is detected from higher-lying states of Sr+.

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