Jon-Paul R. Wells

Invited Review Article: Large ring lasers for rotation sensing

Over the last two decades a series of large ring laser gyroscopes have been built having an unparalleled scale factor. These upscaled devices have improved the sensitivity and stability for rotation rate measurements by six orders of magnitude when compared to previous commercial developments. This progress has made possible entirely new applications of ring laser gyroscopes in the fields of geophysics, geodesy, and seismology. Ring lasers are currently the only viable measurement technology, which is directly referenced to the instantaneous rotation axis of the Earth. The sensor technology is rapidly developing. This is evidenced by the first experimentally viable proposals to make terrestrial tests of general relativistic effects such as the frame dragging of the rotating Earth.

Ultrafast nonlinear response of AlGaAs two-dimensional photonic crystal waveguides

We have used femtosecond pump-probe spectroscopy to study the ultrafast nonlinear response of AlGaAs two-dimensional photonic crystal waveguides in the near-infrared spectral region. The modulation of the reflectivity spectra due to the refractive index change produced by photogenerated carriers was measured. We observed an instantaneous pump-induced shift in the wavelength of a photonic resonance at 882 nm with a fast decay time of ≈8 ps. The magnitude of the reflectivity change was very large at wavelengths close to the photonic resonance, with a maximum value of ΔR/R>30% at 877 nm. These results confirm the excellent potential of photonic crystal waveguides in ultrafast nonlinear switching applications.

Auger recombination in long-wavelength infrared InNxSb1−x alloys

Dilute nitrogen alloys of InSb exhibit strong band gap bowing with increasing nitrogen composition, shifting the absorption edge to longer wavelengths. The conduction band dispersion also has an enhanced nonparabolicity, which suppresses Auger recombination. We have measured Auger lifetimes in alloys with 11 and 15 μm absorption edges using a time-resolved pump-probe technique. We find the lifetimes to be longer at room temperature than equivalent band gap Hg1−yCdyTe alloys at the same quasi-Fermi level separation. The results are explained using a modified k⋅p Hamiltonian which explicitly includes interactions between the conduction band and a higher lying nitrogen-related resonant band.

Band anticrossing in dilute InNxSb1-x

Dilute nitrogen alloys of InSb exhibit extremely strong band gap bowing with nitrogen composition that has been associated with anticrossing between the localized resonant states of the nitrogen within the conduction band and the extended states of the conduction band itself. This also results in the conduction band dispersion having an enhanced nonparabolicity. We have measured the electron effective mass near the anticrossing by cyclotron resonance in InNxSb1−x alloys with absorption edge near 15 μm, using pulsed fields up to 150 T. The results directly demonstrate the band anticrossing and quantitatively confirm the increase of effective mass versus x predicted for InNxSb1−x by a tight binding calculation for low nitrogen concentration (x<0.01).

Crystal growth and optical assessment of Nd3+ :GdAl3(BO3)4 crystal

This paper reports the high-temperature top-seeded solution growth (TSSG) of large single crystal of Nd3+-doped GdAl3(BO3)4 from a K2Mo3O10-LiF flux cooled at a rate of 2 K/day and using a crystal rotation rate of 4.5 rpm. Thepolarized optical absorption and fluorescence spectra of Nd3+ : GdAl3(BO3)4 crystal have been investigated. Polarized absorption cross sections are 4.3x10-20 and 2.6x10-20 cm2 with band widths of 8.7 and 6.1 nm for σ- and π-polarization, respectively. The emission cross sections are 6.3x10-19 and 6.0x10-19 cm2 for σ- and π-polarization, respectively, at 1062 nm corresponding to the 4F3/2→4I11/2 transition. The fluorescence lifetime is 54.3 and 57 μs at 300 and 20 K, respectively.

Picosecond intersubband dynamics in p-Si/SiGe quantum-well emitter structures

We report time-resolved (ps) studies of the dynamics of intersubband transitions in p-Si/SiGe multiquantum-well structures in the far-infrared (FIR) regime, ℏω<ℏωLO, utilizing the Dutch free electron laser, (entitled FELIX—free electron laser for infrared radiation). The calculated scattering rates for optic and acoustic phonon, and alloy scattering have been included in a rate equation model of the transient FIR intersubband absorption, and show excellent agreement with our degenerate pump-probe spectroscopy measurements where, after an initial rise time determined by the resolution of our measurement, we determine a decay time of ∼10 ps. This is found to be approximately constant in the temperature range from 4 to 100 K, in good agreement with the predictions of alloy scattering in the Si0.7Ge0.3 wells.

Transient photoluminescence enhancement as a probe of the structure of impurity-trapped excitons in CaF2:Yb2+

We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF(2):Yb(2+). The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation.

Electron paramagnetic resonance and optical properties of Cr3+ doped YAl3(BO3)4

We report on the electron paramagnetic resonance (EPR) and optical absorption and fluorescence spectroscopy of YAl3(BO3)4 single crystals doped with 0.2 mol% of trivalent chromium. From EPR we determine that the Cr3+ ions reside in sites of essentially octahedral symmetry with an orthorhombic distortion. The ground state 4A 2 splitting is determined to be 2?D2 + 3E2 ? 1.05 ? 0.04 cm?1, where D and E are fine-structure parameters, and we can attribute this splitting to the combined effect of a low-symmetry distortion and spin?orbit coupling. The g-values and fine-structure parameters D and E of the ground state 4A2 are measured to be gx ? gy ? gz = 1.978 ? 0.005, |D| = 0.52 ? 0.02 cm?1 and |E| = 0.010 ? 0.005 cm?1 respectively. From 10 K optical absorption we have measured the position and crystal-field splittings of the 2E, 2T 1, 4T 2, 2T 2 and 4T 1 states with the 4T 2 and 4T 1 levels appearing as vibronically broadened bands.

Laser site selective excitation of KY3F10-doped with samarium

Laser selective excitation has identified three Sm2+ centres in KY3F10 Of these, two exhibit strong up-conversion fluorescence due to sequential absorption from the metastable D-5(0) multiplet and high lying 4f(5)5d states. The observation of Sm2+ in KY3F10 appears to provide further evidence for the presence of F- vacancies as proposed by previous workers. The expected Sm3+ C-4v symmetry centre is also observed. A crystal-held analysis of the 46 measured electronic energy levels of this centre, yields parameters consistent with those of other trivalent rare-earth ions in this material

Bridgman growth and laser excitation of LiYF4 : Sm3+

We report on the Bridgman–Stockbarger growth of LiYF4 crystals doped with trivalent samarium ions. As the Sm3+ ions substitute for Y3+ ions in the LiYF4 matrix and in the absence of significant F− anion vacancies, only the trivalent samarium ion species is present. Attempts to grow KxLi(1−x)Y(1−y)SmyF4 have not been successful due to rejection of KF at the melt–crystal interface. Optical absorption and laser selective excitation reveals the presence of a single Sm3+ ion centre of S4 symmetry.