The intensity, the peak wavelength and the decay time of polaron photoluminescence in congruent lithium niobate are measured versus temperature from 77 K to 290 K. The radiative relaxation shows quasi athermal behaviour (τ R ≈ 9 µs) whereas the nonradiative relaxation follows arrhenius law with activation energy of 220 meV. The crossing point between radiative and nonradiative lifetimes is about 210 K.1 Introduction Lithium niobate (LiNbO 3 , LN) is of great interest for optical applications owing to its large electro-optic and non linear optical coefficients. Several physical properties involved in device operation are sensitive to the concentration of point defects and to the chemical reduction degree of the material. In particular the defect structure of LiNbO 3 is attributed to the presence of Nb in the lithium site (so called niobium antisite). Niobium antisite defects Nb Li 5+ are able to trap an electron on an energy level below the conduction band, giving a small bound polaron Nb Li 4+ [1]. This defect plays a major role in light-induced phenomena (photoconductivity, light-induced absorption, photorefractive effect [2][3][4]. Previous studies show that excitation of congruent lithium niobate in the visible range always gives a photoluminescence (PL) band in the near infrared, which is attributed to polaron defects [5]. Recently the PL efficiency of congruent LN has been measured versus temperature under continuous wave excitation at 355 nm [6]. The main aim of the present work is to study the PL decay after short pulse excitation and to measure the relaxation time versus temperature.

Photoluminescence related to the bound polaron Nb 4+ Li is investigated as a function of temperature and incident light intensity in iron-doped lithium niobate crystals with various iron concentrations. Experiments are done under constant-wave (CW) and pulsed illumination. Its found that the decay time is always monoexponential. The radiative lifetime, the activation energy of the nonradiative lifetime and the quenching temperature are only weakly sensitive to iron concentration. On the other hand, the magnitude of the photoluminescence signal seems strongly correlated to the Fe 2+ concentration, and the superlinear regime evidenced at low CW illumination definitely confirms that polaron excitation in lithium niobate is a two-step process.

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Aim To design and demonstrate dynamic pupils, which react to light for use with ocular prostheses. Methods The realism of ocular prostheses is limited by the immobility of the pupil. Our solution is to use a liquid crystal display (LCD) in the prosthesis to vary the pupil size as a function of the ambient light. Several liquid crystal cells were fabricated and tested for survivability through the ocular prosthesis manufacturing process. The dynamic pupil is controlled by a novel and entirely autonomous, self-powered passive electronic circuit using a solar cell, matching the minimum diameter of the pupil. Results The first LCD surviving the rugged conditions of the ocular prosthesis manufacturing steps and an entirely passive circuit controlling the pupil have been demonstrated for the first time to our knowledge. A design for a complete prosthesis with a dynamic pupil has been proposed. Finally, a standard device for the mass production of ocular prostheses is presented. Conclusion We have shown that a practical solution for an autonomous self-powered dynamic pupil is possible, given the constraints of size, fabrication process, weight, cost and manufacturability on a mass scale. We envision that the LCD could be mass produced, and only the final steps for the integration of the iris matched to a patient would be necessary before assembly using standard processing steps for the production of the prosthesis. Using a clinical trial, we hope to demonstrate that the dynamic pupil will have a positive impact on the quality of life of patients.