Probing Laser Induced Space Charge Fields with Rare Earth Dopants
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Probing Laser Induced Space Charge Fields with Rare Earth Dopants Hosanna Odhner1,2, Greg Stone1,*, and Volkmar Dierolf1 1
Department of Physics, Lehigh University, Bethlehem, PA 18015 USA Department of Physics, Bryn Mawr College, 101 N. Merion Ave, Bryn Mawr, PA 19010 *Current Address: Materials Research Institute, Pennsylvania State University, University Park, PA 16802 USA
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ABSTRACT Spectral shifts of the emission lines of Erbium ions in Lithium Niobate are used to determine the build-up of intrinsic electric fields under intense light irradiation. The observed spectral shifts can be translated into internal electric fields through a calibration using applied external fields. The studies show that a substantial field can be created locally (up to 150kV/mm) with observed occasional electric breakthroughs that have a corresponding field strength of up to 35kV/mm. In addition, a modification of some Erbium incorporation sites is observed suggesting its relationship with a defect that can by photo-ionized, such as Fe2+/Fe3+. INTRODUCTION Due to its favorable properties, LiNbO3 is found in a wide range of advanced photonic and nonlinear optical devices. It is well known that in this material a large number of defects exist, in particular in the congruent composition that forms the basis for most commercially available LiNbO3 based devices. Unfortunately, these defects can serve as charge donors and traps, which result in optical damage and performance instabilities limiting the use of the material in higher power applications. In order to study the underlying build-up of space charge fields in a more detailed way, we have developed spectroscopic methods to characterize intrinsic electric fields [1-3]. In this paper, we present our results obtained using the emission spectra of erbium ions as a probe for space charge fields that are formed under visible laser irradiation. EXPERIMENTAL METHOD We measure emission spectra of erbium ions intentionally doped into congruent and LiNbO3 samples that are placed in a liquid helium cryostat and cooled to 4K. Excitation is achieved by the 488nm light of an Argon laser that is focused into the inside of the sample with a NA=0.4 microscope objective resulting in a tightly focused (100µm). They need to be taken into account whenever precision spectroscopy is being employed.
Our study also reveal that there is a characteristic difference in the local electric space charge field that appears locally (up to 150kV/mm) and a the field that builds up over larger distances within the sample and which can trigger an electric breakthrough (35-50kV/mm). This long-range influence, far away from the laser focus can also be seen by the observation that spectral shifts can still be detected more than 100µm away from the original laser spot. The change of intensity of the emission is present in all peaks suggesting that it is a result (to a large extent) of a reduction of the excitation efficiency due to the build-up of the electric field. A reduction with a longer time scale is observed as w
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