Jacopo Parravicini

Linear and nonlinear optical properties of Hafnium-doped lithium-niobate crystals

Measurements of birefringence, second-harmonic phase-matching conditions, and nonlinear coefficient d(31) are performed for a set of Hafnium-doped congruent lithium niobate (Hf:cLN) crystals as functions of dopant concentration. The data highlight that the threshold concentration, above which there is a change in the Hf incorporation mechanism, is slightly above 2mol% and that, up to this value of concentration, the efficiency of nonlinear processes is not affected by the dopant insertion. Combining these results with those already present in literature, Hf:cLN crystals appear to be very promising candidates for the development of photorefractivity-free wavelength converters working at room temperature.

Zirconium-doped lithium niobate: photorefractive and electro-optical properties as a function of dopant concentration

Measurements of refractive indices, electro-optic coefficients and photorefractivity are performed for a set of Zirconium-doped congruent lithium niobate (Zr:LN) crystals as functions of the dopant concentration in the range 0.0-3.0 mol%. The photorefractive properties are studied by measuring the green-light induced birefringence change and by direct observation of the transmitted-beam distortion. The index of refraction data show that the threshold concentration, above which there is a change in the Zr incorporation mechanism, is about 2.0 mol%, but photorefractivity results suggest that the concentration of ZrO2 required to strongly reduce the photorefractive effect is somewhat larger than the 2.0 mol% “threshold” concentration derived from index-of-refraction data. The electro-optic coefficients are little influenced by Zr-doping. All the reported results confirm that Zr:LN is a very promising candidate for the realization of efficient electro-optic and all-optical nonlinear devices working at room temperature.

Structural and optical properties of zirconium doped lithium niobate crystals

Zirconium doped lithium niobate is a promising candidate as a substrate for nonlinear optical applications, since it does not suffer from the so-called “optical damage.” In order to optimize this aspect, the proper Zr concentration has be used, hence the precise determination of the so-called “threshold concentration,” i.e., the concentration above which the photorefractive effect is markedly reduced, is of great importance. In this work, we prepared by Czochralski growth a series of Zr-doped lithium niobate crystals with various Zr content and studied them using structural (high-resolution x-ray diffraction) and optical (birefringence) measurements as a function of the dopant content in the melt. Both the approaches pointed out a marked change in the crystal characteristics for a Zr concentration between 1.5 and 2 molu2009%, a value which is identified as the threshold concentration.

All-optical technique to measure the pyroelectric coefficient in electro-optic crystals

We report an all-optical method to measure the pyroelectric coefficient p of electro-optic crystals. Through this technique, we first acquire the birefringence variation δΔn of the crystal as a function of its temperature T, both in closed and open-circuit conditions, using a Senarmont phase-compensation configuration. Then the pyroelectric field is deduced from the difference between these two measurements, so it leads to the material spontaneous polarization change, whose derivative with respect to the temperature finally gives p. This technique is applied on congruent and stoichiometric lithium niobate.

Observation of nonlinear Airy-like beam evolution in lithium niobate

We report the observation of Gaussian beam fragmentation into Airy-like waveforms during nonlinear propagation. The effect is supported by the high-intensity photovoltaic nonlinearity arising in unbiased pure congruent lithium niobate. The process is found to occur when the nonlinear response is dominated by the nonlocal effects associated with the charge-displacement process.

Probing phase transitions and stability of organic semiconductor single crystals by dielectric investigation

The characterization of organic crystalline semiconductors in terms of possible phase transitions with temperature may be very important for the general knowledge of the material but also in view of application in devices: such properties may indeed cause variations in the macroscopic behavior of the material, especially relevant at the operation temperatures of few tens of degrees. Here, phase transitions in α-quaterthiophene single crystals are detected and studied by means of dielectric investigation, a powerful tool to go deeper in this matter. After describing the relative dielectric constant and the ac conductivity of the different solid phases, found to display either an insulating or a semiconducting character, quantitative information is provided on the relative stability of the different phases the active material may transform into, by giving an estimate of their entropy content.

Photorefractivity, electro-optical coefficients and refractive indices of Zr-doped LiNbO 3 crystals

Lithium niobate (LN) is an extremely interesting material for the realization of optical devices and circuits, thanks to its good optical and electro-optical properties. At the state of the art, the use of LN crystals for nonlinear optical functions is limited, despite the very large d33 nonlinear coefficient exhibited by this crystal, because LN is severely affected by the so called “optical damage” due to the photorefractive effect [1]. When a high intensity, spatially inhomogeneous, visible light beam impinges on the LN crystal, a significant and non uniform electric field is produced in the crystal, because of the low crystal photoconductivity, and this yields (through the electro-optic effect) a non-uniform modification of the crystal refractive indices. As a consequence, the efficiency of nonlinear interactions in LN can be greatly reduced, and it becomes necessary to operate the device at high temperature (in some cases even > 200°C), in order to get rid of photorefractivity. In order to operate the nonlinear device at room temperature (or close to it), it is possible to use LN crystals doped with a small quantity of appropriate elements able to significantly increase crystal photoconductivity, like Mg, In, or Sc [1], but it is difficult to obtain high-quality crystals when LN is doped with such ions. Recently it was shown that by using tetravalent ions, like Hf or Zr, the dopant concentration required to significantly reduce the crystal photorefractivity is considerably lower than that required for bivalent and trivalent dopants, even if some uncertainty about the exact required concentration is still present due to the discrepancies between the values reported in the literature. The low dopant-concentration, in combination with the fact that the segregation coefficient of the Zr ion is reported to be very close to 1, should allow growing more easily large homogeneous high-optical-quality Zr:LN crystals.

Optical and structural properties of Zirconium doped lithium niobate crystals

We present a careful investigation of the optical properties of Zr-doped lithium-niobate crystals. We also investigate in detail the threshold concentration for Zr-doping, by means of three different optical measurement techniques.

Photovoltaic self-focusing in lithium niobate

Photorefractive media are known to support spatial solitons, i.e. beams that do not spread during propagation because self-focusing may balance natural diffraction [1]. For visible wavelengths this also occurs in lithium-niobate (LN), one of the most important crystals in modern optical technology. Previous studies have shown that when LN is not biased, beams undergo strong self-defocusing. The effect is attributed to the nature of the photovoltaic effect that drives photorefractive charge separation, and in particular to the sign of the relevant Glass coefficient. The result is that in these conditions only one class of solitons can be observed, the so-called dark solitons, consisting in dark notches in an otherwise illuminated wavefront that maintain unaltered their shape [2]. In turn, in unbiased barium-titanate an appropriate geometry can be found for which the photovoltaic nonlinearity is “opposite“ to that of LN and can lead to bright solitons, that is beams that have a bell-like transverse intensity profile centred on the beam axis [3]. Whereas LN is known to support self-focusing and bright solitons through the χ2 nonlinearity [4], or through the screening-photovoltaic nonlinearity, that is when it is biased by an external field [5,6], to date no self-focusing and bright self-trapping is known to occur through the photorefractive effect in an unbiased sample.

Nonlinear Coefficients of Hafnium doped Lithium Niobate Crystals

The study aims to measure the second-order nonlinear coefficient of LiNbO3:Hf crystals as a function of the doping concentration. Second-harmonic generation measurements are performed using picosecond pulses of an optical parametric oscillator. Results show that doping levels significantly affect the wavelength dependence of refractive indices. Crystals with dopant concentration below 3% exhibits a nonlinear efficiency comparable to congruent crystals which shows enhanced resistance to photorefractive damage.