Hassan Akhouayri

Laser induced damage investigation at 1064 nm in KTiOPO4 crystals and its analogy with RbTiOPO4

Bulk laser-induced damage at 1064 nm has been investigated in KTiOPO4 (KTP) and RbTiOPO4 (RTP) crystals with a nanosecond pulsed Nd:YAG laser. Both crystals belong to the same family. Throughout this study, their comparison shows a very similar laser-damage behavior. The evolution of the damage resistance under a high number of shots per site (10,000 shots) reveals a fatigue effect of KTP and RTP crystals. In addition, S-on-1 damage probability curves have been measured in both crystals for all combinations of polarization and propagation direction aligned with the principal axes of the crystals. The results show an influence of the polarization on the laser-induced damage threshold (LIDT), with a significantly higher threshold along the z axis, whereas no effect of the propagation direction has been observed. This LIDT anisotropy is discussed with regard to the crystallographic structure.

Grating Enhanced Second Harmonic Generation Through Electromagnetic Resonances

This paper is concerned with surface enhanced second harmonic generation through delocalized electromagnetic resonances such as surface plasmons or guided waves at bare or coated metallic gratings. The formalisms presented are rigorous in the sense that the groove depth of the grating is not considered as a perturbative parameter. This allows us to show that there exists an optimum groove depth of the grating for which the enhancement of the second harmonic efficiency, as compared to the flat case, is the greatest. Shallow modulated gratings (10 -1) lead to optimum enhancement as high as 105. Emphasis is placed on the effect of the periodicity and of the profile of the grating on this optimum enhancement.

Multipulse laser damage in potassium titanyl phosphate: statistical interpretation of measurements and the damage initiation mechanism

Abstract. Multipulse laser-induced damage is an important topic for many applications of nonlinear crystals. We studied multipulse damage in X-cut KTiOPO4. Using a 6-ns Nd:YAG laser with a weakly focused beam, a fatigue phenomenon was observed. We addressed whether this phenomenon necessarily implies material modifications. Two possible models were tested, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant. The first model, pulse energy fluctuations and depointing, increases the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number; however, this effect is too small to explain the observed fatigue. The second model assumes a constant single-shot damage probability p1, so a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement was found between the 2000-on-1 volume damage data and this statistical multipulse model. Additionally, the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are transient, the presented data explain the experimental results without supposing material modifications.

Catastrophic nanosecond laser induced damage in the bulk of potassium titanyl phosphate crystals

Due to its high effective nonlinearity and the possibility to produce periodically poled crystals, potassium titanyl phosphate (KTiOPO4, KTP) is still one of the economically important nonlinear optical materials. In this overview article, we present a large study on catastrophic nanosecond laser induced damage in this material and the very similar RbTiOPO4 (RTP). Several different systematic studies are included: multiple pulse laser damage, multi-wavelength laser damage in KTP, damage resistance anisotropy, and variations of the laser damage thresholds for RTP crystals of different qualities. All measurements were carried out in comparable experimental conditions using a 1064 nm Q-switched laser and some were repeated at 532 nm. After summarizing the experimental results, we detail the proposed model for laser damage in this material and discuss the experimental results in this context. According to the model, nanosecond laser damage is caused by light-induced generation of transient laser-damage precur...

Statistical interpretation of S-on-1 data and the damage initiation mechanism

Multipulse laser induced damage optical materials is an important topic for many applications of nonlinear crystals. We studied multi pulse damage in X-cut KTiOPO4. A 6ns Nd:YAG laser has been used with a weakly focused beam. A fatigue phenomenon has been observed and we try to clarify the question whether or not this phenomenon necessarily implies material modifications. Two possible models have been checked, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant: (i) Pulse energy fluctuations and depointing increase the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number. However, this effect turned out to be too small to explain the observed fatigue. (ii) Assuming a constant single shot damage probability p1 a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement has been found between the 2000-on-1 volume damage data and the statistical multipulse model. Additionaly the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are either transient or, if they are permanent, irradiation of the precursor above its threshold only causes damage with a small probability, the presented data can be interpreted without supposing material modifications.

Perturbation theory for optical bistability of prism and grating couplers and comparison with rigorous method

A perturbation method for studying the bistable behavior of a prism or grating coupler filled with a Kerr nonlinear medium is developed. A field representation limited to its first space harmonic already gives a satisfactory agreement with the much more complicated rigorous differential method. A better accuracy can be obtained by the use of the two first space harmonics. We then obtain an almost perfect agreement with rigorous theory, always with much lower computation times. All the relevant parameters useful to characterize the bistable behavior are given by this perturbation method.

Nonlinear optical thin films for super-resolved direct laser writing

The diffraction limit is one of the main difficulties in order to achieve nano-resolution. An evolution in this domain can allow a significant advance in the field of photonic circuit fabrication. Surpassing the diffraction limit can be achieved by employing significant optical nonlinearities (nonlinear refraction and/or nonlinear absorption). In this direction we investigate the nonlinear absorption efficiency of annealed Sb2Te3 chalcogenide thin films (20 nm thickness). The studies have been performed using the Z-scan technique, employing 11 ns duration pulses at 1064 nm. The Z-scan technique has been chosen as it allows the simultaneous determination of the nonlinear refraction and the nonlinear absorption of a material, under certain conditions. Our results indicate a significant nonlinear absorption, while the nonlinear refraction is at least one order of magnitude lower. The nonlinear absorption parameter has been found to be in the order of -10-3 m/W under infrared irradiation showing the importance of the prepared materials for laser photoinscription applications. Our values are compared with previous studies, which have been carried out in the visible part of the spectrum. The possibility to perform photonic component fabrication by local modification of the linear or the nonlinear properties of the materials is also discussed.

Taking advantage of acoustic inhomogeneities in photoacoustic measurements

Abstract. This paper proposes a method for improving the localization and the quantification of the optical parameters in photoacoustic (PA) tomography of biological tissues that are intrinsically heterogeneous in both optical and acoustic properties. It is based on the exploitation of both the PA signal, generated by the heterogeneous optical structures, and the secondary acoustic echoes due to the interaction between a primary PA wave generated near the tissue surface and the heterogeneous acoustic structures. These secondary echoes can also be collected through proper measurements of the PA signals. The experimental procedure is presented along with the method to filter the signal and the reconstruction algorithm that includes the account of the acoustic information.

Optical Nonlinearities of Chalcogenide Thin Films for Super-Resolution Applications

Thin films exhibiting high nonlinear optical responses can be promising candidates to achieve super-resolution. Chalcogenide based layers are studied using the Z-scan technique employing 13.5 ns, 1064 nm laser pulses.

About Propagation Modes In Nonlinear Dielectric Waveguides

A numerical study of the propagation in Kerr nonlinear prism and grating couplers is presented based on singularity expansions for the field coefficients. The numerical results show that the change of location of pole and zeros in the complex plane explain the bistable behaviour of the prism-coupler and that an analytic description of the hysteresis loops can be obtained. It is also found that the grating coupler is a more complex case but that the relative position of pole and zeros does not change when the guide index is changed, allowing to define a scalar dynamic detuning parameter.