Zebra Finches are songbirds which constitute a model for neuro-ethologists to study the neuro-mechanisms of
vocal recognition. For this purpose, in vivo and non invasive monitoring of brain activity is required during
acoustical stimulation. MRI (Magnetic Resonance Imaging) or NIRS (Near InfraRed Spectroscopy) are suitable
methods for these measurements, even though MRI is difficult to link quantitatively with neural activity and
NIRS suffers from a poor resolution. In the particular case of songbirds (whose skin is thin and quite transparent
and whose skull structure is hollow), two-photon microscopy enables a quite deep penetration in tissues and
could be an alternative. We present here preliminary studies on the feasability of two-photon microscopy in
these conditions. To do so, we chose to image hollow fibers, filled with Rhodamine B, through the skin of Zebra
finches in order to evaluate the spatial resolution we may expect in future in vivo experiments. Moreover, we used
the reflectance-mode confocal configuration to evaluate the exponential decrease of backreflected light in skin
and in skull samples. Following this procedure recently proposed by S.L. Jacques and co-workers, we planned
to determine the scattering coefficient μs and the anisotropy g of these tissues and make a comparison between
fixed and fresh skin and skull samples for future Monte Carlo simulations of the scattering in our particular
multi-layered structure.
Self-focusing is one of the dramatic phenomena that may occur during the propagation of a high power laser beam in a nonlinear material. This phenomenon leads to a degradation of the wave front and may also lead to a photoinduced damage of the material. Realistic simulations of the propagation of high power laser beams require an accurate knowledge of the nonlinear refractive index γ. In the particular case of fused silica and in the nanosecond regime, it seems that electronic mechanisms as well as electrostriction and thermal effects can lead to a significant refractive index variation. Compared to the different methods used to measure this parmeter, the Z-scan method is simple, offers a good sensitivity and may give absolute measurements if the incident beam is accurately studied. However, this method requires a very good knowledge of the incident beam and of its propagation inside a nonlinear sample. We used a split-step propagation algorithm to simlate Z-scan curves for arbitrary beam shape, sample thickness and nonlinear phase shift. According to our simulations and a rigorous analysis of the Z-scan measured signal, it appears that some abusive approximations lead to very important errors. Thus, by reducing possible errors on the interpretation of Z-scan experimental studies, we performed accurate measurements of the nonlinear refractive index of fused silica that show the significant contribution of nanosecond mechanisms.
The Z-scan method has been widely used for the estimation of the nonlinear refractive index and the nonlinear absorption coefficient of various materials which usually show quite important nonlinear behaviors. However, it still remains difficult to perform accurate measurements of small nonlinear phase shifts since the major drawbacks of the method are an important multiplicative noise and a great sensitivity to the incident beam spatial quality and to the pulses temporal profile. In order to measure accurately the nonlinear refractive index of optical glasses in the nanosecond regime we had to improve the Z-scan method sensitivity and to reduce
drastically the numerous possible errors. We have developed and optimized a Z-scan experimental setup which is well-adapted for the metrology of the nonlinear refractive index.
We present here a Z-scan based experimental setup and an adapted numerical simulation to perform absolute measurements of small nonlinear refractive indexes in the nanosecond regime, where bound electronic, as well as electrostriction and thermal effects, can occur. In order to have a reliable and stable experimental setup and a
better sensitivity, a trimmed Airy beam has been used. An accurate study of the spatio-temporal parameters of the beam allows us to take into account the real nature of the beam in the nonlinear refractive index estimation. In these conditions, measurements have been performed in different types of fused silica at 1064nm and 532nm.
A nonlinear refractive index of 5.2x10-20m2/W has been found at 1064nm and 3.5x10-20 m2/W at 532nm.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.