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An integrated setup allowing high resolution photothermal microscopy and laser damage measurements at the same wavelength has been implemented. The microscope is based on photothermal deflection of a transmitted probe beam : the probe beam (633 nm wavelength) and the CW pump beam (1.06 µm wavelength) are collinear and focused through the same objective. In-situ laser irradiation tests are performed thanks to a pulsed beam (1.06 µm wavelength and 6 nanosecond pulse). We describe this new facility and show that it is well adapted to the detection of sub-micronic absorbing defects, that, once located, can be precisely aimed and irradiated. Photothermal mappings are performed before and after shot, on metallic inclusions in dielectric. Results obtained on gold inclusions of about 600 nm in diameter embedded in silica are presented.
©2003 Optical Society of America
Laser-induced damage in optical materials has long been widely acknowledged as a localized phenomenon associated with the presence of micrometer and nanometer-sized defects, such as scratches, polishing residues and, more generally, impurities, contaminants or bulk inhomogeneities [1–5]. All these defects can be responsible for local variations of optical, thermal or thermo-optical properties. Absorbing defects are suspected to induce thermal effects that lead to damage, but, as many attempts have shown, the correlation between defect absorption and laser damage is not easy to demonstrate.
Photothermal deflection techniques are directly related to optical absorption. They are widely used to map lowly absorbing defects and thermal inhomogeneities with high spatial resolution [6–12]. Many works have been performed to study the correlation between absorption and laser damage thanks to photothermal techniques [13–17], but no clear conclusion could be established. In fact, lots of parameters such as irradiation wavelength, defect absorption wavelength, multilayer stack nature in case of dielectric multilayers, play an important role in damage processes. Moreover, absorption measurements and damage threshold measurements are usually performed on different experimental setups and even in different laboratories, which involves various drawbacks: a precise positioning of samples on the measurement setups is hard to obtain, the use of different lasers complicates the interpretation of results, the contamination of the surface sample during the change of location is possible. So, the behavior of local absorbing defects during laser irradiation has never been clearly experimentally analyzed.
To solve these problems, a new facility has been implemented, allowing in-situ, high resolved investigation of laser damage. It is constituted of an integrated setup of photothermal microscopy and laser damage measurements. The scheme of the setup is shown on Fig. 1.
Photothermal deflection technique is based on measurement of probe beam deflection, due to local heating by a modulated incident pump beam. Part of the energy of the pump beam is absorbed by the sample and transformed in a temporal variation of temperature. This variation of temperature results in both a surface bumping and a refractive index gradient able to deflect the probe beam. The deflection of the probe beam is directly related to the absorbed optical power in the sample. Then, a measurement of this deflection, at modulation frequency, followed by a calibration procedure, permits to precisely determine the absorption of the sample. The calibration procedure is performed by comparing the deflection caused by the absorption of the sample to be measured with that of a sample of well-known absorption [6]. The resolution of the system is related to the size of the pump beam and to the sampling step: one way to magnify the resolution is to focus the pump beam on the surface sample and to reduce the sampling step.
We implemented a photothermal deflection microscope in the collinear configuration: the pump and probe beams are parallel and focused through the same optics. The pump laser is a YAG laser with 1.064 µm wavelength and the probe laser a He-Ne laser. The details of the experimental realization and characterization of the photothermal microscope have already been described [11]. In order to maximize the sensitivity of the setup, we optimized the position and the focus length of the lens which focuses the probe beam on the position sensor (lens L). We showed that with a pump beam diameter of 1 µm, obtained thanks to a microscope objective, a resolution of 1 µm was reached on non-isolated defects.
To link photothermal microscopy studies with laser damage measurements, we combine this photothermal microscope with a laser damage test facility. The high dependency of defect absorption on wavelength has already been shown [10]. The use of the same wavelength for photothermal microscopy measurements and damage threshold measurements is essential to avoid misinterpretation of the results. The laser used for the irradiation procedures is a YAG laser with 1.064 µm wavelength and 6 ns pulse duration. Energy of the incident beam is measured by a calorimeter and a pyroelectric detector. Beam profile of the focused beam is analyzed with an optical system associated to a CCD camera. The laser beam is focused on the sample surface thanks to an adapted objective. The irradiation beam diameter is 12 µm.
The sample is visualized by an imaging system, which permits a real time observation of the irradiated zone. This system is composed of a CCD camera and a set of long working distance objectives. The resolution obtained with the higher magnification is about 1 µm. The sample observation is performed in dark field mode. “Damage” is defined as having occurred when a visible modification is detected.
To demonstrate the practicability of this new setup for the detection of sub-micronic absorbing defects, that, once located, can be precisely aimed and irradiated, we use special samples prepared by CEA/LETI. They are constituted of gold inclusions, covered by an evaporated ultra-pure silica layer. The structure of the samples is described on Fig. 2.
The thickness of the upper layer is 2 µm and the size of the gold inclusions, deduced from the height of domes that cover them and measured by AFM ranges from 400 nm to 800 nm. An example of sample topography by AFM measurement is presented on figure 3.
The low damage threshold of the silica film, measured avoiding gold inclusions, and using 1 on 1 procedure, is evaluated to T=52 J/cm2.
The samples are studied by photothermal microscopy, before and after shot at various fluences, below T. The results obtained on a 600 nm gold inclusion are presented on Fig. 4, and the corresponding photothermal values are summed up in Table 1.
Fig. 4. Photothermal (top) and refraction (bottom) measurement of the same gold inclusion before shot (a.), after shot at 2 J/cm2 (b.) and after shot at 10 J/cm2 (c.), mapped area: 20 µm×20 µm.
Two kinds of mappings are presented: on one hand photothermal mappings representing absorption and on the other hand, “refraction” mappings representing surface profile variations. These mappings are acquired simultaneously to photothermal mappings and the signal is the probe deflection but measured at frequency 0. This signal is proportional to the refraction angle
Table 1. Photothermal values corresponding to Fig. 4, in arbitrary units.
of the probe beam on the sample surface and then is related to the derivative of the surface profile. Refraction mappings are similar to AFM measurements (see Fig. 3): they are a good indicator of the sample surface state and can also be used to detect damage. Photothermal measurements presented here are not calibrated and represent relative variations of absorption. As a reference, the mean value of a noise mapping is 1.8×10-3 A.U.. This noise mapping is obtained with a cut off pump beam, in the conditions of experience (1/e2 pump beam diameter=1 µm, sampling step=1 µm, mapping area=20 µm×20 µm).
The results obtained before shot are presented on figure 4a. We can perfectly detect the gold inclusion and the corresponding dome of the silica layer on the photothermal and refraction mappings. These results show that gold inclusions are highly absorbing: the ratio between maximum signal and noise level is about 103. Thus, the possibility of the melting of gold during photothermal measurement, by interaction between the pump beam and the inclusion is not negligible. To check that gold is not melted during photothermal measurement, we perform two successive photothermal mappings of the same gold inclusion. To quantify the likeness of the 2 mappings, we calculate their correlation coefficient [10]. It is equal to 97.6%, which corresponds to a very good repeatability and shows that no modification of the gold inclusion occurred during photothermal measurement.
A shot with a fluence of 2 J/cm2 is performed on this gold inclusion. Given the aiming error of the irradiation laser, the absolute value of fluence is determined with a 10% error. The results are presented on Fig. 4(b). The comparison with fig. 4(a) shows that we are well positioned since the gold inclusion and the corresponding dome are still visible. However, absorption has partially decreased, though the refraction mapping is not significantly modified, showing no damage of the sample surface. Then, a shot with a fluence of 10 J/cm2 is performed. As shown on figure 4c, absorption is no more detectable, and the structure of the dome has changed, revealing that a surface damage has occurred. The low value of fluence leading to damage on gold inclusion (10 J/cm2 instead of 52 J/cm2 on pure silica), shows that these inclusions behave as laser damage precursors.
In conclusion, we showed that this new facility, constituted of an integrated photothermal microscopy and laser damage setup, is well adapted to the detection of sub-micronic absorbing defects and to the study of their behavior under laser irradiation. Photothermal mappings, performed before and after shot on gold inclusions, permit to study the evolution of localized absorption under irradiation before surface damage appearance. Refraction mappings, performed simultaneously, give information about the surface profile. It is also possible to precisely associate laser damage with the absorption level of gold inclusions. Then, systematic work, taking into account the size and nature of such inclusions will permit to established the correlation between laser damage threshold and defect absorption. Moreover, these inclusions behaving as precursor of laser damage, this new setup appears of great interest for the study of damage initiation mechanisms. Study of smaller inclusions will be possible and further work on inclusions of different size and nature at different wavelength (UV for example) seems really promising.
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