High-resolution photothermal microscope: a sensitive toolfor the detection of isolated absorbing defectsin optical coatings

Bertrand Bertussi, Jean-Yves Natoli, and Mireille Commandré

The photothermal deflection technique allows us to highlight the presence of inhomogeneities of absorp-tion in optical components. This nondestructive tool is of great interest to the study of the role ofcontaminants, inclusions, and impurities in the laser-induced damage process. We show that the detec-tion of nanometer-sized isolated absorbing defects requires the development of an adapted photothermalsetup with high detectivity and high spatial resolution. Thus it is essential to improve the resolving powerup to its theoretical limit. © 2006 Optical Society of America

OCIS codes: 300.1030, 350.5340, 110.0180, 350.5730.

1. Introduction

The recent advances in optical-coatings manufactur-ing have facilitated a considerable increase in thelaser-induced damage threshold (LIDT) of opticalcomponents. However, in high-power laser applica-tions, such as micromachining, surface cleaning, ornuclear fusion,1–3 the slightest defect (crack, scratch,structural defect, contaminant, etc.) can lead to thematerial’s breakdown. Furthermore, considering howdifficult it is to eliminate all the contaminants thatcan appear, especially during polishing and thin-filmdeposition, it is important to have an accurate tool toexhibit these defects, which act as precursor centersof laser-induced damage. For the detection ofmicrometer-sized defects, optical techniques such asdark field or the Nomarski microscope are often used.But different studies4,5 have also established thatlaser-induced damage in optical components is linkedto the presence of nanometer-sized precursor centers.The origin and the nature of these defects could bevery different according to the material and to themanufacturing conditions. In the case of optical sub-strates such as fused silica, precursor centers that are

responsible for surface damage are assumed to becontaminants issued mainly from the stages of pol-ishing and cleaning.6–9 In the case of absorbing de-fects, their characteristic dimension does not exceed afew tens of nanometers.10 For optical thin-film coat-ings, absorbing particles with a submicrometer rangehave also been highlighted.11,12 The density of thesenanometer-sized defects measured by destructivetechniques13 are low at substrate surfaces as well asin thin films. If we assume a homogeneous distribu-tion, the mean distance between defects is high incomparison with the defect size, and the defects canbe considered as isolated. Thus the problem of theirdetection is one of single-particle detection.

Currently, different studies are being conducted todevelop nondestructive techniques adapted for thedetection of these nanometer-sized absorbing defectspresent in thin-film coatings or at the substrateinterface. We can cite, for example, techniques asvaried as luminescence14,15 or photothermal inves-tigations.16,17 Photothermal techniques have clearlypermitted the correlation of the presence of absorbinginhomogeneities in materials, such as in optical mul-tilayer coatings or borosilicate glasses, with the laser-induced damage process.18 These studies have shownthat with this technique it is possible to routinelydetect metallic particles of the order of a few hundrednanometers.17 In the best configurations,19 it is nowpossible to reach a level of sensitivity of the order ofa few nanometers. Nevertheless, we note that in thisconfiguration, this ultimate detectivity is reachedonly for the detection of absorbing defects present atthe surface.

The authors are with the Institut Fresnel, Université PaulCezanne Aix-Marseille III, Ecole Généraliste d’Ingénieurs deMarseille, Université de Provence Aix-Marseille I, Domaine Uni-versitaire de Saint Jérome, 13397 Marseille Cedex 20, France. B.Bertussi’s e-mail address is bertrand.bertussi@fresnel.fr.

Received 1 March 2005; accepted 28 July 2005.0003-6935/06/071410-06$15.00/0© 2006 Optical Society of America

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The purpose of our research is to develop a high-resolution collinear photothermal microscope(HRPTM) for the detection of the nanometer-sizedabsorbing precursors present at the surface as wellas in the bulk of optical materials and to study theevolution of precursors under laser irradiation.

2. High-Resolution Photothermal Microscope

A. Experimental Setup

To obtain a high level of detectivity and the capabilityto probe the surface as well as the bulk of the opticalmaterials, the photothermal microscope described inFig. 1 was developed in the collinear configuration.20

Two laser beams (pump and probe) are parallel andfocused through the same objectives. The pump laseris a continuous Nd:YAG at 1064 nm, and the probelaser is a He–Ne �633 nm wavelength). In the config-uration called “microscope,” both beams are focusedby means of microscope objectives, which allow one toobtain beam diameters from 100 to 1 �m at 1�e2. Thefocused pump beam, modulated by an optical chopper(frequency of modulation of a few kilohertz), heatsthe material on a very localized area. In fact, with amicrometer pump beam, the thermal extension islimited to the absorbing area. The modulatedrefractive-index gradient due to the optical absorp-tion induces a deflection of the probe beam at thesame frequency. Under these conditions, it is possibleto have access to the total deflection of the transmit-ted probe beam by means of a quadrant positionsensor. Because of the small deflection ��10�6 rad�,measurements on the photodiode are performed with

a lock-in amplifier. Finally, the photothermal signalis converted in terms of its optical absorption by anadapted calibration.21 A spectrophotometric mea-surement of an ion-implanted substrate yields an ac-curate value of absorption at 1064 nm. With theconfiguration used for the present experiment, a levelof absorption of 10�5 is detectable. To obtain a map-ping of the absorbing inhomogeneities, we fixed thesamples under study on a motorized translationstage. An in situ Nomarski microscope permits us toobtain an accurate image of the area we want tocharacterize.

A pulsed laser was also added to the setup�Nd:YAG, 1064 nm; pulse duration, 7 ns) to study insitu and under laser irradiation the behavior of theabsorbing defects highlighted in this way. Owing tothe use of different objectives, it is possible to haveaccess to spot sizes ranging from 8 to 50 �m. The localfluence of irradiation is deduced for each shot bybeam-shape analysis. An extended description of thissetup is provided elsewhere.22

B. Theoretical Spatial Lateral Resolution in the Case ofIsolated Defects

Experimentally, the size of the smallest-detectableabsorbing defect is given by the detectivity of thesystem. In the case of a scanning imaging system, theresolution is proportional to the distance between twoareas with a discernible contrast. Here this results inthe smallest scanning step for which the absorptionvariation ��A� between two adjacent measurementsis higher than the noise.

Fig. 1. Experimental setup: high-resolution photothermal microscope coupled with a pulsed laser.

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The absorption measured with the photothermalmicroscope is the convolution between the defect ab-sorption distribution and the pump beam shape (seeFig. 2). In the case of a Gaussian beam (diameter a� 1 �m at 1�e2), which is ten times larger than theabsorbing defect ��100 nm�, the measured absorp-tion is simply given by

Ameas�r� � A�a2

P0��r� � P�r� � A exp��

2r2

a2 �, (1)

where A is the total absorption of the considered de-fect. Experimentally, the best sensitivity is obtainedwhen the measured variation of absorption ��A� ismaximum for the minimum scanning step (R). Re-garding the Gaussian shape of the measured absorp-tion (convolution between the pump beam and theabsorption distribution), we can assume that sucha configuration is realized at 1�e2 �r � �a�2� (seeFig. 2).

Under these conditions, the experimental variationof absorption is linked to the scanning step as follows:

�A � RdAmeas�a�2�

dr , (2)

⇔ R � a�e4

�AA . (3)

Regarding Eq. (3), the experimental detection of ananometer-sized isolated absorbing defect in the op-tical material is limited mainly by parameters suchas the pump beam size and the photothermal frac-tional ratio �A��A�. The ultimate resolution of thesetup is reached when the variation of absorption��A� is equal to the noise. In our configuration (pump-beam diameter fixed at 1 �m at 1�e2) and for a signal-

to-noise ratio (SNR) of 20 �A��A�, it is theoreticallypossible to reach a lateral resolution (R) of 10 nm.With a SNR of 5, the lateral resolution falls to 40 nm.

Consequently, this result means that the use of ascanning step of the same order of magnitude as thelateral resolution ��10 nm� would theoretically per-mit the detection of isolated absorbing particles of theorder of a few tens of nanometers. Experimentally, itis possible in our new photothermal setup to obtainsuch a scanning step with piezoelectric actuators.The sample under study in the photothermal micro-scope is fixed on a three-dimensional (3D) piezo-electric translation stage (PiezoJena System). Thecharacteristics of this piezoelectric motor are summa-rized in Table 1. We note that the resolution in the Ydirection is lower than for the two other axes. Thisdifference comes from the weight of the sample,which is applied mainly on this axis.

In the X–Y direction the piezoelectric stage is usedto realize a photothermal mapping of the sample witha nanometric resolution.

Furthermore, in the Z direction, the use of a motor-ized translation stage allows us to perform automati-cally such a measurement on different subsequentplanes in the material. The measurement in the Zdirection permits, on the one hand, the realization ofan investigation of the inhomogeneities of absorptionin the three dimensions (tomography) and, on theother hand, when the defects have been localized, thismeasurement provides an accurate study of the de-fects. This second point becomes important for the de-

Fig. 2. Scheme of the absorption evolution versus the scanning step.

Fig. 3. Calculation of the ratio of the power absorbed by the goldsphere to that traversing a disk of identical radius as a function ofinclusion diameter (incident beam is a top-hat beam).

Table 1. Experimental Characteristics of the PiezoelectricTranslation Stage

Axis Range (�m) Resolution (nm)

X 80 �X � 10Y 80 �Y � 30Z 80 �Z � 10

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tection of nanometric absorbing defects. Indeed, theabsorptance of a metallic inclusion falls significantlywith the decrease in its size. Mie theory allows us tocalculate theoretically the amount of energy absorbedby a solid gold sphere inserted in silica as a function ofits size. Under these conditions, Fig. 3 represents, foran irradiation at 1064 nm and as a function of thediameter, the ratio of the energy absorbed by thesphere to the incident energy. For this calculation,the laser beam is represented by a top hat whosewidth is equal to the sphere size. For example, forgold spheres of 200 nm, the absorptance is �10% andfalls to 1% in the case of a 40 nm sphere.

This result shows clearly that for a metallic inclu-sion of the order of a few nanometers, the photother-mal signal will stand out from the noise only for theconfiguration in which the defect is localized exactlyin the waist of the pump beam. To illustrate thisproblem, we realized several photothermal mappingsof a single 100 nm gold inclusion embedded in silicalayers. Each measurement is performed with a spe-cific focusing of the pump beam (planes are 5 �mapart in the Z direction). Figure 4 confirms that, inthis case, the photothermal signal is significant onlyover a range of 10 �m in the Z direction. Conse-quently, the detection of such an absorbing particle isproved to be impossible if the focusing is performedmanually. The only solution to detect such defects is

to perform an automatic 3D high-resolution photo-thermal mapping of the component.

3. Experimental Detection of Nanometer-SizedIsolated Absorbing Defects in Optical Coatings

To specify the capability of the photothermal micro-scope to detect automatically nanometer-sized iso-lated absorbing defects in optical coatings, werealized the detection of calibrated gold particles thatare artificially embedded in silica layers (thickness,�2 �m). Statistical measurements permitted us toestimate the size of the gold inclusions present in thesample. Measurements with an atomic force micro-scope permitted access to the size of the silica domerising above the inclusion. In the first approximationthe height of this dome is equal to the particle size.Results range from 50 to 300 nm, with a Gaussiandistribution centered on 230 � 20 nm.

Figure 5 represents two photothermal mappings�30 �m 30 �m with a scanning step of 50 nm) ob-tained automatically on the same region on differentZ planes approximatively 10 �m apart. In the firstmapping it is impossible to reveal any absorbing de-fect, whereas the same mapping obtained 10 �mdeeper highlights two gold inclusions with diametersequal to 100 �SNR � 8� and 60 nm �SNR � 5�. Thismajor result permits the quantification of the detec-tivity of the HRPTM and the validation of the impor-

Fig. 4. Detection of a 100 nm gold particle by three-dimensional mapping.

Fig. 5. Photothermal mappings of the same area obtained in two different Z planes.

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tance of an automatic 3D mapping for the detection ofthe smallest inhomogeneities of absorption in opticalmaterials.

4. Application to the Study of Laser-InducedDamage Initiation

To investigate the initiation process of laser-induceddamage in optical coatings, one must follow the be-havior of the precursor centers through laser irradi-ation. Classical optical techniques (e.g., dark fieldand Nomarski microscope) are not adapted for such astudy. Indeed, the detection of nanometer-sized in-clusions embedded in thin films or substrates is prov-ing to be difficult with the aid of only a classicalmicroscope. In the case of absorbing defects such asmetallic inclusions embedded in optical coatings, theHRPTM coupled with a pulsed Nd:YAG laser is anappropriate tool for this kind of investigation. More-over, it is possible to give the absorption of a defect at

1064 nm and to follow the evolution of this valueversus the fluence of irradiation.

Figure 6 is an example of the measurement real-ized to understand the initiated laser-induced dam-age process. Indeed, we experimentally followed thein situ absorption evolution of an artificial gold par-ticle versus the fluence of irradiation. After each shot,a photothermal mapping of the defect is realized tomeasure its new absorptance accurately.

The high detectivity of the setup permits the real-ization of the same study for the smallest gold inclu-sions. In particular, we highlighted the same kind ofbehavior through laser irradiation for particles assmall as 100 nm �SNR � 8�. Figure 7 summarizesthese two results.

5. Conclusion and Perspectives

The detection of nanometer-sized absorbing defectspresent in optical coatings requires a high level ofdetectivity and a high spatial resolution. The devel-opment of a high-resolution photothermal microscope(HRPTM) permits access to a nondestructive tool thatis capable of detecting nanometer-sized absorbing de-fects responsible for the laser-induced damage. Inparticular, the collinear configuration coupled with avery focalized pump beam is necessary to investigatewith high accuracy the different planes in the bulk ofthe material. Under these conditions, it is clearlypossible to study the defects localized in the coatingor in the substrate independently. Currently, thephotothermal microscope developed at the FresnelInstitut permits the automatic detection of metallicinclusions of the order of a few tens of nanometers.The use of a 3D piezoelectric translation stage allowsus to rapidly obtain a mapping of the optical absorp-tion in the three directions. Moreover, the coupling ofthis very sensitive tool with a laser of irradiationpermits the study of the initiation stage of the laser-

Fig. 6. Evolution of a 250 nm gold inclusion under laser irradiation.

Fig. 7. Evolution of gold inclusion (250 and 100 nm) absorptionunder laser irradiation.

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induced damage process in the optical componentwith a high level of accuracy.

To increase the spatial resolution of the setup, wehave to improve the experimental conditions, i.e., thepump beam size and the SNR. Specifically, we plan touse a 355 nm pump laser instead of a 1064 nm one.This change will allow us to reduce the size of thepump beam. Consequently, we can expect to be ableto detect metallic inclusions of the order of a fewnanometers. Moreover, the capability of obtaining aphotothermal mapping in the three directions willpermit us to acquire information about the real pre-cursor centers and their evolution through laser ir-radiation.

The authors thank Comissariat à l’Energie Atom-ique and Delegation Generale pour l’Armament fortheir support.

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