1. Introduction

The presence of structural and electronic defects can be of critical importance in determining the performance and characteristics of solid state materials of technological importance such as laser components and semiconductors. Optical techniques are particularly suitable for the acquisition of high resolution images of bulk defects in transparent materials. Ogawa et al. first proposed a light scattering tomography technique which is suitable for three-dimensional imaging and detection of structural defects located in bulk [1]. This technique was first utilized in transparent materials [1,2] and it was later expanded using infrared light for application in semiconductor materials [3,4]. Optical coherence tomography (OCT) is an emerging technique for performing high resolution cross sectional imaging in materials and biological tissues [5]. OCT constructs two dimensional, cross sectional images by measuring the echo time delay and magnitude of the backreflected or backscattered light. OCT has been extensively applied in biomedical applications [6] as well as for imaging materials such as polymer composites [7] or waveguides [8]. Image resolutions of 1–2 μm have been achieved. Because OCT can achieve extremely high detection sensitivities, it enables imaging of very small changes in materials associated with near threshold laser damage. More recently, fluorescence microscopy has been employed to detect and image electronic defect formations located in the bulk of crystalline systems [9,11]. Other techniques such as electron and near field scanning microscopy offer higher spatial resolution, but they are suitable only for the detection of defects at or near the surface of the material.

The motivation of this work arises from the need to detect and characterize laser induced defects in optical components for large aperture laser systems such as the National Ignition Facility [12] in the US and the Laser MegaJoule in France which are being built to achieve fusion ignition in the laboratory. The size of these optical components, which is of the order of 50-cm, represents a challenge in the manufacturing process which must persevere the high performance characteristics throughout the element. It is almost unavoidable that a number of imperfections or impurities from the manufacturing process [13] or operational [14,15] conditions will be present. These defects can lead to laser induced damage, which results in losses in the transmission and additional damage to optics downstream due to beam modulation. Although localized damage in optical elements for large aperture laser system may be acceptable as long as the beam characteristics remain within specifications, a secondary process appears at fluences higher than ≈ 5 J/cm² for 355-nm, 3-ns laser irradiation whereby small surface damage sites start growing in size [16]. This effect is observed in both, DKDP nonlinear crystals and SiO₂ optical elements. As a result, the beam obscuration level exceeds specifications within a small number of laser pulses making the optical component unusable. This problem can be addressed by avoiding damage initiation or by applying a damage mitigation process to preclude further growth at damage sites.

The laser-induced damage process is believed to be associated with localized formation of plasma, heating of the material leading to melting and, transient stresses that instigate mechanical damage. These extreme conditions give rise to material modifications where the large band gap of the material is compromised by a large population of defects. A damage site on the surface of an optical material generated using nanosecond laser sources usually appears as a crater with rough surfaces that strongly scatter the incoming laser beam. Most often, cracks originating at the bottom of the damage crater are visible. It has been recently shown that damage growth is due to re-ignition of the damage process due to absorption of the laser light by the defect population formed at damage sites as well as a result to field intensification arising from cracks [17, 18]. To extend the lifetime of optical components and reduce the cost of operation, damage sites may be treated with some kind of post damage mitigation process to eliminate the possibility of damage growth on subsequent exposure to laser irradiation.

The objective of this work was to devise techniques that can provide high resolution images of material modification including defect populations and subsurface cracks. To address these issues, fluorescence microscopy and optical coherence tomography (OCT) are explored. These techniques are suitable for application in large size optics because they can be realized to image the bulk through the surface of the optic.

2. Experimental arrangement

2” diameter fused silica substrates were used in the study. The surfaces of the samples were polished at commercial vendors using processes optimized for high fluence laser applications. Laser damage sites were created using the 355-nm beam from the third harmonic of an Nd-YAG laser. The pulse length is 3-ns and the beam profile is near-Guassian with a 1/e² diameter of 600 μm at the sample plane. Damage sites were created using ≈23 J/cm² pulses. At this fluence, small in size (10–50 μm diameter) damage pits are formed on the surface. These sites were grown to better expose the features of interest using additional irradiation with 10 pulses of 355-nm, 3-ns at fluences of ≈ 10 J/cm².

Localized emission spectra from damage sites under 351-nm excitation were recorded using a single grating spectrograph equipped with a liquid nitrogen cooled CCD detector. The laser beam is focused in the sample using a X25 reflecting microscope objective. The same lens is used to collect the emitted light which is focused into the slit of the spectrograph after passing through a pinhole using a confocal experimental scheme. The lens/pinhole combination provides lateral spatial resolution of ~5 μm and depth resolution of ~10 μm.

The distribution of the defects formed in damage sites was imaged using a fluorescence microscopy system. This system has been described in detail elsewhere (see Ref. 18). In brief, the imaging system is composed of a X10 microscope objective followed by a X5 magnification zoom lens. In this arrangement, images were recorded using a liquid nitrogen cooled CCD detector where 1 μm² of the object is projected to 1 pixel of the detector. Optical filters positioned between the CCD and the zoom lens were used to record emission images within a specified spectral region. A white light source was used to illuminate the sample to obtain light scattering (bright field) images. Fluorescence images of the area of the sample under investigation were also recorded using as the photoexcitation source a CW Argon ion laser operating at 351-nm.

Optical Coherence Tomography was used to obtain three dimensional information on cracks formed around the damage site. The OCT system in this study used a femtosecond Ti:sapphire laser light source and was optimized to support optical spectra of up to 260 nm (FWHM), achieving a 1.5 μm longitudinal resolution in free space, corresponding to ~1 μm in materials. The system was sensitive to reflected signals as small as -110 dB of the incident optical power. In order to achieve high transverse resolution, an achromatic doublet with 10 mm focal length was used as the imaging objective. The transverse resolution can be as fine as 3 to 5 μm depending on the spot size of the focused beam. Imaging was performed by scanning the OCT beam over the region of laser damage in the sample using a micron resolution translation stage. Successive axial scans of backreflected or backscattered light were measured at different transverse positions and the resulting two dimensional data set was displayed as a logarithmic false color or gray scale image.

3. Experimental results

Photoexcitation of a damage site with 351-nm laser light results in a characteristic yellow-red emission clearly visible to the naked eye. Figure 1 shows characteristic emission spectra measured at different locations within a surface damage site obtained using the micro-spectroscopy system discussed above. Both, the total intensity and the relative intensity of individual peaks varied significantly across the damage site. In spite of the variation in the local spectral intensities and spectral profiles, three prominent emission peaks located at 440-nm, 565-nm, and 650-nm are indicated. Investigation of a large number of damage sites created on the surface or in the bulk using varying fluences and number of pulses always indicated the same three emission peaks. In most cases, the 565-nm peak appears as the dominant, the only clearly visible feature in the emission profiles.

The concentration of the defect population formed at damage sites was examined using the imaging system discussed in the previous section. Figure 2a shows a light scattering image of a surface damage site. The fluorescence image using a 420-nm long wavelength pass filter to select most of the emitted light for imaging under 351-nm excitation is shown in Fig. 2b. Comparison of these two images indicates that emission signal originating from within the damage crater is much stronger than that of the cracks formed in the adjacent area. In fact, the cracks located in the upper part of the damage site shown in Fig. 2 are barely visible in the fluorescence image. Even lower by at least one order of magnitude is the “background” emission arising from surface and bulk defects of the pristine material. Electron microscope images of the damage site indicate that the intensity variations are due to the structure of the damage site. The melted during damage material inside the crater was quickly solidified leading to a complex structure where some areas have a thicker layer of the emissive modified material. These areas exhibit higher emission intensity in the fluorescence image. Also, some areas may appear darker in intensity due to partial obscuration of the laser beam due to the three dimensional complex morphology of the crater.

 

Fig. 1. Emission spectra measured at three different locations within a surface damage site obtained using a micro-spectroscopy system and 351-nm excitation.

Download Full Size | PDF

 

Fig. 2. Images of a 930μm × 900 μm section of a surface damage site. a) Light scattering image. b) The fluorescence image under 351-nm excitation using a 420-nm long wavelength pass filter. c) Ratio image obtained from division of the 650-nm over 560-nm emission images.

Download Full Size | PDF

 

Fig. 3. OCT image of a 2000 μm wide by 612 μm deep section of a laser-induced damage site.

Download Full Size | PDF

The non-uniformity of the emission spectra across the damage sites was studied by recording photoluminescence images obtained using 10-nm bandwidth interference filters centered at 650-nm, and 560-nm which coincide with the main peaks observed in the emission spectra shown in Fig. 1. Figure 2c shows an image of the same damage site obtained from the division of the 650-nm over 560-nm emission images. This image best depicts the change in the relative intensity of the two main peaks of the emission spectra.

Due to intense scattering of light at the core of the damage sites, comprehensive imaging of the structural changes can only be achieved using an imaging technique that can discriminate the signal arising from features of interest such as cracks from the dominating signal arising from the highly scattering region. This is particularly important for features located behind the damage site. Optical Coherence Tomography is one of the few such techniques. To test this concept, the OCT system that was described above was utilized to image surface damage sites formed using various conditions. Typical experimental results are shown in Fig. 3. This OCT image was performed over an area of 2000 μm wide by 612 μm deep and is composed of 2000 pixels in the transverse direction and 900 pixels in the axial direction. This image shows that cracks are formed underneath the damage site at various depths and propagate through the material often directed towards the surface.

4. Discussion

The damage site may nominally include three different layers: a layer with modified material that contains a large defect population, a layer of mechanically damaged material (crushed, compressed), and a region of extended cracks. Direct exposure of the host to the extreme conditions occurring during damage leads to the formation of a defect population that is detectable by its fluorescence under photoexcitation. The pressure wave formed as the result of this process leads to the observed mechanical damage. Absorption by defects at the modified material causes re-ignition of the damage process [18]. In addition, field intensification due to the presence of cracks can initiate plasma formation in the bulk. Both processes contribute to the growth of the damage site under subsequent exposure to 355-nm laser pulses. One way to reduce the cost of operation and avoid frequent replacement of the laser components is to apply some type of mitigation process that will stop the damage growth. The imaging techniques we explore in this work provide the means to detect and image both potentially harmful features located in damage sites. This information may be used to assess the susceptibility of a damage site to damage growth or help determine the specific parameters of the mitigation process for each individual damage site. It may also be useful post-mitigation tool for quality control.

The emission peaks at 440-nm and 650-nm observed at damage sites under 351-nm excitation are almost certainly attributed to two well-known defects in SiO₂ [19]. The 650-nm peak is due to the non-bridging oxygen hole center (NBOHC) which is essentially a broken Si-O bond. The 440-nm peak is likely associated with the oxygen deficiency center (ODC). The exact structure of this defect is under debate but is often linked to two-fold coordinated Si or oxygen vacancies. These defect states are known to form in silica by much higher energy radiation (neutrons, gamma rays or deep UV (<300-nm) illumination) [20] or by mechanical stress or damage to the material [21]. For the case of 355-nm laser damage, the defects may result from the plasma created at the damage site rather than from absorption of the 355-nm laser irradiation itself. The nature of the dominant peak in the emission spectrum under 351-nm excitation located at 565-nm is not clear yet. A similar peak that has been reported in γ-irradiated amorphous SiO₂ has been assigned to formation of clusters of silicon [22]. Emission spectra from damage sites created using fs laser pulses [23] indicate the same emission bands to those shown in Fig. 1 although it is known that, compared to damage sites formed with ns laser irradiation, their morphology and the energy deposition mechanisms [24] are different.

Figure 2 shows that the changes in the material integrity were visible in both, the light scattering image and the fluorescence image under 351-nm CW excitation. The later image (shown in fig. 2b) reveals the presence of optical emission from the modified material at the locations where plasma was formed. Cracks that are visible with light scattering in Fig. 2a are barely visible in the fluorescence image. It is difficult to estimate what fragment of the low intensity emission signal observed in the cracks is due to scattering of emitted light originating at the highly emissive damage core as opposed to emission arising from defects located at or near the cracks [21]. The ratio image obtained from the division of the 650-nm over 560-nm fluorescence images shown in Fig. 2c depicts the variation in the relative population of the various defect species formed in the damage site. This may reflect the variation in temperature and/or pressure at different regions in the damage site during laser induced plasma formation. This process is still not well understood.

The OCT images allows the visualization of small backreflections or backscatter within the sample which originates from cracks, defects, or other changes in optical properties. Cross sectional information on damage below the surface of the material can be obtained nondestructively. The OCT images show that cracks originating at different depths under the damage crater propagate towards the surface of the sample. This effect may be due to the weaker mechanical properties of the surface and/or near surface stress fields resulting from the shaping and polishing process of the surface. It has been shown that stress plays an important role in the formation and propagation of cracks [25,26]. Under subsequent laser irradiation and re-ignition of the damage process, these cracks can propagate and reach the surface. In this case, pieces of the material can detach contributing to the growth of the size of the damage crater. The same cracks can also form secondary sites of absorbing modified material through plasma formation by field intensification as discussed above [18]. These new sites may re-ignite under subsequent irradiation which will further enhance the process of crack generation and propagation followed by material removal and enlargement of the damage site. This is the case for surface damage sites. On the other hand, the removal of material in damage sites located in the bulk is not possible and as a result, damage growth in the bulk is not as severe problem as it is on the surface. In addition, cracks formed around the damage site may scatter light and reduce the intensity of the laser light that irradiates the center of the damage site containing the modified material and thus further detain the damage growth process in the bulk. By further increasing the laser fluence, all damage sites will grow to form a network of propagating cracks.

The previous optical tomography technique discussed in the introduction is difficult to implement in large size optical elements because the illumination beam is orthogonal to the direction of the imaging optics and therefore, it requires that the sides of the optics are flat and polished. In contrast, as shown in this work, OCT has subsurface imaging capabilities and therefore, it can be realized through the surface of the optical element. A combination of OCT and fluorescence microscopy can provide information on both, the presence of absorbing modified layer of the material and subsurface cracks, which may both need to be mitigated in order to avoid damage growth under subsequent exposure of the optical element to laser irradiation.