Int J Thermophys (2008) 29:220&2205 DO1 10.1007/~10765-007-0262-3 Photopyroelectric Microscopy of Plant Leaves B. R. Briseiio-Tepepa . J. L. Jimdnez-Pedz R. Saavedra R. GonzBlez-Ballesteros E. Suaste A. Cruz-Om Published online: 25 September 2007 O Springer Science+BusinessMedia. LLC 2007 Abstract The use of photothermal microscopy to obtain superficial and in-depth images, by means of the interaction of a thermal wave with the analyzed material, has reached great interest due to its numerow applications. The application of the photopyroelectric microscopy technique to obtain images of plant leaves is presented in this article. In the experimental setup, a pyroelectric sensor and linear micro-positioners were used to obtain the photothermal signal at each point of the sample. Then it is possible to obtain images of plant leaves through their differences in local thermal properties and thickness. Keywords Ligusmm japonicum leaf . Photoacoustic microscopy . Photopyroelecmc microscopy . Photothermal techniques . Thermal-wave inmging . 'I'hin-film photopyroelectric detection B. R. Briseh-Teppa . J. L. Jim6nez-Per6z Centro de Investigation en Ciencia Aplicada y Tecnologia Avanzada IPN Calz. Legaria 694. Col. irrigac~on.11500 Mdxico. D F . Mthim R. Saavedra Departamento de Flsic& Universidnd de Concepcibn. Cnsilla 1 6 K , Concepcion. Chile R. Gonzdez-Bdlesteros Universidad Polikknica de Pachuca Km 20 de Can. Pachuca-Cd. Sahagiin, Zempoala. Hgo. Mexico E. Suaste Seccion de B~oelectmnica,Deptamento de Ingenieria El&ctricaCMVESTAV-IPN, Av. IPN No. 2508. Col. San Pedro Zacatenco, C.P. 07360 M6xico. D.F.. Mdxico A. Cruz-Orea (W Departamento de Fisica Centro de Investigaci6n y Estudios Avanzados del IPN, Av. IPN M. 2508. Col. S u Perlro Zxaknco, C.P. 07360 Wxico, DF., Mixico e-mail: orea@fis.cinvestav.mx Inr J Thermophys (2008) 29:220%2205 2201 1 Introduction Thermal waves are used nowadays for non-des~ctiveevaluation of various types of surface and sub-surface features. In thermal-wave imaging, macroscopic and microscopic themla1 features on or beneath the surface of a sample can be detected and imaged. Thermal features can be defined as those regioos of an otherwise homogeneous material that exhibit variations, relative to their surroundings, in any of the following three thermal parameters: density, p; specific heat, c; and thermal conductivity, k. Variations in these thermal parameters arise, in the most general sense, from variations in the local structure of the material. Imaging of these local thermal features requires detection of the scattered and reflected thermal waves [ I]. This detection is currently accomplished by several photothermal (PT) techniques. These techniques involve measurements of the heat produced, as an excited species relaxes by a nonradiative path. The excited light is chopped at a suitable frequency, and the resulting modulated heat flow is detected by using a direct or indirect temperature sensor, and a lock-in amplifier. These techniques allow the acquisition of information about samples with inherent difficulties such as high dispersion of light and structures that vary with depth (for example, semiconductors, minerals, vegetables, and animal tissue). PT techniques have been used since 1979 to obtain thermal images from different samples, being a nondestructive technique. Among the PT techniques, the photoacoustic technique was the first to be used to obtain thermal images. Photoacoustic microscopy (PA..) is a well established and useful tool in the nondestructive testing of materials. There has been considerable interest in employing PAM for surface and sub-surface imaging, and microscopy of solid materials. The unique characteristic of photoacoustic imaging lies in the detection of subsurface features or flaws by the interaction of the photoacoustically generated ther1nal waves with these features [?I. This characteristic to detect surface, and subsurface defects makes PAM particularly valuable in locating voids and cracks in metals, ceramics, and semiconductors. Briefly, the technique involves modulating the intensity of a focused laser beam. and slowly scanning the position of the beam across the sample of interest. The thermal waves generated in the sample by the periodic heating scatter off defects below the surface of the sample, and this causes a change in the photoacoustic signal. As the laser beam is scanned across the sample, any change in the photoacoustic signal indicates the presence of defects at that location [3]. Different researchers have used PT techniques to detect inhomogeneous suuctures such as diffused and ion-implanted regions in sen~iconctuctors[4], and integrated circuits [5]. Unlike these, there are few reports on imaging applications using a pyroelectric (PE) sensor. The emergence of thin-film photopyroelecuic (PPE) detection as a convenient photothermal method, with considerably more degrees of freedom than photoacoustic imaging (both microphonic and piezoelectric), and a back--detection character, as opposed lo the photothermal-beam defleclion front --detection characler, has been well documented. Conventional pyroelectric detection of thermal waves used in a scanned, spatially integrated detection mode has also been reported. Imaging of the thermal wave field with a polyvinylidene-dilluoride (PVDF) pyroelectric sensor in the back detection configuration (in this case. the modulated light Int J Thermophys (2008) 29:2200-2205 2205 where a, = CL;l is the thermal diffusion coefficient. Then Eq. I indicates that it is possible to obtain a PPE signal phase shift when the sample varies in its local thermal properties or thickness of its structure. On the other hand. the PPE signal amplitude decreases exponentially when the sample thickness is increased [ I 01. Then, it is possible to obtain a PT image of samples where thermal and thickness variations take place in their structure, as is the case for many biological tissues. It is important to mention that the resolution of this technique is limited by the spot size of the focused laser beam, used to scan the sample, also the resolution of the x-y stage, and the light modulation frequency ( f ) along with the sample thermal diffusivity, which determine the thermal diffusion length in the sample. Also, in the back detection configuration. the thickness of the sample is impoxtam due to the fact that the PPE signal amplitude decreases exponentially when the sample thickness is increased, and the laser intensity is limited in order to avoid damage in the biological sample. 4 Conclusions Photothermal images of plant leaves, tigustrum japottiticum, were obtained by using a PPE technique. The back and front detection configurations were used to obtain PT images of these leaves. In both configurations, it is possible to obtain the sample image by scanning the modulated laser beam on the sanlple, and detecting the PPE signal at each point of the scan. The studied leaves have thermal variations in their structure, and also have significant thickness variations in their main parts (midrib and blade), which makes it possible to obtain their images by using the PPE technique. From the PPE amplitude and phase, it was possible to observe images of the leaf midrib and blade. The images obtained from the back and front PPEdetection configurations show an internal channel in the mifib, which is not possible to observe in the microscopic picture. Acknowledgmeots The authors are thankful to the Mexican Agencies. CONACYT, COFAA, and CGPI for financial support of this wcrk. One of the authors (A. Cmz Orea) is grateful for financial support from CONACYT Project KO.43252-R. We also want to thank Ing. D. Jacinto Mndez. Ing. E. Ayala, Ing. M. Guerrem, and Ing. A. B. Soto for heir Lechnical support at the Physics Depanment. CINVESTAV-IPPN References 1. A. Rosencwaig, Science 218.223 (1982) 2. G. Busse, A. Rosencwaig, Appl. Phys. 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