The Stratakis Lab

The Stratakis Lab

Ultrafast Laser Micro and Nano Processing Laboratory

Applications on non linear imaging in materials

Pixel resolution polarization resolved second harmonic generation (SHG) and two-photon photoluminescence (2P-PL) imaging microscopy of non-linear materials.

The goal of the project is to employ precise polarization measurements of SHG and 2p-PL signals, in order to gain further physical insight into the non-linear properties of nanomaterials.

Contact Person(s):
Dr. Emmanuel Stratakis
Dr. Sotiris Psilodimitrakopoulos

Research Topics
Non-linear optical materials


Despite their similarities, 2P-PL and SHG are based on fundamentally different phenomena. 2P-PL relies on nonlinear multiphoton absorption, followed by fluorescence emission, and hence is not a coherent process. SHG, on the other hand, relies on nonlinear scattering, and hence is a coherent process. The consequences of this basic difference are exploited and utilized in this project as a new contrast mechanism. SHG is a coherent second-order nonlinear phenomenon, and thus its strength is fully determined by the second-order susceptibility tensor of the nonlinear medium. This tensor is non-vanishing only for non-centrosymmetric media and reflects the nonlinear response of materials at the molecular level.
When SHG measurements are performed using laser raster-scanning scheme and fast detection, contrasted pseudo colored images can be created. One pixel corresponds to one measurement. The intensity of the signals is translated into shades inside each pixel, while the pixel size of such images depends on the speed of the scanning mirrors and the frequency of detector measurement (typical axial resolution is in the order of few hundreds of nanometers).
Laser raster-scanning polarization measurements based on the orientation of the excitation laser field and on the orientation of the generated SHG for every pixel of the image provide new contrasted images, which reveal details in the materials non-linear properties at sub-micron level. Finally, differences in the signals intensity after polarization analysis, provide the means of the pixel resolution mapping, of the mean orientation average, of the materials crystallographic axis, inside each pixel. Mapping of the orientation of the crystallographic axis, provides new contrasted images with pixel values based on this orientation.
Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) have lately attracted attention due to their extraordinary non-linear properties. In this project we investigate, both experimentally and theoretically, the physical mechanisms underlying nonlinear optical properties of monolayer TMDs, prepared by mechanical exfoliation and chemical vapor deposition (CVD). Because high resolution polarization SHG and 2P-PL are sensitive to layer thickness, crystallographic orientation and layer stacking, critical information regarding numerous type of defects is expected to be revealed.
Moreover, polarization dependent SHG, in combination with adaptive optics are utilized for the birefringence characterization of local stresses, induced by inhomogeneous energy absorption. Combined SHG and 2P-PL microscopy realizes a versatile and precise technique for the simultaneous surface structuring and in situ investigation of the fundamental mechanisms regarding light-matter interactions, upon ultrashort laser irradiation of solid surfaces.
Nonlinear imaging further contributes in the development of advanced materials with vast applications ranging from modern optical tools, biomimetic functional surfaces, tissue engineering, to microelectronics.


Image: Laser raster-scanning SHG false color image, with white showing the most intense 515nm green signal) of CVD grown SW2 monolayers (λex=1030nm, pulse duration=70fs, repetition rate=80MHz, objective numerical aperture=0.8), scalebar shows 10μm.

Prof. G. Kioseoglou
Dr. Ioannis Paradisanos

⦁ Ultrahigh-resolution nonlinear optical imaging of the armchair orientation in 2D transition metal dichalcogenides
Sotiris Psilodimitrakopoulos, Leonidas Mouchliadis, Ioannis Paradisanos, Andreas Lemonis,
George Kioseoglou and Emmanuel Stratakis, Light: Science & Applications, 7, 18005, (2018).

⦁ Effect of composition and temperature on the second harmonic generation in silver phosphate glasses
I. Konidakis, S. Psilodimitrakopoulos, K. Kosma, A. Lemonis, and E. Stratakis, Optical Materials 75, 796-801, (2018).

⦁ Polarization second harmonic generation discriminates between fresh and aged, starch-based adhesives used in cultural heritage
S. Psilodimitrakopoulos, E. Gavgiotaki, K. Melessanaki, V. Tsafas, G. Filippidis, Microscopy and Microanalysis, 22(5), 1072-1083 (2016).

⦁ Effect of molecular organization on the image histograms of polarization SHG microscopy
S. Psilodimitrakopoulos, I. Amat-Roldan, P. Loza-Alvarez, and David Artigas, “,” Biomedical Opt. Express, 3, 2681 (2012).

⦁ Estimating the helical pitch angle of amylopectin in starch using polarization second harmonic generation microscopy
S. Psilodimitrakopoulos, I. Amat-Roldan, P. Loza-Alvarez, and David Artigas, J. Opt. 12, 084007, (2010).

⦁ Fast image analysis in polarization SHG microscopy
I. Amat-Roldan, S. Psilodimitrakopoulos, P. Loza-Alvarez, and D. Artigas, Opt. Express, 17, 17209 (2010).

⦁ Starch-based backwards SHG for in- situ MEFISTO pulse characterization in multiphoton microscopy
A. K. N. Thayil, E. J. Gualda, S. Psilodimitrakopoulos, I. G. Cormack, I. Amat-Roldan, M. Mathew, D. Artigas, and P. Loza-Alvarez, J. Microscopy, 230, 70 (2008).

Project Members
Dr. Emmanuel Stratakis
Dr. Sotiris Psilodimitrakopoulos
Dr. Leonidas Mouchliadis
Mrs. Ioanna Demeridou
Mr. Andreas Lemonis
MSc. Διονύσης Ξυδιάς