The Stratakis Lab

The Stratakis Lab

Ultrafast Laser Micro and Nano Processing Laboratory

Applications of Non-Linear Imaging in Materials Science

Polarization-resolved second-harmonic generation (P-SHG), two-photon photoluminescence (2PL) and third-harmonic generation (THG) imaging microscopy of non-linear materials.

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

Research Topics
Non-linear optical imaging of nanomaterials


Despite their similarities, 2PL, SHG and THG are based on fundamentally different phenomena. 2PL 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, while THG originates from changes in the samples’ index of diffraction. The consequences of those basic differences are exploited and utilized in this project as new contrast mechanisms for characterizing nanomaterials of interest (see Figure 1).

Specifically, 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 atomic level.

Figure 1: Comparison of non-linear contrast mechanisms (2PL, SHG and THG) in aged atomically thin, CVD grown, WS2 flakes, scalebar shows 5μm.

Additionally, the polarization dependency of SHG (P-SHG) reveals the orientational distribution of armchair orientations and their degree of organization in the two-dimensional (2D) crystal lattices of 2D materials. This enables us to distinguish between different crystallographic domains, locate boundaries and reveal atomic structure. As a consequence, we can calculate the mean orientational average of armchair angle distributions in specific regions of interest and define the corresponding standard deviation as a figure-of-merit for the 2D crystal quality (Figure 2).

Moreover, we use P-SHG to rapidly map the twist angle of the individual layers in stacked multilayered 2D materials and to map the electrons population imbalance in such 2D materials.

Figure 2: PSHG as a crystal quality marker: a) SHG intensity image. Four ROIs of 50×50 pixels, namely ROI-A1, ROI-A2, ROI-A3, ROI-A4, and four POIs are indicated. b) Armchair mapping reveals grains of different crystal orientations, not seen in the SHG intensity image. c) Experimental PSHG modulation for POIs 1-4. d) Image histograms showing the distribution of armchair orientations inside ROIs-A1-4. The crystal quality is reflected in the standard deviation (σ2D) of the mean armchair direction, <θ>. Small σ2D values are indicative of good crystal quality.


  • Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation,” G. M. Maragkakis, S. Psilodimitrakopoulos, L. Mouchliadis, I. Paradisanos, A. Lemonis, G. Kioseoglou and E. Stratakis, Opto-Electronic Advances, 2, 190026 (2019).
  • Twist Angle mapping in layered WS2 by Polarization-Resolved Second Harmonic Generation,” S. Psilodimitrakopoulos, L. Mouchliadis, I. Paradisanos, G. Kourmoulakis, A. Lemonis, G. Kioseoglou and E. Stratakis, Scientific Reports, 9, 14285 (2019).
  • Ultrahigh-resolution non-linear optical imaging of the armchair orientation in 2D transition metal dichalcogenides,”S. Psilodimitrakopoulos, L. Mouchliadis, I. Paradisanos, A. Lemonis, G. Kioseoglou and E. 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).

Contact Person(s):
Dr. Emmanuel Stratakis

Dr. Sotiris Psilodimitrakopoulos

Project Members

Dr. Emmanuel Stratakis

Prof. George Kioseoglou

Dr. Sotiris Psilodimitrakopoulos

Dr. Leonidas Mouchliadis

Mr. George Maragkakis

Mr. George Kourmoulakis

Mrs. Ioanna Demeridou

Mr. Andreas Lemonis


Dr. Ioannis Paradisanos