THG Imaging Microscopy

We use our nonlinear coherent microscope to characterzie and shape the spatial polarization of an ultrashort laser pulse at the focus of a microscope objective. By imaging a 1-D spatial modulator, we create an inhomogeneous polarization state; by collecting the third-harmonic signal of a nano-bead, we map the polarization state at the focus, and retrieve the spatial polarization through a non-iterative algorithm. Sample retrievals for various constant, linear, and quadratic spatial polarization states are shown in Fig. 1.

Enhanced spatial resolution in the THG microscope is demonstrated through switching the polarization of the field from linear at the center, to circular at the beam edges. As the THG scattering is suppressed for circular polarization, the THG signal diameter is reduced.

Enhancements in resolution up to a factor of 2 are observed. Since the spatial polarization is shaped by a 1-D SLM, enhancement in lateral resolution in one dimension is observed.

Our microscopy setup uses 2D galvanometers to scan the incident laser beam over the sample. An relay telescope images the galvanometers into a standard microscope objective. The upright microscope design is adopted for maximum flexibility and to allow for liquid-immersed microscopy images. A PMT collects the emitted light in the forward or epi-direction.

For calibration, we capture images of gold reference meshes. The grids shown in the image have a 64-um pitch. The circular edge of the grid is clearly visible in the left image, and lowering the scanning range of the galvanometer gives a finer-resolution image.

We designed and built a diode-pumped Yb:KGW oscillator for use with microscopy and biomolecular imaging. The laser incorporates a saturable absorber mirror and intracavity prisms (not shown in figure).