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Optical Sensing and Microscopy

Figure 1. Simplified layout of scanning optical frequency-domain reflectometry instrument. Light from the frequency-swept laser is sent to a sample on a 3D stage and the reflection is interfered with a reference. Each reflection in the sample generates interferograms which encode material properties as a function of depth.
Characterization of the devices we fabricate requires precise quantitative measurement of material properties with sub-micron 3D resolution. Thus, we develop custom scanning microscopes optimized for this purpose. Two such instruments are illustrated here.

Optical frequency-domain reflectometry (see Figures 1 & 2) measures the reflections from a sample as the laser wavelength is swept to provide depth-resolved images, much like in optical coherence tomography. We have optimized this method for materials metrology to simultaneously measure sample profile and refractive index to very high precision and accuracy.

A complementary instrument is the differential scanning transmission microscope (see Figures 3 & 4). This measures the absorption and deflection of a scanning focus. Post-processing of this data yields a 3D map of the complex index of refraction. An interesting feature of this microscope is that it can provide depth sectioning like a confocal microscope but without the typical pinhole.

These and other custom instruments are available for academic use. See the Facilities page for more information.
Figure 2. In this example OFDR range return, Fresnel reflections are generated at 3 depths where there are sharp index boundaries in the sample. The amplitude is related to the index change and the shape of the probe beam. The round-trip delay yields the position to approximately 5 microns of resolution with sub-nanometer precision.
Figure 3. The scanning differential transmision microscope measures absorption by the transmitted power and the derivative of refractive index by the deflection of the focused beam. These data are measured simultaneously by a position sensitive detector as the sample scans in 3D. A coherent confocal reflection channel is included for sampmle positioning.
Figure 4. A sample image from the instrument of Figure 3. This shows the refractive index of a stack of multiplexed volume holograms in a diffusive photopolymer. The horizontal lines were generated by scattering from a hard aperture during recording. The absorption (traditional bright field microscope) image of this sample is uniform, revealing no information.

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This work has been generously funded by: