Team III: Advanced Optical Nano-Imaging

Motivation

Light microscopy is one of most widespread microscopy techniques yet its spatial resolution in its conventional implementation is generally limited to the micron scale as determined by the optical wavelength. This so called diffraction limit, could be overcome in the past two decades by several revolutionary concepts, providing light microscopy with deep-subwavelength spatial resolution down to the nanometer scale, corresponding to 1/10,000’th the diameter of a human hair.

One such approach, based on almost conventional optical microscopes, yet a new clever way of using single molecules and their localization in space, was honored with the Nobel Prize in Chemistry in 2014. This technique of ‘super-resolution’ microscopy already provides for cellular imaging with new and unprecedented insight into bio-molecular processes. 

An alternative technique, based on light localization using ultrasharp tips as antennas for light, provides for nanometer spatial resolution without the need for molecular labels. It is applicable with almost any optical spectroscopy at any wavelength with demonstrated applications advancing understanding of nanoscale phenomena in materials science and chemistry. 

However, both of these complementary modalities are nowhere near their fundamental limit in terms of spatial resolution, molecular sensitivity, chemical specificity, time resolution, or even application space from industrial processes to fundamental science across all disciplines just starting to be explored. 

STROBE combines new and emergent concepts to further advance nanoscale imaging into previously inaccessible regimes of controlled and in situ environments, dense molecular, soft, or hard materials, chemical imaging across the UV to THz spectral range, development and implementation of novel light sources, and time resolution from the millisecond to the femtosecond time scales. 

Our approach is based on the integration of fundamental scientific discoveries (e.g., compressive sensing, quantum imaging) and technological achievements (e.g., micro-sensor arrays, nanostructured materials) with which we overcome the traditional limits of diffraction, emitter lifetime, detector sensitivity, shot noise, slow scan speed, and scattering. Augmented by computational imaging techniques, with more sensitive sensor arrays, wavefront engineering, synthetic apertures, metamaterials imaging architectures, or advances in laser development qualitatively new image content can be obtained correlating spatial, chemical, structural, or temporal parameter from different modalities providing the missing link how to map multidimensional image data onto physical models and theory about the sample system. 

We expect the resulting sensing and imaging on the nanoscale to become a commonplace technology, with new applications in biomedical imaging, environmental science, materials science, catalysis, energy conversion technologies, photonics devices, or extraterrestrial science. 

Our Approaches

 II.1 Computational optical nanoscopy   The STROBE center seeks to advance high-speed, three-dimensional (3D), and multispectral image acquisition at multiple scales using engineered point spread functions matched to post-processing algorithms. These systems are designed using unconventional optical elements such as nano/micro-structured phase masks. Information theoretical analyses are carried out to achieve inherent improvement in estimation precision with respect to traditional methods that use just lenses. The systems concurrently incorporate the design of optics, optoelectronics, and signal processing. The greatest potential is in realizing capabilities and performance that are otherwise impossible to achieve. This principle is particularly important in the microscopy domain to attain simultaneously high temporal and 3D-spatial resolution.  

Localization microscopy is an imaging paradigm in which enhanced spatial resolution is obtained at the cost of temporal resolution and the use of a photo-switchable mechanism. STROBE addresses the problem of resolving overlapping images of emitters upon the observation that frame sequences of localization microscopy data are highly compressible. As a result, the information can be reconstructed with far fewer coefficients than its pixel number. Localization microscopy data is reconstructed using different algorithms. Experiments have confirmed that such techniques maintain high localization precision and high resolution in three dimensions while accelerating image data acquisition. 

II.2 Hybrid imaging through complex media  STROBE investigates new hybrid imaging modalities to enable deep sensing inside of and through scattering materials. A common shortcoming of current techniques is the limited penetration beyond the optical transport mean free path. 

Recent advances in adaptive wavefront shaping have made imaging through scattering walls a possibility. By pre-compensating the optical wavefront, light propagation can be controlled through and beyond scattering materials. Most existing techniques, however, are limited by their need to generate feedback from behind or inside of a scattering material with direct invasive access. 
The photoacoustic effect, where acoustic waves are generated by the absorption of light, enables a new feedback mechanism for wavefront optimization because acoustic waves propagate with little scattering.  By combining the spatially non-uniform sensitivity of the ultrasound transducer to the generated acoustic waves with an evolutionary competition among optical modes, it is possible to generate a strong optical focus with a spot size significantly smaller than the acoustic focus used for feedback. This opens up the possibility to achieve exceptional spatial resolution and signal-to-noise ratio in wavefront enhanced photoacoustic imaging.

III.3 Multimodal and hyper-spectral chemical nano-imaging. Through direct measurement of intrinsic vibrational and electronic modes, infrared spectroscopy provides label-free, chemical characterization of molecules and solids. However, the long infrared wavelength severely limited the spatial resolution. To overcome that spatial resolution limit and limited spectral bandwidth of conventional infrared sources, we combine advanced scanning probe microscopies, with ultrabroadband coherent infrared synchrotron radiation of high spectral irradiance, femtosecond lasers, and novel spatio-spectral imaging algorithms for novel and routine chemical and electronic resonant nano-imaging. Extending the reach into in situ and in operando environments, including THz and femtosecond spatio-temporal regime provides for nano-imaging of functional materials at their elementary level. 

III.4 Femtosecond optical nano-focusing for novel ultrafast optical and low-energy electron imaging. Progress in ultrafast electron microscopy relies on the development of efficient laser-driven electron sources with femtosecond electron pulse durations. Gold nanotips with nonlocal excitation and nanofocusing of surface plasmon polaritons provide for the nonlinear emission of ultrashort electron wave packets. With electron pulse durations as short as few femtoseconds we will use this approach for the development and implementation of a range of enhanced novel low-energy electron microscopies, such as femtosecond point-projection microscopy with unprecedented temporal and spatial resolution, femtosecond low-energy electron in-line holography, or new routes towards femtosecond scanning tunneling microscopy and spectroscopy.