1. Complex Nanophotonics
Hui Cao
Dept. of Applied Physics, Yale University
Critical Needs
Nanofabrication (cleanroom)
High-performance computing
• What are the critical needs for your instrumentation work?
Nanofabrication (cleanroom), high performance computing
• How could the Yale instrumentation initiative help your work?
Combine hardware design with software development for photonic devices.
• What can you offer to the Yale instrumentation community?
Optical imaging, sensing and control techniques, design and fabrication of novel light sources.
Contributions
Optical imaging, sensing and control
Design and fabrication of light sources

2. Speckle-Free Laser as Illumination Source
Speckle pattern
Scattering tissue
Coherent laser beam
Develop laser with low spatial coherence to illuminate object
Speckle-free image
Many imaging applications require increasingly bright illumination sources, motivating the replacement of conventional thermal light sources with bright light-emitting diodes, superluminescent diodes and lasers. Despite their brightness, lasers and superluminescent diodes are poorly suited for full-field imaging applications because their high spatial coherence leads to coherent artifacts such as speckle that corrupt image
formation (top panel). We recently developed new laser sources that provide low spatial coherence. Using such a source to illuminating an object, we demonstrated speckle-free full-field imaging in the setting of intense optical scattering. By providing intense laser illumination
without the drawback of coherent artifacts, our new lasers are well suited for a host of full-field imaging applications, including massive parallel confocal microscopy, phase-contrast microscopy and fluorescence microscopy.
CCD AF
Source IP S
Iris
Obj
Lens
Redding, Choma & HC, Nature Photonics 6, 355 (2012)

3. Switch Laser Coherence for Imaging Heartbeat of Tadpole
2mm
Low-coherence illumination
High-coherence illumination
Illumination sources with tunable spatial coherence are often desirable as they can offer both speckled and speckle-free images. We develop an efficient method to switch the coherence of laser from low to high. This technology enables multimodality imaging, where low spatial coherence illumination is used for traditional high-speed video microscopy and high spatial coherence illumination is used to extract dynamic information of flow processes. As an initial example, we perform dynamic multimodality biomedical imaging in Xenopus embryo (tadpole) hearts, an important animal model of human heart disease.
Knitter et al, Optica 3, 403 (2016)

4. Optical Imaging

5. Dispersion using a Grating Dispersion using a Random Medium
δλ ~ 1/L
δλ ~ 1/L2
Detector Array
Detector Array L L
Random Medium
50μm
100 μm
Δλ = 0.75 nm
1500
1505
1510
1515
1520
0.2
0.4
0.6
0.8 1
Wavelength (nm)
Normalized Intensity
If we replace the grating with the random medium, the input light no longer travels in straight path, in other words, the light path is folded by multiple scattering, so the effective path length is significantly longer than the linear dimension of the random medium L. If light transport in the random medium is diffusive, the average path length scales as L^2.
Moreover, light does not travel in a single path due to multiple scattering, light from many paths interfere and forms a speckle pattern at the output. The speckle pattern changes with wavelength, so the speckle pattern provides a sort of fingerprint, that uniquely identifies the wavelength of the probe signal.
The spectral resolution depends on how much the wavelength needs to shift in order to form a different or uncorrelated speckle pattern. In the diffusive regime, the spectral resolution scales as 1/L^2.
Unfortunately, the total transmission through a diffusive system scales as 1/L. For a thick sample, most light is reflected back, that is why the wall paint looks white. This low transmission limits the spectrometer sensitivity.
That is why in the previous demonstration of spectrometers using disordered photonic crystals and random scattering media, the spectral resolution was not high because the scattering was kept weak, otherwise the loss is too large.