Sungsam Kang

High-resolution adaptive optical imaging within thick scattering media using closed-loop accumulation of single scattering

Thick biological tissues give rise to not only the multiple scattering of incoming light waves, but also the aberrations of remaining signal waves. The challenge for existing optical microscopy methods to overcome both problems simultaneously has limited sub-micron spatial resolution imaging to shallow depths. Here we present an optical coherence imaging method that can identify aberrations of waves incident to and reflected from the samples separately, and eliminate such aberrations even in the presence of multiple light scattering. The proposed method records the time-gated complex-field maps of backscattered waves over various illumination channels, and performs a closed-loop optimization of signal waves for both forward and phase-conjugation processes. We demonstrated the enhancement of the Strehl ratio by more than 500 times, an order of magnitude or more improvement over conventional adaptive optics, and achieved a spatial resolution of 600 nm up to an imaging depth of seven scattering mean free paths.Optical imaging deep in biological tissue is difficult due to multiple scattering and specimen induced aberrations of both the incident and reflected light. Here, Kang et al. develop an adaptive closed-loop algorithm to correct tissue aberrations in the presence of multiple scattering for deep tissue imaging.

Focusing of light energy inside a scattering medium by controlling the time-gated multiple light scattering

The efficient delivery of light energy is a prerequisite for the non-invasive imaging and stimulating of target objects embedded deep within a scattering medium. However, the injected waves experience random diffusion by multiple light scattering, and only a small fraction reaches the target object. Here, we present a method to counteract wave diffusion and to focus multiple-scattered waves at the deeply embedded target. To realize this, we experimentally inject light into the reflection eigenchannels of a specific flight time to preferably enhance the intensity of those multiple-scattered waves that have interacted with the target object. For targets that are too deep to be visible by optical imaging, we demonstrate a more than tenfold enhancement in light energy delivery in comparison with ordinary wave diffusion cases. This work will lay a foundation to enhance the working depth of imaging, sensing and light stimulation.The use of a time-gated reflection matrix of a scattering medium, in particular via using singular value decomposition and injecting light into the largest time-gated eigenchannel, can lead to a more than tenfold enhancement in light energy delivery in comparison with ordinary wave diffusion cases.

Electrical Characteristics of All Polymer Based Thin Film Transistor using Poly (3,4-ETHYLENEDIOXYTHIOPHENE) and Polypyrrole

We report the electrical characteristics of all polymer based thin film transistor(TFT). Conducting poly(3,4-ethylenedioxythiophene)(PEDOT)and polypyrrole(PPy)were used for gate electrode and active layer, which were made by photolithographic micro-patterning. Polyvinyle cinnamate and epoxy were used for insulating layer through spin coating. From the current(I)-voltage(V)characteristics of all polymer based TFTs, the source-drain current(Ids)of the devices dramatically decreased with increasing positive gate bias(Vg), indicating p-type TFT. We analyze these results based on the“bottle-neck”effect, which implies the distinct depletion mode of the field effect region on the gate electrode with positive Vg. With negative Vg, the Idsof the devices weakly increases. Depending on the channel length between the contacts of source and drain electrodes, the on-off current ratio(Ion/Ioff)was changed.

Optical imaging featuring both long working distance and high spatial resolution by correcting the aberration of a large aperture lens

High-resolution optical imaging within thick objects has been a challenging task due to the short working distance of conventional high numerical aperture (NA) objective lenses. Lenses with a large physical diameter and thus a large aperture, such as microscope condenser lenses, can feature both a large NA and a long working distance. However, such lenses suffer from strong aberrations. To overcome this problem, we present a method to correct the aberrations of a transmission-mode imaging system that is composed of two condensers. The proposed method separately identifies and corrects aberrations of illumination and collection lenses of up to 1.2 NA by iteratively optimizing the total intensity of the synthetic aperture images in the forward and phase-conjugation processes. At a source wavelength of 785 nm, we demonstrated a spatial resolution of 372 nm at extremely long working distances of up to 1.6 mm, an order of magnitude improvement in comparison to conventional objective lenses. Our method of converting microscope condensers to high-quality objectives may facilitate increases in the imaging depths of super-resolution and expansion microscopes.

Label free assessment of ultra-violet radiation induced damages in skin cells (Conference Presentation)

Changes in the cellular homeostasis in response to a stimuli, disease or therapeutic intervention are multifaceted in nature, and cannot be grasped by routinely employed targeted imaging that focuses on a small set of suspected molecules or genes. Novel approaches relying on global analysis of cellular features, from morphology to the composite biomolecular status (notably chemical composition and molecular conformation), is a pre-requisite for accurate monitoring of cellular processes. In the present study label-free profiling of normal skin fibroblasts (Hs895.Sk) exposed to sub-lethal doses of ultra-violet radiation has been performed using quantitative phase imaging and Raman spectroscopy. Spectral differences in the Raman fingerprint region indicates differences in the protein and nucleic acid composition. These differences were successfully utilized to develop an automated classification model based on principal component analysis. Distinct changes in the cellular morphology were observed and validated through quantitative phase imaging. Significant dose dependent differences in different biophysical parameters such as dry mass and matter density were observed. Combination of these two techniques, one suited for detection of subtle morphological/biophysical alterations while the other appropriate for capturing molecular perturbations, could pave the way to address issues of label-free monitoring of cellular responses in response to an external stimulus. These findings can provide an accurate understanding of different markers associated with radiation damage and would assist in providing a quantitative tool to our future studies on designing alternate diagnostic tools.

Focusing of light energy inside a scattering medium by controlling the time-gated multiple light scattering (Conference Presentation)

To exploit photonics technologies for in vivo studies in life science and biomedicine, it is necessary to efficiently deliver light energy to the target objects embedded deep within complex biological tissues. However, light waves diffuse randomly inside complex media due to multiple scattering, and only a small fraction reaches the target object. Here we present a method to counteract the random diffusion and to focus ‘snake-like’ multiple-scattered waves to the embedded target. To realize this, we experimentally identified time-gated reflection eigenchannels that have extraordinarily large reflectance at a specific flight time where most of the multiple-scattered waves have interacted with the target object. By injecting light to these eigenchannels, we achieved more than 10-fold enhancement in light energy delivery compared to ordinary wave diffusion cases. This method works up to depths of approximately 2 times the transport mean free path at which target objects are completely invisible by ballistic optical imaging. This work will lay a foundation for enhancing the working depth of imaging, sensing, and light stimulation.

Long-range and depth-selective imaging of macroscopic targets using low-coherence and wide-field interferometry(Conference Presentation)

With the advancement of 3D display technology, 3D imaging of macroscopic objects has drawn much attention as they provide the contents to display. The most widely used imaging methods include a depth camera, which measures time of flight for the depth discrimination, and various structured illumination techniques. However, these existing methods have poor depth resolution, which makes imaging complicated structures a difficult task. In order to resolve this issue, we propose an imaging system based upon low-coherence interferometry and off-axis digital holographic imaging. By using light source with coherence length of 200 micro, we achieved the depth resolution of 100 micro. In order to map the macroscopic objects with this high axial resolution, we installed a pair of prisms in the reference beam path for the long-range scanning of the optical path length. Specifically, one prism was fixed in position, and the other prism was mounted on a translation stage and translated in parallel to the first prism. Due to the multiple internal reflections between the two prisms, the overall path length was elongated by a factor of 50. In this way, we could cover a depth range more than 1 meter. In addition, we employed multiple speckle illuminations and incoherent averaging of the acquired holographic images for reducing the specular reflections from the target surface. Using this newly developed system, we performed imaging targets with multiple different layers and demonstrated imaging targets hidden behind the scattering layers. The method was also applied to imaging targets located around the corner.

Deep-tissue imaging with collective accumulation of single scattering microscopy

We present an approach that maintains full optical resolution in imaging deep within scattering media. Imaging depth of 11.5 times the scattering mean free path was achieved with near-diffraction-limit resolution of 1.5 μm.