Zhiwen Liu, Ph.D. - US grants

Liu
0547475

Abstract

Intellectual Merit:
This CAREER program is focused on the development of a new single-particle optical spectroscopy technology using white light supercontinuum optical tweezers. The proposed supercontinuum tweezers can not only trap a particle but also perform broadband optical scattering/absorption and coherent anti-Stokes Raman scattering spectroscopy on the trapped particle, taking advantage of the broad bandwidth of the supercontinuum. Both the physical and chemical features (e.g., size, refractive index, chemical composition) of the trapped particle can therefore be probed in situ. Over the next five years, the PIs research will be focused on the following three tasks: 1) systematically investigate the properties of supercontinuum trap and calibrate the force; 2) investigate broadband optical scattering spectroscopy of a single trapped particle; and 3) investigate broadband coherent anti-Stokes Raman scattering spectroscopy of a single trapped particle.

Broader Impact:
The proposed research can potentially result in major advancement of the current single-particle spectroscopy and optical tweezers technologies, and is expected to have significant impact on many areas ranging from engineering and materials science to biology and biomedicine. In addition, this CAREER program will provide a great opportunity to expose students to the broad spectrum of optics and to train them through the proposed research which includes calibrating the force of supercontinuum tweezers, developing experimental techniques and systems for single particle spectroscopy, and developing simulation tools to analyze the measured spectra. Efforts will be made to foster woman and minority students to participate in the research. Education opportunities will also be provided for high school teachers and K-12 students.

This award is for the development of a faster two-photon fluorescence microscope, with improvements especially in the axial scanning speed. Speed will be increased by wavelength division multiplexing (or chromatic two-photon imaging). Although much progress has been made in improving the lateral scanning speed, a high-speed axial scanning method is still missing. Slow mechanical translation of the objective lens or the specimen itself is still required in order to scan in the axial direction, which limits the 3D imaging speed. As a result, existing instruments have very limited capability to monitor fast biological processes (e.g., transient calcium signals) in 3D and in the x-z or y-z cross-section. The proposed instrument will have a significant impact on 3D microscopy, one of the most important and widely used tools in biology. It will allow researchers to study fast dynamics (e.g., membrane trafficking, chemical waves within or between cells) and provide fundamental understanding of various inter- and intra- cellular processes such as calcium imaging.

In addition to its scientific benefits, the project will involve undergraduate researchers as well as graduate students. Woman and minority students will be particularly encouraged to be involved in the program. The investigators will work closely with Penn State organizations such as Women in Engineering to develop outreach education programs for under-represented groups as well as K-12 students.

Intellectual merit: The goal of the proposed research is to develop a nonlinear nanoprobe for nano-femto scale spatiotemporal characterization of ultrafast optical near fields. The proposed nanoprobe consists of a nonlinear nanoparticle attached to a nanowire, which is in turn attached to a silica fiber taper. The nonlinearity of the nanoparticle enables temporal characterization through autocorrelation or frequency resolved optical gating measurements while the nanoscale spatial resolution is achieved through near field scanning of the nonlinear nanoparticle. We will develop two-photon fluorescent and second harmonic nanoprobes, develop and optimize nanoprobe based spatiotemporal characterization technique, and investigate the precision of the proposed nanoprobe based method. With the unique capabilities of the proposed nonlinear nanoprobes, we also plan to investigate their applications to probing several interesting ultrafast optical near fields.

Broad impact: The proposed nanoprobe can significantly advance the state of the art of nano & ultrafast technology, which can in turn create far-reaching impacts in many scientific disciplines in which they play a central role. With its unique capability in providing nano-femto scale spatiotemporal mappings, the proposed nonlinear nanoprobe can find many important applications. Fundamental questions with regard to light-matter interaction in the ultrafast regime, ultrafast dynamics of complex nanostructures, and nonlinear optics in nanoscale plasmonic structures, can all benefit from the development of the proposed nanoprobe. As a result, the proposed research can have considerable impact on areas such as nonlinear optical microscopy and nanophotonics. The proposed research is highly interdisciplinary and can also provide excellent education opportunities for students.

0968650 Pennsylvania State University; Mohsen Kavehrad
0968651 Tufts University; Valencia Joyner
0968662 University of California-Riverside; Zhengyuan Xu

The Center for Optical Wireless Applications (COWA) will focus on developing new devices using White Light Emitting Devices (WLEDs). Pennsylvania State University (PSU), Tufts University (TU) and the University of California-Riverside (UCR) are collaborating to establish the proposed center, with PSU as the lead institution.

The primary goals of this planning project are to initiate formal partnership with various industry partners and national laboratories that have an interest in optical wireless applications designs, and to discuss fundamental issues and topics for research. The main objective of the envisioned research projects at the proposed Center is to develop a new generation of environment-friendly extremely wideband optical wireless technology applications. The PIs' effort will involve work in relevant device designs, in optical wireless communication systems (physical layer), in networking, sensing, and in imaging.

The proposed Center has the potential to improve the profitability of US manufacturing by developing new optical wireless devices that will improve communication systems, reduce energy consumption and pollution. The proposed Center will offer a series of short courses to update the knowledge of the current workforce and will help universities to tailor new course offerings and to modify existing course offerings to better provide instruction for related areas and industry needs. The Center plans to promote diversity, building on all three universities' high ranking in education of minorities. In addition, the Center will work with existing university resources to recruit and build strong relationships with minority and women-owned companies and provide a collaborative community, within which these companies can contribute expertise, expand their networks and become more globally competitive.

The objective of this program is to develop a new type of optoelectronic nano-hand by integrating fiber optics, electronics, and NEMS/MEMS (nano- or micro- electromechanical systems) in a unified platform. The PIs plan to carry out four core research tasks. First, they will demonstrate a prototype optoelectronic nanohand. Second, they will investigate its optical waveguiding properties. Third, they will investigate optical force exerted by laser beam delivered by the nanohand. Finally, they will investigate in situ electrical and optical characterization, and explore the application of the nanohand.

The intellectual merit is the demonstration and development of new capability to manipulate, control, and characterize individual nanoscale objects. The proposed nanohand has several unique features. It has a large aspect ratio and is free of substrate support. The nanohand has multiple independently controlled nano-actuators or fingers. An object can also be potentially controlled by auxiliary optical force. Finally, electric measurements and optical characterization can be performed to probe a single clenched nanoscale object in situ.

The broader impacts are as follows. The proposed nanohand represents a new class of multi-functional nano-manipulators and can have significant impact on several areas including NEMS/MEMS, optics, and bioengineering. It can open new possibilities for realizing nano-opto-electro-mechanical and nano-robotic systems, for assembling complex three-dimensional nanostructures, and for probing the properties of nanoscale systems and devices at a single nanoparticle or single nanowire level. The multi-disciplinary nature of the proposed research also makes this program an excellent opportunity to educate and train students.

I/UCRC for Optical Wireless Applications

1160924 Pennsylvania State University; Mohsen Kavehrad
1161010 Georgia Tech; Gee-Kung Chang

The Center for Optical Wireless Applications (COWA) will focus on generating technology that enables manufacturing of specific devices with larger communications capacity, employing integrated opto-electronics device design with interfaces necessary to facilitate collaborative device, system and network design. Pennsylvania State University (PSU) and Georgia Tech (GT) are collaborating to establish the proposed center, with PSU as the lead institution.

The objective of this proposal is to establish an NSF-sponsored Industry & University Cooperative Research Center on Optical Wireless Applications (COWA), in order to explore Optical Wireless Technology and economic potentials of energy efficient light sources through innovative designs and applications of solid-state optical sources and detectors for a wide range of practices that include optical imaging, remote sensing, communications and networking. The envisioned Center is based on the integration of interdisciplinary expertise at PSU, and GT with devices and systems-based engineering design and networking concepts.

The envisioned Center will include efforts to instill the cultural paradigm shift associated with promoting research programs of interest to both industry and universities; exploring and extending the interface between engineering systems design, networking and integrated electro-optic device designs; improving the intellectual capacity of the workforce through industrial participation and conduct of high-quality research projects; and developing curriculum in components, systems and networks design aspects of optical wireless applications. The results of this research are expected to contribute to the business competitiveness, energy security, environmental protection, and climate forecast. The proposed Center will make every effort to promote diversity. In addition, it will work with existing university resources to recruit and build strong relationships with minority and women-owned companies.

1264997/1264750
Yang/Liu

The PIs propose to develop a new class of nanoparticle sensors based on ultrahigh quality factor (Q) optical micro-resonators on a silicon wafer to achieve not only ultra-sensitive detection but also molecule-specific identification and measurement of nanoparticles and molecules in air and in liquid environments at single particle resolution. The merit of this new technique lies in enhancing Raman and Rayleigh scattering and integrating them in an ultrahigh-Q Whispering Gallery mode (WGM) micro resonator to achieve multi-function sensors. On one hand, Rayleigh scattering in resonators leads to self-referencing mode-splitting, which can be utilized to detect and measure nanoparticles (e.g., size) down to several nm in size. On the other hand, Raman scattering, which is greatly enhanced by several orders of magnitude in an ultrahigh-Q microresonator, provides identification of molecules/particles through recognition of their spectral fingerprints of molecular vibrations. By integrating mode-splitting and microresonator-enhanced Raman spectroscopy, they can achieve a multifunction sensing unit capable of ultra-sensitive real-time, in-situ detection, identification and measurement of single nanoparticles.

Non-technical Description: The goals of this project are to synthesize atomically thin two-dimensional (2D) semiconducting layers, which possess novel properties often unavailable in their bulk counterparts, and incorporate them onto devices for novel photonic applications. Once demonstrated, these active and nonlinear photonic devices using 2D materials can potentially create a new paradigm of optoelectronics and may lead to numerous optical information and quantum information related applications. A multidisciplinary team from four academic institutions (Rensselaer Polytechnic Institute, Pennsylvania State University, Virginia Polytechnic Institute and State University, and Washington University in St. Louis) is formed to investigate key research areas from 2D material synthesis, condensed matter theory, and optical engineering. This project also includes a comprehensive education and outreach plan at all levels, from encouraging underprivileged K-12 students into the exciting field of material sciences and optical engineering all the way up to faculty mentoring.

Technical Description: This project aims to predict, synthesize, characterize, and engineer semiconducting transition metal dichalcogenides (STMD) such as molybdenum and tungsten disulfides and diselenides, and utilize them to develop a new class of potentially transformative active and nonlinear photonic devices. A key challenge to develop high-quality 2D STMD materials is the control of these materials with predetermined thickness (number of layers), stacking and composition, and to tailor their optical properties for specific photonics applications. The synthesis strategy of this project is based on chemical vapor deposition (CVD), which has been proved suitable for synthesizing large-area (cm) 2D crystals of STMD, and for controlling the growth, clustering, doping, alloying and stacking of different STMD. The research team aims to establish a comprehensive first-principles framework for modeling the doping, alloying and stacking of a few layered STMD. The team develops more realistic approaches for the band structure (GW approximation) and excitonic behaviors by solving the Bethe-Salpeter equation. Such studies directly inform material design and CVD synthesis, and enable new photonic applications and devices based on STMD dressed fiber optics, plasmonic nanostructures, and micro-resonators. The research can potentially lead to new paradigms of STMD functionalized optoelectronics, with examples including parity-time symmetric systems for unidirectional invisibility, nonreciprocal light transmission, novel low-power optical switching and photon routing, ultra-low threshold lasers and amplifiers, TMD-plasmonic systems with nonlinear efficiency enhanced up to several orders of magnitude, and high-efficiency nonlinear/quantum optical devices.

DESCRIPTION (provided by applicant): Three-dimensional optical imaging of biological cells and tissues has become increasingly critical for biomedical studies and medical diagnosis. Although images in the lateral direction (i.e., parallel to the sample surface) can be routinely acquires in parallel (e.g., using light-sheet microscopy) or at high speed (e.g., using high-speed scanning microscopy), no current imaging modality can image an axial slice (i.e., perpendicular to the sample surface) in parallel, limiting our ability to visualize fast biological processes happening across different depths. In this proposed study, we aim to overcome this limitation by developing a new technology capable of imaging axial slices of a sample in parallel. The key concept is the proposed use of an array of 45¿- tilted high-aspect-ratio micro mirrors arranged along the axial direction, which can demultiplex image signals emitted from different depth positions by exploiting their different wavefronts. Each micro mirror has a dimension comparable to the diffraction limited beam width. Consequently it behaves effectively as a confocal pinhole or slit, providing optical sectioning capability. Building upon our preliminary study, in the proposed program we will first design the proposed high-aspect-ratio micro mirror array and fabricate it by using micromolding. We will then develop the proposed axial slice light-sheet imaging system by incorporating the high-aspect-ratio micro mirror array device. The prototype system will be characterized and optimized. High- resolution axial slice light sheet imaging of fluorescent microspheres and 3D spheroid tumor cell culture complex systems will be demonstrated.

Advances in optical nanotechnology have enabled a wide range of applications, such as increased sensitivity for the detection of just a few molecules with plasmonic nanoparticles. To quantify the performance of optical devices and develop new capabilities, it is essential to measure the behaviors of ultrafast optical fields with nanoscale spatial resolution and femtosecond scale temporal resolution. To address the challenge, this program engenders a novel type of nanoprobe, integrated with a custom near-field scanning microscope system, for comprehensive characterization of the ultrafast optical near field. The research program will enhance understanding by combining the expertise of three researchers in different research areas at three universities. The exciting areas in ultrafast optics and nano-optics will provide excellent education opportunities for graduate and undergraduate as well as K-12 students in the lab, in the classroom, and through outreach activities. Graduate and undergraduate students will be trained through the research activities such as nanoprobe fabrication, application of near-field scanning optical microscope system, optical measurements, and numerical simulation and retrieval in a collaborative setting across the three universities. Results will be incorporated into courses. In keeping with prior projects of the researchers, women and underrepresented groups will be encouraged and expected to participate in the program.

The goal of this program is to develop a nanoprobe based characterization method that can map the spatiotemporal evolution of ultrafast optical vector near field in nanometer-femtosecond scale. A nanoprobe, which consists of a second order nonlinear nanocrystal perched on a nanowire or a near-field scanning optical microscope (NSOM) probe, will be integrated with a custom-built NSOM system to achieve sample-probe distance control and nanoscale spatial resolution. The nonlinear response of the nanocrystal (i.e., second harmonic generation -SHG) can be exploited to characterize both the amplitude and the phase profiles of the local ultrafast field as well as the spatiotemporal evolution through the collinear SHG frequency resolved optical gating (FROG) holography. Due to the presence of a strong "local oscillator" and the reliance on homodyne detection, FROG holography will also improve the measurement sensitivity. Finally, polarized SHG from the nanoprobe is utilized to probe the polarization of the local ultrafast optical field. Since the second harmonic signal has a distinct wavelength, it is insensitive to any background noise generated by the reflection or scattering of the fundamental field. Further, the second order nonlinear tensor is determined by the material properties such as the crystal structure and is largely independent of the particle morphology, leading to a more controllable nanoprobe sensor. Knowledge of the local spatiotemporal fields enhances the capability to quantify spectroscopic signals from plasmonic structures, a long-standing challenge in nanospectroscopy.

Chromatic two-photon fluorescence microscopy using band-shifting imaging probes Two-photon fluorescence microscopy has emerged as one of the most important imaging modalities for biological research and medical diagnosis. Although many techniques exist for realizing high speed two-photon imaging in the lateral directions, the axial imaging speed is still often limited by the slow mechanical scanning of the objective lens or the specimen, presenting a significant challenge for monitoring fast biological processes at multiple depths as well as in 3D and the development of miniature endoscopy. To overcome this limitation, here a new spectrally encoded two-photon imaging technique is proposed using band-shifting imaging probes, which can enable parallel axial imaging. Specifically, different excitation wavelengths are focused onto different axial positions (through purposely introduced chromatic aberration) to excite two-photon fluorescence from the band-shifting imaging probes, which shift the emission band when the excitation wavelength varies. As such, the fluorescence signals at different axial positions are spectrally encoded to exhibit different spectral bands, and can thus be imaged in parallel by using a spectrometer or arrayed wavelength-resolving detectors. The proposed band-shifting imaging probes will be synthesized, optimized, and used for cellular labeling. Systematic characterization on the molecular structures, molecular weight, photophysical and two-photon properties will be performed. The proposed chromatic two-photon imaging system will be designed, developed, and optimized. System metrics including the mapping relationship between the axial position and fluorescence band shift, the axial imaging range, and spatial resolutions will be characterized. The proposed method will be demonstrated and validated by performing imaging of cells and tissue phantoms as well as by in vivo imaging studies of a colorectal cancer xenograft model.

Multi-line coherent Raman imaging Coherent Raman microscopy, by merging coherent Raman vibrational spectroscopy with optical microscopy, has been demonstrated as a powerful platform for label free imaging. However, traditional coherent Raman imaging techniques usually fall into one of the two categories: single line vs. broadband. The former is fast but has limited specificity, while the latter provides a wealth of spectral information and thus has high specificity, but suffers from much slower imaging speed. To address these challenges, we propose a multi-line coherent Raman imaging method that essentially splits the difference between single line and broadband coherent Raman, allowing for simultaneous excitation of multiple vibrational peaks in order to achieve both high specificity and high speed imaging. The key enabling technical innovation is our method of coupling pulse division and recombination techniques with the soliton self frequency shift to achieve a fiber-based, power scalable, broadly tunable, multi-color ultrashort excitation source, which makes the practical implementation of the proposed imaging methodology possible. In this proposed program, we will first develop a Yb:fiber laser based divided pulse soliton self frequency shift source, which can allow the simultaneous excitation of four vibrational lines. Next, we will develop a multi-line coherent Raman imaging system by using the developed multi-color source. We will perform systematic characterization and optimization to meet the design goals, and use the system to monitor synapse formation in the developing nigrostriatal pathway in vitro in order to demonstrate the power of the proposed imaging technique for enabling new and important studies of dynamic processes in brain tissues in their natural state.

Capturing ultrafast phenomena in their actual time of occurrence plays a central role in the discovery of new scientific principles and the development of new technologies. For example, the abilities to monitor transient molecular states using the pump-probe spectroscopy have provided new insight into chemical reactions. The premise for a typical pump-probe method is that identical phenomenon will be induced by identical pump pulses, affording the opportunity to study it repeatedly. However, many ultrafast phenomena are either non-repeatable or irreversible, and thus they cannot be imaged using the conventional pump-probe techniques. Compact, affordable, and single-shot ultrafast imaging technology beyond trillions of frames per second remains an unmet need for the observation of non-periodic and irreversible transient events. Merging an artificially engineered synthetic surface, called metasurface, and the compressive sensing technology, the research aims to develop a compact, metasurface-enabled, single-shot imaging system for capturing the dynamic properties of ultrafast phenomena and uncovering the unknown or hidden laws that govern such dynamics. The program will closely integrate diverse research, teaching, and outreach activities together to enhance photonics education infrastructures at the university and will form a unique platform that will make broad impacts on human resource and education. The proposed research will also generate opportunities at both the college and K-12 levels and the related results will be used for designing exhibitions for local educational events.

The overarching goal of this research program is to integrate two cutting-edge technologies – the optical metasurface and compressive sensing – together to develop a compact, cost-effective, single-shot imaging system enabling ultrahigh-speed imaging beyond trillions of frames per second. The system can be used for capturing the dynamic properties of ultrafast phenomena and uncovering the unknown or hidden laws that govern such dynamics. Empowered by the metasurface – a synthetic surface consisting of subwavelength-sized elements (meta-atoms) that locally engineer the electromagnetic response on the nanoscale – the ultrafast imaging system encodes the incoming optical information using a metasurface-enabled “pixelized” encoder both in space and in time. The encoded information is then captured by a normal camera. By leveraging the compressive sensing algorithm and the high temporal resolution of the metasurface encoder, a three-dimensional ultrafast movie revealing information in (x, y, z, t) dimensions is computationally reconstructed for an ultrafast non-periodic event, which is previously not possible.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.