Qiaochu Li

Printed photonic elements: Nanoimprinting and beyond

In order to manufacture large-scale photonic devices of various dimensions at a low cost, a number of patterning techniques have been developed. Nanoimprint lithography is among the most promising given its unique advantages, such as high resolution, fast processing speed, high throughput, compatibility with diverse materials, and low cost. This review covers various aspects of nanoimprint lithography, including its operational principles, material requirements, and different ways of implementation. Nanoimprint lithography facilitates numerous high-performance and low-cost photonic elements, including optical interconnects, sensors, solar cells, and metamaterials. In addition, other related patterning techniques, together with their utilization for photonic device fabrication and their integration with nanoimprint lithography, are briefly discussed.

Reconfigurable thermo-optic polymer switch based true-time-delay network utilizing imprinting and inkjet printing

Reconfigurable true-time-delay lines, comprising of 2×2 thermo-optic polymer switches and rib waveguides are fabricated utilizing a combination of roll-to-roll (R2R) compatible UV imprinting and ink-jet printing, which promises high throughput and low cost photonic devices.

Out-coupling of Longitudinal Photoacoustic Pulses by Mitigating the Phase Cancellation

Waves of any kinds, including sound waves and light waves, can interfere constructively or destructively when they are overlapped, allowing for myriad applications. However, unlike continuous waves of a single frequency, interference of photoacoustic pulses is often overlooked because of their broadband characteristics and short pulse durations. Here, we study cancellation of two symmetric photoacoustic pulses radiated in the opposite direction from the same photoacoustic sources near a free surface. The cancellation occurs when one of the two pulses is reflected with polarity reversal from the free surface and catches up with the other. The cancellation effect, responsible for reduced signal amplitudes, is systematically examined by implementing a thin transparent matching medium of the same acoustic impedance. By changing the thickness of the transparent layer, the overlap of the two symmetric pulses is controlled. For optimized matching layers, the cancellation effect can be significantly reduced, while the resulting output waveform remains unchanged. Similar to the planar absorber, different dimensional absorbers including cylinders and spheres also exhibit the cancellation between the outward and inward waves. This work could provide further understanding of photoacoustic generation and a simple strategy for increasing photoacoustic signal amplitudes.

Laser-Induced Focused Ultrasound for Cavitation Treatment: Toward High-Precision Invisible Sonic Scalpel

Beyond the implementation of the photoacoustic effect to photoacoustic imaging and laser ultrasonics, this study demonstrates a novel application of the photoacoustic effect for high-precision cavitation treatment of tissue using laser-induced focused ultrasound. The focused ultrasound is generated by pulsed optical excitation of an efficient photoacoustic film coated on a concave surface, and its amplitude is high enough to produce controllable microcavitation within the focal region (lateral focus <100 µm). Such microcavitation is used to cut or ablate soft tissue in a highly precise manner. This work demonstrates precise cutting of tissue-mimicking gels as well as accurate ablation of gels and animal eye tissues.

High optical coupling efficiency quasi-vertical taper for polymer waveguide devices

Quasi-Vertical tapers are designed to enable high coupling efficiency from a conventional single mode fiber into a single mode polymer rib waveguide. A triangular region fabricated under the single mode waveguide is adopted to adiabatically transform the fiber mode into the polymer rib waveguide mode. This structure works as an optical mode transformer. Because the trenches are deeper at the facets than at the active regions of the waveguide, the waveguide mode size in vertical direction becomes larger at the facets and can better match the input and output fiber mode. A coupling efficiency of 82.95% is achievable with a tip width of 1 μm.

Characterizing cellular morphology by photoacoustic spectrum analysis with an ultra-broadband optical ultrasonic detector

Photoacoustic spectrum analysis (PASA) has been demonstrated as a new method for quantitative tissue imaging and characterization. The ability of PASA in evaluating micro-size tissue features was limited by the bandwidth of detectors for photoacoustic (PA) signal acquisition. We improve upon such a limit, and report on developments of PASA facilitated by an optical ultrasonic detector based on micro-ring resonator. The detectors broad and flat frequency response significantly improves the performance of PASA and extents its characterization capability from the tissue level to cellular level. The performance of the system in characterizing cellular level (a few microns) stochastic objects was first shown via a study on size-controlled optically absorbing phantoms. As a further demonstration of PASAs potential clinical application, it was employed to characterize the morphological changes of red blood cells (RBCs) from a biconcave shape to a spherical shape as a result of aging. This work demonstrates that PASA equipped with the micro-ring ultrasonic detectors is an effective technique in characterizing cellular-level micro-features of biological samples.

Printed polymer photonic devices for optical interconnect systems

Polymer photonic device fabrication usually relies on the utilization of clean-room processes, including photolithography, e-beam lithography, reactive ion etching (RIE) and lift-off methods etc, which are expensive and are limited to areas as large as a wafer. Utilizing a novel and a scalable printing process involving ink-jet printing and imprinting, we have fabricated polymer based photonic interconnect components, such as electro-optic polymer based modulators and ring resonator switches, and thermo-optic polymer switch based delay networks and demonstrated their operation. Specifically, a modulator operating at 15MHz and a 2-bit delay network providing up to 35.4ps are presented. In this paper, we also discuss the manufacturing challenges that need to be overcome in order to make roll-to-roll manufacturing practically viable. We discuss a few manufacturing challenges, such as inspection and quality control, registration, and web control, that need to be overcome in order to realize true implementation of roll-to-roll manufacturing of flexible polymer photonic systems. We have overcome these challenges, and currently utilizing our inhouse developed hardware and software tools, <10μm alignment accuracy at a 5m/min is demonstrated. Such a scalable roll-to-roll manufacturing scheme will enable the development of unique optoelectronic devices which can be used in a myriad of different applications, including communication, sensing, medicine, security, imaging, energy, lighting etc.

Photoacoustic spectrum analysis for microstructure characterization using ultra-broad bandwidth optical ultrasonic detector

Photoacoustic spectrum analysis (PASA) has been found to have the ability to characterize microstructures in phantoms. The ability of PASA technique which is subjected to the ultrasound detector has been reported in identify tissue with hundreds of microns in size. This paper demonstrated the feasibility of micro-ring, an ultrasonic detector with ultra-broad bandwidth, in characterizing microspheres’ sizes ranging from few microns to 100 microns using PASA technique and the results were compared with a commercial hydrophone. In order to further verify the capability of micro-ring, spectrum of single micro-spheres with sizes of tens of microns were measured and compared to simulation result. Our work proves that micro-ring based PASA technique has the ability of differentiating the particles with different size in phantoms.

Laser-generated focused ultrasound for micro-cavitation and its application to high-precision cavitation treatment

Histotripsy is considered a non-invasive therapeutic modality capable of mechanically fractionating tissue using cavitation bubbles induced by high intensity focused ultrasound pulses. Histotripsy-induced cavitation region over relatively large focal volume is effective in treating sizable lesion, while compromising its treatment precision. We demonstrate high-precision cavitation treatment using laser-generated focused ultrasound.

Printed large-area flat optical component: Metasurfaces for cylindrical vector beam generation

We develop a cost-effective and highly reproducible method for manufacturing large-area dielectric metasurfaces. A high-performance silicon metasurface for generating cylindrical polarized vector beam is demonstrated as an example.