Research
Metasurfaces & Flat Optics
Structured interfaces with sub-wavelength features enable modulations of phase, amplitude, polarization, dispersion, etc. on demand, leading to a plethora of flat optics and metasurfaces. By controlling the phase-shifting of each meta-atom at wavelength scale, one can realize metasurfaces that would enable complex wavefront engineering. Looking back upon the 12 years development of metasurfaces, several functions and physics of elements or meta-atoms were investigated and used as metasurfaces to improve commercial optical components (lenses, waveplates, filters, and the like).
超穎介面 & 平面光學
在更輕更薄的元件上更高效率操控光一直是人們的願景，無論在顯微生醫影像、通訊技術、光電元件提升效率或消費光電產品等應用領域上，找尋更高效率的光調控元件為極重要。而隨著奈米科技與奈米光學的發展，我們已可利用次波長的奈米結構形成的「介面」，來任意操控光的相位、振幅、偏振、色散等，這個領域就叫做「超穎介面 (metasurfaces)」。超穎介面提供人們超越以往對於「介面」的認知，可以達成各種看似「異常」的光操控。這與「超穎材料 (metamaterials)」相似，是近10年蓬勃發展的研究領域，然而不同的是，超穎材料偏重結構的「週期性」，利用集體共振的形式來操控光；而超穎介面則著重於結構的「二維分佈」以及各個奈米結構的「獨立性」，利用奈米結構近場的相位、振幅與偏振來調控遠場的光，有著繞射光學的性質，又不侷限於傳統繞射光學元件的限制。藉由這些高度只有波長尺度的奈米結構形成的光學與光學元件，我們統稱為平面光學 (flat optics)。
超穎介面是如何在介面上操控光的呢？我們在光子學有學到惠更斯-菲涅耳原理 (Huygens-Fresnel principle)：波前的每個點會形成新的點波源，相干疊加後在空間中重新建構出下一個波前，又稱為二次波。當光從一個材料傳播到另一個不同折射率的材料，在介面上也會重新建構二次波，因為光在兩材料折射率不同、走的速度不同，折射光的波前就會往不同方向前進，此原理可以解釋折射 (refraction) 與反射 (reflection) 現象，也就有了司乃耳定律 (Snell’s law)。而超穎介面的原理相似，超穎介面在介面上有各種不同的奈米結構，光在不同結構中有的走得快，有的走得慢，形成所謂的「相位差」，這些不同相位的新點波源也會相干疊加，在空間中形成各種波形的二次波。如果我們將空間相位分佈設計成透鏡所需相位，就會是超穎透鏡(metalens)，我們也可以設計各種不同空間相位分佈，製作超穎全像片 (metahologram)、光束轉向 (beam steering)，甚至可以產生光學渦旋 (vortex beam)。而這些利用超穎介面形成的光學與光學元件，我們統稱為平面光學 (flat optics)。
近年來，平面光學在健康檢測、自動車、穿戴裝置、測距應用、雷射光等領域都有前沿的發展。隨著更輕更薄更多元的超穎介面平面光學推陳出新，加快了傳統光學的演進，先進科技與下一世代智慧光學系統會一直不斷革新，期待將來帶給人們更優質便利的生活。
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Research Topics
INNOVATIVE AREA THAT ARE OF INTEREST TO US
Imaging & Sensing
Compact depth sensing and facial recognition
3D surface imaging is a common tool used in smartphone facial recognition, as well as in computer vision and autonomous driving. These systems typically consist of a dot projector that contains multiple components: a laser, lenses, a light guide and a diffractive optical element (DOE). The DOE is a special kind of lens that breaks the laser beam into an array of about 32,000 infrared dots. So, when a person looks at a locked screen, the facial recognition system projects an array of dots onto most of their face, and the device’s camera reads the pattern created to confirm the identity. However, dot projector systems are relatively large for small devices such as smartphones. So, our group set out to develop a more compact facial recognition system that would be nearly flat and require less energy to operate.
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Press release from ACS PressPacs and others "A sleeker facial recognition technology tested on Michelangelo’s David"
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中文簡介 from 光學工程 LiDAR 光達特輯
Structured light
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dot projector
Time-of-flight (ToF) and structured light (SL) technology are examples of depth sensors that use active light sources. SL technology stands out for its high resolution, large field-of-view (FOV), and low computational demands, making it ideal for applications like smartphones and single-shot facial recognition. The core of SL-based systems comprises miniaturized dot projectors and cameras. Current commercial dot projectors still rely on diffractive optical elements (DOEs) and vertical-cavity surface-emitting laser (VCSEL) array. The size and optical power of each VCSEL is small. But when arranged in an array, they collectively provide sufficient output power. However, the divergence angle of each spot from each VCSEL is about 20 degrees. Therefore, it needs additional collimation lens and light guide. This makes the whole dot projector bulky and makes the large notch on top screen of smart phones.
We develop a metasurface-based dot projector, incorporating metasurface with photonic crystal surface-emitting laser (PCSEL). The divergence angle of a beam from PCSEL is only 0.3 degree. So, there is no need for further collimation lens or light guide. The metasurface area is much smaller than the DOE and light guide area in commercial one, which is about 233 times smaller. And the power consumption of a PCSEL is much lower than that of VCSEL array in commercial one, which is about 10 times lower.
References:
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Press release from ACS PressPacs and others "A sleeker facial recognition technology tested on Michelangelo’s David"
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中文簡介 from 光學工程 LiDAR 光達特輯
Compact depth camera inspired by spiders
Metalens sensor could be used for microrobotics, augmented reality, wearable devices. The device combines a multifunctional, flat metalens with an ultra-efficient algorithm to measure depth in a single shot.
Many of today’s depth sensors, such as those in phones, cars and video game consoles, use integrated light sources and multiple cameras to measure distance. Face ID on a smartphone, for example, uses thousands of laser dots to map the contours of the face. This works for large devices with room for batteries and fast computers, but what about small devices with limited power and computation, like smart watches or microrobots? Evolution, as it turns out, provides a lot of options.
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Press release from SEAS Harvard News
Biomedical imaging:
Nano-optic endoscopes
Improvement in the accuracy of endoscopic biopsy for small peripheral lesions is necessary if bronchoscopy will play a major role in lung cancer diagnosis. Endoscopic optical coherence tomography (OCT) with commercial catheters that rely on graded-index (GRIN) lenses or ball lenses, however, exhibit strong astigmatism and spherical aberration and thus deviate from diffraction-limited focusing. Shown is an artistic impression of the nano-optic endoscope that uses a metalens, with the ability to modify the phase of incident light at subwavelength level, to enable high-resolution endoscopic imaging at extended depth-of-focus by avoiding monochromatic aberrations. High-resolution three-dimensional images are captured by inserting the nano-optic endoscope into the lungs endo-bronchially visualize airway tissue microstructures. The combination of the superior resolution and higher imaging depth of focus of the nano-optic endoscope is likely to increase the clinical utility of endoscopic optical imaging.
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Algorithm
High-Q colors
The high-Q metasurface demonstrates exceptional color selectivity with angular multifunctionality, producing vivid colors at narrow angles and four narrow-band wavelengths. The high-Q phenomenon, illustrated by the distinct spectral peaks and colored samples, is exemplified by resonant waveguide gratings. Efficiency improvements, achieved through advanced topology optimization, result in significant enhancements.
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Algorithm enables the phenomenon
Through adjoint-based topology optimization, while considering nonlocal effects, we significantly enhance its diffraction efficiency, achieving numerical efficiencies up to 78% and Q-factors as high as 1362. Experimentally, we demonstrated efficiencies of up to 59% with a Q-factor of 93. Additionally, we applied our topology-optimized metasurface to color selectivity, producing vivid colors at 4 narrow-band wavelengths.
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New physics from machine intelligence
We demonstrate a metasurface that can be continuously tuned from linear to elliptical birefringence, opening up the entire space of polarization control with just one device. We are able to tailor broad polarization behavior of a material beyond what naturally exists, which has a lot of practical benefits. What used to require three separate conventional birefringent components now only takes one.
Machine learning and computational algorithm play an important role in metasurface design. Topological optimization, one kind of machine intelligence, is an inverse approach using adjoint method. We start with what we want the metasurface to do and then the algorithm explores the huge parameter space to develop a pattern that can best deliver that function.
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Press release from SEAS Harvard News
Algorithms empower metalens design
We’ve been guided by intuition-based design, relying heavily on one’s training in physics, which has been limited in the number of parameters that can be considered simultaneously, bounded as we are by human working memory capacity. Inverse design aims at optimizing meta-optics design but has been currently limited by expensive brute-force numerical solvers to small devices, which are also difficult to realize experimentally.
To overcome those limitations, we teach a computer program the physics of metasurface design. The program uses the foundation of physics to generate metasurface designs automatically, designing millions to billions of parameters simultaneously.
This is an inverse design process, meaning the process starts with a desired function of the metalens — such as a lens that can correct chromatic aberration — and the program finds the best design geometries to achieve that goal using its computational algorithms.
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Press release from SEAS Harvard News
Structured Light
Backward
The first Janus metasurfaces demonstrate directionally asymmetric orbital angular momentum generation from arbitrary spin. Titania-based meta-atoms enable operation in the visible range, extending beyond prior demonstrations in microwave and mid-infrared regions. Meta-atoms provide local/global rotation and width control, achieving global directional multifunctionality tailored by the Jones matrix. This multifunctionality in a single-layer design surpasses previous approaches based on spatial multiplexing or vertical stacking, expanding its applicability across diverse domains.
Reference:
Advanced Optical Materials early view (2024) - LINK & PDF
Forward
Janus metasurfaces
J-plates: Asymmetric OAM generator
Structured light refers to the tailoring or shaping of light in all its degrees of freedom, which can be used for micromanipulation and enhancing the capacity of optical communication channels. Chiral light is foremost among the family of structured light fields which carries spin angular momentum and orbital angular momentum (OAM). The metasurface J-plate is a metasurface converter for optical states that couples between arbitrary spin and optical angular momentum states of light in a compact planar. J-plates overcome a key limitation in alternative technologies such as Q-plates and spatial light modulators: conjugate symmetry of light’s angular momentum. And the relatively small pixel size of J-plates improves the beam quality, efficiency, and capability of generating higher OAM states.
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Metasurface vortex laser
Using a J-plate, a new metasurface laser that produces high-purity and non-symmetric super-chiral light never yet observed from lasers, creating of arbitrary spin-orbital chiral states of structured light at the source. Our laser conveniently outputs in the visible, offering a compact and power-scalable source that harnesses intra-cavity structured matter for the creation of arbitrary chiral states of structured light. The metasurface-enhanced laser is a new milestone in the history of structured light lasers as it breaches spin-orbit coupling symmetry as well as sets up a novel record for what high-purity and high-order OAM states can be created from a laser.
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AR/VR Technology
Virtual reality (VR)
Virtual and augmented realities are rapidly developing technologies, but their large-scale penetration will require lightweight optical components with small aberrations. Our presented VR platform is based on a meta-eyepiece and a laser back-illuminated micro-LCD, which offers many desirable features, including compactness, light weight, high resolution, wide color gamut, and more.
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Augmened reality (AR) and VR
Unlike electronics that has rapidly evolved and shrunk in size following the Moore’s law over the past decades, the appearance and the underlying physics of today’s optical lenses are similar to the Nimrud lens dating back to ~3000 years ago. This challenge has caused a bottleneck in the development of next-generation optical systems such as wearable displays for virtual reality, which require compact, lightweight, and cost-effective components.
We demonstrate a meta-eyepiece that is capable of achromatic focusing of blue, green, and red light through the exploration of new design physics – co-engineering of dispersion and zone interference. We also developed a miniaturized full-color fiber scanning near-eye display inspired by bio-medical endoscopic imaging techniques. This display exhibits high resolution, high brightness, high dynamic range, and a wide color gamut. The combined system comprising meta-optics and a novel near-eye display may find future deployment in VR/AR headsets and other consumer electronics.
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