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Significance We demonstrate, with simulations corroborated by experiments, that broadband long-wave infrared (LWIR) imaging is possible with a single flat lens with a thickness of 10 μm and a weight that is over 100 times less than conventional refractive optics. Reducing the weight and thickness of LWIR optics is crucial for increasing the range of camera-carrying drones as well as for reducing head and neck injuries among camera-borne soldiers. The technology discussed herein will be extremely useful not only to optics specialists but also to camera designers and users in general., We experimentally demonstrate imaging in the long-wave infrared (LWIR) spectral band (8 μm to 12 μm) using a single polymer flat lens based upon multilevel diffractive optics. The device thickness is only 10 μm, and chromatic aberrations are corrected over the entire LWIR band with one surface. Due to the drastic reduction in device thickness, we are able to utilize polymers with absorption in the LWIR, allowing for inexpensive manufacturing via imprint lithography. The weight of our lens is less than 100 times those of comparable refractive lenses. We fabricated and characterized 2 different flat lenses. Even with about 25% absorption losses, experiments show that our flat polymer lenses obtain good imaging with field of view of 35° and angular resolution less than 0.013°. The flat lenses were characterized with 2 different commercial LWIR image sensors. Finally, we show that, by using lossless, higher-refractive-index materials like silicon, focusing efficiencies in excess of 70% can be achieved over the entire LWIR band. Our results firmly establish the potential for lightweight, ultrathin, broadband lenses for high-quality imaging in the LWIR band.

It is generally assumed that correcting chromatic aberrations in imaging requires optical elements. Here, we show that by allowing the phase in the image plane to be a free parameter, it is possible to correct chromatic variation of focal length over an extremely large bandwidth, from the visible (Vis) to the longwave infrared (LWIR) wavelengths using a single diffractive surface, i.e., a flat lens. Specifically, we designed, fabricated and characterized a flat, multi-level diffractive lens (MDL) with thickness {\pounds} 10{\mu}m, diameter ~1mm, and focal length = 18mm, which was constant over the operating bandwidth of l=0.45{\mu}m (blue) to 15{\mu}m (LWIR). We experimentally characterized the point-spread functions, aberrations and imaging performance of cameras comprised of this MDL and appropriate image sensors. We further show using simulations that such extreme achromatic MDLs can be achieved even at high numerical apertures (NA=0.81). By drastically increasing the operating bandwidth and eliminating several refractive lenses, our approach enables thinner, lighter and simpler imaging systems., Comment: arXiv admin note: substantial text overlap with arXiv:1907.06251

We demonstrate 10 cm achromatic flat multilevel diffractive lens with a focal length and thickness of 200mm and 2.4 µm, respectively operating across the entire visible band, which is the largest flat lens ever reported till date.

The f-number (f#) and numerical aperture (NA) of a conventional lens are inherently coupled. We demonstrate a free-form broadband flat lens that de-couples the f# and NA.

Inspired by compound eye structures found in insects, we demonstrate an ultrathin arrayed camera enabled by a flat multilevel diffractive microlens array for super-resolution visible imaging, with resolution improvement factor of 1.4 compared to the diffraction limit.

Treating phase in the image plane to be a free parameter, we demonstrate multilevel diffractive lenses via inverse design enabling functionalities like achromaticity, depth-of-focus, wide-angle imaging, etc. for various imaging applications in the IR and beyond.

We demonstrate efficient ultra-compact power splitters designed via machine learning algorithm viz. binary-additive reinforcement learning algorithm. Two different splitter geometries (Y- and T- junctions) are shown; each with an area footprint of 1.2 x 1.2 µm2. The simulated insertion loss is < 1dB for both.

In conventional imaging, the information transfer from the object to the image plane is accomplished either with the help of a traditional lens that performs a one-to-one mapping or an unconventional lens that performs a one-to-many mapping. In the first case, the image is formed directly, whereas in the second case, the image is formed after a computation. The conventional lens approach is preferred in most cases due to the high signal-to-noise ratio achievable at each image pixel. By appealing to the fact that for most of the imaging applications, it is only the intensity, which is measured by the detector, the phase of the field in the image or focal plane is a free parameter, something that comes from the inverse diffraction transform. Therefore, it is easy to visualize that this phase of the plane wave after it transmits the lens can have multiple forms. Hence, the final choice can be made based upon specific application tailored requirements like achromaticity, depth-of-focus, wide-angle imaging, etc. This concept was exploited to design an achromatic MDL via inverse design across almost the entire electromagnetic spectrum (λ = 450 nm to 15 μm). Furthermore, a MDL with a Field Of View (FOV) up to 50° for wide-angle imaging as well as a MDL to enable an extreme Depth of Focus (EDOF) imaging of up to 6 m in the NIR were also designed.

We present the design, fabrication, and response of a low-power, polymer-based VOC sensor based on the self-leveling of mechanically leveraged structures. The device utilizes folded polymer-coated microcantilevers to achieve passive temperature compensation without the need for additional compensating sensors or electronics. We demonstrate that a self-leveling vapor sensor provides the same gas response as a simple microcantilever geometry, showing ~20% change in device capacitance when subjected to 35-85 %RH change while showing nearly-zero baseline drift due to changes in ambient temperature when the temperature is increased from 23-72{\deg}C which is ~52-fold better than a simple microcantilever geometry. The response of the VOC sensor was measured using three polymers (Polyimide, Polyurethane, and PDMS) against five different analytes (Ethanol, Acetone, Benzene, Hexane, and Water) and an SVM-based model was used to show target specificity. The sensor also showed an absorption response time ({\tau}90) of ~138s. We propose that the self-leveling vapor sensor geometry is a significant improvement to a simple microcantilever vapor sensor as it offers the same performance but shows near-complete elimination of temperature-induced baseline drift., Comment: 15 pages, 13 figures. Manuscript accepted for publication in Sensors and Actuators B: Chemical

The resolution of a conventional lens is governed by its numerical aperture (NA) while its light-concentration ability is governed by its f-number (f/#), both of which are dependent on the diameter and focal length of the lens. Hence, these properties for a conventional lens are inherently coupled. Here, we show that by utilizing the principle of free-form broadband diffractive optical elements (BDOEs), we can create ultra-flat optical elements that are able to concentrate incoming light over a fixed aperture to different sized focal spots engineered in the shape of a square. Hence, we are able to decouple the NA and f/# of a lens by being able to independently control the size of the input aperture and the resolution of these devices. Specifically, we designed, fabricated, and characterized three BDOE lenses operating in the visible band (450nm–750nm), each having f/# of 11.25, but with NAs of 0.00075, 0.0067, and 0.054, respectively. We further use simulations to emphasize that such decoupling is possible even at much higher NAs. Experiments confirm achromatic focusing and broadband imaging. One of the lenses exhibited a depth-of-focus almost 2 orders of magnitude larger than the diffraction limit. Such BDOE lenses can eliminate the need for anti-aliasing filters and could also be very useful in focal-plane arrays with large pixel sizes, where light collection efficiency needs to be maintained. Furthermore, by abandoning rotational symmetry, one can achieve free-form geometries in the focal spot, such as a square that can more closely match the geometry of the sensor pixel.

We demonstrate an achromatic multi-level diffractive lens, which has a focal length of 25 mm, the aperture diameter of 8.93 mm, and thickness of 2.6 pm operating in the SWIR band (875 nm - 1675 nm).

By utilizing a single multi-level diffractive lens coupled along with a conventional monochrome image sensor, we demonstrate a ~1mm thick near-infrared camera with a Field Of View up to 50° and on-axis focusing efficiency > 90%.

We designed an ultra-compact and low-loss T-junction via a reinforcement learning algorithm coupled with finite difference time domain (FDTD) simulations. Simulated insertion loss is < 1dB with a device footprint of 1.2 μm x 1.2 μm.

With judicious design of a multi-level diffractive lens (MDL), it is possible to drastically enhance the depth of focus by over ~4 orders of magnitude while maintaining focus for objects that are separated by ~6m.

By allowing phase in the image plane to be a free parameter, we experimentally demonstrate multilevel diffractive lenses via inverse design in almost the entire electromagnetic spectrum for applications like achromaticity, depth-of-focus, wide-angle imaging, etc.

Liquid-filled tunable-focus lenses have been demonstrated to be suitable for autofocus eyewear applications. Traditionally, these lenses are constructed using an elastomeric polymer chamber filled with a high-index liquid. In this work, we investigate the effect of elastomeric creep on the deformation and eventual degradation of these tunable lenses. We use numerical analysis of a deformable circular disk representative of the lens and provide rigorous experimental results testing the creep property of a number of elastomers. Finally, we provide a comparative study of different elastomeric materials and select the best one for this application.

In this presentation, we will describe the design, fabrication and characterization of flat lenses that operate in any desired spectral regime. Specifically, we will describe flat lenses in the visible band, in the visible and near-IR band, and also in the LWIR band. We have shown that multi-level diffractive optics, when designed properly can enable high efficiency broadband imaging [1]. Here, we will extend the performance to the IR and show experimental results. We further compare our optics with metalenses, and emphasize that metalenses offer no additional advantage [2]. In fact, metalenses are far more challenging to fabricate. Please refer to papers below for details. [1] M. Meem, A. Majumder and R. Menon, “Full-color video and still imaging using two flat lenses,” Opt. Exp. 26(21) 26866-26871 (2018). [2] S. Banerji, M. Meem, B. Sensale-Rodriguez and R. Menon, “Imaging with flat optics: metalenses or diffractive lenses?,” arXiv:1901.05042 [physics.optics]

Recently the field of metalenses has experienced an upsurge as evidenced by the growing number of publications in the area with each passing year. In-turn diffractive lenses have been used as thin-lenses since the late 19th century. Here, we show that multi-level diffractive lenses (MDLs), when appropriately designed, can perform comparably to metalenses. Furthermore, MDLs can be designed and fabricated with larger constituent features, making them accessible to low-cost, large area / volume manufacturing, which is generally challenging for metalenses in particular at IR and visible wavelengths. Based on this approach, we demonstrate computationally designed MDLs in real-time still and video imaging in the long-wave infrared regime (8 - 12 μm).

This paper discusses the design space exploration for an ammonia optical gas sensor based on a silicon microring resonator that is covered by graphene. A comparative analysis of the transmittance sensitivity of its fundamental quasi-TE and quasi-TM modes is presented. Results show that a large fraction of the fundamental quasi-TM mode’s electrical energy is propagating outside the waveguide, which makes this geometry more sensitive to ammonia induced changes in Fermi energy of graphene. The overall transmittance intensity difference, for the fundamental quasi-TE mode is less than 1 ​dB, independent of ring radius, when the ammonia gas concentration is varied from 0.5 to 1000 ​ppm. For the fundamental quasi-TM mode, results show that a ring radius ~8.4 ​μm can provide critical coupling condition and lead to >20 ​dB modulation of transmittance intensity when the ammonia gas concentration is varied from 0.5 to 1000 ​ppm. Overall, proper selection of mode excitation as well as ring geometry is essential to attain practical sensitivity.

We demonstrate three ultra-compact integrated-photonics devices, which are designed via a machine-learning algorithm coupled with finite-difference time-domain (FDTD) modeling. By digitizing the design domain into “binary pixels,” these digital metamaterials are readily manufacturable using traditional semiconductor foundry processes. By showing various devices (beam-splitters and waveguide bends), we showcase our approach's generality. With an area footprint smaller than λ02, our designs are amongst the smallest reported to-date. Our method combines machine learning with digital metamaterials to enable ultra-compact, manufacturable devices, which could power a new “Photonics Moore's Law.”

Recently, we have shown that allowing the phase in the image plane of a flat lens to be a free parameter enables imaging properties of unprecedented versatility in flat, multilevel diffractive lenses through careful design of different diffractive surfaces. The properties we have shown in such flat lenses include super-achromaticity, extreme depth-of-focus and high numerical aperture. These flat lenses can be easily fabricated using one-step grayscale lithography.

Compound eyes found in insects provide intriguing sources of biological inspiration for miniaturized imaging systems. Inspired by such insect eye structures, we demonstrate an ultrathin arrayed camera enabled by a flat multi-level diffractive microlens array for super-resolution visible imaging. We experimentally demonstrate that the microlens array can achieve a large fill factor (hexagonal close packing with p i t c h = 120 µ m ), thickness of 2.6 µm, and diffraction-limited ( S t r e h l r a t i o = 0.88 ) achromatic performance in the visible band (450 to 650 nm). We also demonstrate super-resolution imaging with resolution improvement of ∼ 1.4 times by computationally merging 1600 images in the array.

It is generally thought that correcting chromatic aberrations in imaging requires multiple surfaces. Here, we show that by allowing the phase in the image plane of a flat lens to be a free parameter, it is possible to correct chromatic aberrations over a large continuous bandwidth with a single diffractive surface. In contrast to conventional lens design, we utilize inverse design, where the phase in the focal plane is treated as a free parameter. This approach attains a phase-only (lossless) pupil function, which can be implemented as a multi-level diffractive flat lens that achieves achromatic focusing and imaging. In particular, we experimentally demonstrate imaging using a single flat lens of diameter > 3 mm and focal length = 5 mm (NA = 0.3, f/1.59) that is achromatic from λ = 450 nm (blue) to 1 μm (NIR). This simultaneous achievement of large size, NA, and broad operating bandwidth has not been demonstrated in a flat lens before. We experimentally characterized the point-spread functions, off-axis aberrations, and broadband imaging performance of the lens.

In contrast to conventional bulk optics, achromatic planar diffractive lenses operating in the LWIR (8um-12um) regime has been demonstrated in this work, which provides an efficient way for the development of compact, ultra-lightweight LWIR devices.

Flat lenses enable thinner, lighter, and simpler imaging systems. However, large-area and high-NA flat lenses have been elusive due to computational and fabrication challenges. Here we applied inverse design to create a multi-level diffractive lens (MDL) with thickness < 1 . 35 µ m , diameter of 4.13 mm, and N A = 0.9 at wavelength of 850 nm (bandwidth ∼ 35 n m ). Since the MDL is created in polymer, it can be cost-effectively replicated via imprint lithography.

A lens performs an approximately one-to-one mapping from the object to the image plane. This mapping in the image plane is maintained within a depth of field (or referred to as depth of focus, if the object is at infinity). This necessitates refocusing of the lens when the images are separated by distances larger than the depth of field. Such refocusing mechanisms can increase the cost, complexity, and weight of imaging systems. Here we show that by judicious design of a multi-level diffractive lens (MDL) it is possible to drastically enhance the depth of focus by over 4 orders of magnitude. Using such a lens, we are able to maintain focus for objects that are separated by as large a distance as ∼ 6 m in our experiments. Specifically, when illuminated by collimated light at λ = 0.85 µ m , the MDL produced a beam, which remained in focus from 5 to 1200 mm. The measured full width at half-maximum of the focused beam varied from 6.6 µm (5 mm away from the MDL) to 524 µm (1200 mm away from the MDL). Since the side lobes were well suppressed and the main lobe was close to the diffraction limit, imaging with a horizontal × vertical field of view of 40 ∘ × 30 ∘ over the entire focal range was possible. This demonstration opens up a new direction for lens design, where by treating the phase in the focal plane as a free parameter, extreme-depth-of-focus imaging becomes possible.

Polarization controlling devices such as polarization splitters and rotators are critical elements in integrated-photonic circuits that function via polarization-diversity schemes. Here, we present the design of an ultra-compact nanophotonic-polarization rotator (NPR) that rotates the polarization state from TE to TM with a simulated extinction ratio of 23dB over a coupling length of 5µm and an operating bandwidth of 40nm. This all-silicon device can be fabricated in a single lithography step and we have fabricated and characterized a preliminary device exhibiting 9dB extinction ratio. To emphasize the generality of our methodology, we also designed a NPR that can rotate the polarization state from TM to TE as well. A small device footprint is enabled by the evanescent coupling of guided modes enabled by computationally designed digital metamaterials.

We demonstrate visible and near-IR image projection via non-absorbing, multi-level broadband diffractive-optical elements (BDOEs) in one or more planes. By appropriate design of the BDOE topography, we experimentally demonstrate (1) different images in different spectral bands, (2) different images in different image planes, (3) image magnification by changing the distance between the illumination source and the BDOE, (4) completely flat BDOE via an index-contrast top coating, and (5) reflective BDOEs. All of these are accomplished with broadband illumination. Furthermore, the BDOEs are highly efficient and versatile and can be inexpensively mass manufactured using imprint-based replication techniques.

We designed and fabricated a flat multi-level diffractive lens (MDL) that is achromatic in the SWIR band (875 nm to 1675 nm). The MDL had a focal length of 25 mm, aperture diameter of 8.93 mm, and thickness of only 2.6 µm. By pairing the MDL with a SWIR image sensor, we also characterized its imaging performance in terms of the point-spread functions, modulation-transfer functions, and still and video imaging.

Recently, there has been an explosion of interest in metalenses for imaging. The interest is primarily based on their subwavelength thicknesses. Diffractive gratings have been used as thin optical elements since the late 19th century. Here, we show that multilevel diffractive lenses (MDLs), when designed properly, can exceed the performance of metalenses. Furthermore, MDLs can be designed and fabricated with larger constituent features, making them accessible to low-cost, large-area volume manufacturing, which is generally challenging for metalenses. The support substrate will dominate overall thickness for all flat optics. Therefore, the advantage of a slight decrease in thickness (from ∼2λ to ∼λ/2) afforded by metalenses may not be useful. We further elaborate on the differences between these approaches and clarify that metalenses have unique advantages when manipulating the electromagnetic fields, rather than intensity.

Absorbance-modulation-optical lithography (AMOL) uses the opposing reactions between two isomeric states (absorbing and transparent) of a photochrome to confine light to sub-diffraction limited dimensions. The extent of light confinement is controlled by the ratio of the intensities of the confining and the exposing beams. Traditionally, high intensity in the confining beam is required due to the low quantum yield of the corresponding photo-reaction. Here, we report AMOL using low-light intensities, enabled by a novel photochrome with well-matched quantum yields. We provide rigorous simulations and experiments to demonstrate ∼ λ/4.5 feature-sizes at approximately 1/4th the light intensities required in conventional AMOL.

We report on our latest developments in computational metamaterials based nanophotonics devices. Non-linear optimization is used to design passive metamaterials for ultracompact on-chip polarization rotator and waveguide cloaks. We also report a novel method of implementing active control to such devices and present the design of an all-optical modulator. Our devices exhibit efficient performance at a much smaller footprint compared to similar conventional devices.

Computationally designed metamaterials are used to realize a number of novel nanophotonic devices. We report on the development of ultra-compact on-chip polarization rotators, waveguide cloaks and all optical modulators made possible by the use of a non-linear optimization algorithm.

This paper presents a comprehensive model and simulation results of the performance and governing factors of the photochromic layer in Absorbance-Modulation-Optical-Lithography (AMOL) and examines the possibility of performing AMOL at low light intensities.

This paper reports two new techniques of performing the sub-wavelength lithographic process - Absorbance Modulation Optical Lithography (AMOL) without barrier layers and using separable substrates and its most recent developments and characterization.

Absorbance-Modulation-Optical Lithography (AMOL) has been previously demonstrated to be able to confine light to deep sub-wavelength dimensions and thereby, enable patterning of features beyond the diffraction limit. In AMOL, a thin photochromic layer that converts between two states via light exposure is placed on top of the photoresist layer. The long wavelength photons render the photochromic layer opaque, while the short-wavelength photons render it transparent. By simultaneously illuminating a ring-shaped spot at the long wavelength and a round spot at the short wavelength, the photochromic layer transmits only a highly confined beam at the short wavelength, which then exposes the underlying photoresist. Many photochromic molecules suffer from a giant mismatch in quantum yields for the opposing reactions such that the reaction initiated by the absorption of the short-wavelength photon is orders of magnitude more efficient than that initiated by the absorption of the long-wavelength photon. As a result, large intensities in the ring-shaped spot are required for deep sub-wavelength nanopatterning. In this article, we overcome this problem by using the long-wavelength photons to expose the photoresist, and the short-wavelength photons to confine the “exposing” beam. Thereby, we demonstrate the patterning of features as thin as λ/4.7 (137nm for λ = 647nm) using extremely low intensities (4-30 W/m2, which is 34 times lower than that required in conventional AMOL). We further apply a rigorous model to explain our experiments and discuss the scope of the reverse-AMOL process.

Optical lithography is the most prevalent method of fabricating micro-and nano-scale structures in the semiconductor industry due to the fact that patterning using photons is fast, accurate and provides high throughput. However, the resolution of this technique is inherently limited by the physical phenomenon of diffraction. Absorbance-Modulation-Optical Lithography (AMOL), a recently developed technique has been successfully demonstrated to be able to circumvent this diffraction limit. AMOL employs a dual-wavelength exposure system in conjunction with spectrally selective reversible photo-transitions in thin films of photochromic molecules to achieve patterning of features with sizes beyond the far-field diffraction limit. We have developed a finite-element-method based full-electromagnetic-wave solution model that simulates the photo-chemical processes that occur within the thin film of the photochromic molecules under illumination by the exposure and confining wavelengths in AMOL. This model allows us to understand how the material characteristics influence the confinement to sub-diffraction dimensions, of the transmitted point spread function (PSF) of the exposure wavelength inside the recording medium. The model reported here provides the most comprehensive analysis of the AMOL process to-date, and the results show that the most important factors that govern the process, are the polarization of the two beams, the ratio of the intensities of the two wavelengths, the relative absorption coefficients and the concentration of the photochromic species, the thickness of the photochromic layer and the quantum yields of the photoreactions at the two wavelengths. The aim of this work is to elucidate the requirements of AMOL in successfully circumventing the far-field diffraction limit.

Absorbance-Modulation-Optical Lithography (AMOL) enables super-resolution optical lithography by simultaneous illumination of a photochromic film by a bright spot at one wavelength, λ1 and a node at another wavelength, λ2. A deep subwavelength region of the transparent photochromic isomer is created in the vicinity of the node. Light at λ1 penetrates this region and exposes an underlying photoresist layer. In conventional AMOL, a barrier layer is required to protect the photoresist from the photochromic layer. Here, we demonstrate barrier-free AMOL, which considerably simplifies the process. Specifically, we pattern lines as small as 70nm using λ1 = 325nm and λ2 = 647nm. We further elucidate the minimum requirements for AMOL to enable multiple exposures so as to break the diffraction limit.

This paper reports the application of absorbance modulation optical lithography (AMOL) in maskless photon-based patterning of non-periodic two-dimensional arbitrary patterns by using a micromechanical-mirror array-based optical system and absorbance modulation.