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We demonstrate the implementation of a low-power, low-profile, varifocal liquid-crystal Fresnel lens stack suitable for tunable imaging in smart contact lenses. The lens stack consists of a high-order refractive-type liquid crystal Fresnel chamber, a voltage-controlled twisted nematic cell, a linear polarizer and a fixed offset lens. The lens stack has an aperture of 4 mm and thickness is ∼980 µm. The varifocal lens requires ∼2.5 VRMS for a maximum optical power change of ∼6.5 D consuming electrical power of ∼2.6 µW. The maximum RMS wavefront aberration error was 0.2 µm and the chromatic aberration was 0.008 D/nm. The average BRISQUE image quality score of the Fresnel lens was 35.23 compared to 57.23 for a curved LC lens of comparable power indicating a superior Fresnel imaging quality.

We utilized inverse design to engineer the point-spread function (PSF) of a low-f-number, freeform diffractive microlens in an array, so as to enable extended depth of focus (DOF). Each square microlens of side 69 µm and focal length 40 µm (in a polymer film, n∼1.47) generated a square PSF of side ∼10 µm that was achromatic over the visible band (450 to 750 nm), and also exhibited an extended DOF of ∼ ± 2 µm. The microlens has a geometric f/# (focal length divided by aperture size) of 0.58 in the polymer material (0.39 in air). Since each microlens is a square, the microlens array (MLA) can achieve 100% fill factor. By placing this microlens array (MLA) directly on a high-resolution print, we demonstrated integral imaging with applications in physical security. The extended DOF preserves the optical effects even with expected film-thickness variations, thereby increasing robustness in practical applications. Since these multi-level diffractive MLAs are fabricated using UV-nanoimprint lithography, they have the potential for low-cost large volume manufacturing.

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.

Supplementary Information

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.

We discuss recent progress on the design of passive integrated photonic devices and phase-change material based active devices enabled by digital metamaterials. This is a promising approach offering advantages in terms of efficiency and footprint.

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.

In Opt. Lett. 44, 5450 (2019)OPLEDP0146-959210.1364/OL.44.005450, there were errors in the author listing and in one figure.

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.

Supplementary Information

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.

This paper reports an inexpensive method for stiction suppression in MEMS devices using the sublimation of camphor. Suspended microcantilever structures which would otherwise collapse to the substrate due to stiction, were successfully released using this method. The technique uses deposition of camphor nanoparticles acting as temporary supports under suspended microstructures using an immersion process. The sample is then dried, and the camphor sublimates under heating, preventing device failure due to capillary forces. This new method does not require any specialized equipment or additional processing steps; therefore, it is simpler and more economical than alternate conventional release methods.

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).

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.

This erratum contains corrections to Optica 7, 252 (2020).OPTIC82334-253610.1364/OPTICA.388697

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.

A conventional motorized wheelchair has been fitted with sensors and programmed with an intelligent guidance system to efficiently maneuver itself automatically from one point to another in a facility equipped with a grid of sensors that provide the wheelchair with the basic map of its course. The device described in this paper has been conceptualized such that once the wheelchair is given information regarding the starting and stopping point in a controlled facility, the wheelchair with this pre programmed information can efficiently construct a path towards its destination and automatically drive to that point from its present position while avoiding obstacles in its path and negotiating any turns and bends that it encounters in its course. This is achieved by means of sensors (IR and sonic) located at strategic points on the chair, circuits that control the speed of the motors, and a set of microcontrollers programmed to execute the different functions of the wheelchair. The facility in which the wheelchair works has been fitted with a set of sensors that form the basis of the network which is used by the program governing the wheelchair’s automated movement to provide guidance to it by means of a course map.