Space-time wave packets (STWPs) are pulsed fields with non-separable spatio-temporal structure that yields exotic and useful behavior. To date, STWPs have been synthesized using bulky free-space optical systems that require precise alignment. Here, we describe a compact system based on an alternative strategy for spatially resolving the spectrum, which makes use of a volume chirped Bragg grating with Bragg structure rotated by 45degree with respect to device facets. Consequently, the field exits spectrally-resolved from a facet orthogonal to the entrance. We synthesize STWPs using a pair of such devices occupying a volume of 25×25×8mm3, which is orders-of-magnitude smaller than previously.
Recently proposed nonvolatile chalcogenide phase change materials Sb2Se3 and Sb2S3 exhibit low loss and significant refractive index modulation in the visible and NIR, which paves the way for the development of novel reconfigurable on-chip nanophotonic devices and programmable optical devices. Here, we discuss our recent investigations in terms of such devices, in particular, the realization of a compact (3 μm × 3 μm) integrated silicon nanophotonic 1 × 2 optical switch with phase change material Sb2Se3, and programmable multilevel diffractive optical lenses and holograms with phase change material Sb2S3.
Traditional refractive lenses are bulky owing to their curvature. Flat diffractive lenses can overcome this difficulty, but traditional diffractive optics have limited reach, primarily due to chromaticity. Recently, we have shown that by treating the “imaging” phenomenon as simply information transfer from the object to the image plane, the spatial distribution of the phase in the focal plane can be an arbitrary function. Using this concept, 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 (MDLs). Our research group has demonstrated multi-level diffraction lenses in multiple high performance categories: unchromatic lenses with dramatically improved operating bandwidths, high NA and large aperture sizes, and extreme depth of focus.Furthermore, these can be combined with advanced machine-learning algorithms to enhance inferencing.
Experimental measurements related to the opto-mechanical stability of thin film diffractive beam-riders are discussed. Our theoretical predictions of radiation pressure forces indicate that these structures allow a perturbed laser-driven light sail to remain in the beam path. Radiation pressure forces of both a liquid crystal polymer bi-grating and an etched photoresist axicon diffraction grating will be described. Our experiments made use of a vacuum torsion oscillator having sub-nano-Newton sensitivity. The parametric damping of both systems will also be described. Our measurements validate the technical feasibility of a laser-driven light sail based on diffractive thin films.
Space-time (ST) wave packets are propagation-invariant pulsed beam endowed with a tight correlation between the underlying spatial and temporal frequencies. We present a theoretical formulation and experimental demonstration of sub-millimeter incoherent ST light sheets traversing 110 m without significant diffraction. We synthesize ST fields of beam width ~ 0.7 mm from a superluminescent diode with a bandwidth of ~20 nm centered at 840 nm and observe propagation-invariant behavior across a distance of over 100-m distance in free-space. Finally, we study the effect of the aperture on the propagation distance and far-field beam size of ST light sheets.
KEYWORDS: Modulation transfer functions, Point spread functions, Near infrared, Long wavelength infrared, Lenses, Infrared radiation, Infrared imaging, Diffraction, Visible radiation
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.
Imaging is the transfer of information from the object to the sensor. This transfer is typically mediated by a lens. However, this is not the only option. In this presentation, we will describe alternatives that can enable unusual imaging systems with concomitant advantages including a needle microscope, hyperspectral cameras and optics-less cameras. These imaging systems produce data that are not easily interpretable by humans, but nevertheless could be valuable for inferencing. We will explore the co-optimization of algorithms with hardware to enable previously impossible imaging tasks.
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]
Micropatterning on oblique and multiplane surfaces remains a challenge in microelectronics, microelectromechanics, and photonics industries. We describe the use of numerically optimized diffractive phase masks to project microscale patterns onto photoresist-coated oblique and multiplane surfaces. Intriguingly, we were able to pattern a surface at 90 deg to the phase mask, which suggests the potential of our technique to pattern onto surfaces of extreme curvature. Further studies show that mask fabrication error of below 40-nm suffices to conserve pattern fidelity. A resolution of 3 μm and a depth-of-focus of 55 μm are essentially dictated by the design parameters, the mask generation tool, and the exposure system. The presented method can be readily extended for simple and inexpensive three-dimensional micropatterning.
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