Two-photon microscopy is a key imaging technique in biological sciences because of its superior deep tissue imaging capabilities in addition to high transverse and axial resolution. In recent years, development of low-weight miniature two-photon microscopes has been of great interest for in vivo imaging of brain activity. Limited by these mechanical constraints, most of the developed miniature two-photon microscopes utilize graded index objective lenses that usually have inferior optical characteristics compared to conventional refractive objective lenses.
Dielectric metasurfaces, a recent category of diffractive optical elements with enhanced capabilities, have proven versatile in various applications ranging from lensing to holography and polarization control. Their ultrathin form factor and potentially extremely low-weight make them very attractive for applications with stringent size and weight constraints. However, despite their success in various types of microscopy and imaging applications, they have not been previously utilized for multi-photon fluorescence microscopy. The main barrier for using metasurface lenses in multi-photon microscopy arises from their large chromatic dispersion that effectively makes them single-wavelength. Here we will present a double-wavelength metasurface lens especially designed to have the same focal length at 820 and 605 nm, corresponding to the excitation and emission wavelengths of a certain fluorophore. After characterizing the poly-silicon metasurface lens at both wavelengths, we used it in a two-photon microscopy setup and demonstrated its capability to capture two-photon images qualitatively similar to images taken with a conventional objective lens. We will also discuss the effects of chromatic dispersion of the metasurface lens on its two-photon imaging performance.
Polarization is an important degree of freedom of light carrying information that is usually missing in other degrees of freedom. Polarimetric imaging is the process of measuring the state of polarization of light over an extended scene. It has several applications ranging from remote sensing to biological and medical imaging because it provides various pieces of information about the light source or the objects with which the light has interacted. So far polarization cameras have been made using polarization filters, and therefore suffer from two major drawbacks. First, there is a theoretical 50% upper limit on the efficiency of devices based on polarization filters. Second, to fully determine the state of polarization, multiple layers should be integrated in order to make polarization filters for circular or elliptical polarization states. Here, we present a polarization camera made using dielectric metasurfaces that operates based on separating and focusing orthogonal polarization states instead of polarization filtering. This allows for overcoming both drawbacks of current polarization camera designs. At the core of the design lies the capability of dielectric metasurfaces to fully control the polarization and phase of light. This enables designing and fabricating superpixels that separate and focus orthogonal polarization states of light on adjacent pixels on an image sensor over a single metasurface layer. Using this technique we have demonstrated full-Stokes polarization cameras with experimental efficiencies surpassing 60%, and superpixel dimensions reaching 4.8 µm×7.2 µm. We have also used this camera to form polarization images of custom-designed polarization targets.
Despite the great advances, potentials of augmented reality to fundamentally transform the way people use computers is partially hindered by the size and weight of the AR headsets. In waveguide-based devices, the light engine constitutes a significant portion of the total volume and weight. Dielectric metasurfaces have in recent years been used to demonstrate various high performance optical elements like blazed gratings and wide field of view lenses with small thicknesses, high efficiencies, and little stray light. Here, we report our work on the design of a compact light engine based on multi-metasurface optics with wide fields of view, integrated with three monochrome μ-LED displays for red, green, and blue. The metasurfaces image the μ-LEDs on the prism or grating couplers. This design avoids an important shortcoming of μ-LEDs and metasurface lenses, i.e., each work well for a single wavelength. As an example, we present a design for 532 nm, with over 3000 resolved angular points in an 8-mm-diameter field of view, and a total volume less than 0.65 cc (<2 cc for the three wavelengths). Limited by the total internal reflection region inside a waveguide with a 1.78 refractive index, the light engine can produce an image with over 1500x1500 points over a field of view slightly larger than 85°x85° in air. To the best of our knowledge, this is the first proposal and demonstration of such a system and therefore opens the path towards exploring the potentials of the metasurface diffractive optics technology for compact AR headsets with enhanced optical capabilities.
Miniaturized optical systems with planar form factors and low power consumption have many applications in wearable and mobile electronics, health monitoring devices, and as integral parts of medical and industrial equipment. Flat optical devices based on dielectric metasurfaces introduce a new approach for realization of such systems at low cost using conventional nanofabrication techniques. In this talk, I will present a summary of our recent work on dielectric metasurfaces that enable precise control of both polarization and phase with large transmission and high spatial resolution. Optical metasurface components such as high numerical aperture lenses, efficient wave plates, components with novel functionalities, and their potential applications will be discussed. I will also present the results of our efforts on optimizing and increasing the diffraction efficiency of metasurfaces. Furthermore, by using metasurface cameras and planar retroreflectors as examples, I will discuss a vertical on-chip integration platform that introduces a new architecture for the on-chip integration of conventional and novel optical systems and enables their low-cost manufacturing.
Metasurfaces are two-dimensional arrangements of nano-scatterers that enable control of phase, amplitude, and polarization of light with high efficiency and subwavelength resolution. They have enabled diffractive optical elements with enhanced functionalities and performance. Nevertheless, metasurface diffractive optical elements share many of the properties of regular diffractive optical elements. One of these properties is the response of diffractive elements to changing the angle of illumination: if the beam incident on a grating is rotated by an angle, all diffraction orders will rotate by corresponding angles in the same direction. More precisely, because of the constant grating momentum, the change in the sine of all diffraction angles will be equal to the change in the sine of the illumination angle.
Many optical devices of interest, however, do not require this type of behavior, which makes their implementation using metasurfaces very challenging. For instance retroreflectors, which reflect light incident from any angle to the same direction, or collimators, that deflect light coming from any angle to a single given direction, do not follow the regular diffractive optics angular response. We investigate properties of single-layer metasurfaces that enable devices like retroreflectors and collimators. We show that such metasurfaces should have the ability to control the phase, as well as the derivative of phase with respect to angle. We demonstrate designs that provide such control, and use them to show devices that defy the regular response of diffractive optical devices to changes in the illumination angle.
Diffractive optical devices have many applications in various fields of optics. A fundamental property of all diffractive devices is their negative chromatic dispersion: a diffractive grating always disperses light in the opposite order compared to a refractive prism made of a material with positive (normal) dispersion. Unlike refractive devices, chromatic dispersion in diffractive devices stems from geometrical features, and cannot be controlled via the intrinsic material dispersion. In addition to the always negative sign, the amplitude of diffractive chromatic dispersion is set only by the function of the device. For instance, the angular dispersion of a grating is always given by dθ/dλ=tan(θ)/λ (where θ is the deflection angle and λ is wavelength), or the focal distance dispersion of a diffractive lens is given by df/dλ=-f/λ. Therefore, the chromatic dispersion of diffractive devices has always been set by their function (e.g. by the deflection angle for a grating or the focal distance for a lens), and could not be controlled separately. Here, we present our work on breaking this fundamental relation between the function and chromatic dispersion of diffractive devices using metasurfaces providing independent control over phase and group delays. We use a reflective dielectric metasurface to experimentally demonstrate gratings and lenses that have positive, zero, and extraordinary negative chromatic dispersion. Apart from its fundamental scientific value, this concept expands the applications of diffractive devices as it enables various types of chromatic dispersions. For instance, a special case would be a dispersionless lens operating over a wide bandwidth with the same focal distance.
Miniaturized optical systems with planar form factors and low power consumption have many applications in wearable and mobile electronics, health monitoring devices, and as integral parts of medical and industrial equipment. Flat optical devices based on dielectric metasurfaces introduce a new approach for realization of such systems at low cost using conventional nanofabrication techniques. In this talk, I will present a summary of our recent work on dielectric metasurfaces that enable precise control of both polarization and phase with large transmission and high spatial resolution. Optical metasurface components such as high numerical aperture lenses, efficient wave plates, components with novel functionalities, and their potential applications will be discussed. I will also present the results of our efforts on developing multi-wavelength and dispersion engineered metasurfaces, as well as conformal, flexible, and tunable metasurfaces. Furthermore, by using metasurface cameras and planar retroreflectors as examples, I will introduce a vertical on-chip integration platform enabled by vertical stacking of multiple metasurfaces and active optoelectronic components. This vertical integration scheme introduces a new architecture for the on-chip integration of conventional and novel optical systems and enables their low-cost manufacturing.
Diffractive optical devices based on dielectric metasurfaces have recently attracted significant attention. Small size, low weight, planar form factor, and potential for low-cost manufacturing using semiconductor fabrication techniques are some of the main features that make metasurfaces ideal candidates for implementation of low-cost miniaturized optical systems. However, to become competitive for practical applications, metasurfaces should also offer specifications (e.g. efficiency, bandwidth, and wavefront error) comparable to their refractive counterparts. We have recently demonstrated diffraction-limited metasurface lenses with high efficiency using high refractive index nano-posts. Low numerical aperture (NA) metasurface lenses have more than 90% focusing efficiency, but the efficiency of the lenses with NA>0.5 decreases with increasing NA and drops to ~40% for NA=0.9, thus resulting in a trade-off between the NA and efficiency. Here we identify the main physical origin of this trade-off as the low transmission of large diameter nano-posts for transverse-magnetic (TM) polarized light incident at large angles, and show that the low transmission is caused by the excitation of undesired high order modes in these nano-posts. To overcome this issues, we present a novel approach for evaluating different metasurface designs in implementation of high NA metasurface components. The approach is based on adiabatic approximation of aperiodic metasurfaces by periodic gratings, and considers the effect of large deflection angles. Using the proposed design approach, we experimentally demonstrate more than 75% focusing efficiency for metasurface lenses with NA=0.7, and more than 70% deflection efficiency for 50-degree beam deflectors for unpolarized light at 915 nm.