The rapid development of display technologies has raised interest in arrays of self-emitting, individually controlled light sources atthe microscale. Gallium nitride (GaN) micro-light-emitting diode (LED) technology meets this demand. However, the current technology is not suitable for the fabrication of arrays of submicron light sources that can be controlled individually. Our approach is based on nanoLED arrays that can directly address each array element and a self-pitch with dimensions below the wavelength of light. The design and fabrication processes are explained in detail and possess two geometries: a 6 × 6 array with 400 nm LEDs and a 2 × 32 line array with 200 nm LEDs. These nanoLEDs are developed as core elements of a novel on-chip super-resolution microscope. GaN technology, based on its physical properties, is an ideal platform for such nanoLEDs.

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Directly addressable GaN-based nano-LED arrays: fabrication and electro-optical characterization

In this Perspective, we will introduce possible future developments on group III-nitride nano-LEDs, which are based on current achievements in this rapidly arising research-technological field. First, the challenges facing their fabrication and their characteristics will be reported. These developments will be set in a broader context with primary applications in lighting, display technology, biology, and sensing. In the following, we will center on advanced applications in microscopy, lithography, communication, and optical computing. We will discuss unconventional device applications and prospects for emerging photon source-based technologies. Beyond conventional and current achievements in optoelectronics, we will present hybrid nano-LED architectures. Novel device concepts potentially could play an essential role in future photon source developments and serve as a key component for optical computing. Therefore, forefront fully photon operated logic circuits, photon-based computational processors, and photon driving memories will be discussed. All these developments will play a significant role in a future highly secure, low energy consuming green IT. Besides today's environmentally friendly terrestrial industrial and information technologies, an enormous potential of nano-LED technology for a large range of applications especially in the next stage of space research is envisaged.

Cutting-edge nano-LED technology

Nano-LED driven phase change evolution of layered chalcogenides for Raman spectroscopy investigations

GaN-based light emitting diodes (LEDs) have been shown to effectively operate down to nanoscale dimensions, which allows further downscaling the chip-based LED display technology from micro- to nanoscale. This brings up the question of what resolution limit of the illumination pattern can be obtained. We show two different approaches to achieve individually switchable nano-LED arrays. We evaluated both designs in terms of near-field spot size and optical crosstalk between neighboring pixels by using finite difference time domain (FDTD) simulations. The numerical results were compared with the performance data from a fabricated nano-LED array. The outcome underlines the influence of geometry of the LED array and materials used in contact lines on the final illumination spot size and shape.

Individually Switchable InGaN/GaN Nano-LED Arrays as Highly Resolved Illumination Engines

Power improvement in ridge bent waveguide superluminescent light-emitting diodes based on GaN quantum dots

Lensless microscopy requires the simplest possible configuration, as it uses only a light source, the sample and an image sensor. The smallest practical microscope is demonstrated here. In contrast to standard lensless microscopy, the object is located near the lighting source. Raster optical microscopy is applied by using a single-pixel detector and a microdisplay. Maximum resolution relies on reduced LED size and the position of the sample respect the microdisplay. Contrarily to other sort of digital lensless holographic microscopes, light backpropagation is not required to reconstruct the images of the sample. In a mm-high microscope, resolutions down to 800 nm have been demonstrated even when measuring with detectors as large as 138 μm × 138 μm, with field of view given by the display size. Dedicated technology would shorten measuring time.

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A Compact Raster Lensless Microscope Based on a Microdisplay.

This work presents a first prototype for a new approach to microscopy: a system basing its resolving power on the light emitters instead of the sensors, without using lenses. This new approach builds on the possibility of making LEDs smaller than current technology sensors, offering a new approach to microscopy we plan on developing towards superresolution. The microscope consists on a SPAD based camera, a 8x8 LED array with 5x5 μm LEDs distributed with a pitch of 10 μm, and discrete driving electronics to control them. We present simulations of the system, as well as the first microscope prototype implementing the method, and the results obtained through it.

Towards a super-resolution structured illumination microscope based on an array of nanoLEDs

Recent research into miniaturized illumination sources has prompted the development of alternative microscopy techniques. Although they are still being explored, emerging nano-light-emitting-diode (nano-LED) technologies show promise in approaching the optical resolution limit in a more feasible manner. This work presents the exploration of their capabilities with two different prototypes. In the first version, a resolution of less than 1 µm was shown thanks to a prototype based on an optically downscaled LED using an LED scanning transmission optical microscopy (STOM) technique. This research demonstrates how this technique can be used to improve STOM images by oversampling the acquisition. The second STOM-based microscope was fabricated with a 200 nm GaN LED. This demonstrates the possibilities for the miniaturization of on-chip-based microscopes.

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Pursuing the Diffraction Limit with Nano-LED Scanning Transmission Optical Microscopy.

This work presents a compact low-cost and straightforward shadow imaging microscopy technique based on spatially resolved nano-illumination instead of spatially resolved detection. Independently addressable nano-LEDs on a regular 2D array provide the resolution of the microscope by illuminating the sample in contact with the LED array and creating a shadow image in a photodetector located on the opposite side. The microscope prototype presented here is composed by a GaN chip with an 8x8 array of 5μm-LEDs with 10 μm pitch light sources and a commercial CMOS image sensor with integrated lens used as a light collector. We describe the microscope prototype and analyze the effect of the sensing area size on image reconstruction.

Sergio Moreno

Papers

A Compact Raster Lensless Microscope Based on a Microdisplay.

Towards a super-resolution structured illumination microscope based on an array of nanoLEDs

Pursuing the Diffraction Limit with Nano-LED Scanning Transmission Optical Microscopy.

Individually Switchable InGaN/GaN Nano-LED Arrays as Highly Resolved Illumination Engines

A shadow image microscope based on an array of nanoLEDs