Devices that capture light from quantum dots, like chip-scale lasers and optical amplifiers, have made their way from the lab to the commercial market. The transition for newer quantum dot-based devices has been slower due to the extreme level of accuracy needed in the alignment of the individual dots and the optics that extract and guide the emitted radiation. When localization microscopy of quantum emitters is used to guide lithographic placement of photonic structures, microscopy and lithography measurement errors can easily occur. These errors degrade registration accuracy, limiting device performance and process yield. To address this bottleneck, researchers at the National Institute of Standards and Technology (NIST) and the University of Maryland have developed standards and calibrations for the optical microscopes used to guide the centering of quantum dots within photonic chips. The method enables a precision down to 10-20 nm across the entire image from an optical microscope, allowing the correction of many individual quantum dots. The model predicts that if microscopes are calibrated with the new standards, the number of high-performance devices could increase by as much as a hundred-fold. The standards and calibrations created by the collaborators are traceable to the International System of Units (SI). “The seemingly simple idea of finding a quantum dot and placing a photonic component on it turns out to be a tricky measurement problem,” said NIST researcher Craig Copeland. In a typical measurement, errors begin to accumulate as researchers use an optical microscope to locate the quantum dots, which reside at random points on the surface of a semiconductor material. If the researchers ignore the shrinkage of semiconductor materials at the ultracold temperatures at which quantum dots operate, the errors grow larger. Further complicating matters, these measurement errors are compounded by inaccuracies in the fabrication process that researchers use to make their calibration standards, which also affects the placement of photonic components. Accurate alignment of quantum dots with photonic components is critical for extracting the radiation emitted by the dots. In this illustration, a quantum dot centered in the optical “hotspot” of a circular grating (center dot in the inset) emits more light than a dot that is misaligned (off-center dot in the inset). Courtesy of S. Kelley/NIST. Addressing these errors, the NIST team created two types of traceable standards to calibrate optical microscopes — first at room temperature to analyze the fabrication process, and then at cryogenic temperatures to measure the location of quantum dots. Building on previous work, the room temperature standard consisted of an array of nanoscale holes spaced a set distance apart in a metal film. The researchers measured the precise positions of the holes with an atomic force microscope and ensured that the positions were traceable to the SI. They compared the apparent positions of the holes when viewed using the optical microscope with the actual positions of the holes to assess errors caused by magnification calibration and image distortion from the optical microscope. Once the researchers completed this process, they used the calibrated optical microscope to rapidly measure additional standards they had developed, which enabled them to perform a statistical analysis of the accuracy and variability of the process. “Good statistics are essential to every link in a traceability chain,” NIST researcher Adam Pintar said. Working at cryogenic temperatures, the researchers then calibrated an ultracold optical microscope for imaging quantum dots. To create this standard, they built an array of pillars on a silicon wafer. The researchers chose to work with silicon because the shrinkage of silicon at low temperatures has already been accurately measured. The researchers found that cryogenic optical microscopes tended to have worse image distortion than microscopes operating at room temperature. The optical imperfections in the cryogenic microscopes bent the images of straight lines into curves, which the calibration straightened out. If left uncorrected, this image distortion caused large errors in determining the position of quantum dots and in aligning the dots within targets, waveguides, and other light-controlling devices. “These errors have likely prevented researchers from fabricating devices that perform as predicted,” researcher Marcelo Davanco said. The team believes that its methodology could be a key enabler of the lab-to-fab transition for quantum information technologies. Nanoparticle characterization, microsystem tracking, and semiconductor metrology applications could also benefit. The traceable standards could also be used for photonic structures, such as broadband waveguides, that require registration errors of less than 10 nm to achieve high coupling efficiency. “A researcher might be happy if one out of a hundred devices works for their first experiment, but a manufacturer might need ninety-nine out of a hundred devices to work,” researcher Samuel Stavis said. “Our work is a leap ahead in this lab-to-fab transition.” Beyond quantum dot devices, traceable standards and calibrations under development at NIST may improve accuracy and reliability in other demanding applications of optical microscopy, such as imaging brain cells and mapping neural connections. Additionally, scientists may need to coordinate position data from different instruments at different temperatures, as is true for quantum-dot devices. The research was published in Optica Quantum (www.doi.org/10.1364/OPTICAQ.502464).
