Traceable microscopy could improve the reliability of quantum information technologies, biological imaging, and more.
Devices that capture the sensible light from hundreds of thousands of quantum dots, including chip-scale lasers and optical amplifiers, have made the transition from laboratory experiments to industrial products. But newer kinds of quantum-dot devices have been slower to return to market because they require extraordinarily accurate alignment between individual dots and the miniature optics that extract and guide the emitted radiation.
Researchers on the National Institute of Standards and Technology (NIST) and their colleagues have now developed standards and calibrations for optical microscopes that allow quantum dots to be aligned with the middle of a photonic component to inside an error of 10 to twenty nanometers (about one-thousandth the thickness of a sheet of paper). Such alignment is critical for chip-scale devices that employ the radiation emitted by quantum dots to store and transmit quantum information.
For the primary time, the NIST researchers achieved this level of accuracy across the complete image from an optical microscope, enabling them to correct the positions of many individual quantum dots. A model developed by the researchers predicts that if microscopes are calibrated using the brand new standards, then the variety of high-performance devices could increase by as much as a hundred-fold.
That recent ability could enable quantum information technologies which can be slowly emerging from research laboratories to be more reliably studied and efficiently developed into industrial products.
In developing their method, Craig Copeland, Samuel Stavis, and their collaborators, including colleagues from the Joint Quantum Institute (JQI), a research partnership between NIST and the University of Maryland, created standards and calibrations that were traceable to the International System of Units (SI) for optical microscopes used to guide the alignment of quantum dots.
“The seemingly easy idea of finding a quantum dot and placing a photonic component on it seems to be a tough measurement problem,” Copeland said.
In a typical measurement, errors begin to build up as researchers use an optical microscope to seek out the situation of individual quantum dots, which reside at random locations on the surface of a semiconductor material. If researchers ignore the shrinkage of semiconductor materials on the ultracold temperatures at which quantum dots operate, the errors grow larger. Further complicating matters, these measurement errors are compounded by inaccuracies within the fabrication process that researchers use to make their calibration standards, which also affects the position of the photonic components.
The NIST method, which the researchers described in an article posted online in Optica Quantum on March 18, identifies and corrects such errors, which were previously neglected.
The NIST team created two kinds of traceable standards to calibrate optical microscopes — first at room temperature to investigate the fabrication process, after which at cryogenic temperatures to measure the situation of quantum dots. Constructing on their previous work, the room-temperature standard consisted of an array of nanoscale holes spaced a set distance apart in a metal film.
The researchers then measured the actual positions of the holes with an atomic force microscope, ensuring that the positions were traceable to the SI. By comparing the apparent positions of the holes as viewed by the optical microscope with the actual positions, the researchers assessed errors from magnification calibration and image distortion of the optical microscope. The calibrated optical microscope could then be used to rapidly measure other standards that the researchers fabricated, enabling a statistical evaluation of the accuracy and variability of the method.
“Good statistics are essential to each link in a traceability chain,” said NIST researcher Adam Pintar, a coauthor of the article.
Extending their method to low temperatures, the research team calibrated an ultracold optical microscope for imaging quantum dots. To perform this calibration, the team created a brand new microscopy standard — an array of pillars fabricated on a silicon wafer. The scientists worked with silicon since the shrinkage of the fabric at low temperatures has been accurately measured.
The researchers discovered several pitfalls in calibrating the magnification of cryogenic optical microscopes, which are likely to have worse image distortion than microscopes operating at room temperature. These optical imperfections bend the pictures of straight lines into gnarled curves that the calibration effectively straightens out. If uncorrected, the image distortion causes large errors in determining the position of quantum dots and in aligning the dots inside targets, waveguides, or other light-controlling devices.
“These errors have likely prevented researchers from fabricating devices that perform as predicted,” said NIST researcher Marcelo Davanco, a coauthor of the article.
The researchers developed an in depth model of the measurement and fabrication errors in integrating quantum dots with chip-scale photonic components. They studied how these errors limit the power of quantum-dot devices to perform as designed, finding the potential for a hundred-fold improvement.
“A researcher may be glad if one out of 100 devices works for his or her first experiment, but a manufacturer might need ninety-nine out of 100 devices to work,” Stavis noted. “Our work is a leap ahead on 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, reminiscent of imaging brain cells and mapping neural connections. For these endeavors, researchers also seek to find out accurate positions of the objects under study across a complete microscope image. As well as, scientists might have to coordinate position data from different instruments at different temperatures, as is true for quantum-dot devices.
