Single-photon converter expands IR spectrometry

Marie Freebody, mariefreebody@physics.org

A cleverly designed single-photon detector has enabled scientists at the National Institute of Standards and Technology (NIST) to develop a highly sensitive and low-cost spectrometer that operates in the infrared.

The technique could be used in many areas that require ultrasensitive spectrum measurement in the region, such as biomedical research, nanotechnology and quantum information. What’s more, the detector can measure – at the single-photon level – weak infrared light given off by fragile bio- and nanomaterials.

Until now, steady progress had been made to increase the efficiency and sensitivity of detectors operating in the visible and ultraviolet regimes. However, such detectors have proved too inefficient and slow to detect single photons in the near-infrared. The performance of today’s infrared detectors also has limited the sensitivity of the spectrometers in which they are used.

In a study published in the August issue of Optics Express, professor Xiao Tang and colleagues at NIST have found a way to use existing detectors to measure infrared photons that have been converted up to the visible.

Xiao Tang, Oliver Slattery and Lijun Ma, left to right, make up the NIST research team that developed a highly sensitive and low-cost spectrometer that operates in the IR region. Courtesy of Xiao Tang.

“Silicon avalanche photodiodes [Si-APDs] work very well, but only for wavelengths shorter than 1000 nanometers,” Tang explained. “So we converted photons at 1310 nm to 710 nm, where Si-APDs have highest detection efficiency.”

In the NIST approach, a 1550-nm narrowband pump laser scans the infrared signal photons and converts to visible light only those that have the desired polarization and wavelength.

Conventional spectrometers use a dispersion element such as a prism, a grating or a tunable narrow bandpass filter to separate light into its component wavelengths. The NIST team, however, uses the law of energy conservation in the wavelength conversion process. In this way, the spectrum of signal light can be obtained by sweeping the pump laser wavelength without the need for dispersive optical elements.

“When we tune the pump wavelength, a spectrum of the signal photons is obtained,” Tang said. “Since our up-conversion detector has such a low noise level, the sensitivity of the spectrometer is as high as –126 decibels above 1 mW at 1310 nanometers, with a signal-to-noise ratio of 10. The sensitivity is three orders of magnitude higher than any commercial optical spectral analyzer.”

This result opens up the possibility that scientists in fields ranging from forensics to quantum communications can deal with infrared photons almost as easily as with visible photons.

As government employees responsible for helping US industries with new technologies, Tang and colleagues are prepared to cooperate with any company that wishes to commercialize this technology.

The next step for the NIST team is to use the newly developed spectrometer to obtain more advanced results in its quantum communications research.