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  • Ultrafast spectroscopy reveals new uses for flawed gems

Photonics Spectra
Dec 2013
TUCSON, Ariz. – Ultrafast laser pulses have provided the first detailed observation of how energy travels through diamonds with specific flaws that provide interesting optical and electronic properties.

A team led by University of Arizona assistant professor Vanessa Huxter observed how energy travels through diamonds containing nitrogen-vacancy centers – defects in which two adjacent carbon atoms in the diamond’s crystal structure are replaced by a single nitrogen atom and an empty gap.

Because the nitrogen-vacancy defects can be manipulated with optical methods such as lasers, they could be used for computing, data storage, sensing and even advanced imaging techniques capable of revealing the structure of molecules, Huxter said. She led the research during a postdoctoral fellowship funded by the Natural Sciences and Engineering Research Council of Canada. Her team, which included Lawrence Berkeley National Laboratory and the University of California, Berkeley, is the first to study the ultrafast dynamics in these crystals in real time.

Ultrafast spectroscopy
Vanessa Huxter uses ultrafast laser pulses to study physical processes in diamonds that happen on a timescale of a few nanoseconds. Photo courtesy of Beatriz Verdugo, UANews.

“To use these systems for quantum computing, you want to have some idea of what we call vibrational modes, because they determine the local environment and may possibly be used for information processing,” she said.

Wherever a nitrogen-vacancy defect interrupts the gem’s uniform carbon lattice, the vibrational properties change in ways that can be manipulated by laser pulses, for example. The pulse knocks the electrons in the nitrogen-vacancy centers into an excited state. Over time, the electrons fall back into their ground state in a process called relaxation, while dissipating the energy into their surroundings.

To watch how vibrations influence the ultrafast relaxation of the system, Huxter’s team used ultrafast laser pulses, because the relaxation occurs on a timescale of a few nanoseconds. Exactly how that energy moves through the crystal and how it influences the vibrations around the nitrogen-vacancy centers are crucial to figuring out how to take advantage of its properties, but this process has been elusive until now. “This is the first time we have been able to directly observe the vibrational spectrum of the system in real time,” Huxter said.

The researchers used two-dimensional electronic spectroscopy, basically a way of creating two-dimensional correlation “maps” that allow them to watch the system as it relaxes to the ground state.

“Think of it as ultrahigh-speed photography to freeze the action on a scale of atoms and molecules,” Huxter said. “We can watch the energy flow through the system in real time and take snapshots along the way. We can see where the energy is going in and where it is coming out.”

They observed vibrations local to the defect with femtosecond time resolution. “Being able to directly follow these vibrations led to some surprising new results, including that these vibrations are quantum mechanically coherent for thousands of femtoseconds,” she said. “The question we ask is: What happens when you start replacing the atoms in the crystal? Will you get a change in the elastic properties? Each nitrogen-vacancy center is like a softer region you can poke at. They absorb the laser energy where there was previously no absorption, and we see all these extra vibrational modes we don’t see in the rest of the crystal.

“In our scenario, the diamond is like a clear window. We look straight through it and only see the defects. We tailor our laser pulse to the absorption of the defects.”

The work was published in Nature Physics (doi: 10.1038/nphys2753).

A charged elementary particle of an atom; the term is most commonly used in reference to the negatively charged particle called a negatron. Its mass at rest is me = 9.109558 x 10-31 kg, its charge is 1.6021917 x 10-19 C, and its spin quantum number is 1/2. Its positive counterpart is called a positron, and possesses the same characteristics, except for the reversal of the charge.
femtosecond laser
A type of ultrafast laser that creates a minimal amount of heat-affected zones by having a pulse duration below the picosecond level, making the technology ideal for micromachining, medical device fabrication, scientific research, eye surgery and bioimaging.
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