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Review of Scientific Instruments

A compact, diode laser based excitation system for microscopy of NV centers

Thomas Oeckinghaus, Rainer Stöhr, Roman L. Kolesov, Julia Tisler, Friedemann Reinhard, Jörg Wrachtrup

Published Jul 1, 2014• 15 citations
Publisher

Abstract

We demonstrate that a recently introduced family of direct-emitting green laser diodes is a simple yet efficient light source for excitation of NV centers in diamond. Thanks to their fast (sub-ns) response time, these sources are suitable for a broad variety of measurements, including pulsed optically detected magnetic resonance (ODMR) and fluorescence lifetime imaging. This feature, together with a drastically simplified design, is a significant advantage over the traditional excitation system comprising an Nd: YAG laser switched by an acousto-optic modulator. We introduce a simple design for such a compact laser system and experimentally verify that it enables simultaneous lifetime and ODMR measurements on NV centers. In particular, we find that the NV(-) charge state remains stable in spite of the short excitation wavelength of the new source.

DOI
10.1063/1.4885469

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We demonstrate that a recently introduced family of direct-emitting green laser diodes is a simple yet efficient light source for excitation of NV centers in diamond. Thanks to their fast (sub-ns) response time, these sources are suitable for a broad variety of measurements, including pulsed optically detected magnetic resonance (ODMR) and fluorescence lifetime imaging. This feature, together with a drastically simplified design, is a significant advantage over the traditional excitation system comprising an Nd: YAG laser switched by an acousto-optic modulator. We introduce a simple design for such a compact laser system and experimentally verify that it enables simultaneous lifetime and ODMR measurements on NV centers. In particular, we find that the NV(-) charge state remains stable in spite of the short excitation wavelength of the new source.
We demonstrate the use of two-photon excitation for observing the ground state optically detected magnetic resonance (ODMR) of nitrogen-vacancy centers in diamonds at room temperature. An ultrafast femtosecond laser at 1040 nm was used for excitation, while fluorescence signal read out was achieved through a combination of a PMT and a lock-in amplifier. The imaging capability of two-photon excitation fluorescence (2PEF) was utilized to map the distribution of NV centers in a bulk diamond and micro-sized diamonds. For the first time, ODMR traces of the nitrogen-vacancy center are observed with two-photon excitation, providing a promising tool for fast 3D quantum sensing and imaging.
The nitrogen-vacancy (NV) center is a defect in the structure of diamond, roughly the size of a single atom, that possesses properties that make it applicable to quantum information processing, high-resolution magnetic resonance imaging, and probing biological systems. When an NV center is excited by green light of wavelength 523 nm, it emits red light of in the range from 600-800 nm tens of nanoseconds later. A confocal microscope can be used to excite and collect emitted photons from NV centers. By detecting the intensity of the emitted red photons, we can determine the quantum state of a single NV center, which is needed to build various quantum computing, sensing and imaging devices. The autocorrelation method is used to detect antibunching in the emission of red photons and ascertain that the observed photons are being emitted from a single NV center. We have built an autocorrelation device that measures, with sub-nanosecond accuracy, the statistical distribution of the arrival times of the red photons emitted from a diamond sample at a photon counter. The device can measure fixed time values with a time resolution of 120 ps FWHM. For uncorrelated sources measured for 120 seconds at the TAC full range of 100 ns, the average counts in each histogram bin were about 0.01% of the predicted value. The autocorrelation device that we have built costs about 10 times less than commercial units used by other NV researchers.
We demonstrate the use of two-photon excitation for observing the ground-state optically detected magnetic resonance (ODMR) of nitrogen-vacancy centers in diamonds at room temperature. An ultrafast femtosecond laser at 1040 nm was used for excitation, while fluorescence signal readout was achieved through a combination of a PMT and a lock-in amplifier. The imaging capability of two-photon excitation fluorescence (2PEF) was utilized to map the relative distribution of NV centers in a bulk diamond and micro-sized diamonds. For the first time, to the best of our knowledge, ODMR traces of the nitrogen-vacancy center are observed with two-photon excitation, providing a promising tool for fast 3D quantum sensing and imaging.
Nitrogen vacancy (NV) centres are a type of defect in diamond that exhibit a number of interesting properties. NV centres can be optically excited around 450-650 nm and emit fluorescence at around 550-800 nm [1]. The optical excitation also spin polarises the negatively charged NV centres, permitting optical detection of magnetic resonance [2]. This allows nanotesla scale sensitive magnetometry to be performed with nanometre scale spatial resolution [3].To date, a direct measurement of stimulated emission from ensembles of NV centres in diamond has yet to be demonstrated experimentally. Despite this, stimulated emission depletion microscopy has been utilised in superresolution imaging that relies on the stimulated emission of single NV centres to quench fluorescence [4]. Recently, there has also been similar indirect experimental evidence of stimulated emission in NV ensembles [5]. We thus aimed to show both optical gain of a probe beam in a pumped NV-rich diamond sample and a modulation of this gain via microwave excitation of its ground level spin transitions. The experimental set-up consisted of a pump (532 nm) and probe (723-950 nm) laser beam combined with a dichroic mirror and focused into a bulk diamond specimen to a spot-size of around 90 microns. Pump excitation powers of up to 3 W and probe powers up to 0.5 W were used. The beams were then collimated and separated with a prism and the probe power measured with a photodiode. The diamond specimen was placed on an antenna to allow simultaneous excitation with microwaves. A double modulation scheme was employed to measure either the change in probe power due to the pump laser or the microwave excitation during any one experiment.
Abstract We developed a compact, portable quantum sensor module for pulsed measurements using diamond nitrogen-vacancy (NV) centers, an excitation laser diode of 3 mW and a microwave source of 15 dBm. This module allowed the evaluation of ensemble diamond NV centers for measuring magnetic fields by continuous wave— optically detected magnetic resonance (ODMR), Rabi oscillation, pulsed ODMR, Ramsey measurement and spin-echo measurement. The sensitivity through pulsed measurements reached 40 nT/ Hz at 1 Hz, which achieved higher sensitivity by one order, especially at a frequency around 1 Hz compared with previously reported compact modules using diamond NV centers.
We study the contrast for electrical and optical readout of NV centers in diamond in dependence of the optical excitation wavelength using different excitation schemes. While the optically detected magnetic resonance (ODMR) works efficiently between 480 and 580 nm, electrically detected magnetic resonance (EDMR) shows a strong dependence on the excitation dynamics. The highest, electrically detected contrast of -23% is achieved by resonantly exciting the zero-phonon line of the neutral charge state of NV at 575 nm. EDMR is also enhanced at 521 nm, possibly due to a further excited state of NV−^-−.