Manipulating the Quantum Coherence of Optically Trapped

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Manipulating the quantum coherence of optically trapped nanodiamonds Lachlan W. Russell, Simon G. Ralph, Kazuma Wittick, JeanPhilippe Tetienne, David A. Simpson, and Peter J. Reece ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00946 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Manipulating the quantum coherence of optically trapped nanodiamonds Lachlan W. Russell,† Simon G. Ralph,† Kazuma Wittick,† Jean-Philippe Tetienne,‡ David A. Simpson,‡ and Peter J. Reece∗,† †Photonics & Optoelectronics Group, School of Physics, The University of New South Wales, Sydney, NSW 2052, Australia ‡School of Physics, The University of Melbourne, Parkville, VIC 3010, Australia E-mail: [email protected] Abstract The use of optical tweezers as a tool to facilitate non-destructive nanoscale sensing has been a growing area of research, particularly in the biological sciences. The nitrogen-vacancy (NV) centre in diamond has attracted significant interest in this area due to the array of sensing modalities available and the bio-compatibility of the material itself. Many of the diamond sensing modalities rely on the measurement and characterisation of the NV spin. Recent work has demonstrated the utility of the spin-lattice relaxation time (T1 ) of NV centres in nanodiamond for nanoscale magnetic sensing and spectroscopy. Here, we demonstrate spin relaxometry with optically trapped nanodiamonds. The all-optical sensing protocol we developed eliminates the spin decoherence effects of the trapping laser and can determine spin lattice relaxation times on the order of ms. Moreover, the protocol requires relatively low trapping powers < 50 mW, making it particularly applicable to biological systems.

Keywords: Nanodiamond, NV Centre, Optical Tweezers, Nanoscale Sensing, Spin Relaxometry 1

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Optical tweezers (OT) have been demonstrated to be an exceedingly useful tool for the micromanipulation of micro and nanoscale particles, they have been particularly successful in the study of biological systems. By optically trapping nanodiamond a host of new research avenues across the physical and life sciences are made possible by real-time control of nanodiamond location and orientation in three dimensions through the manipulation of trap position and trapping laser polarisation. 1 The negatively charged NV centre is a naturally occurring defect in diamond, it has increasingly become the subject of proposed and implemented applications ranging from quantum information 2 to nanoscale sensing, 3,4 particularly magnetometry 5–7 and thermometry. 8–10 The viability of these applications stems from the electronic structure of the NV centre, namely that the non-radiative transitions to and from a metastable singlet state allow the NV to become spin polarised under optical excitation, and yield an optical contrast between the ms = 0 and ms = ±1 spin states. 11 The NV centre also exhibits long spin coherence times at room temperature and photo-stable luminescence. 3 Further, diamond is an attractive material for these nanoscale sensing applications as it is both bio-compatible and chemically inert. 12 To date optical tweezing demonstrations with NDs have been limited to optically detected magnetic resonance (ODMR) studies on NV centre ensembles 13 and single defects. 1 Limiting the practicality of nanodiamond sensing in OT is the influence of the infrared (IR) illumination on the NV photoluminescence, which has been found by multiple previous studies to cause significant quenching, 14–17 hence reducing contrast in optical measurements. The extent of the quenching scales exponentially with trapping power, making it particularly troublesome for OT which require trapping powers on the order of tens of mW for well confined and stable trapping. Optical traps in atmosphere/vacuum face a similar issue, however Pettit et al. 18 in a 2017 study of spin-spin relaxation (T2 ) were able to show on short timescales (µs) the loss of contrast can be mediated by modulating the IR trapping laser. The utility of T2 sensing in nanodiamonds is however limited by the short coherence times

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due to the high levels of nitrogen present in commercial nanodiamond material. Therefore, the spin-lattice relaxation time (T1 ) represents an alternate sensing approach which can be applied to the detection of very weak fluctuating magnetic signals. The T1 time of NV centre in nanodiamonds is orders of magnitude longer than T2 , 19 therefore the trapping laser has significantly more time to interact with the NV spin population. Here, we study the impact of the trapping laser on the spin dynamics and demonstrate a modulated trapping protocol which allows direct readout of the spin-lattice relaxation time, whilst maintaining a stable trapping environment. Performing T1 measurements on optically trapped nanodiamonds, combines the high spatial resolution of ND based sensors, the confined range of T1 sensitivity (∼ 100 nm), and the dynamic control offered by optical tweezers to create a unique magnetic probe. We show that under usual measurement protocols, measurement of the NV spin lattice relaxation time is compromised by the infrared trapping beam which not only reduces the optical contrast, but also influences NV spin dynamics on the longer timescales. Circumventing this issue with the introduction of intra-measurement laser modulation we demonstrate, for the first time, readout of the NV centre T1 in an optical tweezers.

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Methodology

The nanodiamonds used in this work were sourced from FND Biotech Inc. (Taiwan) (brFND100). The nanodiamonds are dispersed in water at a concentration of 1 mg/ml. The average particle size is 100 nm and the average number of NVs per particle was ∼ 500. For the trapping experiments the concentration of nanodiamonds were reduced by a factor of 1000 to ensure stable trapping of individual nanodiamonds. The experimental setup used for this study is presented in Figure 1, the optical tweezers consist of a linearly polarised Nd:YAG infrared trapping laser (Laser Quantum, IR Ventus) with λ = 1064 nm, TEM00 , focused using a 1.3 NA oil-immersion microscope objective

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Figure 1: Schematic diagram of the optical setup used for these experiments, split into three major sections: (A) Trapping, (B) Excitation and (C) Detection. (Nikon CFI Plan Fluor 100x) to a diffraction limited spot in the front focal plane. The power and position of the beam are selected via a two-axis acousto-optic deflector (2DAOD, Gooch & Housego 45035 AOBD) which is controlled with two digitally addressable digital frequency synthesisers (DFS, Gooch & Housego, 64020-200-2ADMDFS-A). A spatial light modulator (SLM1, Hamamatsu LCOS-SLM x10468-03) is incorporated to correct for aberrations in the beam profile. Unless otherwise specified, the trapping laser power at the focus was ∼ 48 mW for all measurements. The optical excitation is provided by a 10 mW green diode laser (Melles Griot) with λ = 561 nm, this beam is switched using an acousto-optic modulator (AOM, Gooch & Housego 3110-120) and is directed into the objective such that it is collinear with the trapping laser and hence forms a focus coincident with the trap position. This collinearity is achieved via a second spatial light modulator (SLM2, Hamamatsu LCOS-SLM x10468-03). Detection of ND photoluminescent intensity is achieved by passing the light collected by

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the 0.65 NA condenser lens (Nikon CFI Plan Fluor ELWD 40X) through a bandpass filter (Semrock FF02-675/67-25), to remove the excitation and trapping laser light, then to an avalanche photo-diode (APD, PicoQuant τ -SPAD) for time correlated single photon detection and is binned by a multi-channel scalar (MCS, FastComTec MCS64A). The Brownian dynamics of a trapped nanodiamond are tracked by forward scattered infrared light from the trapping laser which is collected by the condenser and the resulting back focal place interference pattern is projected onto a Position Sensitive Detector (PSD, Pacific Sensor DL16-7PCBA). For spectroscopic analysis the photoluminescence is directed into a spectrometer (Princeton Instruments, Acton SP2300 & PIXIS 256). In order to co-ordinate the control of trapping, excitation and detection, a high speed programmable pulse generator (SpinCore PulseBlasterESR-PRO) is used. This is able to trigger the MCS as well as modulate the trapping and excitation lasers via the 2D-AOD & AOM respectively.

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Results

To study the effect of the trapping laser on the photoluminescence (PL) of the nanodiamond we compare simultaneous illumination of IR and green lasers with alternating illumination (with a 50% duty cycle), under increasing IR laser power, these illumination sequences are shown in Figure 2(a). Figure 2(b) shows that even at low trapping powers the PL with IR illumination is ∼ 0.5× the PL without IR. As trapping power increases, under simultaneous illumination the PL intensity decreases exponentially, whereas for the alternating case the PL intensity is roughly constant, with higher trapping powers reducing the fluctuations by confining the ND more strongly. The time required for the population to return to steady state conditions after IR is switched off is on the order of a few microseconds, 20 as such the alternating pulse sequence is made up of 100 µs alternate green and IR pulses, separated by a 8 µs buffer period of

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no illumination to ensure a steady state population. The variation in the PL intensity at each trapping power, represented by the error bars, can be attributed to the motion of the particle in the trap. As expected, the trap is more stable when the IR is constantly on and both sequences are more stable at higher trapping powers. While turning the trapping laser off during PL readout is a simple solution to the quenching problem in optical tweezers, it does restrict any continuous readout to ∼ 3 ms, the time before the particle is likely to diffuse away from the trap location. Modulating the IR laser during PL measurement has been employed successfully in atmospheric optical traps to improve the contrast in ODMR and electron spin transient measurements. 18 In these measurements the IR illumination resulted in a reduction in the spin contrast, however, the ESR position and width was unchanged in ODMR under continuous and modulated illumination and in electron spin transient measurements the extracted T2 values were comparable in both cases. We performed measurement of the spin-lattice relaxation time and found that there were significant changes in spin population dynamics beyond that of simple quenching on the longer timescales of T1 . The transverse spin relaxation time of NV centres in NDs is short, on the order of a microseconds, 21 which means that readout can be performed with the trapping laser off before the ND is lost from the trap. However, the long T1 times in ND are more challenging as the duration of pulse sequence required for readout is often on the order of milliseconds, increasing the risk of losing the particle if the trap is modulated during measurement. Here, we overcome this problem and demonstrate an all-optical method to determine the T1 time of an individual optically trapped nanodiamond. Measurement of T1 is achieved by an optical pump-probe sequence using 561 nm excitation, outlined in green in Figure 3(a). The sequence begins with a 10 µs 561 nm pulse which spin polarises the NV ensemble into the ms = 0 sublevel. Over time the energy from the spins is transferred to the lattice nonradiatively eventually resulting an equal distribution of spins in the ms = 0 and ms = ±1 sublevels. The instantaneous population is probed using a 3 µs 561 nm pulse which reads out

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( s) Figure 3: (a) IR and 561 nm laser pulse sequences used for (b) a comparison of T1 relaxation curves under different pulse sequences, all measurement are performed on the same ND. (I) IR on: simple T1 without any modulation of the IR trapping laser, the expected exponential decay is not seen. (II) IR off: IR laser off for the duration of measurement, analogous to T1 measurements external to an optical trap, sequence duration is limited to a few ms before the ND diffuses away from the trap position. An exponential fit yields T1 = 422.1 ± 168.2µs. (III) IR interleaved: An exponential fit yields T1 = 641.7±558.6µs. The relative intensity of all sequences is scaled by the intensity per sweep of the first readout pulse.

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and then re-polarises the spins. This readout and re-polarisation is repeated at increasing evolution times τ and the spin population is inferred by taking the ratio of the integrated intensity of the first 300 ns of the pulse (IA ) normalised by the integrated intensity of the last 300 ns (IB ). The measurement can be performed as a full sequence, where each readout (RO) pulse resets the spin population before the next evolution time and subsequent readout (Figure 3(a) I & II), or it can be performed as a split sequence, where there is a pump-probe pair for every τ point (Figure 3(a) III). We present the results from both measurement schemes taken with a trapping laser power of ∼ 48 mW, and an average acquisition time of ∼ 5 minutes depending on the individual PL strength of each ND. When probing an ensemble of NV defects, the form of the spin-lattice relaxation is a stretched exponential decay, due to the variable local spin environment experienced by each individual NV, which is given by y = A exp(−( Tt1 )p ) + c where A is the amplitude, c is the offset and p is the stretching parameter. For our fitting we used p = 0.7 which is consistent with our previous work 22 and et the value of c to be = (1 − A) because of how the data were normalised, A and T1 were left as free parameters. The uncertainty value for the extracted T1 is taken from the fitting coefficient standard error. For the case of continuous IR illumination (Figure 3(b) I), our observed PL intensity does not show the expected decay behaviour with increasing τ , however its behaviour was consistent over many trials. As τ increases the intensity increases over ∼ 200µs before reaching a stable maximum. This behaviour was seen at several different IR powers, and across many different NDs indicating that IR has some unaccounted for effect on the spin dynamics of the system. Previous work has shown a spin dependent effect on the ODMR under simultaneous IR and green excitation. 23 Further, we found that when the trapping laser was restored during the dark evolution time, the spin population was still affected and T1 could not be measured. In contrast, Figure 3(b) II shows the expected exponential decay achieved by chopping the IR during measurement, this, to the best of the authors’ knowledge, is the first measurement of T1 time of an individual ND in optical tweezers. A stretched exponential fit yields a

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relaxation time of = 422.1 ± 168.2µs. This all-optical readout of T1 is limited to a total sequence duration of ∼ 3 ms before the particle is lost, which restricts the number of data points which can be collected. In order to extend the measurement time indefinitely we modified the sequence such that we and restore the trapping laser between each pair of polarisation and RO pulses which yielded the similar T1 decay in Figure 3(b) III (T1 = 641.7 ± 558.6µs) from the same ND. The relative intensities of the three sequences show that while IR on (I) produces significant quenching, the IR off (II) and IR interleaved (III) readout methods are of comparable intensity, this combined with their similar T1 fits leads us to conclude that in terms of spin dynamics these latter two methods are identical. For noisy environments the interleaved method is most suitable as the full decay can be mapped with many data points, at the cost of longer acquisition times. For tracking changes in T1 , a single-τ or few-τ measurement may be suitable, 22 where the intensity few evolution times is tracked as a function of some external change, these types of measurement can be most efficiently implemented using the IR off sequence described in Figure 3(a)II. Further effects of the IR laser on the T1 readout can be seen in Figure 4, which compares the behaviour of the decay curve when IR is restored during the dark time between RO pulses. The measurement was performed as a single sequence, split into three sections, during sections I and III the IR laser was off during the dark time and the T1 exponential decay was observed. During section II, however, the trapping laser was restored between individual RO pulses and the PL intensity drops significantly (∼ 78% of it’s maximal value) and is roughly constant, until section III begins where the original decay behaviour is restored. The behaviour of the PL in section II emphasises the significant effects of IR laser illumination on NV spin dynamics which clearly extend beyond simple quenching, though appear to be completely reversible as shown by the restored decay in section III. Figure 5(a) shows a series of T1 decays from three nanodiamonds yielded from the split sequence interleaved IR method. With the interleaved technique we are able to perform readout sequences with many more data points and extending out into greater evolution

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( s) Figure 4: Effect of the trapping laser restoration during the dark time (evolution time between individual readout pulses). During sections I and III the IR laser is off during the dark time, during section II the IR laser is on during the dark time.

Table 1: Lateral trap stiffnesses, diameters and T1 times of trapped nanodiamonds ND 1 2 3

κx (pN/µm/mW ) 0.28 ± 0.01 0.48 ± 0.01 0.66 ± 0.02

κy (pN/µm/mW ) 0.23 ± 0.01 0.33 ± 0.01 0.45 ± 0.02

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Dx (nm) Dy (nm) 184 ± 4 244 ± 7 375 ± 8 457 ± 11 286 ± 7 355 ± 8


T1 (µs) 196.63 ± 44.75 73.99 ± 21.41 227.41 ± 63.57

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ND 2: T1 = 74.0 ± 21.4 s ND 3: T1 = 227.4 ± 63.6 s

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times where the spin populations have reached their equilibrium value. This method allows more reliable determination of the spin-lattice relaxation time of nanodiamonds in optical tweezers and opens up the possibility of using T1 as an in-situ probe of local fluctuating magnetic fields, such as those caused by unpaired electron spins. 7 Both sequences II and III have a 50% IR duty cycle, sequence II has constant IR modulation frequency (set by the sequence duration) of ∼ 500 Hz and sequence III has a variable IR frequency (set by τ ) of > 1000 Hz, it has been shown from studies of time sharing optical tweezers 24 at these switching frequencies the trap stiffness is roughly constant and the particle simply feels a weaker effective trapping power. Further, we can analyse the Brownian motion of the each nanodiamond in the optical trap, by interpreting the lateral positional histograms and power spectral densities, the trap stiffness and diameter can be inferred. The methods used to collect and interpret this data can be found in our previous publications. 25,26 Table 1 shows the lateral trap stiffnesses and diameters of the four NDs shown in Figure 5(a) and Figure 5(b-d) shows example Brownian motion data for ND 1. While the average size of our NDs is specified as 100 nm, the optical tweezers preferentially traps larger particles, hence the typical range of particles sizes found in our experiments is ∼ 200 − 400 nm. The NDs are also generally asymmetric in their axial diameters, allowing the possibility of orientation control via polarisation. 1

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Conclusion

Optical tweezers are inherently suited to use in biological settings and to other studies in solution, by combining them with NV centre based sensing and extending the repertoire of sensing techniques to include T1 , this work presents a sizeable step forward in the development of non-destructive nanoscale sensing protocols. Our results enable the possibility of performing spin relaxometry based sensing using individual “directed” nanodiamonds, combining the localised sensitivity of spin-lattice relaxation with the high spatial resolution of

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tweezers. Further, our measurement scheme is relatively simple, as it does not require auxiliary microwave excitation, and requires relatively low trapping powers, making it an ideal sensing platform for biological studies and lending itself well to other practical avenues for further research.

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ND 1: T1 = 196.6 ± 44.8 s

(a)

1.2

ND 2: T1 = 74.0 ± 21.4 s ND 3: T1 = 227.4 ± 63.6 s

IA/IB

1.15 1.1 1.05 1 0.95 0.9

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400

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( s)

1000

Histogram (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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X Axis Y Axis

800 600 400 200 0

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Figure 6: Table of Contents Figure.

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Manipulating the Quantum Coherence of Optically Trapped

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