Visible to Infrared Diamond Photonics Enabled by Focused Femtosecond Laser Pulses

micromachines

Article Visible to Infrared Diamond Photonics Enabled by Focused Femtosecond Laser Pulses

Belén Sotillo 1,2, Vibhav Bharadwaj 1, John Patrick Hadden 3, Stefano Rampini 2, Andrea Chiappini 4, Toney T. Fernandez 1, Cristina Armellini 4, Ali Serpengüzel 5, Maurizio Ferrari 4, Paul E. Barclay 3, Roberta Ramponi 1,2 and Shane M. Eaton 1,2,* 1 Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci 32, Milano 20133, Italy; [email protected] (B.S.); [email protected] (V.B.); [email protected] (T.T.F.); [email protected] (R.R.) 2 Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche (IFN-CNR), Piazza Leonardo da Vinci 32, Milano 20133, Italy; [email protected] 3 Institute for Quantum Science and Technology, University of Calgary, Calgary, AB T2N 1N4, Canada; [email protected] (J.P.H.); [email protected] (P.E.B.) 4 Institute of Photonics and Nanotechnology of the National Research Council (IFN-CNR), Characterization and Development of Materials for Photonics and Optoelectronics (CSMFO) and The Centre for Materials and Microsystems (FBK-CMM), Trento 38123, Italy; [email protected] (A.C.); [email protected] (C.A.); [email protected] (M.F.) 5 Microphotonics Research Laboratory, Department of Physics, Koç University, Rumelifeneri Yolu, Istanbul 34450, Turkey; [email protected] * Correspondence: [email protected]; Tel.: +39-02-2399-6123

Academic Editor: Maria Farsari Received: 18 January 2017; Accepted: 13 February 2017; Published: 17 February 2017

Abstract: Diamond’s nitrogen-vacancy (NV) centers show great promise in sensing applications and quantum computing due to their long electron spin coherence time and because they can be found, manipulated, and read out optically. An important step forward for diamond photonics would be connecting multiple diamond NVs together using optical waveguides. However, the inertness of diamond is a significant hurdle for the fabrication of integrated optics similar to those that revolutionized silicon photonics. In this work, we show the fabrication of optical waveguides in diamond, enabled by focused femtosecond high repetition rate laser pulses. By optimizing the geometry of the waveguide, we obtain single mode waveguides from the visible to the infrared. Additionally, we show the laser writing of individual NV centers within the bulk of diamond. We use µ-Raman spectroscopy to gain better insight on the stress and the refractive index profile of the optical waveguides. Using optically detected magnetic resonance and confocal photoluminescence characterization, high quality NV properties are observed in waveguides formed in various grades of diamond, making them promising for applications such as magnetometry, quantum information systems, and evanescent field sensors.

Keywords: diamond; nitrogen-vacancy (NV) center; femtosecond laser; laser micromachining; optical waveguide; magnetometry

1. Introduction Apart from its remarkable beauty when cut appropriately, diamond is the hardest naturally occurring bulk material, has a record high thermal conductivity and offers outstanding transparency from the ultraviolet to far infrared. However, it is actually a defect in diamond’s tetrahedral lattice of carbon atoms which has scientists excited. In both naturally found and synthetically fabricated diamond, the nitrogen-vacancy (NV) center is present, where a nitrogen sits next to an empty site

Micromachines 2017, 8, 60; doi:10.3390/mi8020060 www.mdpi.com/journal/micromachines Micromachines 2017, 8, 60 2 of 10 in the carbon lattice. The optically active spin defects boasts long room temperature spin coherence time [1], making them attractive for efficient nanoscale magnetic field sensing and for quantum information systems. An integrated optics platform in diamond would be beneficial for magnetometry, due to the enhanced interaction provided by waveguides, and quantum computing, in which NV centers could be optically linked together for long-range quantum entanglement. However, despite some previous attempts [2,3], it remains a challenge to fabricate optical waveguides in diamond due to its hardness and chemical inertness. Recently, a disruptive technology based on femtosecond laser microfabrication was proposed to enable a 3D photonics toolkit for diamond [4,5]. Since focused ultrashort laser pulses damage the crystalline lattice [6], an indirect approach was applied: optical mode confinement was achieved between two closely spaced and parallel laser-inscribed modification lines. We discuss the mechanisms for waveguiding using this type II fabrication method and the role of the repetition rate. We show for the first time the fabrication of optical waveguides in ultrapure diamond that can address single NV centers in quantum information systems. In addition, we show shallow waveguide formation in the less pure high-pressure high-temperature (HPHT) diamond, of relevance for magnetometry. For practical implementation of quantum information systems, an important requirement is the deterministic placement of NVs. The common method to place NV centers in diamond relies on ion implantation techniques to insert single ions of nitrogen [7]. Although this method offers submicron spatial accuracy, it has limitations with respect to the depth that NVs can be created. Recently, on-demand single NVs in the bulk of ultrapure diamond were demonstrated using static femtosecond laser exposures, aided by a spatial light modulator (SLM) to correct for spherical aberrations and produce symmetric modifications [8]. In the present work, we demonstrate the laser writing of single NVs in quantum grade diamond without the use of special beam shaping techniques. The ability to write waveguide devices that exploit diamond NV centers would be compelling for quantum information and spin-based sensors, as it would enable novel 3D device configurations for interacting with NVs.

2. Materials and Methods The femtosecond laser used for optical waveguide writing in diamond was a regeneratively amplified Yb:KGW system (Pharos, Light Conversion, Vilnius, Lithuania) with 230-fs pulse duration, 515-nm wavelength (frequency doubled), focused with a 1.25-NA oil immersion lens (RMS100X-O 100× Plan Achromat Oil Immersion Objective, Olympus, Tokyo, Japan). The repetition rate of the laser was variable from 500 kHz to single pulse. Computer-controlled, 3-axis motion stages (ABL-1000, Aerotech, Pittsburgh, PA, USA) were used to translate the sample relative to the laser to form the optical circuits in synthetic diamond. Polished synthetic single-crystal HPHT (3 mm × 3 mm × 0.3 mm, nitrogen impurities 100 ppm), optical grade (5 mm × 5 mm × 0.5 mm, type II, nitrogen impurities 100 ppb), and quantum grade (dimensions 2 mm × 2 mm × 0.3 mm, type II, nitrogen impurities <5 ppb) diamond samples were acquired from MB Optics (Velp, Netherlands). For waveguide transmission measurements, high-resolution 3-axis manual positioners (Nanomax MAX313D, Thorlabs, Newton, NJ, USA) were used. The four-axis central waveguide stage (MicroBlock MBT401D, Thorlabs) enabled transverse displacement between sets of diamond waveguides. Light sources at 1550 nm (TLS001-1550, Thorlabs), 808 nm (S1FC808, Thorlabs), 635 nm (TLS001-635, Thorlabs), and 532 nm (4301-010, Uniphase) were coupled to the waveguides using the Thorlabs single-mode fibers (P1-SMF28E-FC-2 for 1550 nm, 780HP for 808 nm, SM600 for 635 nm, and 460HP for 532 nm). At the output, light was coupled to an optical power meter (818-IG-L or 818-SL, Newport, Irvine, CA, USA) to measure the power transmitted through the waveguide. To measure the near-field waveguide mode profile, a 60× asphere (5721-H-B, Newport) was used to image the light to a charge-coupled device (CCD) (SP620U, Spiricon, North Logan, UT, USA). Micromachines 2017, 8, 60 3 of 10

Micro-Raman spectra were recorded using a Labram Aramis Jobin Yvon Horiba microRaman Micromachines 2016, 7, 60 3 of 10 system with a DPSS laser source of 532 nm and equipped with a confocal microscope and an air-cooled CCD. A 100Micro-Raman× objective spectra was used were to recorded focus the using laser a on Labram the sample Aramis as Jobin well asYvon to collectHoriba themicroRaman Raman signal, with asystem spatial with resolution a DPSS oflaser about source 1 µ m.of 532 A wavenumber nm and equipped accuracy with ofa confocal about 1 cmmicroscope−1 can be and achieved an air- with a 1800cooled line/mm CCD. grating.A 100× objective Photoluminescence was used to focus measurements the laser on the were sample also as done well as with to collect this systemthe Raman using a 600 line/mmsignal, with grating. a spatial resolution of about 1 µm. A wavenumber accuracy of about 1 cm−1 can be Forachieved confocal with photoluminescencea 1800 line/mm grating. measurements, Photoluminescence nitrogen-vacancy measurements defectswere also were done excited with this with a system using a 600 line/mm grating. DPSS 532-nm laser (CL532-500-L, CrystaLaser, Reno, NV, USA) focused on to the sample with a For confocal photoluminescence measurements, nitrogen-vacancy defects were excited with a × 0.8 NADPSS objective 532-nm (100laser (CL532-500-L,CFI60 TU Plan CrystaLaser, Epi ELWD, Reno, Nikon, NV, USA) Tokyo, focused Japan). on to Photoluminescencethe sample with a 0.8 was collectedNA throughobjective the(100× same CFI60 objective, TU Plan filtered Epi ELWD, from theNikon, excitation Tokyo, lightJapan). using Photoluminescence a dichroic beamsplitter was (ZT 532collected RDC, through Chroma, the Bellow same objective, Falls, VT, filtered USA) from and long-passthe excitation filters light (ET using 555 a dichroic LP Chroma, beamsplitter FELH 0650 Thorlabs)(ZT 532 and RDC, focused Chroma, into Bellow a single Falls, mode VT, fiber USA) which and long-pass provided filters the confocal (ET 555 LP aperture. Chroma, Photon FELH 0650 counting of theThorlabs) filtered lightand wasfocused performed into a single using mode an avalanche fiber which photodiode provided (SPQR-14, the confocal Perkin-Elmer, aperture. Photon Waltham, MA, USA).counting of the filtered light was performed using an avalanche photodiode (SPQR-14, Perkin-Elmer, AnnealingWaltham, MA, of USA). the diamond samples was performed in a tubular horizontal furnace Lenton LTF15/50/450.Annealing The of samples the diamond were placedsamples in was a quartz performe boatd andin a coveredtubular horizontal with diamond furnace grit Lenton to protect LTF15/50/450. The samples were placed in a quartz boat and covered with diamond grit to protect the surface. In order to purge the furnace chamber, oxygen was extracted using a diaphragm pump the surface. In order to purge the furnace chamber, oxygen was extracted using a diaphragm pump and aand nitrogen a nitrogen flow flow for for 1 h. 1 Thermalh. Thermal treatments treatments werewere carried out out in in a nitrogen a nitrogen atmosphere atmosphere following following ◦ ◦ threethree steps: steps: first, first, temperature temperature was was raised raised to to 10001000 °CC in in 3 3 h h and and then then kept kept at 1000 at 1000 °C forC another for another 3 h, 3 h, and theand furnace the furnace was was finally finally switched switched off off and and allowed allowed to cool down down to to room room temperature. temperature.

3. Results3. Results and and Discussion Discussion

3.1. Optical3.1 Optical Waveguides Waveguides Previously,Previously, we demonstratedwe demonstrated that that high high repetition repetition rates (~500 (~500 kHz) kHz) were were favorable favorable for avoiding for avoiding graphitegraphite during during femtosecond femtosecond laser laser modification modification of diamondof diamond [4 ].[4]. The The highest highest repetition repetition raterate ofof 500500 kHz availablekHz available from our from laser our was laser thus was thus applied applied to writeto write two two closely closely spaced modification modification lines, lines, which which yieldedyielded the first the demonstration first demonstration of optical of optical waveguiding waveguiding in diamond in diamond using femtosecondusing femtosecond laser inscription laser inscription (Figure 1). Using a low 5 kHz repetition rate, we could not detect optical waveguiding (Figure1). Using a low 5 kHz repetition rate, we could not detect optical waveguiding between the between the closely spaced modifications tracks. We attributed the lower damping losses at 500 kHz closelyto spacedthe lower modifications amount of graphite tracks. formed, We attributed as confirmed the lower by µ damping-Raman spectroscopy losses at 500 and kHz absorption to the lower amountmeasurements. of graphite formed, as confirmed by µ-Raman spectroscopy and absorption measurements.

Figure 1. (Left) Type II waveguide geometry in diamond where optical mode confinement is achieved Figure 1. (Left) Type II waveguide geometry in diamond where optical mode confinement is achieved between two closely spaced and parallel laser-inscribed modification lines. (Right) Transverse view between two closely spaced and parallel laser-inscribed modification lines. (Right) Transverse view optical microscope image of type II modification in diamond, with modification tracks separated by optical microscope image of type II modification in diamond, with modification tracks separated by 13 µm. The laser processing conditions were 500 kHz repetition rate, 100 nJ pulse energy, and 0.5 13 µm.mm/s The laserscan processingspeed. The conditionsdepth of werethe waveguide 500 kHz repetition track (measured rate, 100 from nJ pulse surface energy, to andcenter 0.5 of mm/s scan speed.modifications) The depth was of40 theµm. waveguide Overlaid mode track profile (measured measured from at surface 635 nm towavelength. center of modifications)Guiding could was 40 µm.also Overlaid be observed mode by translating profile measured the input atfiber, 635 a nmfew wavelength.microns above Guidingand below could this central also beand observed highest by translatingtransmission the input mode. fiber, Note a few that microns the color above linear and scale below varying this from central 0 (violet) and highest to 1 (red) transmission indicates the mode. Noterelative that the intensity color linear of the scale mode varying profile, normalized from 0 (violet) to the to peak 1 (red) intensity. indicates the relative intensity of the mode profile, normalized to the peak intensity.

Micromachines 2016, 7, 60 4 of 10

In parallel, a group from Oxford demonstrated that optical waveguides could indeed be formed in diamond using a low (1 kHz) repetition rate [5]. Using an SLM to produce more symmetric modifications compared to our highly elliptical modifications (Figure 1), the authors used a gentler interaction with lower energy pulses (30 nJ) to modify the bulk of diamond. To achieve optical waveguiding, however, a multiscan writing geometry was required with 6 symmetrical Micromachines 2017, 8, 60 4 of 10 modifications spaced 3 µm apart vertically, to achieve similar elliptical confinement regions as our work. It is possible that the lower fluence interaction resulted in reduced graphite formation at this low repetitionIn parallel, rate, a group yielding from optical Oxford wavegu demonstratedides with that reasonably optical waveguides low damping could loss. indeed be formed in diamondThe lowest using measured a low (1insertion kHz) repetition loss for our rate type [5]. II Using waveguides an SLM is to11 producedB including more 1.4 symmetric dB/facet couplingmodifications loss (0.5-cm compared sample to our length). highly The elliptical insertion modifications loss is lower (Figure than previously1), the authors reported used due a gentler to the useinteraction of a PM withfiber to lower launch energy TM po pulseslarized (30 light nJ) into to modify the waveguides. the bulk of diamond. To achieve optical waveguiding, however, a multiscan writing geometry was required with 6 symmetrical modifications 3.1.1spaced µ-Raman 3 µm apart Spectroscopy vertically, toand achieve Photolum similarinescence elliptical inconfinement the Waveguiding regions Region as our work. It is possible that theThe lowerrepetition fluence rate interaction used not only resulted affects in reducedthe graphite graphite formation formation inside at the this modification low repetition tracks, rate, butyielding also the optical quality waveguides of the crystal with reasonablyin the guiding low region. damping To loss. study this effect, we wrote double-line structuresThe lowest at 5, 25, measured and 500 insertionkHz repetition loss for rates, our typekeeping II waveguides other parameters is 11 dB co includingnstant (0.5 1.4 mm/s dB/facet scan speed,coupling 200 loss nJ pulse (0.5-cm energy, sample 13 length). µm separation The insertion betwee lossn lines, is lower 40 µm than depth) previously to obtain reported similarly due tosized the modificationuse of a PM fiber tracks to launch(Figure TM 2a). polarized In µ-Raman light spec intotroscopy, the waveguides. a shift of the diamond peak to higher (lower) wavenumber is associated with a compressive (tensile) stress. Mapping the shift of the 3.1.1. µ-Raman Spectroscopy and Photoluminescence in the Waveguiding Region diamond peak around the laser-modified lines gives information about the spatial distribution and type ofThe stress repetition that is rateproduced used notin the only guiding affects region. the graphite formation inside the modification tracks, but alsoThe theµ-Raman quality spectra of the in crystal the guiding in the regions guiding reveal region.ed a To shift study of the this diamond effect, we peak wrote towards double-line higher wavenumbers,structures at 5, an 25, indication and 500 kHz of compressive repetition rates, stress. keeping As shown other in parameters Figure 2a, the constant compressive (0.5 mm/s stress scan is greaterspeed, 200for lower nJ pulse repetition energy, 13rates.µm Previous separation studie betweens have lines, shown 40 thatµm depth)compressive to obtain stress similarly in diamond sized resultsmodification in a decrease tracks (Figure of the2 a).refractive In µ-Raman index spectroscopy, [9], which is a detrimental shift of the diamond for optical peak waveguiding. to higher (lower) The mechanismswavenumber for is associatedwaveguiding with in a compressivediamond are (tensile) still under stress. investigation, Mapping the but shift could of the be diamond the result peak of increasedaround the polarizability laser-modified [10], lines a givesreduced information refractive about index the in spatial the modification distribution lines, and type or loss-guiding of stress that quantumis produced effects in the [11]. guiding region.

FigureFigure 2. 2. EffectEffect of the of therepetition repetition rate on rate the on stress the and stress the and photoluminescence the photoluminescence (PL) of the (PL) nitrogen- of the vacancynitrogen-vacancy (NV) centers (NV) for centers double-line for double-line waveguides. waveguides. (a) Shift of (thea) Shift Raman of the diamond Raman peak diamond at the peak center at ofthe the center waveguide of the waveguide for different for repetition different rates. repetition (b) Map rates. of the (b) Mapshift of of the the Raman shift of peak the Raman in a waveguide peak in a fabricatedwaveguide with fabricated 5 kHz. with The 5laser kHz. modification The laser modiﬁcation tracks, where tracks, the whereamorphous the amorphous carbon phase carbon is formed, phase is areformed, shown are as shown black. as(c) black. PL of (thec) PL NVs of recorded the NVs recordedat the center at the of centerwaveguides of waveguides fabricated fabricated with 500 withkHz and500 kHz5 kHz and repetition 5 kHz repetitionrates. The rates.inset shows The inset a zoomed shows in a view zoomed of the in spectra view of near the spectrathe zero near phonon the zeroline (ZPL)phonon transition. line (ZPL) transition.

The µ-Raman spectra in the guiding regions revealed a shift of the diamond peak towards higher wavenumbers, an indication of compressive stress. As shown in Figure2a, the compressive stress is greater for lower repetition rates. Previous studies have shown that compressive stress in diamond results in a decrease of the refractive index [9], which is detrimental for optical waveguiding. Micromachines 2017, 8, 60 5 of 10

The mechanisms for waveguiding in diamond are still under investigation, but could be the result of increased polarizability [10], a reduced refractive index in the modification lines, or loss-guiding quantum effects [11]. At 500 kHz repetition rate, the peak width is about 2 cm−1 and similar to the pristine value, showing that the crystalline structure is preserved. However, this peak width increases as the repetition rate is lowered, reaching a value of 3 cm−1 at 5 kHz repetition rate, which indicates a slight disordering of the crystal. Further evidence of this disorder in the diamond crystal lattice is the decrease in the Raman intensity for lower repetitions rates. A closer inspection of the Raman peak shows that it is not only shifted but also split into two components (one located at the position of the pristine peak, the second one at higher wavenumber). This effect is most evident at the lowest 5 kHz repetition rate. This split is associated with the presence of biaxial or uniaxial stress between the two damage lines [12] and could explain why only the TM polarization of light can be detected in our diamond waveguides. The stress in the guiding region in not only higher for the lower repetition rates but also more nonuniform, as shown in the Raman shift map of Figure2b. The shift observed for the guiding region formed with 5 kHz repetition rate varies from 1 to 4 cm−1, whereas for 500 kHz repetition rate the shift shows a uniform value of about 1.5 cm−1 [4]. Therefore, the higher losses observed for lower repetition rates could be due to several effects: increased graphite formation within the modification lines, higher compressive stress leading to less optical confinement and increased disorder in the guiding region. The good quality of the diamond that we have observed for 500 kHz may be associated with increased heat accumulation that is achieved for higher repetition rates [13]. The quality of the diamond crystal can affect the properties of the NV centers that can be found in the guiding region. Having this in mind, we have measured the photoluminescence at room temperature of the NVs at the center of the guiding region of the structures formed with 5 kHz and with 500 kHz repetition rates. The excitation wavelength was 532 nm, and the PL spectra are presented in Figure2c. For 500 kHz, the zero phonon line (ZPL) of the NVs remains similar to that of pristine diamond (not shown), whereas for 5 kHz the ZPL is broader and its intensity is decreased. We attribute this broadening of the ZPL to the higher stress and disorder [14] in the guiding region for a 5 kHz repetition rate. Therefore, the elevated repetition rate of 500 kHz is not only beneficial for waveguiding but also for preserving the functionality of the NV centers.

3.1.2. Shallow Waveguides for Sensing and Magnetometry The type II waveguide in Figure1 exhibited multiple coupling locations vertically within the guiding region. To achieve coupling to a single mode at only one location, we used a four line type II modification, where tracks were also written above and below the guiding region shown in Figure1. In the present work, we explored a different range of waveguide depths, with emphasis on shallower waveguides, which can interact with near-surface NV centers for applications in magnetometry. We found that for depths shallower than 30 µm, the two line type II modifications revealed single mode behavior at only a single, central location. We attribute this transition to single mode guiding at shallower depths to the reduced spherical aberration, which results in less vertically elongated modifications. As a result, the guiding region has a less vertical extent, allowing for the coupling to only a single vertical location. For waveguides characterized at visible wavelengths (532–800 nm), we found the minimum waveguide depth to be ~12 µm, as shown in Figure3. Depths shallower than 12 µm resulted in surface laser ablation and very high insertion losses. We also explored a range of separation distances between the modification tracks for extending the guiding capabilities to longer wavelengths, namely 1550 nm, which is of interest for evanescent coupling to surface photonics such as microspheres [15,16]. For this longer wavelength, the optimum separation distance was 19 µm, for which we achieved single mode operation (Figure4). Micromachines 2017, 8, 60 6 of 10

SimilarMicromachines waveguide 2016, 7, 60 loss and mode proﬁle were found at 1550 nm compared to waveguides optimized6 of 10 Micromachines 2016, 7, 60 6 of 10 for visible wavelength operation.

Figure 3. Side view optical microscope image of type II modificationmodification in diamond, with modificationmodification Figure 3. Side view optical microscope image of type II modification in diamond, with modification tracks separated by 15 µµm.m. The lase laserr processing conditions were 500 kHz repetition rate, 60 nJ pulse tracks separated by 15 µm. The laser processing conditions were 500 kHz repetition rate, 60 nJ pulse energy, andand 0.50.5 mm/smm/s scan scan speed. speed. The The waveguide waveguide depths depths (left (left to to right) right) were were and and 20, 20, 15, 15, and and 12 12 µm.µm. energy, and 0.5 mm/s scan speed. The waveguide depths (left to right) were and 20, 15, and 12 µm. Coupling toto aa single single vertical vertical position position was was possible possible with with depths depths below below 30 µ m,30 withµm, thewith minimum the minimum depth Coupling to a single vertical position was possible with depths below 30 µm, with the minimum beingdepth 12beingµm for12 µm waveguide for waveguide operation. operation. Mode profile Mode at profile 635 nm at characterization 635 nm characterization wavelength wavelength shown for depth being 12 µm for waveguide operation. Mode profile at 635 nm characterization wavelength theshown shallowest for the shallowest waveguide. waveguide. Note that theNote color that linear the co scalelor linear varying scale from varying 0 (violet) from to0 (violet) 1 (red) to indicates 1 (red) shown for the shallowest waveguide. Note that the color linear scale varying from 0 (violet) to 1 (red) theindicates relative the intensity relative ofintensity the mode of the profile, mode normalized profile, normalized to the peak to intensity.the peak intensity. The 15 µm The scale 15 barµm isscale the indicates the relative intensity of the mode profile, normalized to the peak intensity. The 15 µm scale samebar is forthe thesame mode for the and mode microscope and microscope image. image. bar is the same for the mode and microscope image.

Figure 4. Side view of type II waveguide for 1550 nm guiding. The separation between the Figure 4.4.Side Side view view of typeof type II waveguide II waveguide for 1550 for nm 1550 guiding. nm Theguiding. separation The betweenseparation the between modiﬁcation the modification tracks was 19 µm and the depth was 20 µm. The laser processing conditions were 500 tracksmodification was 19 tracksµm and was the 19 depth µm and was the 20 µdepthm. The was laser 20 processingµm. The laser conditions processing were conditions 500 kHz repetitionwere 500 kHz repetition rate, 60 nJ pulse energy, and 0.5 mm/s scan speed. Single mode guiding was obtained rate,kHz repetition 60 nJ pulse rate, energy, 60 nJ andpulse 0.5 energy, mm/s and scan 0.5 speed. mm/s Single scan speed. mode Single guiding mode was guiding obtained was at 1550obtained nm. at 1550 nm. Note that the color linear scale varying from 0 (violet) to 1 (red) indicates the relative Noteat 1550 that nm. the Note color that linear the scale color varying linear scale from varying 0 (violet) fr toom 1 (red)0 (violet) indicates to 1 the(red) relative indicates intensity the relative of the intensity of the mode profile, normalized to the peak intensity. The 10 µm scale bar is the same for modeintensity proﬁle, of the normalized mode profile, to thenormalized peak intensity. to the peak The 10intensity.µm scale The bar 10 is µm the scale same bar for is the the mode same and for the mode and microscope image. microscopethe mode and image. microscope image.

All of the above reported waveguide results were achieved in optical grade diamond. In All ofof thethe above above reported reported waveguide waveguide results results were we achievedre achieved in optical in gradeoptical diamond. grade diamond. In ultrapure In ultrapure quantum grade diamond and lower purity HPHT diamond, similar transmission quantumultrapure gradequantum diamond grade and diamond lower purityand lower HPHT purity diamond, HPHT similar diamond, transmission similar characteristicstransmission characteristics were observed for the same laser processing parameters. Waveguides operational in werecharacteristics observed were for the observed same laser for processingthe same laser parameters. processing Waveguides parameters. operational Waveguides in quantum operational grade in quantum grade and HPHT diamond will be beneficial for optical devices in quantum information andquantum HPHT grade diamond and HPHT will be diamond beneﬁcial will for be opticalbenefic devicesial for optical in quantum devices information in quantumsystems information and systems and magnetometry, respectively. magnetometry,systems and magnetometry, respectively. respectively.

3.2. NV Centers Centers in Quantum Grade Diamond For practicalpractical implementation implementation of aof diamond-based a diamond-based magnetometer magnetometer or quantum or quantum information information system, ansystem, important an important requirement requirement is the deterministic is the deterministic placement placement of NV of defects, NV defects, while while preserving preserving the NV’s the NV’s excellent spin properties. Applying the same femtosecond laser to fabricate both the optical waveguides and NV centers would enable a true diamond photonics fabrication platform, ensuring excellent alignment of NVs with optical circuitry.

Micromachines 2017, 8, 60 7 of 10 excellent spin properties. Applying the same femtosecond laser to fabricate both the optical waveguides and NV centers would enable a true diamond photonics fabrication platform, ensuring excellent alignment of NVs with optical circuitry. It has been shown previously that a femtosecond laser can create color centers in various materials [17–19]. In the case of diamond, we have seen from photoluminescence measurements within the modification lines [4] that the femtosecond laser creates amorphous carbon as well as a large amount of defects, which are mainly vacancies and vacancy complexes. Therefore, by using a single focusedMicromachines femtosecond 2016, 7, 60 laser pulse using an energy below the amorphization threshold,7 of 10 it may be possible to produceIt has been vacancies shown previously within the that focal a femtosec volume.ond Inlaser quantum can create grade color diamond,centers in various there are up to 3 1000 nitrogenmaterials impurities [17–19]. In present the case within of diamond, the ~1weµ havem focalseen from volume photoluminescence of the focused measurements femtosecond laser beam, so thewithin probability the modification of creating lines [4] a that vacancy the femtosec nextond to an laser impurity creates amorphous is low. The carbon sample as well is subsequentlyas a annealed inlarge order amount to of create defects, NV which centers. are mainly Vacancies, vacancies and which vacancy become complexes. mobile Therefore, above bythe using temperature a ◦ single focused femtosecond laser pulse using an energy below the amorphization threshold, it may of 600 Cbe are possible captured to produce by the vacancies substitutional within the focal nitrogen volume. impurities In quantum tograde form diamond, NV centers there are [up20 ]. Higher temperatureto 1000 annealing nitrogen mayimpurities be used present to annealwithin the out ~1 otherµm3 focal vacancy volume complexes,of the focusedwhich femtosecond can be laser detrimental to the NVbeam, centers’ so the properties probability [of21 creating]. a vacancy next to an impurity is low. The sample is subsequently This strategyannealed in was order recently to create applied NV centers. successfully Vacancies, which by Chen become et al. mobile [8], usingabove the single temperature static femtosecond of 600 °C are captured by the substitutional nitrogen impurities to form NV centers [20]. Higher laser pulsestemperature focused annealing in the may bulk be of used ultrapure to anneal out quantum other vacancy grade complexes, diamond. which can A SLMbe detrimental was applied by the researchersto the NV to centers’ achieve properties a symmetric [21]. modification, resulting in the formation of single NVs after annealing. HereThis we strategy show was that recently single applied NVs successfully can be produced by Chen et withoutal. [8], using spatial single static beam femtosecond shaping methods. Using thelaser same pulses 1.25-NA focused oil in immersion the bulk of ultrapure microscope quantum objective grade diamond. applied A forSLM optical was applied waveguide by the writing, researchers to achieve a symmetric modification, resulting in the formation of single NVs after µ we found thatannealing. single Here NVs we couldshow that be single formed NVs in can quantum be produced grade without diamond spatial beam at depths shaping less methods. than 30 m. To produceUsing the vacancies same 1.25-NA at theoil immersion desired position,microscope we objective used applied single for femtosecond optical waveguide laser writing, pulses focused 25 µm belowwe found the surface that single with NVs energies could be formed between in qu 10antum and 30grade nJ, diamond well below at depths the less amorphization than 30 µm. threshold (~50 nJ). The regionsTo produce selected vacancies for at the laser desired processing position, we contained used single no femtosecon pre-existingd laser NVpulses centers. focused For each 25 µm below the surface with energies between 10 and 30 nJ, well below the amorphization threshold pulse energy,(~50 10nJ). trials The regions were performed.selected for laser After processing the laser contained exposure, no pre-existing no visible NV modification centers. For waseach observed, ◦ as shown inpulse Figure energy,5a. 10 The trials sample were wasperformed. then annealedAfter the laser at 1000 exposure,C for no 3vi hsible in amodification nitrogen atmosphere was to avoid oxidationobserved, at as the shown diamond in Figure surface. 5a. The sample This temperature was then annealed was at selected 1000 °C for based 3 h in on a previousnitrogen studies that haveatmosphere shown that to avoid high oxidation annealing at the temperatures diamond surface. improve This temperature the properties was selected of thebased resulting on NV previous studies that have shown that high annealing temperatures improve the properties of the centers [8,21resulting,22]. NV centers [8,21,22].

Figure 5. (a) Overhead microscope image of the static femtosecond laser exposure area. The black Figure 5. (a) Overhead microscope image of the static femtosecond laser exposure area. The black marker spots are written with higher energy and multiple laser pulses to identify the positions of the marker spotslower are energy written single with static higher laser exposures energy andwhich multiple are not visible. laser ( pulsesb) Overhead to identify photoluminescence the positions of the lower energyintensity single map static and (c laser) spectrum exposures for single which NV center are produced not visible. by focused (b) Overhead femtosecond photoluminescence laser pulse intensity map(24 nJand pulse ( cenergy)) spectrum followed for by single annealing. NV (d center) Intensity produced autocorrelation by focused (corrected femtosecond for background laser pulse (24 nJ pulseon energy) left y-axis followed [23], raw byuncorrected annealing. correlations (d) Intensity counts autocorrelation on right y-axis) revealing (corrected single for photon background on emission. left y-axis [23], raw uncorrected correlations counts on right y-axis) revealing single photon emission.

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After annealing, the photoluminescence emission from non-irradiated diamond was the same as before the treatment. However, a photoluminescence (PL) signal showing an NV signature was found at the static laser exposure conditions, for pulse energies of 16 and 30 nJ. Below 16 nJ pulse energy, no PL signal was detected between 630 to 800 nm. In the energy range where NVs are formed, we performed cross correlation measurements, which revealed that pulse energies between 20 and 26 nJ had the highest probability of producing single NV centers. In particular, for 24 nJ, in our best sample, we found the probability of creating single NVs to be 80% (4 out of 5 trials), although this value is typically approximately 50% (average of ﬁve samples), similar to the success probability reported by Chen et al. [8]. The maximum of the NV emission is always located inside a 1 µm radius around the center of the static exposure. In Figure5b–d, we present the results for an NV obtained with a 24 nJ static femtosecond laser pulse exposure. In Figure5b, the overhead photoluminescence intensity map shows that the emission is well localized in a 1 µm2 area near the static exposure. The spectrum recorded at room temperature for this NV is shown in Figure5c, where the characteristic ZPL at 637 nm and the broadband phonon side band are clearly visible. An intensity autocorrelation measurement of the photons detected is shown in Figure5d, showing an antibunching dip with g(2)(0) well below 0.5, characteristic of single photon source emission. From these results, we can conclude than it is possible to create single NV centers with submicron precision using a single femtosecond laser without the use of beam shaping techniques. Future work will seek to integrate optical circuits and NV centers in diamond, all formed using femtosecond laser microfabrication.

4. Conclusions In summary, we have demonstrated that femtosecond laser microfabrication can inscribe buried and single mode optical waveguides in diamond using type II geometry. The waveguides can be tuned to operate between 532 and 1550 nm wavelength by increasing the separation from 13 to 19 µm between the modiﬁcation lines. The highest repetition rate of 500 kHz was found to produce less graphite in the modiﬁcation lines and a higher quality guiding region, leading to lower damping losses. However, more work is required to better understand the mechanisms for waveguiding in diamond. We demonstrated the possibility of forming single NV centers in the bulk of diamond without the use of special beam shaping methods. The laser-written NV centers and optical circuits will serve as the building blocks for a diamond photonics platform that could enable both quantum information systems with optically connected entangled qubits and ultrasensitive and high-resolution optical magnetometers.

Acknowledgments: This work has been supported by the FP7 DiamondFab CONCERT Japan project, DIAMANTE MIUR-SIR grant, FemtoDiamante Cariplo ERC reinforcement grant and the Scientific and Technological Research Council of Turkey (TUBITAK) Grant No. 114F312. We are grateful to Guglielmo Lanzani and Luigino Criante for access to the FemtoFab facility at CNST–IIT Milano for the laser fabrication experiments. We thank Paolo Olivero, Federico Bosia, and Federico Picollo from the University of Torino for helpful scientific discussions. Author Contributions: S.M.E. and P.E.B. conceived the idea of forming waveguides in the bulk of diamond. V.B., B.S., S.R. and S.M.E. performed femtosecond laser writing experiments in diamond. B.S., V.B., T.T.F., S.R. and A.S. characterized the transmission properties of the diamond waveguides. B.S., A.C. and C.A. performed annealing experiments to produce NV centers from static laser exposures. J.P.H., P.E.B. and S.M.E. characterized the properties of the NV centers. B.S., A.C., C.A., R.R. and M.F. characterized the structure of femtosecond laser written lines using microRaman spectroscopy. All authors discussed the experimental implementation and results and contributed to writing the paper. Conflicts of Interest: The authors declare no conflict of interest.

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