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We report graphite-free laser etching of diamond surfaces using 266 nm laser pulses for a wide range of incident fluences below the threshold for ablation. The etching rate is proportional to the (fluence)x where x = 1.88 ± 0.16 over the range 10−6 - 10−2 nm per pulse for incident pulse fluences 1 – 60 J/cm2. Surface sensitive near edge x-ray fine absorption structure measurements (partial electron yield NEXAFS) reveal that etching does not significantly alter the surface structure from the initial oxygen terminated and graphite-free state. The etching process, which is consistent with a mechanism involving the desorption of carbon species via the decay of 2-photon excited excitons near the surface, appears to have no threshold and is promising for creating a range of high resolution structures.
©2011 Optical Society of America
1. Introduction
There is currently intense interest in diamond micro- and nano-structured devices for use in applications as diverse as quantum information science [1,2], nano-electromechanical systems [3] and diamond Raman lasers [4]. Diamond is one of the most challenging materials to process due to its extreme mechanical hardness and chemical inertness, and further complicated by crystal planes that behave differently under mechanical and plasma processing. A variety of methods for diamond surface structuring have been investigated including inductively-coupled plasma etching [5,6], ion beam milling [7,8], plasma reactive ion etching [9], ion-implantation with chemical etching [10], and laser ablation [11]. Laser ablation has the convenience of being a direct write process, but there are major challenges in creating high resolution and graphite-free features. An optical etching process that can offer increased resolution without causing graphitization and underlying damage will allow much greater flexibility than currently available.
Laser induced desorption (LID) may be distinguished from laser ablation by lower incident pulse fluences and intensities, and by a mechanism of mass removal that involves bond breaking and ejection of surface atoms or molecules via the decay of electronically or vibrationally excited quanta rather than via thermo-mechanical or electrostatic forces (e.g [12,13].). LID has been observed in a wide variety of materials with desorption rates dependent on many variables in addition to the photon energy and fluence such as the substrate temperature and the presence of chemical reactants at the surface. Material removal rates are typically less than an atomic layer per pulse and can be achieved without substantial heating of the substrate. As a result, the technique is well suited to a range of surface atomic and molecular diagnostics such as laser mass spectrometry (including biomolecular analysis using matrix assisted laser desorption and ionisation) and surface chemistry studies in addition to micro and nano-structuring applications [14]. For diamond, LID has been observed recently by using pulsed near band-gap laser irradiation [15,16]. Kononenko et al [15] showed that diamond was etched in air at rates 0.1-1x10−3 nm/pulse, which is orders of magnitude slower than rates typical of ablation (> 1 nm/pulse [17]), when using sub-ablation threshold laser pulses of wavelength 248 nm. In studies of UV pumped diamond Raman lasers, pits approximately 50 nm deep were etched into the uncoated facets by repeated exposure to 266 nm laser pulses of duration 30 ps, with etch rates as low as tens of carbon atoms per pulse [16].
Improved knowledge on the desorption mechanism is crucial for assessing the flexibility and utility of LID in creating diamond micro- and nano-structures. In etching reports to date [15,16], the rate is a nonlinear function of the incident pulse fluence. Kononenko et al [15] also noted that the presence of oxygen at the surface was crucial to the etching process and proposed that the likely mechanism for material ejection was by enhanced oxidation of the surface due to two-photon excitation of electron hole pairs near the surface. The fine control afforded by LID-based etching heralds substantial promise for expanding the range of diamond structures possible using versatile all-optical processes. However, the characteristics of diamond LID are not well known and further investigation is required in order to determine resolution limits of the process and the properties of the etched surface.
In this paper, we report a detailed characterisation of the etch rate and etched surface structure upon exposure to pulsed deep ultraviolet radiation. We show that the etch rate closely follows a square relationship with the incident pulse fluence, as expected for a 2-photon process, and that this relationship holds across the incident beam profile and over a large range of incident fluences up to the ablation threshold. Surface sensitive partial electron yield near edge x-ray absorption fine structure (NEXAFS) spectroscopy shows that etched surface is oxygen terminated and graphite free. We deduce that the process is thresholdless and highly promising for creating a range of smooth structures with spatial resolution at least down to the micron scale.
2. Experimental Description
Single crystal Type IIa (Electronic grade, Element 6) and polycrystalline (Diafilm, Element 6) diamond samples were studied. The surfaces were cleaned using a hot oxidizing acid solution to ensure an oxygen terminated surface. The samples were irradiated using a 266 nm laser (JDSU Q-series Q-201HD 532 nm with custom second-harmonic conversion) of pulse duration 11 ns, pulse repetition rate 7-14 kHz and incident powers up to 300 mW. The linearly-polarized output beam was focused using a 25 mm focal length objective (LMU-5X-UVB, Thorlabs) to a ω0 = 4.8 ± 0.2 µm radius spot on the sample surface. Standard definitions for ω0 and fluence which refer to the radius at the 1/e2 value of the maximum with spot area taken as πω0 2/2 were used. In order to investigate etch rates as a function of fluence for fixed spatial properties of the incident beam, it was important to operate the laser at a fixed output power and attenuate the beam prior to the focusing using a combination of reflective filters with low residual absorption.
The depth, shape and roughness of the etched surface were measured using an interferometric profiler (VEECO, Wyko NT3300). Surface composition and electronic structure were investigated using NEXAFS using the high resolution soft x-ray spectroscopy beamline at the Australian Synchrotron (Melbourne, Australia). The background pressure during these measurements was 10−10 mb. The measurements were performed in partial electron yield mode, to maximise the surface sensitivity of the technique [18].
3. Results
The morphology of laser treated diamond surfaces was investigated as a function of incident fluence both above and below the ablation threshold for [100] Type IIa samples. The threshold is defined here as the minimum laser fluence for ablation damage when using a burst of 60 laser pulses with a duration of approximately 50 ms (corresponding to the shortest exposure time available from the shutter) and corresponds to 37 ± 3 µJ of incident energy or pulse intensities of I p = 9.2 GW/cm2 (101 J cm−2 in 11 ns). Figures 1(a) –1(c) shows microscope images of the exposed surface for laser intensities 1.1, 0.6 and 0.07 times the threshold. Just above the ablation threshold, pits approximately 10 times the diameter of the incident beam were ablated within the minimum exposure time. The ablated pit typically exhibits cleavage along major crystal planes (eg., [111]) and dross typical of explosive mechanical ejection of large particles. The depth created during the 60 pulse burst corresponds to an approximate ablation rate of 1500 nm/pulse. The ablation rate and morphology is consistent with the strong laser interaction of the beam with the sample via the production of graphite characteristic of nanosecond ablation reported previously (see e.g [19].).
Fig. 1 Microscope images of the diamond surface subsequent to exposure at a) 1.1, b) 0.6 and c) 0.07 times the ablation threshold. The exposure times and depths were 0.05 s, 30 s, 14083 s and 30 µm, 0.60 µm, 0.67 µm respectively, and the pulse rate 7.5 kHz. In a), the beam size is indicated by the dashed circle. For b) and c), the contrast was enhanced using differential interferometric contrast mode. d) 3-D rendered optical interferometric profile image of the pit in b). e) An example of an arbitrary pattern created by the direct write process using a scan speed of 9 µm/s and 120 repetitive scans. Note that in d) and e), the colour bar spans a range of 600 nm and 350 nm respectively, and the raised section on one side of ablated regions where there is discontinuity in the gradient is a known artifact of the measurement system [20].
Below the ablation threshold, the material removal rate is orders of magnitude slower and pits are only observable using a microscope when using much longer exposure times (> 5s). Etched pits of depth less than 1 micron and aspect ratio less than 0.15 were investigated in order to avoid side-wall effects on the etch rate and shape. In this regime, the depth is proportional to the number of cumulative pulses as shown in Fig. 2 for the case of the incident fluence 34 J cm−2 and yielding an average etch rate of 6.4x10−4 nm/pulse. Incubation effects on the etch rate during the initial exposure period, if present, are not observable for the typically large number of pulses used (>105). The etched-pit morphology is in stark contrast to ablation; the pits are of similar diameter to the incident beam, are smooth and appear on the front and back sides of the sample. All analysis presented herein is on the microstructures of the first surface to avoid complications in the analysis arising from the subsequent defocusing of the transiting beam.
Fig. 2 Etch depth as a function of exposure time. The open circles are for [100] single crystal and the closed circles for polycrystalline material. The dashed line indicates an average etch rate of 6.4x10−4 nm/pulse. Conditions: 170 mW at 14 kHz.
Vertical scanning interferometric profile measurements (see for example Fig. 1(d)) reveal that the pits are approximately Gaussian shaped. For pits of Figs. 1(b) and 1(c), the maximum depths are 600 nm and 680 nm respectively and full-width half maxima are 4.3 µm and 5.3 µm. The slightly wider pit of Fig. 1(c), which was created over a period of 2-3 hours, is attributable to drift in the focal spot position that occurs for the system over extended periods. The pit in Fig. 1(b) was etched in a 30 s period and represents the minimum width observed. Figure 3 shows a comparison of the pit etched with incident laser beam profile. The pit width is narrower than the beam and more closely follows the square of the beam profile, i.e., the pit width is approximately equal the beam width divided by √2. Taking into consideration uncertainties in the beam and pit profiles, the pit and beam widths are consistent with an I p 2 relationship between the etch rate and the incident intensity as discussed in more detail below.
Fig. 3 Comparison of the etch profile with an inverted Gaussian profile of width equal to the measured beam waist. Conditions: 14 kHz pulse rate in air.
The etch rate as a function of pulse fluence varies nonlinearly with fluence up to the ablation threshold as shown in Fig. 4 . Rates were measured for the range 1-60 J cm−2 by varying the time period for etching to etch pits of depth in the range (100-800 nm) and using the fact that the etch depth is proportional to the number of incident pulses for all fluences (refer Fig. 2). This procedure ensured that etch rates could be measured accurately over several orders of magnitude whilst maintaining a pit aspect ratio sufficiently small (<15%) to avoid side-wall effects during the etching process and depth measurement procedure. Just below the ablation threshold (60 J cm−2), the etch rate was 2.5 × 10−3 nm/pulse. As the fluence was decreased, the etch rate decreased with approximately the square of the incident power. A least squares regression to the data yields a dependence of I p x where x = 1.88 ± 0.16 with uncertainty taken as twice the variance. The power factor is consistent with the I p 2 relationship expected for a two-photon process underpinning the desorption rate. At low fluences (1.1 J cm−2), the etch rate is less than 10−6 nm/pulse which corresponds to approximately 27 000 atoms/pulse over the area of the beam or approximately 0.001% of the exposed surface atoms.
Fig. 4 Etch rate as a function of laser fluence. The data point shown for the fluence above the ablation threshold corresponds to the average ablation rate per pulse for 60 pulses of fluence 110 J/cm2. Pulse rate 7.5 kHz.
Analysis of the etched surface smoothness and chemical structure was undertaken using the optical interferometric profile and NEXAFS spectroscopy measurements. For both these procedures it was important to create a larger etched region, which was achieved by placing the sample on a computer controlled compound stage and slowly translating the sample (at rates typically < 10 µm s−1). In this manner arbitrary shaped etch patterns could be created as shown in Fig. 1(e). After etching to 300 nm deep, the rms roughness of the surface increased from 1 nm to 3 nm as measured using the phase shift interference mode of the optical profilometer.
The NEXAFS measurements were undertaken in the range 280-305 eV (C K-edge) in partial electron yield mode with an electron analyzer energy of 165 eV, for which the mean free path of Auger electrons, and thus the surface sensitivity, corresponds to a few (<5) atomic layers. A rectangular area 430 µm x 200 µm wide was etched to match the expected spot size of the synchrotron beam. The etch depth was 50 nm. An ablated 500 µm rectangular grid was placed around the etched area to enable the etched surface to be conveniently located in the NEXAFS spectrometer. The NEXAFS spectrum for the etched surface is presented in Fig. 5 (blue curve). The bulk x-ray absorption edge occurs at 289 eV, with the observation of a strong C 1s core-hole exciton resonance. The pre-edge region contains information on unoccupied surface electronic states within the band gap. Two clear features of differing origin are observed. Feature A is associated with sp 2 hybridized carbon [18,21]. The spectrum for freshly cleaved highly oriented pyrolytic graphite (also included in Fig. 5) clearly shows the peak energy and magnitude corresponding to a graphite surface. The small sp 2 signal appearing in the spectrum of the etched surface is not attributed to generation of etching-induced sp 2 but due to slight overlap of the x-ray beam with the graphite-containing locating grid as confirmed by comparison with the spectrum of the virgin surface containing an ablated grid of twice the density (red curve). Feature B is associated with oxygen termination and in particular the ‘top’ bonded (ketone) oxygen [22]. The similar areas of B in the etched and un-etched spectra indicate no major change in the level of ‘top’ bonded oxygen termination between the initial chemically oxidized surface and the laser etched one.
Fig. 5 NEXAFS spectrum for the laser etched surface compared with spectra for high oriented pyrolitic graphite (HOPG) and the un-etched surface. Feature A indicates sp 2 hybridized carbon which appears in the etched and un-etched spectra due to presence of graphite containing locator grids.
3. Discussion
The observed etch rate and surface analysis of the treated surface allow us to make the following conclusions about the etching process:
- 1. Etching is observed at pulse fluences and peak powers at least two orders of magnitude lower than characteristic fluences for ablation. The etch rate scales with approximately the square of intensity and this relationship is reproduced as a function of position on the surface in accordance with the incident beam profile.
- 2. The intrapulse etch rate, defined here as the etch depth per pulse divided by the pulse duration FWHM, has a similar dependence on pulse irradiance for ns and ps pulses. When plotted alongside the picosecond measurements of ref [16] (20 ps pulses at 78 MHz repetition rate) and the 248 nm results in ref [15] (15 ns pulses at 100 Hz) as shown in Fig. 6 , it is evident that the intrapulse etch rates have a similar dependence on incident intensity suggesting that the rate is largely independent of pulse duration in the range 10−10 – 10−8 s and pulse repetition rate in the range 102 - 108 Hz. The I p 2 dependence observed in this study reveals a proportional relationship between the energy absorbed by the two-photon process (proportional to βI p 2Δt where β is the TPA coefficient and Δt is the pulse duration) and the number of ejected atoms. The etch rate for 15 ns pulses at 248 nm at pulse repetition rate <100 Hz [15] also shows similar dependence but at a systematically higher rate. The higher rate may partially result from the higher TPA coefficient for the shorter wavelength (β248 = 1.60 cm GW−1 and β266 = 1.48 cm GW−1 [17]), although this is not enough to completely explain the difference. The observed independence with pulse repetition rate from 78 MHz to less than 100 Hz is expected given that multi-pulse effects are unlikely to be significant due to the short relaxation time of free carriers and surface thermal gradients (less than a nanosecond [23,24]) compared to the interpulse period.
- 3. The surface roughness increases from 1 nm to 3 nm upon etching to a depth of 300 nm. Low roughness is crucial in many applications, to reduce scattering losses in optical applications for example, and to improve device performance in electronics and MEMS. For waveguides in Si, scattering losses less than the order of the absorption losses are typically obtained for roughness values the order of 5-10 nm (see for example [25]). Thus 2-photon etching is promising for creating a range of low scatter loss optical surfaces.
- 4. The etched surface is oxygen terminated and free from graphite. In contrast, graphite formation is intrinsic to laser ablation [11,17,26–28], and ablated features are often characterized by cracking, spalling, crater-halo and debris [28,29]. Although ultrafast machining has enabled reduced formation of graphite [17], and improved resolution [17,30], the absence of graphite in the etched surface observed herein is crucial to enabling linear cumulative etching (as shown in Fig. 2) and an advantage for avoiding post processing (such as ozone, plasma, annealing or chemical treatments).
- 5. Etching is found to proceed at rates largely independent of the facet direction. We observe etch rates for [100] and polycrystalline surfaces within approximately 20% (as shown in Fig. 2). Etching was also reported [16] for Brewster angled surfaces (63.7 degrees from [110]) at rates similar to the present study when taking into consideration the shorter pulse duration (see Fig. 6). We have also undertaken preliminary etching investigations of polished [110] surfaces which suggest that etching also proceeds at a rate similar to that seen for other surfaces.
These observations reveal that UV etching of diamond in air is a promising and highly versatile method for slow and controlled removal of surface atoms. The most striking observation is the I p 2 dependence of the etch rate over a large range of pulse durations and pulse rates. Such a well-defined etch rate dependence spanning 3 orders of magnitude, which has not been seen in any other material as far as we are aware, is indicative of a mechanism of particle ejection that is a direct consequence of TPA. TPA by the surface atoms and in the bulk needs to be considered. We discount TPA in the air adjacent the surface due to the small cross-section for TPA in any major air constituents and due to the weak influence on etch rate we observe in the pressure range 1-1000 mb. Kononenko et al deduced that thermal processes were too small to account for any observed enhancement of surface oxidation and proposed that surface oxidation was mediated by electron-hole recombination in the diamond bulk [15]. At room temperature, electron hole pairs in diamond form Wannier-Mott excitons with binding energy 80 meV and potential energy just below the indirect band gap [31]. These excitons may diffuse to the surface and give up the stored energy (5.2 eV) to break surface bonds. The exciton energy is sufficient to desorb CO from [100] surfaces, for example, which has an activation energy of desorption of 1.67 eV [32]. If exciton decay at the surface is the dominant cause of desorption, the diffusion length of excitons will be a crucial parameter determining the etching rate and the minimum spatial resolution of etched structures. One would expect that the minimum feature size would be comparable to, or longer than, the diffusion length L ex. Our results suggest that L ex is less than a few microns to account for the observed diameter of the etch pit of less than 5 µm. An upper bound of 5 µm for L ex was deduced on the basis of photo-electron emission yield measurements [33]. However, there seems to be some uncertainty in the value with other reports suggesting values from 200 nm [31] to 200 µm [34]. We note that if L ex < 200 nm, the calculated probability for an exciton created by TPA within L ex of the surface to eject a single carbon atom exceeds unity. If L ex is longer than 5 µm, it may be necessary to consider absorption in the adsorbed oxygen layer to account for the sub-5 µm resolution observed herein.
A notable corollary of the two-photon desorption mechanism at sub-ablation fluences is that etching persists for very low UV fluences, and that even under incoherent illumination diamonds will steadily lose mass. However, the effect under ambient light conditions is rather insignificant. For example, continuous wave Hg lamp illumination at 253 nm for typical irradiances (0.1 W cm−2) would require approximately 1010 years to desorb a significant mass (e.g. 1 µg) from a surface a few millimetres square. Under sunlight conditions the etching rate is even slower due to the reduced irradiance (10−4 W cm−2; 300-350 nm) and the reduced probability for TPA at longer wavelengths. Of more practical significance, the I p 2 dependence is of interest for enabling etching using methods other than laser direct-write and sources other than Q-switched and ultrafast lasers. The field enhancement provided by wavelength scale probe structures (such as those used in scanning near field microscopy) can provide adequate etch rates at small pulse energies (eg., [35]). Projection of patterns using broad area mask imaging and interferometric methods are also interesting avenues for creating high resolution structures. Progress in AlGaN quantum well diode lasers emitting in the deep UV regime (see. e.g [36].) may lead to highly compact sources suitable for etching small areas. Depending on the aforementioned diffusion characteristics of the two-photon excited state, the combination of high resolution scanning methods and pulse fluences corresponding to near single atom removal rates may be an attractive and flexible technique for atomic removal from diamond surface atoms with near single atom precision.
In conclusion, etching of diamond in air is observed at etch rates 10−6-10−3 nm/pulse using nanosecond pulses at 266 nm. The oxygen termination of the surface is preserved and free from sp 2 hybridized carbon and graphite. Carbon removal is proportional to TPA of the incident beam and occurs over a large range of fluences up to the ablation threshold. The absence of a threshold indicates substantial promise for enabling a wide range of high resolution structures using diverse techniques such as direct write processing, near-field, masking and interferometric illumination.
Acknowledgments
The authors thank Prof Vitaly Konov for interesting discussions on this subject, Adam Joyce, Carlo Bradac and Törsten Gaebel for their expert assistance in the surface profile analysis methods, and Dr Bruce Cowie for his expert assistance in performing the x-ray measurements. This material is based on research sponsored by the Australian Research Council Future Fellowship Scheme (FT0990622), and the Air Force Research Laboratory under agreement number AOARD-10-4078.
References and links
1. T. Gaebel, M. Domhan, I. Popa, C. Wittmann, P. Neumann, F. Jelezko, J. R. Rabeau, N. Stavrias, A. D. Greentree, S. Prawer, J. Meijer, J. Twamley, P. R. Hemmer, and J. Wrachtrup, “Room-temperature coherent coupling of single spins in diamond,” Nat. Phys. 2(6), 408–413 (2006). [CrossRef]
2. A. Faraon, P. E. Barclay, C. Santori, K.-M. C. Fu, and R. G. Beausoleil, “Resonant enhancement of the zero-phonon emission from a colour centre in a diamond cavity,” Nat. Photonics 5(5), 301–305 (2011). [CrossRef]
3. L. Sekaric, J. M. Parpia, H. G. Craighead, T. Feygelson, B. H. Houston, and J. E. Butler, “Nanomechanical resonant structures in nanocrystalline diamond,” Appl. Phys. Lett. 81(23), 4455–4457 (2002). [CrossRef]
4. R. P. Mildren, J. E. Butler, and J. R. Rabeau, “CVD-diamond external cavity Raman laser at 573 nm,” Opt. Express 16(23), 18950–18955 (2008). [CrossRef] [PubMed]
5. M. Karlsson, K. Hjort, and F. Nikolajeff, “Transfer of continuous-relief diffractive structures into diamond by use of inductively coupled plasma dry etching,” Opt. Lett. 26(22), 1752–1754 (2001). [CrossRef] [PubMed]
6. E. Gu, H. W. Choi, C. Liu, C. Griffin, J. M. Girkin, I. M. Watson, M. D. Dawson, G. McConnell, and A. M. Gurney, “Reflection/transmission confocal microscopy characterization of single-crystal diamond microlens arrays,” Appl. Phys. Lett. 84(15), 2754–2756 (2004). [CrossRef]
7. M. Tarutani, Y. Takai, and R. Shimizu, “Application of the focused-ion-beam technique for preparing the cross-sectional sample of chemical vapor deposition diamond thin film for high-resolution transmission electron microscope observation,” Jpn. J. Appl. Phys. 31(Part 2, No. 9A), L1305–L1308 (1992). [CrossRef]
8. S. Castelletto, J. P. Harrison, L. Marseglia, A. C. Stanley-Clarke, B. C. Gibson, B. A. Fairchild, J. P. Hadden, Y.-L. D. Ho, M. P. Hiscocks, K. Ganesan, S. T. Huntington, F. Ladouceur, A. D. Greentree, S. Prawer, J. L. O’Brien, and J. G. Rarity, “Diamond-based structures to collect and guide light,” N. J. Phys. 13(2), 025020 (2011). [CrossRef]
9. W. J. Zhang, Y. Wu, W. K. Wong, X. M. Meng, C. Y. Chan, I. Bello, Y. Lifshitz, and S. T. Lee, “Structuring nanodiamond cone arrays for improved field emission,” Appl. Phys. Lett. 83(16), 3365–3367 (2003). [CrossRef]
10. P. Olivero, S. Rubanov, P. Reichart, B. C. Gibson, S. T. Huntington, J. R. Rabeau, A. D. Greentree, J. Salzman, D. Moore, D. N. Jamieson, and S. Prawer, “Ion-beam-assisted lift-off technique for three-dimensional micromachining of freestanding single-crystal diamond,” Adv. Mater. (Deerfield Beach Fla.) 17(20), 2427–2430 (2005). [CrossRef]
11. M. Rothschild, C. Arnone, and D. J. Ehrlich, “Excimer-laser etching of diamond and hard carbon films by direct writing and optical projection,” J. Vac. Sci. Technol. 4(1), 310–314 (1986). [CrossRef]
12. A. Piqué and D. B. Chrisey, Direct-Write Technologies for Rapid Prototyping Applications: Sensor, Electronics, and Integrated Power Devices (Academic Press, 2002), p. 440.
13. D. J. Ehrlich and J. Y. Tsao, “A review of laser–microchemical processing,” J. Vac. Sci. Technol. B 1(4), 969–984 (1983). [CrossRef]
14. I. P. Sytov, “Estimation of the capabilities of maskless micropatterning by laser-induced chemical etching,” Appl. Phys., A Mater. Sci. Process. 61(1), 75–80 (1995). [CrossRef]
15. V. V. Kononenko, M. S. Komlenok, S. M. Pimenov, and V. I. Konov, “Photoinduced laser etching of a diamond surface,” Quantum Electron. 37(11), 1043–1046 (2007). [CrossRef]
16. E. Granados, D. J. Spence, and R. P. Mildren, “Deep ultraviolet diamond Raman laser,” Opt. Express 19(11), 10857–10863 (2011). [CrossRef] [PubMed]
17. S. Preuss and M. Stuke, “Subpicosecond ultraviolet laser ablation of diamond: nonlinear properties at 248 nm and time-resolved characterization of ablation dynamics,” Appl. Phys. Lett. 67(3), 338–340 (1995). [CrossRef]
18. A. Stacey, B. Cowie, J. Orwa, S. Prawer, and A. Hoffman, “Diamond C 1s core-level excitons: surface sensitivity,” Phys. Rev. B 82(12), 125427 (2010). [CrossRef]
19. V. V. Kononenko, T. V. Kononenko, S. M. Pimenov, V. I. Konov, P. Fischer, V. Romano, H. P. Weber, A. V. Khomich, R. A. Khmelnitskiy, and V. N. Strekalov, “Laser-induced structure transformations of diamonds,” Proc. SPIE 5121, 259–270 (2003). [CrossRef]
20. A. Harasaki, J. Schmit, and J. C. Wyant, “Improved vertical-scanning interferometry,” Appl. Opt. 39(13), 2107–2115 (2000). [CrossRef] [PubMed]
21. Y. Muramatsu, K. Shimomura, T. Katayama, and E. M. Gullikson, “Total electron yield soft x-ray absorption spectroscopy in the CK region of the mixtures of graphitic carbons and diamond for quantitative analysis of the sp2/sp3-hybridized carbon ratio,” Jpn. J. Appl. Phys. 48(6), 066514 (2009). [CrossRef]
22. J. C. Zheng, X. N. Xie, A. T. S. Wee, and K. P. Loh, “Oxygen-induced surface state on diamond (100),” Diamond Related Materials 10(3-7), 500–505 (2001). [CrossRef]
23. H. Jeschke and M. Garcia, “Theoretical description of the ultrafast ablation of diamond and graphite: dependence of thresholds on pulse duration,” Appl. Surf. Sci. 197–198, 107–113 (2002). [CrossRef]
24. B. Luther-Davies, A. V. Rode, N. R. Madsen, and E. Gamaly, “Picosecond high-repetition-rate pulsed laser ablation of dielectrics: the effect of energy accumulation between pulses,” Opt. Eng. 44(5), 051102 (2005). [CrossRef]
25. M. J. A. de Dood, A. Polman, T. Zijlstra, and E. W. J. M. van der Drift, “Amorphous silicon waveguides for microphotonics,” J. Appl. Phys. 92(2), 649–653 (2002). [CrossRef]
26. J. Smedley, C. Jaye, J. Bohon, T. Rao, and D. A. Fischer, “Laser patterning of diamond. Part II. Surface nondiamond carbon formation and its removal,” J. Appl. Phys. 105(12), 123108 (2009). [CrossRef]
27. J. Smedley, J. Bohon, Q. Wu, and T. Rao, “Laser patterning of diamond. Part I. Characterization of surface morphology,” J. Appl. Phys. 105(12), 123107 (2009). [CrossRef]
28. S. K. Sudheer, V. P. M. Pillai, and V. U. Nayar, “Characterization of laser processing of single-crystal natural diamonds using micro-Raman spectroscopic investigations,” J. Raman Spectrosc. 38(4), 427–435 (2007). [CrossRef]
29. D. Ramanathan and P. A. Molian, “Micro- and sub-micromachining of Type IIa single crystal diamond using a Ti:sapphire femtosecond laser,” J. Manuf. Sci. Eng. 124(2), 389–396 (2002). [CrossRef]
30. M. Shinoda, R. R. Gattass, and E. Mazur, “Femtosecond laser-induced formation of nanometer-width grooves on synthetic single-crystal diamond surfaces,” J. Appl. Phys. 105(5), 053102 (2009). [CrossRef]
31. C. Bandis and B. B. Pate, “Electron emission due to exciton breakup from negative electron affinity diamond,” Phys. Rev. Lett. 74(5), 777–780 (1995). [CrossRef] [PubMed]
32. M. Frenklach, D. Huang, R. E. Thomas, R. A. Rudder, and R. J. Markunas, “Activation energy and mechanism of CO desorption from (100) diamond surface,” Appl. Phys. Lett. 63(22), 3090–3092 (1993). [CrossRef]
33. J. Ristein, W. Stein, and L. Ley, “Defect spectroscopy and determination of the electron diffusion length in single crystal diamond by total photoelectron yield spectroscopy,” Phys. Rev. Lett. 78(9), 1803–1806 (1997). [CrossRef]
34. J. Cui, J. Ristein, and L. Ley, “Low threshold electron emission from diamond,” Phys. Rev. B 60(23), 16135–16142 (1999). [CrossRef]
35. D. Zeisel, S. Nettesheim, B. Dutoit, and R. Zenobi, “Pulsed laser-induced desorption and optical imaging on a nanometer scale with scanning near-field microscopy using chemically etched fiber tips,” Appl. Phys. Lett. 68(18), 2491–2492 (1996). [CrossRef]
36. H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, “Demonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diode,” Appl. Phys. Lett. 93(24), 241106 (2008). [CrossRef]
