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Intense ultrashort light pulses induce three dimensional localized phase transformation of diamond. Photoinduced amorphous structures have electrical conducting properties of a maximum of 64 S/m based on a localized transition from sp3 to sp 2 in diamond. The laser parameters of fluence and scanning speed affect the resultant electrical conductivities due to recrystallization and multi-filamentation phenomena. We demonstrate that the laser-processed diamond with the periodic cylinder arrays have the characteristic transmission properties in terahertz region, which are good agreement with theoretical calculations. The fabricated periodic structures act as metallo-dielectric photonic crystal.
©2009 Optical Society of America
1. Introduction
Coherent photon pulses with duration in femtosecond regime have opened new frontiers in material research of light-matter interactions [1]. The ultrafast feature of femtosecond laser pulses has been widely used for direct imaging of chemical reactions in gases [2] and terahertz spectroscopy based on photoconductive emitters [3] excited by femtosecond lasers. As a source of localized energy deposition, intense ultrashort light pulses have become key technologies for direct modification in transparent materials due to new applications and phenomena ranging from 3D optical waveguides [4], micro-explosions [5] and laser nano-surgery [6] to photonic crystals [7] and 3D self-organized subwavelength structures [8]. The process, initiating by a multiphoton ionization, exhibits a highly nonlinear dependence on the intensity of the light beam. The light is absorbed by photoelectrons and the optical excitation ends before the surrounding lattice is perturbed, which results in highly localized breakdown without collateral damage of material [9]. In recent years the existence of nonthermal ultrafast phase transitions has been observed on the surface of several materials such as silicon [10,11], gallium arsenide [12,13], indium antimonide [14], germanium antimonide [15], and carbon [16–19]. In many cases, these transient phases are metal-like disordered or glassy phases resulting from laser-induced electron-hole plasma. Recent discoveries based on such phase transitions span from source and detector of terahertz radiation [20] and ultrafast X-ray generation [21,22] to femtosecond optical switch and high-dense rewritable optical memory [1]. More recently, a variety of permanent and metastable phase transformation, which are based on the structural rearrangement due to thermal accumulation and the shock wave generation [23,27,28], has been observed [23–26]. Although nonequilibrium dynamics of electrons and lattice during graphitization on diamond surface in several theoretical studies [29–31], the three-dimensional structuring in diamond and the electrical conductivities of the spatially modified structures are still not fully understood.
Here we report the observation of a localized phase transformation in diamond induced by intense ultrashort light pulses. Apparent continuous structures consisting of a phase-transformed to amorphous carbon were formed inside diamond along with the laser spot moving. Such photoinduced amorphous structures have electrical conducting properties of a maximum of 64 S/m, which can be changed, based on a localized transition from sp3 to sp2 in diamond depending on the laser irradiation conditions. The laser parameters such as pulse energy and writing speed affected the resultant electrical conducting properties of the amorphous regions due to recrystallization and multi-filamentation phenomena. We demonstrated that the laser-processed diamond with the periodic array of cylinders composed of amorphous carbon have the characteristic transmission properties, which are good agreement with the finite-difference time-domain (FDTD) calculations, in terahertz region and act as metallo-dielectric photonic crystal. We anticipate that such metallo-dielectric periodic structures will open new opportunities in left-handed metamaterial [32].
2. Localized phase transformation in diamond
The laser radiation, in a Gaussian mode, produced by a regeneratively amplified mode-locked Ti:Sapphire laser (230 fs pulse duration, 1 kHz), operating at 780 nm, was focused via a 20× (NA=0.45) objective into the synthetic diamond sample (Sumitomo Electric Hard metal Corp., bland of Type Ib diamond with nitrogen impurities of ~ 100 ppm ) of 3 mm × 3 mm × 1.5 mm size. A typical beam waist diameter at the focus and the laser energy fluence were estimated to be ~ 2 μm and 28.5 J/cm2, respectively. A series of cylindrical structures with certain spacing were directly written by scanning along the laser irradiation direction from the bottom to the top surface. After writing, the sample surfaces were processed by using a focused ion beam (JEOL, JIB-4500), and then the cross-sectional pieces including both the laser-irradiated and non-irradiated regions were measured by using scanning electron microscope (JEOL, JSM-6700F) and micro-Raman spectroscopy (Nicolet, Almega XR). A black and unbroken cylindrical structure was formed inside synthetic diamond sample by moving a tightly focused laser spot at 20 μm/s (Fig. 1 (a), (b)), however, no apparent continuous structures were observed when the scanning speed is higher than 100 μm/s. To estimate structural changes in the laser-irradiated regions, a small cross-sectional piece was extracted by focused ion beam techniques. Figure 1 (c) indicates a typical secondary electron image on the cross-sectional surface extracted from the laser irradiated sample. An area indicated by a double-headed arrow d with width of 18 μm represents a laser-induced area. The points marked P 1 and P 2 in Fig. 1 (c) represent the typical points of the laser-irradiated and the unirradiated region inside diamond sample, respectively.
Fig. 1. Optical microphotograph being taken in an oblique direction (a) and a side view (b). Secondary electron image of the cross-sectional surface including the laser-processed region was also shown (c). The double-headed arrow d and the points marked P 1 and P 2 represent the laser-induced area, the typical points of the laser-irradiated and the unirradiated region, respectively. (d) Micro-Raman spectra at P 1 and P 2 points on the cross-sectional surface. The components of the G peak (broken line), D peak (dotted line), and diamond peak (dashed-dotted line) deduced from the peak fittings (solid line) are also shown.
Although the laser-irradiated sample surface was ablated to the depth of 23 μm, it should be noted that the morphology within a laser-irradiated region in the examined cross section obviously remain densified, namely, a void does not exist. In order to reveal the microscopic structural change induced by laser irradiation, we carried out micro-Raman spectroscopy on the same cross-sectional surface with 1 μm spatial resolution (Fig. 1(d)). These Raman spectra were normalized by the diamond peak intensity and the linear background has been subtracted. It is well known that the Raman spectrum from single crystalline diamond consists of a single narrow peak at 1332 cm-1, whereas for disordered carbon structure there are two Raman peaks observed: the G peak around 1500 ~ 1630 cm-1 and the D peak around 1350 cm-1, which are normally assigned to optical zone center phonons of E2g symmetry involving the in-plane bond-stretching motion of all pairs of sp2 sites and K-point phonons of A1g breathing mode of C sp2 atoms in rings, respectively [33]. The broad D peak is indicative of the existence of disordered carbon in a carbon network and the intensity ratio of ID/IG increases with increasing disorder [34]. A sharp intense Raman peak at 1330 cm-1 and apparent broad two peaks centered at 1555 cm-1 and 1350 cm-1, which respectively correspond to G and D peak, were observed in the laser-induced region (P1). On the other hand, in the non-irradiated region (P2), one can see the intense diamond peak remains, whereas no apparent D peak was observed. Based on the fact that the element constituting the initial sample is only sp3-coordinated carbon, we speculated that sp2-bonded carbon clusters were locally formed inside diamond without morphological changes. As a result, the laser-induced cylindrical structures have an electrical conductivity. To test this suggestion, we measured the electrical conductivities of the modified cylindrical structures induced by various laser energy densities.
3. Characterization of the localized modified structure
The laser-processed samples with various energy fluence (28.5 ~ 427 J/cm2) were coated on the bottom surface by Au evaporation (thickness of 150 nm) in order to connect electrically between each lines of modified cylindrical structures. The absolute electrical resistance of two distinct cylinders penetrating from the bottom to the top of the sample was measured by the four-terminal method (Fig. 2(a), (c)), which can eliminate the effects of any contact resistance, in a scanning electron microscope (JEOL, JFAS-7000BT Beam Tracer). The electrical resistance of two pairs of cylinders (2R) can be obtained from I-V measurements (2R = (VA -VC)/Isup), where VA, VC is the electric potential of the anode and cathode, Isup is the supply current (Fig. 2(b)), respectively. From the measured resistance (R) and the diameter (d) of the cylinder, and the thickness of the diamond sample (t), the specific electrical conductivities (σ) were obtained (σ = 4t/πRd 2).
Fig. 2. Schematic illustration (a) and equivalent circuit (b) of the four-terminal method, where R is the electrical resistance of a modified cylindrical structure, VA, VC is the electric potential of the anode and cathode, Isup is the supply current, respectively. Scanning electron micrograph during the measurements with four-point probe was also shown in (c). (d) Specific electrical conductivity (unfilled red circle) and radius (unfilled blue triangle) of the cylinder as a function of the laser energy fluence.
Figure 2(d) indicates electrical conductivity and diameter of the modified cylinder as a function of the energy fluence of the femtosecond laser pulses. The diameter increased in proportion to the laser fluence, whereas the electrical conductivities decreased when the laser energy fluence is higher than 200 J/cm2. The specific electrical conductivities reached a maximum of 64 S/m at laser energy fluence of 28.5 J/cm2. The following explanation of why the lower electrical conductivities were obtained at higher laser energy fluence is proposed. When the femtosecond laser pulses are tightly focused inside diamond, the intensity in a focal volume becomes high enough to produce self-focusing arising from Kerr effect. Simultaneously, a high free electron density is produced by multiphoton ionization and avalanche, the material has the properties of plasma, which defocuses the light pulses. As a result, the femtosecond laser pulses are alternatively self-focused and defocused, self-guiding the light on a straight path [35]. A large number of regularly located filaments can be formed when the peak power of the light pulses is sufficiently large [36]. Indeed, the spatially periodic modulation of the G peak with a period of 8.1 μm due to the multiple filaments in the directions transverse to laser propagation has been observed at the higher laser energy fluence of 427 J/cm2 (Fig. 3(d)), the divided structures of thinner cylinders were formed, compared with the lower laser fluence. Simultaneously, sp2-bonded carbon clusters were discretely formed, namely the cylindrical structures were partially torn in the laser propagation direction corresponding to the refocusing. On the other hand, the continuous structures consisting of sp2-bonded carbon clusters, which have electrical conducting properties, were formed at lower laser energy fluence than 200 J/cm2 (Fig. 3(b)). We have also observed the modified structures inside diamond have a graphitic structure consisting of sp2-bonded carbon clusters and these dendritic structures are extend in the laser incident direction, regardless of the laser energy fluence (Fig. 3(a), (c)). This is may be due to the extent of diffraction of the laser as it propagates through the plasma [37].
Fig. 3. Optical microphotograph and Raman spectra mapping on the cross-sectional surface after the laser writing with laser energy fluence of 200 J/cm2 (a, b) and 427 J/cm2 (c, d) at the same scanning speed of 20 μm/s. Scale bars are 5 μm. Arrows show the direction of the laser incidence (kph) and the scanning of the focus (ks). Color bar indicates the normalized Raman G peak intensity.
Fig. 4. (a) Sequential Raman spectra taken as a function of laser scanning speed. (b) Profiles of electrical conductivity (red filled triangle) and Raman peak intensity ratio of ID/IG (blue filled circle) in the modified structures as a function of laser scanning speed. The Raman spectra are vertically displayed for clarity. The laser energy fluence was set at 28.5 J/cm2 in all experiments.
Another interesting phenomenon was observed in the scanning speed dependence of electrical conductivity. Figure 4(a) shows the evolution of the Raman spectra of the cross-sectional sample surface after the laser irradiation with different scanning speeds. Electrical conductivity and Raman peak intensity ratio of ID/IG in the modified structures as a function of laser scanning speed are also shown in Fig. 4(b). The laser energy fluence was set at 28.5 J/cm2 in all experiments. At slower scanning speeds than 20 μm/s, the G peak position shifts downward from 1615 to 1590 cm-1, while ID/IG peak intensity ratio increases with increasing scanning speed. These tendencies indicate a larger amount of sp2 content with a higher degree of disorder was formed at slower scanning speeds [33]. Such sp2 structure acts as a conduction part of the amorphous carbon network. It should be noted that ID/IG peak intensity ratio reached its maximum at the scanning speed of 20 μm/s, corresponding to the maximum electrical conductivity. Furthermore, the increase of the D peak intensity and the narrowing of the G peak at slower scanning speeds than 20 μm/s indicate the reduction of bond angle disorder at sp2 aromatic rings [33], namely graphitization of the initial diamond structure occurs, which is in agreement with the theoretical studies [29–31]. From these results, the lower electrical conductivities at the slower scanning speed than 20 μm/s are assumed to be due to the recrystallization from amorphous structure to nanodiamond and/or the formation of tetrahedral amorphous carbon structure. On the other hand, for scanning speed above 20 μm/s, the G peak position keeps around 1590 cm-1 and the intensity ratio of ID/IG is approximately constant, however, a steep decrease in conductivity can be observed (Fig. 4). This may be due to the discontinuous structures, which were formed between the bottom and the top of the sample. Indeed, no apparent continuous structures consisting of sp2-bonded carbon clusters were observed when the laser scanning speed is higher than 100 μm/s. As a result, the electrical conductivity of the spatially modified structures in diamond reached a maximum of 64 S/m, which is between the typical value of crystalline graphite (~ 104 S/m) and of amorphous carbon (~ 10-2 S/m), at the scanning speed of 20 μm/s.
Fig. 5. Comparison between measured (red line) and calculated (blue line) transmission intensity and phase shift of the square metallo-dielectric photonic crystal with lattice constant of Λ = 80 μm. The calculation model (enclosed area with dotted line) and the two polarization configurations (E) are shown on the side of the graphs, where (εg, σg) and (εd, σd) represent a set of the specific permittivity and the specific electrical conductivity of the modified cylindrical structure and the initial diamond, respectively. The direction of incidence, which was polarized parallel (a, b) or perpendicular (c, d) to the longitudinal axis of a cylinder with a diameter of d = 25 μm, was set along the <10> direction of the square lattice.
4. Metallo-dielectric photonic crystal structure
Apart from the fundamental importance of the observed phenomenon as the first direct evidence of the localized transition from sp3 - to sp2-bonded structure embedded in diamond, the observed phenomenon could be useful for the fabrication of metallo-dielectric photonic crystal in terahertz region. It is well known that diamond exhibits a low absorption and a high refractive index of 2.35 with a low dispersion in the terahertz region. To evaluate the availability of these techniques, we carried out terahertz time-domain spectroscopy (THz-TDS) characterization of the periodically modified diamond with embedded electrically conducting structures created by intense femtosecond light pulses. We measured the transmission and phase-shift spectra in THz region (Fig. 5). In this measurement, the two-dimensional square lattice of cylinder array with lattice constant (Λ) of 80 μm and diameter (d) of 25 μm inside diamond, which was placed in 10 rows and 30 columns, was used. The incident THz wave passed through an aperture with a diameter of 1 mm was focused on a sample by a paraboloidal mirror. The directions of incidence and detection of THz wave were set along the <10> direction of the square lattice. The finite-difference time-domain (FDTD) simulations were also performed on an 32 × 184 two-dimensional square grid with cell size 1 μm and a time step of 5 fs using the relative permittivity and the specific electrical conductivity of the graphitized amorphous carbon (εg = 5.5, σg = 300 S/m) and the diamond (εd = 5.5, σd = 0 S/m). The excitation was a plane wave and line source of 160 μm placed 100 μm away from the cylindrical structure. Transmission intensity was corrected according to Fresnel’s equation, matching the experimental conditions. The configuration of incidence, which was polarized parallel (TM) or perpendicular (TE) to the longitudinal axis of cylinder array, was also shown.
Fig. 6. Transmission spectra of the square metallo-dielectric photonic crystal with lattice constant of Λ = 60 μm (red), 80 μm (blue), and 100 μm (green), respectively. The horizontal axis is a normalized frequency (nωΛ/2πc), where n is the refractive index of diamond (= 2.35). Dotted lines indicate the characteristic peak positions.
At the frequency below 0.46 THz, the transmission exceeded the geometric transmission of 69 %. This phenomenon of transmission enhancement for TM polarization was similar to the extraordinary transmission through metal hole arrays [38]. In the case of TM-polarized THz wave, an apparent bandpass characteristic was observed at 1.6 THz corresponding to the period of the cylinder array (Fig. 5(b)). Furthermore, phase shift also showed a sharp change in the bandpass region (Fig. 5(a)). This increase in the phase shift indicates the decrease in the group velocity due to the multiple scattering between the cylinders. On the other hand, no transmission in frequency range over 1.4 THz was observed for TE polarization (Fig. 5(d)). This monotonic decreasing behavior of phase shift (Fig. 5(c)) is under investigation. From comparison of experimental and theoretical results, although disturbance in the measurement spectra may be due to the multireflection between the aperture and the front surface of diamond, the measured transmission and phase shift are good agreement with those of FDTD calculations. The phenomenon could be interpreted in terms of the simultaneous negativity of the electric permittivity and magnetic permeability [39]. Additionally we have confirmed that the bandpass frequency can be controlled based on the lattice constant. Indeed, when the lattice constant increased from 60 μm to 100 μm, the bandpass region shifted from 143, 188, to 231 μm corresponding to the period of the cylinder array in diamond (Peaks 2 in Fig. 6). Furthermore, the bandpass frequencies corresponding to twice the lattice constant were also observed (Peaks 1). Although the other weak transmissions (Peaks 3) at the normalized frequency of 1.27 were observed, the details are under investigation.
5. Summary
In summary, we have demonstrated how a new localized structural-phase transformation, and then this technique can be useful for the manufacture of a metallo-dielectric photonic crystal. Apart from the fundamental importance of the observed phenomenon as the first direct evidence of the localized transition from sp3 to sp2 in diamond, the fabricated periodic structures will open new opportunities ranging from wire-grid polarizer, and plasmonic metamaterials, to integrated electrical circuit.
Acknowledgments
We would like to thank Masayuki Nishi from Kyoto University, Jiarong Qiu from Zhejiang University, and T. Nakaya form Namiki Precision Jewel Co., Ltd. for helpful discussions. This research was partially supported by the New Energy and Industrial Technology Development Organization (NEDO) and the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (A).
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