Low energy-density recording with a high- repetition-rate laser beam in gold-nanorod- embedded discs

In this paper, we report on the low energy-density recording with a high-repetition-rate femtosecond pulsed beam in homogenous goldnanorod-dispersed discs by using low numerical aperture (NA) microoptics. By focusing a femtosecond pulsed beam at a repetition rate of 82 MHz using a low NA DVD optical head, the spatially-stretched energy density introduces a temperature rising of the polymer matrix. This temperature rising facilitates the surface melting of gold nanorods, which leads to over one-order-of-magnitude reduction in the energy-density threshold for recording, compared with that by focusing single pulses through a high NA objective. Applying this finding, we demonstrate the dual-layer recording in gold-nanorod-dispersed discs with an equivalent capacity of 69 GB. Our results demonstrate the potential of ultra-high density three-dimensional optical memory with a low-cost and DVDcompatible apparatus. ©2012 Optical Society of America OCIS codes: (210.0210) Optical data storage; (210.4680) Optical memories, (210.4810) Optical storage-recording materials. References and links 1. C. Sönnichsen, B. M. Reinhard, J. Liphardt, and A. P. Alivisatos, “A molecular ruler based on plasmon coupling of single gold and silver nanoparticles,” Nat. Biotechnol. 23(6), 741–745 (2005). 2. X. Huang, I. H. El-Sayed, W. Qian, and M. A. El-Sayed, “Cancer cell imaging and photothermal therapy in the near-infrared region by using gold nanorods,” J. Am. Chem. Soc. 128(6), 2115–2120 (2006). 3. J. L. Li, D. Day, and M. Gu, “Ultra-low energy threshold for cancer photothermal therapy using transferrinconjugated gold nanorods,” Adv. Mater. (Deerfield Beach Fla.) 20(20), 3866–3871 (2008). 4. J. W. M. Chon, C. Bullen, P. Zijlstra, and M. 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Introduction
The size-and shape-dependent surface plasmon resonance in gold nanorods has been of great interest recently.Their unique photophysics such as large two-photon (2P) absorption crosssections of ~3*10 −42 cm 4 .s.photon −1 and photothermal shape transition, have mediated broad applications in bio-imaging [1][2][3], optical data storage [4][5][6][7] and photothermal therapy [2,3,8].Gold nanorods can absorb incoming photons and heat up the lattice through the electronphonon relaxation process and transform into spherical shapes, once the laser energy density exceeds the threshold for complete melting [9,10].Dispersing gold nanorods into a proper matrix and manipulating the heat generation and dissipation in the focal region without damage of the polymer matrix are keys towards the application in high-density volumetric optical memory based on the photothermal reshaping mechanism.The complete-melting temperature of gold nanorods of ~1300 K [11] requires a high femtosecond-pulsed energy density of ~230 mJcm −2 and a high numerical aperture (NA) objective in the previous demonstration of optical memory in gold-nanorod-dispersed photopolymers [5].Such a high energy density of femtosecond pulses is close to the damage threshold of many optical elements.For example the damage threshold of fused silica is 600 ~1700 mJcm −2 [12].To avoid the damage to both optics and the polymer matrix by the successive laser pulses, reducing the energy density in time by single pulse modulation is necessary [5].However, the requirement for a high NA objective and a single pulse modulator makes the optical system extremely bulky and less feasible in the development of a compact micro-optics system.
Even though there has been an intense investigation of photothermal melting of gold nanorods with a temporally-stretched energy density by varying pulse widths [13], the photothermal response of gold nanorods excited by the spatially-stretched energy density with a low NA objective is less understood.Unlike complete melting where nanorods are transformed into spherical shapes, surface melting can lead to the shape transition of gold nanorods in solution into shorter and fatter rods at a laser-energy-density threshold of 2 mJcm −2 [13].Further, the surface-melting induced shape transition of gold nanorods has been observed by constant heating in a hot atmosphere at ~400 K [14].Therefore, the temperature of the surrounding matrix under high-repetition-rate pulsed laser illumination can play an important role on the photothermal response of gold nanorods.Thus, by judicious control of the doping concentration and the spatial energy density, the temperature rising of the surrounding matrix under high-repetition-rate pulsed laser illumination can facilitate the surface melting of gold nanorods.Therefore, the low-threshold energy density for optical recording is possible by using a low NA micro-optics apparatus at a high repetition rate.Here, we study the recording performance using low NA micro-optics in gold-nanorod-dispersed optical discs.Indeed, we show a one-order-of-magnitude reduction in the recording threshold of the laser energy density using a low NA DVD optical head to focus a high-repetition-rate femtosecond laser beam.Our results open the potential of ultra-high density optical memory with the DVD-compatible micro-optics apparatus.

Shape transition by surface melting with a low energy density
The principle of spatially stretching the energy density by an objective of NA = 0.6 is shown in Fig. 1(a) [15].It is clearly shown that the full width at half maximum (FWHM) is doubled and the peak energy density is reduced to 3% of that for an objective of NA = 1.4.To gain a better insight into the temperature rise of the surrounding matrix on surface melting of gold nanorods excited by the spatially-stretched energy density at a high repetition rate, the rising of the focal temperature in the medium is modeled by considering gold nanorods as the heat sources.
The position and time dependent temperature change in the focus in the medium is given by [16,17] 2 ) ( ) Here ( ) o F r is the energy-density distribution given by the objective, a is the absorption cross-section of gold nanorods (~1 2 6 10 − × cm 2 ) [18], ρ is the density of the polyvinyl alcohol (PVA) polymer (1200 kgm −3 ), p c is the heat capacity of PVA (1650 Jkg −1 K −1 ), k is the thermal conductivity of the matrix (0.2 Wm −1 K −1 ) [19], t is the time interval between two successive pulses (12 ns for the repetition rate of 82 MHz), n is the number of pulses, o r r − is the relative distance from the gold nanorods.The nanorods are modeled as a homogeneous distribution in the PVA matrix with a particle separation of 80 nm to simulate the experimental condition.The initial temperature of 293 K is assumed.The average focal temperature is obtained by superposing the solutions of every gold nanorod inside the focus.Therefore, at a given energy density and a repetition rate, the rising of the focal temperature is dependent on both the thermal conductivity of the surrounding matrix and the surface-tovolume ratio of the focal region by the spatial stretching.
Figure 1(b) shows the calculated focal temperature of the gold-nanorod-dispersed PVA matrix and the glass matrix excited by a spatially-stretched energy density given by objectives with different values of NA.At a laser energy density of 0.4 mJcm −2 which is 25 times lower than the complete-melting threshold of 10 mJcm −2 under the temporally-stretched excitation [20].The energy of photons absorbed by a single gold nanorod is approximately 2.4 fJ given the absorption cross-section of ~12 6 10 − × cm 2 , which is far below the complete-melting energy threshold of ~60 fJ for a similar sized gold nanorod [11].The focal temperature under a spatially-compressed excitation by an objective of NA = 1.4 rises to ~315 K within the first 1000 pulses, and then saturates.The surrounding matrix has a negligible influence on the photothermal response of gold nanorods at this temperature.The 2P fluorescence experiment confirms that there is no distinguishable fluorescence reduction after exposure to a laser energy density at this level (Fig. 1(c)).In the focal region of an objective with NA = 0.6, the temperature rising of the surrounding matrix plays a significant role in determining the photothermal shape transition of nanorods.The calculation shows that the focal temperature rises quickly to ~370 K within the first 1000 pulses and then slowly increases after 2000 pulses.The ultimate focal temperature is estimated ~400 K after 6 2.1 10 × pulses corresponding to an exposure time of 25 ms, which is the typical experimental condition.This difference in the focal temperature can be attributed to the different surface-to-volume ratios of the focal regions given by different values of NA.The surface-to-volume ratio given by an objective with NA = 1.4 is ~2.6 times larger than that given by a spatially-stretched case for an objective with NA = 0.6; thus the heat can dissipate out of the focal region much efficiently before the arrival of the successive pulses and the focal temperature is significantly lower.These numerical results physically imply that at a low energy density of 0.4 mJcm −2 , the use of a high NA objective may not produce a surface-melting condition under the highrepetition-rate pulsed illumination.However, under the spatially-stretched excitation by a low NA objective, the temperature rising of the polymer matrix exposed at the same energydensity level can be up to ~400 K, which is comparable to constant heating in a hot environment at similar temperature [14].Therefore, the shape transition of gold nanorods excited by an objective of NA = 0.6 at the high repetition rate is now possible.To verify the discovery shown in Fig. 1(b), we have conducted the 2P fluorescence contrast reduction experiment at a variety of energy-density levels.Gold nanorods with an extinction peak at the wavelength of 790 nm and an optical density of 130 were mixed with 10 wt.% PVA solution, and then dried at room temperature.The femtosecond pulsed laser beam at the wavelength of 780 nm with a repetition rate of 82 MHz was employed as the excitation source.After intense irradiation, the shape transition of nanorods can shift the plasmon resonance and significantly reduce the 2P fluorescence intensity of illuminated gold nanorods.
The 2P fluorescence contrast reduction experiment in Fig. 1(c) shows that the energydensity threshold for NA = 0.6 is indeed 2.5-fold lower than that for NA = 1.4,which is consistent with the difference in surface-to-volume ratios.To verify the influence of the temperature rising of the matrix on the surface melting of gold nanorods, a comparison experiment was performed in a sample where gold nanorods were distributed on the cover glass (ρ = 2500 kgm −3 , c p = 840 Jkg −1 K −1 and k = 1 Wm −1 K −1 ) [21].Owing to the relatively high thermal conductivity of glass, the heat can efficiently dissipate out of the focal region before the arrival of the successive pulses and the focal temperature rises only to 306 K after 2000 pulses under spatially-stretched excitation at an energy density of 0.4 mJcm −2 (Fig. 1(b)).Consistent with the calculation, the energy-density threshold in the gold-nanoroddispersed PVA sample is reduced compared with that in gold nanorods distributed on cover glass.At the same energy-density level, the fluorescence contrast reduction in the goldnanorod-dispersed PVA sample, indicating the strength of the surface melting, is significantly stronger than that in gold nanorods distributed on cover glass.The reduced threshold and enhanced strength of surface melting can be attributed to the influence of the temperature rising in the surrounding matrix, which facilitates the photothermal shape transition of gold nanorods.This feature may be explained as softened elastic properties at the surface of gold nanorods by the temperature rise in the matrix [22,23], which may reduce the surface tension to keep the original shape after the successive femtosecond pulses are absorbed.
Figure 1(d) shows the focal temperature of the gold-nanorod-dispersed PVA sample after 2000 pulses at a variety of energy density levels and repetition rates.It should be pointed out that the increase in the repetition rate of the pulsed laser beam might lead to a further reduction in the energy-density threshold for surface melting of gold nanorods facilitated by the matrix temperature rising, enabling ultra-low energy-density recording under spatiallystretched excitation by low NA micro-optics.

Dual-layer optical recording
The low-energy-density recording under spatially-stretched excitation by a high-repetitionrate pulsed laser beam indicates the feasibility of its application in high density optical memory using a low NA micro-optics system.The DVD optical head (NA = 0.6) [24] is one of low NA micro-optics systems and a key component of the DVD technology.Figure 2 shows the experimental configuration to characterize low-energy-density recording in goldnanorod-dispersed optical discs.A linearly polarized Ti:sapphire ultrafast pulsed laser (Spectra-Physics Tsunami) (pulse width of 100 fs and repetition rate of 82 MHz) tunable from 700 nm to 1000 nm was employed as the excitation source.An automated computercontrolled shutter was used to modulate the intensity of the beam for bit-by-bit recording.The excitation beam was focused by a compact DVD optical head into the volume of the optical disc.The 2P fluorescence signal was collected by the same optical head and directed to a photo-multiplier tube (PMT).
Gold nanorods with an extinction peak at the wavelength of 790 nm were prepared by using a well-developed seed mediated growth method [25].To fabricate homogeneously gold-nanorod-dispersed optical discs, the aforementioned gold-nanorod mixture was spincoated and sandwiched between two DVD substrates, as illustrated in the inset of Fig. 2. The thickness of the recordable layer containing gold-nanorods was measured to be 30 3 ± µm.To prevent the recordable layer peeling off from the DVD substrates, a spacer layer with a thickness of 10 µm was uniformly spincoated on the surface of the DVD substrates.
To characterize the volumetric recording capability of such a DVD optical head by coupling the femtosecond laser beam, we performed the point spread function measurement.The axial resolution of the optical head was characterized as a function of the excitation wavelength from 700 nm to 900 nm, as shown in Fig. 3(a).The axial resolution of the optical head was found to be in the range from 3 μm to 9 μm, which indicates the feasibility of volumetric recording with such low NA DVD micro-optics.The lateral resolution of the optical head was characterized by 2P fluorescence imaging of micro-beads with an average size of 1 µm.The lateral resolution defined by the FWHMs was found to be in the range from 0.8 μm to 1.3 μm for the excitation wavelength from 700 nm to 900 nm, as shown in Fig. 3(b).The measured lateral resolution is consistent with the diffraction limited case as d = 1.22λ/NA [15], where λ is the wavelength of the laser beam and NA is the numerical aperture of the DVD optical head.Now we demonstrate the reduction of the energy-density threshold of recording in the gold-nanorod-embedded disc using the DVD optical head to focus the high-repetition-rate femtosecond pulsed beam.The laser wavelength at 780 nm was chosen to match the resonance of the synthesized gold nanorods.The 2P fluorescence of gold nanorods can be significantly reduced due to the plasmonic resonance shift as a consequence of the photothermal reshaping [20].Figure 3(c) shows the 2P fluorescence readout images of recorded bits in the gold-nanorod-dispersed disc.Each image comprises a pattern of 3 3  × bits.To prevent any interference between the adjacent bits, the bit spacing was kept at 10 m μ .The exposure time of 25 ms was optimized to balance the recording speed and the signal-noise-ratio.Figure 3(d) shows the readout bit size as a function of the excitation power.Increasing the writing power leads to a gradual increase in the size of recorded bits.Owing to the temperature rising of the surrounding polymer matrix, indeed, the threshold power for recording is significantly reduced to 0.25 mW (corresponding to a focal energy density of ~0.4 mJcm −2 ).Compared with the complete-melting energy-density threshold of ~10 mJcm −2 with single femtosecond pulses [20], this result yields over one-order-of-magnitude reduction.Once the power exceeds 1.5 mW, corresponding to a laser energy density of ~2.4 mJcm −2 , deformation of the PVA matrix is observed where the accumulative heating raises the temperature above ~540 K [26].The minimum lateral size of the recorded bit was found 1.08 μm at the threshold recording power of 0.25 mW.The image contrast was calculated from the readout fluorescence images of the recorded bits.The contrast is defined as |(I bit -I background )/ (I bit + I background )|, where I bit is the readout intensity of the bit and I background is the readout intensity of the background.Figure 3(e) shows the 2P fluorescence image contrast of the recorded bits as a function of the recording power.Reducing the exposure time from 25 ms to 5 ms improves the recording speed at a cost of degradation of the contrast of recorded bits.The exposure time was optimized at 25 ms to balance the speed and the contrast.The contrast was found to be ~0.14 at the threshold recording power of 0.25 mW and an exposure time of 25 ms.
As a demonstration of the volumetric recording capability, we show the 2P fluorescence readout images of two patterns recorded at two layers, as shown in Figs.4(a) and (b).Letters B and M were recorded in the first and second layer, respectively, with a layer separation of 15 μm.The recording was conducted at the threshold power of 0.25 mW with an exposure time of 25 ms.The images can be readout distinctly without any cross talks from the neighboring layer, as shown in the axial scanning in Fig. 4(c).The cross section of the recorded bits indicated by the dashed line is shown in Fig. 4(d) with a bit separation of 1.25 μm.According to the bit separation of 1.25 μm and a layer separation of 15 μm, the equivalent storage capacity of 69 GB per disc is achieved with low NA DVD micro-optics.

Conclusion
In conclusion, we have demonstrated both numerically and experimentally that the temperature rising of the surrounding polymer matrix can play a significant role in the photothermal shape transition of gold nanorods, when using a low NA micro-optics to focus a high-repetition-rate pulsed beam.Consequently, we have characterized the volumetric recording capability of DVD optical heads by coupling a high-repetition-rate femtosecond pulsed laser beam.Compared with that for a high NA objective under single pulse illumination, the laser-energy density is reduced by over one order of magnitude when a DVD optical head is employed to focus a femtosecond pulsed beam at a repetition rate of 82 MHz.As a result, femtosecond-laser-beam induced dual-layer recording in a gold-nanorodembedded optical disc has been successfully demonstrated by using the DVD optical head.With the recent advance in the development of low-cost femtosecond laser systems, our results demonstrate the potential of ultra-high density and low-cost 3D storage devices that comprise DVD compatible micro-optics systems.

Fig. 1 .
Fig. 1.(a) Calculated focal energy-density distribution in the lateral direction for objectives of NA = 1.4 (red) and NA = 0.6 (blue) (the latter is magnified by 31 times in the plot).(b) Calculated focal temperature as a function of the number of pulses at a repetition rate of 82 MHz and an energy density of 0.4mJcm −2 .The blue squares and red circles present the calculated temperature rising in gold-nanorod-dispersed PVA matrix by objectives with NA = 0.6 and NA = 1.4,respectively.The green triangles represent data for gold nanorods distributed on cover glass.(c) Experimental characterization of the 2P fluorescence contrast reduction as a function of the energy density by surface melting of gold nanorods in the PVA matrix with an objective NA = 0.6 (blue), in the PVA matrix with an objective NA = 1.4 (red) and distributed on cover glass with an objective NA = 0.6 (green), respectively.(d) The focal temperature of the PVA matrix after 2000 pulses at different energy-density levels and laser repetition rates.

Fig. 3 .
Fig. 3. (a) Axial resolution and (b) lateral resolution of the DVD optical head as a function of the wavelength.(c) Fluorescence readout image of the recorded pattern at different recording power levels.The scale bar is 10 μm.(d) Size defined by the FWHM and (e) the readout contrast of the recorded bits as a function of the recording power and the exposure time.

Fig. 4 .
Fig. 4. 2P fluorescence readout images of two letters recorded in the first layer (a) and the second layer (b).The inset shows the zoom in view of the recorded bits indicated by the dashed square.The scale bar is 10 μm.(c) the axial response of the two recorded layers with a layer separation of 15 μm.(d) the cross section plot of the recorded bits as indicated by the dashed line.