DOI: 10.1007/s00339-007-4112-8 Materials Science & Processing
y.-f. chau1,u y.-s. sun1 d.p. tsai2 t.-j. yang3
The optical properties between an incident wave
and the active layer of a bubble-pit AgO
x
-type
super-resolution near-field structure
1Department of Electronic Engineering, Ching Yun University, Chien-Hsin Rd., Jung-Li, 320 Taiwan, R.O.C. 2Department of Physics and Center for Nanostorage Research, National Taiwan University,
Taipei, Taiwan, R.O.C.
3Department of Electrophysics, National Chiao-Tung University, HsinChu 300, Taiwan, R.O.C.
Received: 12 April 2007/Accepted: 2 May 2007
Published online: 21 June 2007 • © Springer-Verlag 2007 ABSTRACTThe deformation and plasmon effects of collective localized surface plasmons between incident light and bubble-pit AgOx-type super-RENS structure have been studied using finite-difference time-domain (FDTD) method. We find that the polarization, wavelength of incident light, and particle sizes of Ag nanoparticles are sensitive to the plasma resonance. The Ag nanoparticles inside the bubble-pit AgOx-type super-RENS structure give the additional outer boundaries to the motion of the Ag nanoparticles, and excite more evanescent field which located in the far edge of the bubble from the optical axis of the incident beam. The optical properties between active layer and incident light with polarization direction, different wave-lengths, and varied particle sizes of Ag nanoparticles exhibits nonlinear optical behavior in the near field. The far-field signals of different wavelength of incident light confirm the relation between highly localized near-field distributions and enhanced resolution of far-field signals. The subwavelength recording marks smaller than the diffraction limit were distinguishable since the Ag nanoparticles with high localized fields transferred evanescent waves to detectable signals in the far field.
PACS42.79.Vb; 71.15.Rn; 72.15.Rn; 73.22.-f; 73.22.Lp; 78.67.Bf; 73.20.Mf
1 Introduction
A high-density optical storage system with more than 100 GB capacity is required for multimedia systems such as internet broadband servers and high-definition video systems (HDTV). The super-resolution near-field structure (super-RENS) [1–5] disc is a promising candidate for the next-generation optical disc to provide high data storage capacity. Today, the novel emergent technique of near-field optical data storage, allows the writing and/or reading of in-formation bits, which are smaller than the resolution limit of the optical head, without increasing the complexity of the detecting system. Such a technique is very attractive be-cause of its compatibility with existing players/recorders. Near-field optical data storage techniques depend on the evanescent coupling of light to the data layer of the opti-u E-mail: [email protected]
cal storage medium to read and write data marks that are smaller than the diffraction limit of conventional optical data storage systems. In recent years, there has been in-creased attention given to techniques that may overcome this limit.
The discovery of optical near-field generation by plas-mons excited in thin AgOxlayers has made these a key
com-ponent in advanced optical applications. An AgOx
Super-RENS structure is composed of an active layer (AgOx thin
film) that is sandwiched between two dielectric films (ZnS-SiO2), and this three-layer stack is deposited directly onto the phase-change data layer (Ge2Sb2Te5) of a disk. One of the di-electric layers is directly adjacent to a polycarbonate layer act-ing as an insulatact-ing layer; the other dielectric layer is the cover layer. If a storage medium incorporates a Super-RENS struc-ture, the light spot that probes the data layer is smaller than the size of the focused spot that is incident on the cover layer of the Super-RENS structure. The optical aperture serves as a probe on the disk like that for scanning near-field opti-cal microscopy (SNOM). After recording on a phase-change layer, small marks are reproduced through the aperture. The particles, instead of the aperture, function as the scattering center, which is the key to inducing the super-resolution phe-nomenon. The 15 nm AgOx thin films are considered to be
nonlinear optical layers which control the near-field optical aperture. Recently, several studies have explored this nonlin-ear nnonlin-ear-field optical property; [6–9] however, the influence of the deformed Super-RENS structure does not seem to be appropriately considered. The active layer (AgOxfilm) of the
AgOx-type Super-RENS structure is deformed after a writing
process, as observed in experimental work [10, 11]. The sil-ver (Ag) nanoparticles spread out in the bubble pit (deformed AgOxfilm) that is, in addition, coincidentally formed at the
focus spot.
To examine this nonlinear near-field optical property and match it to a realistic situation, we study the relationship between the active layer (bubble pit) and incident light for various diameters of Ag nanoparticles in the deformed AgOx
-type Super-RENS structure using the two-dimensional finite-difference time-domain (FDTD) method. In addition, the far-field signals of different wavelengths of incident light confirm the relation between highly localized near-field distributions and enhanced resolution of far-field signals. Our results indi-cate a close connection between the surface plasmon effects of
Ag nanoparticles and the nonlinear near-field optical property observed in experiments.
2 Simulation models
Figure 1 shows bubble pit marks formed in the active layer (deformed AgOx film) and Ag nanoparticles
distributed randomly inside the bubble. We assume that the Ag particles are of the same size and randomly dis-tributed in the deformed AgOx layer. The distribution
dens-ity of Ag nanoparticles near the top surface of the bub-ble pit is lower than that near the base of the bubbub-ble pit. The layer structure is, from bottom to top, polycar-bonate (n= 1.5)/ZnS-SiO2 (n= 2.25 + i0.01)/AgOx (n=
2.270 + i0.08)/ZnS-SiO2/Ge2Sb2Te5 (GST, crystal phase,
nc= 4.01 + i3.61)/ZnS-SiO2/air (n = 1). The figure shows that the layer structure between the insulating layer (the sec-ond ZnS-SiO2) and the record mark (GST) is deformed after the writing process. The nanosize Ag nanoparticles inside the bubble pits are embedded in the AgOxlayer and are
as-sumed to move under the influence of the readout laser beam, as shown by previous experimental work [12]. The disper-sive behavior of Ag and GST is simulated by the Lorentz model [13]. The AgOx layer is sandwiched between the
in-sulating ZnS-SiO2layers and a GST-alloy phase-change data layer is located directly above this trilayer structure. The other parameters that were used were polarization (transverse magnetic (TM)-polarization, the electric field component nor-mal to the Ag nanoparticle surface), cell size (∆x = ∆y = 0.5 nm), and the refractive index of Ag (n = 0.055 + i4.44). The incident Gaussian beams FWHM was adjusted such that FWHM= 0.51λ/NA [14]. The phase-change data layer is in
FIGURE 1 Scheme of bubble pit marks formed at the active layer (de-formed AgOx film), and Ag nanoparticles distributed randomly inside the bubble. The Ag particles are of the same size and randomly distributed in the deformed AgOxlayer. The distribution density of Ag nanoparticles near the top surface of the bubble pit is lower than that near the base of the bubble pit. The layer structure is, from bottom to top, polycarbonate (n= 1.5)/ZnS-SiO2(n= 2.25 + i0.01)/AgOx (n= 2.270 + i0.08)/ZnS-SiO2/Ge2Sb2Te5 (GST, crystal phase, nc= 4.01+ i3.61)/ZnS-SiO2/air (n = 1)
an on-mark situation. The on-mark situation means that the in-cident light focuses on the recording mark, and the off-mark situation means that the incident light focuses between two marks. The numerical aperture (NA) of the objective lens used is 0.85.
To study the far-field optical properties induced by the en-hancements of localized surface plasmons, a near-to-far field transformation was used to obtain the far-field response from near-field results [15, 16]. The method is accurate within the limits of the Kirchoff approximation. A Fourier transform is applied to the fields in the sampling plane to obtain the angu-lar distribution of the radiation in the far-field. As described in [15], the expression of an efficient transformation to get the angular distribution of the electric fields in the far-field is given by the integral
Escaty (r, θ) = 1
2√λr
S
d yexp[ jk0sin(θ)y] × [Ey(x, y)
+ ηHz(x, y) cos θ] , (1)
where S is the sampling plane,θ is the discrete angle defined by the normal to the incident plane and the observation point at the detector on a circle of radius r. The intensities are in-tegrated within the numerical aperture of the objective lens. In order to correlate the simulated results with the experi-mental results, differences of the far-field intensity between on-mark and off-mark cases were calculated for various mark sizes. The differences of far-field intensity between on-mark and off-mark situations was calculated by 10× log(| Ion−
Ioff| /Ino-mark), where Ionis the far-field intensity for the on-mark case, Ioffis the far-field intensity for the off-mark case, and Ino-markis the far-field intensity for the case without any marks. The far-field difference signals could demonstrate the capability of super-RENS disks to distinguish subwavelength recording marks.
3 Results and discussion
As studied by our research group [17], the dens-ity of the Ag nanoparticles can be controlled by increasing or decreasing the incident laser power in the photodissociation process. A lower nanoparticles density shows less interaction among the nanoparticles. If the density of nanoparticles is increased, the local fields are coupled together and produce more evanescent fields from adjacent nanoparticles. Figure 2a and b show the near-field distributions of Ag nanoparticles of deformed and normal (underformed) AgOx-type Super RENS
structures, respectively. The total number of nanoparticles is 82 in the deformed type and 57 in the normal type, and each nanoparticle has a diameter of 4 nm. The wavelength of inci-dent light isλ = 650 nm for TM polarization. In Fig. 2a and b, highly localized enhancements and interactions are pro-duced between adjacent Ag nanoparticles in the near-field zone. When the free electrons of an Ag nanoparticle oscil-late collectively in resonance with the incident light, the in-teraction may constitute a surface plasmon polariton (SPP). The field enhancement is due to SPPs. Each embedded Ag nanoparticle behaves like a dipole and a very strong scatter-ing center. The local fields from adjacent nanoparticles are
FIGURE 2 Calculated results of near-field distributions of Ag nanoparti-cles in deformed and normal (underformed) AgOx-type Super-RENS struc-ture. The total number of Ag nanoparticles is 82 in the deformed type and 57 in the normal type, each with a diameter of 4 nm. The wavelength of the inci-dent light isλ = 650 nm and the numerical aperture (NA) of the lens is 0.85 for TM polarization
coupled together and produce highly enhanced evanescent fields. Since the boundary conditions of Maxwell’s equations stipulate that induced surface charge density is proportional to the discontinuity of the electric field component normal to the surface, only TM-polarized light can excite SPPs. These near-field optical signals of the Ag nanoparticles correspond to the nonlinear near-field optical properties previously found in experiments [18].
The Ag nanoparticles inside the bubble pits are assumed to be under the influence of a readout incident beam. The donut shape aperture (deformed AgOxfilm) is formed only
in the on-mark situation of the readout beam and provides the additional length (or path) longer than the normal length. In comparison with the evanescent fields for the normal type (Fig. 2b), the bubble pits of the deformed type (Fig. 2a) give additional outer boundaries to the motion of the Ag nanoparticles, and excite more evanescent fields, which are located at the far edge (top surface) of the bubble from the optical axis of the incident beam. It can be clearly seen in Fig. 2a that more evanescent fields extend in the direc-tion of the y-axis (propagadirec-tion direcdirec-tion). This indicates that
FIGURE 3 Calculated results of field intensity as a function of the diam-eter of Ag nanoparticles from 1 to 8 nm, in increments of 1 nm, measured at the top surface of the deformed AgOxfilm at various wavelengths of incident light. The wavelengths of the incident light are 400, 450, 500, 550, 600 and 650 nm, respectively
the bubble and particles tend to increase the readout signal intensity.
The optical responses of Ag nanoparticles are compli-cated, however, one key method for understanding these ef-fects is to distinguish the contributions from each nanoparticle and particle–particle interactions as well as the gap between two nanoparticles. Surface plasmon polaritons and the scat-tering efficiency of Ag nanoparticles are usually sensitive to and controllable by particle size, the surrounding medium and the wavelength of the incident light. From experimental ob-servation, the exact shape of an Ag nanoparticle is between a circle and a square. When the diameter of a nanoparticle is increased from 3 nm to 7 nm, the shape becomes closer to a circle. With the same distribution of Ag nanoparticles as shown in Fig. 1, the field intensity measured at the sur-face of the deformed AgOx layer varied considerably when
they were illuminated by incident light at various frequen-cies (wavelengths). To verify the effect of the Ag nanoparticle size, the variation in near-field intensity measured at the top surface of the deformed AgOxfilm with increasing diameter
is tested. Figure 3 shows the field intensity as a function of the diameter of Ag nanoparticles from 1 nm to 8 nm, in incre-ments of 1 nm, measured at the top surface of the deformed AgOxfilm at various wavelengths of incident light. As the n
and k values at different wavelengths of Ag and AgOxcan be
found in [19–21], and corrected by the Drude model, which includes the size effect [22], we chose the wavelengths of the incident light to be 400, 450, 500, 550, 600 and 650 nm as ex-amples, and the surface plasmon effects of other wavelengths can be verified by the same procedures. In the finite area, the height of the deformed AgOxlayer shown in Fig. 1 is 15 nm.
A smaller size of Ag nanoparticles means a lower density be-cause the distance between two particles is larger. In the same manner, a larger size means a higher density cases because the gap between two particles is smaller.
The results in Fig. 3 clearly show that the size variations of Ag nanoparticles may yield different field intensities and
indicate that the intensities of local fields exhibit a nonlin-ear optical response with increasing nanoparticle diameter. When the particle diameter is less than 1 nm (the lower dens-ity cases), shorter wavelengths (e.g.,λ = 400 nm) can excite more evanescent fields from the surface of Ag nanoparti-cles than longer wavelengths. When the particle size is larger than 1 nm (the higher density cases), the gaps between Ag nanoparticles are smaller and more local fields from adja-cent nanoparticles are coupled together to produce highly enhanced evanescent fields. It can be seen in Fig. 3 that the field intensity at a fixed particle diameter of 2 nm exhibited peak values at the longer wavelengths, and these were the points where surface plasmon resonance occurred. This in-dicates that, at these wavelengths, the particle diameter of 2 nmprovides a major contribution to plasmon coupling with the dipole radiation between the particles being the evanes-cent field of near-field light. Since the field propagation is predominantly taken as the near-field light, super-resolution exceeding the diffraction limit is made similar to that of near-field scanning optical microscopy. When the particle size is larger than 2 nm, the Ag nanoparticles form aggregative sil-ver clusters which act as a silsil-ver bulk. The intensity decreases owing to absorption in this metal bulk and the destructive in-terference of SPPs in particle-to-particle interactions.
In order to compare the simulation results to the meas-ured carrier-to-noise ratio (CNR) in an optical disk tester, the different of far-field intensity between the on-mark and the off-mark situations of the deformed super-RENS disks were computed for the various mark sizes. Figure 4 shows the differences of far-field intensity for the Ag nanoparticles of random distribution as shown in Fig. 1, with different wave-lengths at a particle diameter of 2 nm. These results were nor-malized against the far-field signals of the Ge2Sb2Te5 record-ing layer without recordrecord-ing marks, and varied considerably when they were illuminated by incident light at various fre-quencies (wavelengths).
In the observation of our simulations, if there was no Ag nanoparticle in the deformed active layer, the far-field sig-nals dropped rapidly and became indistinguishable when the size of the recording marks were smaller than the optical diffraction limit λ/(4NA). At shorter wavelengths, the min-imum distinguishable size of recording marks was smaller. When there were Ag nanoparticles distributed randomly in the deformed active layer, the far-field difference signals were several magnitudes higher and the recording marks were as small as 50 nm and were still distinguishable, i.e., recording marks smaller thanλ/10 could be read. Similar results have been obtained from the normal type (see Fig. 2b) from our previous results [6]. It can be observed in Fig. 4 that the far-field difference signals did not decrease monotonously with decreasing recording mark size or increasing incident wave-length. The far-field difference signals with smaller incident wavelengths (i.e.,λ = 400, 450, 500 nm) had a higher inten-sity than those of higher incident wavelengths (i.e.,λ = 550, 600, 650 nm) when the mark sizes were less than 100 nm. The random distribution of embedded Ag nanoparticles in the deformed active layer influenced the near-field enhance-ments as well as the far-field signals, so the far-field signals had some complex variations, especially for the recording marks smaller than the diffraction limit. A distinct
nonlin-FIGURE 4 Far-field difference signals of the deformed AgOx-type super-RENS disk with Ag nanoparticles. The wavelengths of the incident light are 400, 450, 500, 550, 600 and 650 nm, respectively
ear response of the deformed AgOx thin film was shown in
the results of our simulation, which agreed with our previous near-field and far-field measurements.
In further research, we will consider the model where not just a single bubble pit but two or more pits are in one spot. However, owing to the varying power of the incident light, the distribution and density of the nanoparticles may not be uni-formly random in the area of the realistically deformed AgOx
layer. Further research into the effects of different distribu-tion patterns may yield a better understanding of the optical properties of the deformed AgOx-type Super-RENS.
4 Conclusion
In conclusion, the deformation and plasmon effects of collective localized surface plasmons in a deformed AgOx
-type super-RENS structure have been studied using the FDTD method. We found that the polarization direction, the wave-length of the incident light, and the size of the Ag nanoparti-cles are sensitive to the plasma resonance. The Ag nanopar-ticles inside the deformed AgOx-type super-RENS structure
provide an additional length (or path) that is longer than the normal length (underformed case), and give additional outer boundaries to the motion of the Ag nanoparticles, and ex-cite more evanescent fields that are located in the far edge of the bubble pit from the optical axis of the incident beam. The far-field signals of different wavelengths of incident light confirm the relation between highly localized near-field dis-tributions and enhanced resolution of far-field signals. Our results can help us to design a super-RENS with superior reso-lution, as well as an alternative approach to near-field optical design for plasmon device, biological sensors and nanopho-tonic devices.
ACKNOWLEDGEMENTS The authors are thankful for the fi-nancial support from the National Science Council, Taiwan, R.O.C., under grant number NSC 95-2112-M-231-001, NSC-95-2119-M-009-029 and the Ministry of Economic Affairs, Taiwan, R.O.C., under grant number 95-EC-17-A-08-S1-0006.
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