Fabrication of three-dimensional plasmonic cavity by femtosecond laser-induced forward transfer

We fabricated a three-dimensional five-layered plasmonic resonant cavity by low-cost, efficient and high-throughput femtosecond laser-induced forward transfer (fs-LIFT) technique. The fabricated cavity was characterized by optical measurements, showing two different cavity modes within the measured wavelength region which is in good agreement with numerical simulations. The mode volume corresponding to each resonance is found to be squeezed over 10 smaller than the cube of incident wavelength. This property may facilitate many applications in integrated optics, optical nonlinearities, and luminescence enhancement, etc. © 2013 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (220.4241) Nanostructure fabrication; (160.3918) Metamaterials; (240.6680) Surface plasmons; (140.4780) Optical resonators; (250.5403) Plasmonics References and links 1. Y. Akahane, M. Mochizuki, T. Asano, Y. Tanaka, and S. 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Introduction
Optical cavity has found many applications in integrated photonics [1,2], micro/nano-laser [3][4][5][6][7], luminescence enhancement [8][9][10] and nonlinear optics [11,12]. To achieve high performances on above-mentioned applications, an optical cavity with a high Purcell factor Q/V m is desired, where Q and V m denotes the quality factor and mode volume.  [39], bio-materials [40], and powders [41]. In particular, we recently demonstrated that, with tightly contacted donor-receiver pair, multilayer plasmonic structures can be fabricated on donor by LIFT process [35]. Since the processing rate [34] and controllability [42] of LIFT can be much better in comparison with other maskless fabrication techniques, LIFT technique is very fascinating for the throughput and fast prototyping of various nanophotonic devices.
In this paper, we implement the femtosecond LIFT (fs-LIFT) technique to fabricate square-shaped multilayer plasmonic resonant cavities, and study their optical properties by both experiments and numerical simulations. We chose to fabricate this type of plasmonic cavity because such a system was shown to exhibit extremely small mode volume. Using the fs-LIFT technique, the illuminated materials of multilayer thin films on donor are ablated and partially transferred to the tightly contact receiver, which would leave the uniform squared multilayer plasmonic cavities which are surrounded by the cooled-down laser-ablated materials on the donor. We found that the fabricated plasmonic cavity indeed exhibits two cavity modes with mode volume ~10 −4 λ 3 , which is highly desired in many applications. Figure 1 shows the schematic illustration of the experimental process. By using a magnetron sputtering system (Shibaura Mechatronics Corp.), multilayer thin films composed of Au(20 nm) / ZnS-SiO 2 (30 nm) / Au(20 nm) / ZnS-SiO 2 (30 nm) / Au(20 nm) are sputtered on a cleaned BK7 glass substrate (Matsunami cover glass, 22 × 22 mm 2 , thickness = 0.15 mm) in Ar atmosphere (5 × 10 −1 pa). In the fs-LIFT process, the multilayer thin films are treated as the donor. In our previous work we found, for fabricating the multilayered nanostructures using fs-laser beam, it is necessary to put another substrate on the top of the multilayered films to prevent the nonuniform edges of fabricated structures caused by laser-induced explosion [30]. Another glass substrate acting as the receiver is tightly contacted with the donor substrate. Subsequently, the donor-receiver pair is mounted on a computer controlled stage (Mad City Labs Inc., Nano-LP200). A Ti:Sapphire fs-laser laser (Coherent Inc., Chameleon ultra II, λ = 800 nm, pulse duration = 140 fs, repetition rate = 80 MHz) is used as a light source. An attenuator is used for controlling the incident laser fluence precisely. The laser beam is focused on the donor side by a high-numerical-aperture oil-immersion objective lens (Zeiss Plan-Apochromat, 100 × , N.A. = 1.4, working distance = 0.17 mm) through the glass substrate on the donor. The illuminated materials in multilayer films are locally melted and mixed after the laser treatment. Non-illuminated parts of the laser-processed thin films are surrounded by the cool-down laser-ablated materials. Therefore, the multilayer cavities are fabricated after the fs-LIFT process with a designed laser raster path. The designed multilayer structure as single unit-cell is depicted in Fig. 1(b). The feature sizes of each a square-shaped cavity is 500 nm length and 120 nm height, and the periodicity is 1100 nm and 650 nm in x-direction and y-direction, respectively.

Results and discussions
Figure 2(b) shows the square patterns surrounded with laser-ablated materials transferred onto the receiver (indicated with red-dashed square). The laser fluence is set at 165 mJ/cm 2 (raster speed is fixed as 53.3 μm/s). The square patterns can be transferred to receiver by the laserablated materials, which are impressed on the receiver during the fs-LIFT process. Moreover, as shown in the green dashed square in Fig. 2(b) and the corresponding magnified SEM image shown in Fig. 2(c), multilayer films of fabricated patterns can be observed, indicating that the multilayer structures of laser-fabricated pattern are not damaged by the lateral heat dissipation during laser illumination. Since the structures shown in Fig. 2(a) are not completed and uniform, the laser processing condition should be modified.   Figure 4 shows the experimental (red solid line) as well as simulated (blue solid line) transmittance spectra of the plasmonic cavity illuminated by a y-polarized light at normalincidence. The transmittance spectra from λ = 1000 nm to λ = 2500 nm were measured using a Bruker VERTEX 70 Fourier-transform infrared spectrometer with a Bruker HYPERION 1000 infrared microscope (15 × Cassegrain objective, numerical aperture N.A. = 0.4, nearinfrared polarizer, and an InGaAs and MCT detector). An iris was used to collect the incident light to a square area of about 50 × 50 µm 2 . The transmittance spectra are normalized by those of the rare fused silica wafer. The simulation spectra were obtained by the commercial software (Comsol Multiphysics) based on the finite-element method. The dimension of simulated squared plasmonic cavity is 500 nm with its periodicity as 1100 nm and 650 nm in x-and y-direction. Periodic boundary conditions are used to mimic the plasmonic cavity array. The refractive index (n) of the sandwiched ZnS-SiO 2 layer was taken as 2.3, considering the ZnS-SiO 2 mixture made of 80% ZnS (n = 2.5) with 20% SiO 2 (n = 1.5). The permittivity of gold was described by the Drude-Lorentz model with a damping constant of 0.35 eV and a plasma frequency of 8.997 eV [43]. We found that two transmittance dips appear around 1280 nm (Mode II) and 1800 nm (Mode I) in both experimental and simulated spectra. The discrepancy between simulation and experiment is due to the fabrication defect in the sample and the inaccuracies of the dielectric parameter taken in simulations. Fig. 4. A comparison transmittance spectra between experimental result (red curve) and simulation result (blue curve). Two resonance modes are marked by I and II from longer to shorter wavelength.
To understand the origin of these resonance modes, we depicted the electromagnetic field patterns for two modes in Fig. 5. To gain a complete picture of the two cavity modes, we showed the E z field at the y = 0 nm plane, the E z and H x fields at the z = 85 nm plane (center symmetry plane of upper ZnS-SiO 2 layer), respectively. For mode I, half-wavelength standing wave behaviors along three dimensions demonstrate that it is the resonance mode with index (1,1,1) [44]. Recalling that the size of the plasmonic cavity is 500 nm × 500 nm × 120 nm, we found that such a resonance mode (with λ ~1800 nm) exhibits subwavelength property with high energy storage ability. Similar analysis revealed that mode II is a resonance mode with index (1,2,1). Considering the distances of plasmonic cavities are different along x and y direction, the case of electric field polarized along x direction has also been studied. It was found that the electromagnetic field distributions of the plasmonic resonant modes are similar with the above-mentioned y-polarized case due to the symmetry of square-shaped cavity, except for about 200 nm blue shift of the plasmonic resonant wavelengths. For a given resonance mode, the quality factor Q is determined by the photon life time, and the mode volume V m quantifies the electromagnetic field strength per photon. According to our simulation spectra, we calculated the quality factors of the two modes and found that they are 3.2 and 8.3, respectively. Indeed, the quality factor Q of such a plasmonic cavity is very low due to the absorption loss of metal. However, our plasmonic cavities have smaller mode volumes (V m ) compared to dielectric micro-cavities. The mode volume V m is calculated by [44,45] where W av is the time averaged electromagnetic energy density calculated by [46,47] where e Γ , ε ′ and ε ′′ are the damping frequency, the real part and image part of permittivity.
The max (W av ) is chosen at the maximum energy density excluding the sharp peak at the corner, since it strongly depends on the mesh quality at the corner. Our calculations revealed that the mode volume V m can go down to 1.68 × 10 −4 λ 3 and 2.8 × 10 −4 λ 3 for mode I, and mode II, in consistency with our expectation that a plasmonic cavity can confine electromagnetic energy far below the diffraction limit. Together with the quality factor Q of the cavity resonance, this enables Q/V m in the case of the fs-laser fabricated multilayered plasmonic cavity to be comparable with dielectric optical cavities. Nevertheless, the multilayered plasmonic cavity fabricated by fs-LIFT has relatively smaller geometrical size, and therefore shows the feasibility to be highly integrated in a given chip with low-cost, high throughput and efficiency.

Conclusion
We successfully demonstrated a low-cost, efficient and simple fabrication technique for manufacturing multilayered plasmonic resonance cavity by femtosecond laser-induced forward transfer technique. We have found the optimized laser fluence (170 mJ/cm 2 ) and laser raster speed (53.3 μm/s) on the multilayer films for making the fabricated multilayer structure uniform and smooth. Two resonance modes are found in near infrared region, showing electromagnetic energy mainly stored in the sandwiched dielectric layer with subwavelength property. The optical properties of laser-fabricated plasmonic cavity quantitatively agree with simulations results. These unique properties of multilayered cavity increase the optical density of state and therefore could be applied to light-matter interactions, such as integrated optics, optical nonlinearities, and luminescence enhancement, etc. This work demonstrates the potential of fs-LIFT technique in fabrications of various kinds of three-dimensional nanostructures: People can readily find the optimization of processing parameters for the desired layered structures, and fabricate the designed photonic devices on the arbitrary substrates. Our strategy can be further improved by the integration with other laser-direct writing technique such as laser micro-lens array photolithography [48] and laser ablation by a laser beam of flat-top profile [49].