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We demonstrate a metal nano-aperture GaAs vertical cavity surface emitting laser (VCSEL) for sub-wavelength optical near-filed probing, which exhibits the strong plasmon enhancement of both optical near-fields and voltage signals with forming a metal nano-particle in the nano-aperture. The threshold current is as low as 300µA, which shows a potential of nano-probing with low power consumption. We achieved the first demonstration of a plasmon enhanced VCSEL near-field probe. The spatial resolutions of the VCSEL probe with 400 nm and 200 nm apertures are 240nm and 130 nm, respectively. The enhancement factors of the optical near-field and voltage signal with a Au particle are 1.8 and 2, respectively. Our FDTD simulation shows that localized plasmon with a Au particle is very helpful for increasing optical near-field intensity and signal voltage in the VCSEL nano-probing.
©2004 Optical Society of America
1. Introduction
High-density optical data storages with Tera-bytes capacity have been attracting much interest. The storage density of conventional optical memories such as CD and DVD is determined by the spot size of light, which is limited by optical diffraction. An optical near-field technology is one of candidates to make a breakthrough for future optical storages [1, 2]. A storage density of Tera bit/inch2 is expected when we reduce the spot size to be in the range of 10 nm.
A high-density optical disk system using a vertical cavity surface emitting laser (VCSEL) array was proposed [3] and we started research on a metal nano aperture VCSEL for use in VCSEL-based near-field optical storages [4].
The evanescent wave is emitted from a metal nano-aperture formed on a VCSEL surface to an optical disk. The refractive index change on the disk may affect the threshold condition of a VCSEL, resulting in the change of an operating voltage under a constant current operation. We can use the same optical head for writing and reading out the data in this simple setup. We demonstrated a metal nano-aperture VCSEL and near-field probing by using a nano-aperture VCSEL [5, 6]. A high spatial resolution can be expected by reducing the physical size of the metal aperture. However, the optical near-field intensity through a metal aperture is decreased with decreasing the diameter of the aperture, which is a common difficulty in near-field optics. It is a challenge to enhance the optical near-field from nano-aperture VCSELs.
An interesting approach for increasing an optical near-field is to use surface plasmon in metallic nano-structures [7, 8, 9]. Surface plasmon excited on a nano particle results in the increase of near field intensity. We proposed and demonstrated a nano aperture VCSEL with a metal nano-particle placed in the center of the aperture [10].
In this paper, we demonstrate the near-field probing of a nano-aperture VCSEL, which shows the strong plasmon enhancement of optical near-fields around a Au nano-aperture VCSEL with a Au particle. We show a possibility of both increasing the optical near-field and the voltage signal by exciting localized surface plasmon in metal nano-structures.
2. Device structure
Fig. 1. (a) Schematic structure and (b) top view of a fabricated nano-aperture VCSEL with a Au particle.
The schematic structure of a nano-aperture VCSEL with nano-matal particle is shown in Fig. 1(a). The device consists of 12 pair p-type distributed Bragg reflector (DBR) and 34.5 pair n-type DBR with a 2.8 µm oxide aperture. The number of the p-type DBR pair was designed to be about a half of a standard VCSEL design for increasing the near-field intensity through the nano-aperture. The details of the fabrication process of our metal-aperture VCSELs are described in ref. [11]. Conventional VCSEL fabrication processes were used except making a metal-nano-aperture. The diameters of an aperture and a metal particle are 400 nm and 100 nm, respectively, which were formed by using a focused ion beam etching system (Seiko Instruments Inc.: model SMI 9200)) as shown in Fig. 1(b).
We inserted a SiO2 layer underneath the aperture in order to enhance the plasmon effect [8]. The thickness of SiO2 and Au layer is 320 nm and 100 nm, respectively. The diameters of an aperture and a metal particle are 400 nm and 100 nm, respectively, which were formed by using a focused ion beam etching system (Seiko Instruments Inc.: model SMI 9200)) as shown in Fig. 1(b). The lasing wavelength is 850 nm. The structure except the top mirror design and the metal nano-aperture is the same as conventional GaAs VCSELs.
Figure 2 shows the far-field output power from the fabricated nano-aperture VCSEL with and without a Au nano-particle in the nano-aperture. The far-field output power was measured by using a large area Si detector, which was calibrated with assuming point source radiation from the nano-aperture. Also, the small part of far-field output intensity through the Au film was subtracted from the measured data. We achieved a low threshold current of below 300 µA, showing a possibility of low power consumption in our near-field VCSELs. The output power from the nano-aperture was increased by a factor of 1.8 by inserting the Au particle. The result shows the enhancement due to localized plasmon.
Fig. 2. Far-field output power versus injection current for a nano-aperture VCSEL with and without a Au particle.
3. Characterization
We measured the localized optical near-field distribution and voltage change by using an experimental setup with a scanning near-field optical microscope (SNOM) as shown in Fig. 3. Our measurement system is based on a commercially available SNOM head (Seiko Instruments Inc.: model SPA300). An aluminum-covered, sharpened fiber probe was scanned in the X, Y direction just above the nano-aperture of the fabricated VCSEL. The distance between the fiber probe and the VCSEL was controlled to be constant and to be less than 20 nm by feedback control in the non-contact AFM mode of SNOM system. When the fiber probe is approaching to the nano-aperture, the scattering from the fiber probe results in threshold changes of the VCSEL. This causes the change in a diode voltage under operating at a constant current. The voltage change was amplified by a low noise 40dB-gain amplifier followed by a 10 Hz-low pass filter to improve signal-to-noise ratios. The spatial resolution of the SNOM is 100 nm. This setup also enables us to measure the topography of the device surface at the same time.
Fig. 4. Measured surface topography (a), optical near-field intensity (b), and voltage change (c) of nano-aperture VCSEL with metal nano-particle.
Figure 4 show the measured surface topography, optical near-field intensity, and voltage change of the nano-aperture VCSEL with a Au nano-particle. The measured full width at half of maximum of optical near-field distribution is 340 nm and thus the net spot size is estimated to be 240 nm. The power density is estimated to be as large as 7.7 mW/µm2. The power density is much larger than that of conventional VCSELs and is approaching to an enough power level for optical recording. The scanning image of operating voltages shows that the voltage decreases when the fiber probe is in the aperture. We achieved the near-field probe demonstration by using the voltage signal of the nano-aperture VCSEL. The maximum voltage change in the VCSEL probe with a Au particle was 2.8 mV, which is 2 times larger than that without a Au particle. The increase in injection current resulted in the decrease of voltage signals. This is due to the appearance of higher order transverse modes. Thus, we have to keep a single mode operation for getting higher voltage signals.
In order to achieve a higher resolution optical probe, the optical spot size of the present device is still large and we have to reduce the nano-aperture. We carried out 3D-FDTD simulations by using our home-made code software. We used a Yee cell size of 10 nm and the metal was expressed by the Drude model. A steady state solution is obtained after the temporal intensity fluctuation falls within 1%. The optical near-field intensity was calculated on the plane placed 20 nm far above the metal-nano-aperture. We define a spot size as the FWHM of calculated near-field intensity. Figure 5 shows the relative near-field intensity averaged over time and the spot size as a function of the metal aperture size. The result shows that the spot size can be reduced in proportional to the metal aperture diameter. The power density decreases especially for the case without a Au particle when the metal aperture size is reduced. We found that a Au particle is very helpful for keeping the power density.
Fig. 5. Calculated spot size and relative near-field intensity as a function of the diameter of metal aperture.
We fabricated near-field VCSELs with a smaller aperture. The diameter of the aperture was reduced to be 200 nm and the diameter of the Au particle was the same as 100 nm. The measured optical near-field and voltage signal are shown in Fig. 6. The spot size and power density are estimated to be 130 nm and 1 mW/µm2, respectively. The signal voltage is 0.85 mV. We could reduce the spot size by reducing the nano-aperture. We expect further enhancement and localization of optical near-fields and signal voltages by optimizing the diameter and the position of a Au nano-particle.
5. Conclusion
We demonstrated the plasmon enhancement of optical near-field VCSELs, showing that the optical near-field intensity and the signal voltage of nano-aperture VCSELs are increased to show record high values by exciting localized surface plasmons in metal nano-structures. We expect further enhancement and localization of optical near-fields and signal voltages by reducing the diameter and by optimizing the position of a Au nano-particle. We achieved the first demonstration of a plasmon enhanced VCSEL probe. The enhancement factors of the optical near-field and voltage signal are 1.8 and 2 respectively. We reduced the optical resolution of the VCSEL probe from 240 nm to 130 nm by reducing the nano-aperture. Plasmon enhancement is very helpful for realizing high resolution optical near-field VCSEL probes.
Acknowledgments
The authors acknowledge Professor Emeritus Kenichi Iga of Tokyo Institute of Technology for his encouragement. This work was supported by the Grant-in-Aid for Creative Scientific Research from the Ministry of Education, Science, Sport and Culture (#14GS0212”).
References and links
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