Investigation of the Near-field Distribution at Novel Nanometric Aperture Laser

Transcription

1 Investigation of the Near-field Distribution at Novel Nanometric Aperture Laser Tiejun Xu, Jia Wang, Liqun Sun, Jiying Xu, Qian Tian Presented at the th International Conference on Electronic Materials (IUMRS-ICEM, Xi an, China, 1 1 June ) 51

2 Advanced Nanomaterials and Nanodevices (IUMRS-ICEM, Xi an, China, 1 1 June ) Investigation of the Near-field Distribution at Novel Nanometric Aperture Laser Tiejun Xu, Jia Wang, Liqun Sun, Jiying Xu, Qian Tian Department of Precision Instruments, State Key Laboratory of Precision Measurement Technology and Instruments, Tsinghua University, Beijing 1, P. R. China The near-field distribution at a novel aperture of a very small aperture semiconductor laser is calculated by FDTD method. The calculation results reveal that the output power intensity of novel aperture laser is or orders higher of magnitude than conventional round and square aperture. The power throughput reaches maximum when the aperture size is one thirds of the incident wavelength. The thickness of the metal film coated on the laser front facet is optimized as 5 ~ 1 nm. The spot size is comparable with that from conventional nano-aperture. The mechanism of electromagnetic field enhancement and some effects in local near field close to the aperture is discussed. 1. Introduction Theory and method of the near-field optics lead people to enter a new research category. The research revolves in the generation and transform of the evanescent wave in local near field at the sub-wavelength or nano-scale. The optical resolution based on near-field optical microscope is beyond the optical diffraction limit and down to tens nanometers or even less [1]. The probe tip is one of the most important elements in near-field optical system. A lot attention was attracted to fabrication and improvement of near-field optical probe in the past years. Many novel probes with excellent performances have been proposed and fabricated. The concept of the active probe was proposed by Motoichi Ohtsu in 1991[]. He described a near-field optical system with a reflection resonant cavity probe. A laser diode can be used as an active probe to realize the function as an illuminating laser source and a feedback resonant cavity in the near-field optical system A. Partovi proposed firstly that a very small aperture laser diode (VSAL) can be used as a light source in near-field optical data storage[]. He demonstrated that the output power of VSAL is orders higher of magnitude than the taper fiber metal coated probe while the aperture size is optimized appropriately. It improves carrier noise ratio and data transfer rate in the storage system. Shinada et al proposed a method to fabricate nano-aperture vertical cavity surface emitting laser (NA-VCSEL). Small aperture on Au film, with dimension of 1 nm in both diameter and thickness was fabricated by focus ion beam (FIB), but the output power was too low. In order to increase optical storage density and data transfer rate, Goto proposed and used array of VCSEL[] to increase the output power. The new array of VCSEL was used as light source and realized multi-beam parallel writing and reading. A vital shortage of the laser above is that the power throughput was too small to be used as a light source of an optical head in an applicable storage system although the demonstrations were performed experimentally. Recently, Xiaolei Shi proposed a novel aperture laser with abnormity shape aperture. The power throughput was increased 1 times[5]. But the dependence on the aperture configuration and throughput has not been discussed in detail for a good understanding as well as the operation mechanism. The attempt is made to calculate near-field distribution in vicinity of novel nano-aperture of a semiconductor laser by Finite-Difference Time-Domain method (FDTD) and characterize the optical properties. The mechanism of electromagnetic field enhancement and some influences of configuration, polarization and separation in local near field close to the aperture will be discussed. A layer of metal film is deposited on the front facet of a semiconductor laser diode. A very small 5

3 Advanced Nanomaterials and Nanodevices (IUMRS-ICEM, Xi an, China, 1 1 June ) aperture, with diameter of 5 1 nm, is fabricated on the film. When the laser is brought in close proximity to the recording media, laser beam from the nano-aperture can illuminate the media and realize writing operation. Based on effect of laser feedback the laser can also be employed as a detector to read signal from the media.. Modeling Fig.1 shows the system schematically. In the following calculation the aperture is employed as a nanometric light source to illuminate locally recording media in its vicinity. metallic strip nano-aperture metallic coating layer p+ GaAs layer p GaAs layer n GaAs layer Having more interest in intensity, the matching dielectric material following discussion will be focused on intensity distribution. The spot size will be Fig.1 The schemetic diagram of nano-aperture determined by intensity calculation results semiconductor laser as well. In order to make comparison, calculation of novel nano-aperture with square shape and C shape is also discussed in this paper. Intensity distribution of the two kinds of apertures is analyzed and characterized. For calculation convenience the aperture geometry is normalized. A square aperture and a C-aperture are shown in Fig.. The square aperture is a foursquare aperture with unit length in each side, which locates at the center of an opaque screen. The C-aperture consists of 5 foursquare apertures with dimension of unit length in its size. So the side size of square aperture equals to unite length, the long and wide side sizes of the C-aperture are times and times unite lengths respectively. The field distribution of VSAL with nano-aperture is very complicated and subtle. In order to make the discussion close to practical situation in near-field optical experiments the spot size is measured in the plane 5 nm apart from the metal film and its size is decided by the FWHM. The spot size is denoted as dx and dy in x direction and y direction respectively. Intensity maximum is one of the most important physical parameters characterizing the emitting field. Intensity maximum, square of field modulus, is measured on the same plane. Efficiency of throughput is defined as the ratio of total output power to input total power in the range of the aperture. The total power is determined on the plane from the metal film 5 nm as evanescent field would decay rapidly with distance increase in propagating direction. Calculation was performed by the software package, XFDTD5.1, from REMCOM.. Results a) result comparison The results of the square apertures laser and C-aperture laser are schematically shown in Fig.. The intensity maximum and throughput efficiency are shown in Table.1. Unit lengths are both 7nm. Fig. u u Geometry of the square aperture and the C-aperture u=unit length Spot size nm Intensity Throughput Aperture dx dy maximum(a.u.) efficiency(a.u.) C-aperture Square aperture Table.1 Calculation results of C- and square aperture with the same unit length size. u 5

4 Advanced Nanomaterials and Nanodevices (IUMRS-ICEM, Xi an, China, 1 1 June ) The intensity maximum of C aperture is enhanced orders of magnitude than that of square aperture, but spot size is comparable with square aperture. The throughput efficiency of C aperture is bigger than one. This means the total power transmitted through the aperture is greater than the total incident power over the physical area of the aperture, indicating that power is transmitted from areas outside the physical aperture boundary. It shows that the field is enhanced greatly by near-field local effects []. b) C aperture C-aperture size takes the unit length 1 intensity maximum changed from to 1nm sequentially, the throughput efficiency thickness of the metal film is optimized as 1nm. The near-field intensity distributions E of C-aperture with the different sizes are achieved. In Fig., the x-axis is the unit length, the long side of C-aperture is times unit length and 1 1 the wide side is times unit length. When the Unit length of C-aperture/(nm) unit length of C-aperture is nm, intensity is very weak; When the unit length of the Fig. The Intensity maximum and Throughput efficiency C-aperture is between to nm, namely, the of C-aperture Laser in different unit length aperture size is nearly nm and one third of wavelength, the throughput efficiency suddenly increases, reaches its maximum; When aperture size increases further close to wavelength, it appears far field diffraction property. Energy of optical field diverges gradually and throughput efficiency decreases rapidly to one. The other curve is the intensity maximum of measured plane vs. the unit length of C-aperture. Before the unit length of C-aperture is 1 nm, namely, the side size of aperture is about nm and half of wavelength, the intensity maximum increases gradually which reached to peak when the unit length is about 1 nm, and then the intensity maximum also decreases along with the increase of aperture size. Intensity maximum/(a.u.) throughput efficiency/(a.u.). Discussion a) Distribution comparison between C- and square aperture What is the mechanism to enhance power peak of C-aperture up to orders in magnitude and what enables the total power through the aperture to be greater than the total incident power over the aperture? Effort was made to explain the effect by means of analysis and comparison of field components of square aperture and C-aperture. As shown in Fig., the distribution of the square aperture and C-aperture gives some characteristics in the near-field zone. For the distribution of C-aperture, (1) E y is much smaller than E t, and distributes are mainly in the two ends of the C-aperture and appears as two symmetry spots. () E z presents the field distribution enhancement property on the edges. E z generates on the edges perpendicular to x direction in (a) (b) (c) (d) Fig. Components of square aperture and C- aperture in x, y and z direction and total intensity. Component distribution of square aperture(up) and C-aperture(down): (a) E x ; (b) E y ; (c) E z ; (d) E t. 5

5 Advanced Nanomaterials and Nanodevices (IUMRS-ICEM, Xi an, China, 1 1 June ) the C-aperture and forms two separate abnormality spots symmetry to y axis in the cross section. The field enhancement effect is apparent on the edges of the aperture. () E z has much effect on the total field distribution although E x dominates in the range of the C-aperture. In the plane at z = 5 nm the field appears an elliptical spot enhanced by the evanescent field enhancement in the z direction although size is slightly extended in x direction. Fig. reveals that the power peak of C-aperture is increased greatly and the spot size is still comparable to the square aperture after the field energy is redistributed. The size of the C-aperture is larger, so it will be easier to fabricate and also as a result, with better reproducibility. b) Polarization affects (a) (b) With calculating square aperture and C-aperture in unit length of 7 nm, the polarization TM mode TE mode x 1 Square - x 1 effects to field distribution are achieved. (a) and (b) - in Fig.5 shows intensity distributions of a square aperture aperture and C-aperture with TM and TE polarization respectively. x 1 It is obvious that the square aperture is C aperture - symmetric, and there is not that much affect to field distributions and throughput efficiency except for rotation at 9 degrees by different polarization illumination. Fig.5 Optic field distribution under TM and TE Under the situation of the C-aperture TM mode light illumination mode, a spot is formed which is similar to a square with very strong intensity as the field enhancement effect. For TE mode, there is no effective way to form single spot with relative weak intensity. The incident beam is TM mode and generates Ey and Ez components after going through the sub-wavelength aperture due to depolarization effect. Ez component is along in incident direction. According to near-field optics theory, a definite nanometric object, particle or aperture, can stimulate optical field to generate evanescent field, and result in Ez component. It is strongly confined in vicinity of near-filed of object. It has higher intensity and dominates enhancement energy in near-field zone. Calculation results of the square aperture reveals that the E z starts increasing at the aperture size of 15 nm, about one fourth incident light wavelength and the throughput increases steeply almost 5 times and reaches its maximum at nm, about one third wavelength. Similarly, in the situation of C-aperture the E z begins increasing with the unit length 5 nm, namely the side of the C-aperture is 15 nm, one fourth of wavelength and the enhancement in the z direction is much stronger than in x direction. The very strong z component generates and forms two separate spots. The throughput increases steeply at the unit length of -7 nm, namely the side size is nm, one third of wavelength. It is likely to be explained as the near-field enhancement. Because boundary condition changes abruptly or a sub-wavelength object enters the optical field, the light beam generates components different from incident light directions. They could interact each other, and the resonance and interference will result in the local field enhancement. 5. Conclusion A new VSAL with novel aperture is investigated and simulated. Theoretical calculation results reveal that the throughput power from the novel aperture laser increases or orders in magnitude. The throughput reaches maximum when the aperture size is about one third wavelength. 55

6 Advanced Nanomaterials and Nanodevices (IUMRS-ICEM, Xi an, China, 1 1 June ) The novel nano-aperture laser could be suitable to optical storage and used as an active probe in near-field optical system. The research is sponsored by 'National Research Fund for Fundamental Key Projects No.97 (G1999) and 95 optical data storage program at Tsinghua University. Reference [1] D.W. Pohl, W. Denk, M. Lanz, 19 Appl. Phys. Lett. 51. [] M.Ohtsu, et al, 1991 Phtoton scanning tunnel microscope, Laser Research, Vol.19, No. (Japanese) [] Afshin Partovi, David Peale, Matthias Wuttig, 1999 Applied Physics Letters. Vol.75,No.11, [] K.Goto, 1997 Proc. SPIE, vol.19, [5] Xiaolei Shi, Robert L. Thornton, et.al.1, ODS1 /Postdeadline Papers/WC7 [] Xiumei Liu, Jia Wang, 1 Journal of Tsinghua University, Science and Technology, Vol.1, No., - Tiejun Xu xutj@post.pim.tsinghua.edu.cn Tel: 1-71 Fax: