JAIST Repository: 波長可変光第二高調波発生法による固体表面の研究
7
0
0
全文
(2) Study of the Solid Surfaces by OpticalSpectroscopy Second Harmonic Hideki Tanaka. School of Materials Science, Japan Advanced Institute of Science and Technology. (Supervised by Goro Mizutani) Keywords:. SHG, surface, second-order nonlinear susceptibility, Au, GaAs. Introduction Optical second-harmonic generation (SHG) is a powerful tool for investigating surfaces and interfaces. Its intensity is enhanced when one or two photon resonance takes place. Thus surface or interface electronic states can be found by SHG observation. However, there are few experimental studies concerning SH intensity spectrum of metal surfaces or interfaces[1]. The purpose of this study is to develop the SHG measurement system with a good performance using a tunable light source, and to establish the method for characterizing metal surfaces by SHG.. SHG measurement system Fig. 1 shows the surface SHG measurement system constructed in this study. The excitation photon energy range of this system is from 1.0 to 2.5eV. This range is the widest among those that have ever appeared in the literature. The S/N ratio of this system is very high as 1000:1. This S/N ratio was achieved by detecting only the output SH light pulse that is synchronized with the excitation laser pulse, and using photomultiplier (PMT) having a very low dark count, and measuring the peak height of output current from the PMT. The system sensitivity was calibrated in the entire wavelength range by measuring the re ected SH intensity from an -SiO2 plate.. Results In this study, I have achieved four experimental
(3) ndings. 1. A new resonance of the surface SHG from GaAs(001) was found at h!=1.45eV. (2) 2. The ratio of the second-order nonlinear susceptibility elements (2) xzx and zxx of the glass-Ag and glass-Au interfaces has been determined experimentally for the
(4) rst time.. 1.
(5) Prism. B.S. Q-switched Nd:YAG laser 355nm. OPO. Polarizer Reference System. Computer. Lens GaAs 2ω cut filter ω cut filter Polarizer. Amp PMT. Sample. Lens. Lens. Amp. Peak-hold circuit & A/D conberter. Monochromator. Figure 1.. SHG measurement system.. 3. An excitation energy dependence of the second order nonlinear susceptibility was obtained on a glass-Au interface. A peak at 2h!=2.5eV was discovered. 4. The SH intensity spectrum from the Au
(6) lm on NaCl(100) was measured in UHV. The resonance energy of SHG of the
(7) lm of thickness 60 A was 2h!=2.5eV and that of the
(8) lm of thickness 27A was higher than this.. 1 A New Resonance of the Surface SHG from GaAs(001) Fig. 2 shows the SH intensity from GaAs(001) in air as a function of the incident photon energy. A resonance enhancement peak at h!=1.45eV has been found in the p-polarized input and p-polarized output (p-in/p-out) con
(9) guration. Because the sum frequency generation (SFG) shows a one-photon resonance at the same photon energy, the observed structure in the SH intensity curve is also due to a one-photon resonance. The bulk SH response of GaAs was separately observed in the p-in/s-out polarization con
(10) guration and does not show a resonance at h!=1.45eV. Thus it was concluded that the observed resonance at h!=1.45eV originates from the oxidized surface of GaAs(001). A possible candidate origin of this peak is a resonance with the inter-surface state transition. These surface states emerge due to the surface bond strain. In this experiment also the performance of my SH measurement system was demonstrated.. 2 Second-Order Nonlinear Optical Susceptibility of the GlassMetal Interface (2) The ratio of the second-order nonlinear susceptibility elements (2) xzx and zxx of the glass-Ag and the glass-Au interfaces have been determined at the fundamental photon. 2.
(11) SH Intensity (a. u.). A. B. Fundamental Photon Energy (eV). The SH intensity from GaAs(001) in air as a function of the excitation photon energy h!. , The SH intensity for =135o(curve A); and , the SH intensity for =45o (curve B). is the angle between the plane of incidence and the [100] direction on the GaAs(001) face. Figure 2.. . . energy h !=1.13eV. SH intensities have been measured in two polarization con
(12) gurations. One is the con
(13) guration with s-polarized input and p-polarized output (s-in/p-out con
(14) guration). The other is the con
(15) guration with linearly-polarized input containing 50% of p-polarized light and 50% of s-polarized light, and with s-polarized output (s-p-mixedin/s-out con
(16) guration). In s-p-mixed-in/s-out and s-in/p-out polarization con
(17) gurations, (2) the nonlinear susceptibility elements (2) xzx and zxx contribute to the harmonic intensity, respectively. From the ratio of the SH intensities in these two con
(18) gurations, I have (2) obtained the ratio of the nonlinear susceptibility elements (2) xzx : zxx =1.0:-0.28 for the (2) glass-Ag interface, and (2) xzx : zxx =1.0:-0.24 for the glass-Au interface. It has been also found that the SH intensity from the glass-Au interface deviated from the theoretically expected intensities for incident angles smaller than 20o . It is suggested that this is either due to the higher order nonlinear optical e ects of the metal bulk or due to the e ect of interface steps and grain boundaries.. 3 Excitation energy pro
(19) le of the SH intensity from the glassAu interface The SHG from a glass-Au interface was observed, and the second order nonlinear susceptibility as a function of the photon energy was obtained. The sample was a Au
(20) lm of thickness 100nm on a glass (BK7) substrate prepared by evaporation. Fig. 3 shows the nonlinear susceptibility as a function of the excitation photon energy h!. In calculating the SH intensity, it was assumed that the zzz element dominantly contributes to the SH intensity.. 3.
(21) (a.u.) (2). S,zzz. 25. Surface Nonlinear Susceptibility. 20 15 10 5 0 1. 1.2 1.4 1.6 (eV) Fundamental Photon Energy. The SH intensity from the glass-Au interface as a function of the excitation photon energy h!. Figure 3.. A peak is seen near 2h!=2.5eV. Three candidate origins of this resonance are considered: 1. The enhancement in the electric
(22) eld strength by the excitation of surface plasmon polaritons, 2. The resonance with the local e ective plasma frequency of free electrons at the interface, and 3. The resonance with the interband transition in which the d-electron of the interface or bulk is involved. Each of these possibilities was checked. 1. At the rough surface, the incoming and outgoing optical light
(23) elds is enhanced because of the local-
(24) eld enhancement resulting from the local-plasmon excitation. Boyd et al measured the SH intensity from several kinds of metal
(25) lms on smooth and rough glass substrates. They showed that the SH intensity from the Au
(26) lm is stronger than that of the Ag
(27) lm on a smooth glass plate, and that the enhanced SH intensity from Ag
(28) lm is much stronger than that of the Au
(29) lm on a rough glass plate. Comparing my results with those by Boyd et al, I can conclude that the interface prepared in this study corresponds to the metal surface on a smooth glass plate prepared by Boyd et al. Thus we consider that there is no e ect of local-
(30) eld enhancement. 2. At the metal surface in a vacuum the electron density is low. The average plasma frequency in this region is the local e ective plasma frequency. At the gold surface in a. 4.
(31) 2. 1.25eV 0. 2.5eV. EF surface state. 1.25eV. -2. Μ Figure 4.. Κ. The SH calculated band structure of the Au(111) surface.. vacuum the local e ective plasma frequency[2] is 2.4eV by calculation. The observed resonance in this study was located at 2h!=2.5eV in the experiment and it is close to this local e ective plasma frequency. However, I can exclude this candicate for the following two reasons. Firstly, the absolute value of the nonlinear susceptibility calculated by Liebsch and Schaich[3] does not have a sharp structure like that in Fig.3. Second, the local e ective plasma frequency on the glass-metal interface must be di erent from that of the metal surface in a vacuum. Hence Origin (2) is also excluded. 3. Fig. 4 shows the calculated band structure of the Au (111) surface[5]. We see a point in the surface band gap of energy approximately equal to 2.5eV near the M Brillouin zone. If we assume that the band gap energy of a glass-Au interface is similar to that of the vacuum-Au interface in Fig. 4, the surface interband transition is a possible candidate origin of the SHG enhancement in Fig. 3.. 4 Excitation energy pro
(32) le of the SH intensity from the Au
(33) lm on NaCl(100) in UHV The excitation energy dependence of the peak photon energy of the SH intensity from the Au
(34) lm of thickness from 2.4 A to 60 A on NaCl(100) in UHV has been measured. The peak photon energy of the SH enhancement depended on the
(35) lm thickness. The crystallinity and the surface topography of Au
(36) lms were investigated by RHEED and AFM observation. The
(37) lm of thickness 27 A had a single crystal face (100). The
(38) lm of thickness 60A was a polycrystal with a crystal face (111). The resonance energy of SHG of the
(39) lm of thickness 60 A was the same as that of the glass-gold interface. On the other hand, the SH resonance energy of the
(40) lm of thickness. 5.
(41) 27 A was higher than that of the
(42) lm of thickness 60 A. This
(43) nding indicates that these
(44) lms have di erent surface states. I suggest that the di erence of the resonance energies of the two
(45) lms re ects the di erence of the lattice e ect on the surface states.. References [1] L. E. Urbach, K. L. Percival, J. M. Hicks, E. W. Plummer, and H. -L. Dai, Phys. Rev. B45, 3769 (1992). [2] J. E. Sipe, V. C. Y. So, M. Fukui and G. I. Stegeman, Phys. Rev. [3] A. Liebsch and W. L. Schaich, Phys. Rev.. B40. , 4389 (1980).. B21. , 5401 (1989).. [4] N. Takeuchi, C. T. Chan, and K. M. Ho, Phys. Rev.. B45. , 3769 (1992).. Publication list [1] H. Tanaka, H. Kurokawa, E. Kobayashi, H. Sano, G. Mizutani and S. Ushioda, Progress in Cryst. Growth & Char. of Mater. 33, 129 (1996). [2] H. Tanaka, T. Miyazaki, G. Mizutani and S. Ushioda, CLEO/Paci
(46) c Rim '97 TECHNICAL DIGEST 181 (1997). [3] H. Tanaka, G. Mizutani and S. Ushioda, Surf Sci, in press. [4] H. Tanaka, G. Mizutani and S. Ushioda, Surf Sci, submitted. [5] H. Tanaka, T. Miyazaki, G. Mizutani and S. Ushioda, to be published. Note: the thesis is written in Japanese. February 20, 1998. Copyright c 1998 by Hideki Tanaka. 6.
(47)
図
関連したドキュメント
Eskandani, “Stability of a mixed additive and cubic functional equation in quasi- Banach spaces,” Journal of Mathematical Analysis and Applications, vol.. Eshaghi Gordji, “Stability
Finally, we give an example to show how the generalized zeta function can be applied to graphs to distinguish non-isomorphic graphs with the same Ihara-Selberg zeta
As with subword order, the M¨obius function for compositions is given by a signed sum over normal embeddings, although here the sign of a normal embedding depends on the
The torsion free generalized connection is determined and its coefficients are obtained under condition that the metric structure is parallel or recurrent.. The Einstein-Yang
We show that a discrete fixed point theorem of Eilenberg is equivalent to the restriction of the contraction principle to the class of non-Archimedean bounded metric spaces.. We
The main problem upon which most of the geometric topology is based is that of classifying and comparing the various supplementary structures that can be imposed on a
It turns out that the symbol which is defined in a probabilistic way coincides with the analytic (in the sense of pseudo-differential operators) symbol for the class of Feller
Then it follows immediately from a suitable version of “Hensel’s Lemma” [cf., e.g., the argument of [4], Lemma 2.1] that S may be obtained, as the notation suggests, as the m A
