Single Frequency Ince Gaussian Mode

Research Express@NCKU - Articles Digest

Research Express@NCKU Volume 5 Issue 10 - October 3, 2008 [ http://research.ncku.edu.tw/re/articles/e/20081003/1.html ]

Single-frequency Ince-Gaussian mode operations of laser-diode-pumped microchip solid-state lasersShu-Chun ChuAssistant Professor of Department of Physics, College of Sciences, National Cheng Kung University [email protected] OPTICS EXPRESS 15, pp. 10705-10717 (August, 2007)

T

wo transverse lasing modes, Hermite-Gaussian modes (HGMs)

and Laguerre-Gaussian modes (LGMs) have been widely investigated with analytical, numerical, and experimental techniques. These two modes separately form two complete families of exact and orthogonal solutions of the paraxial wave equation (PWE) in rectangular and cylindrical coordinates. Researchers have recently predicted a third complete family of transverse modes, PWE solutions in elliptic cylindrical coordinates, namely the InceGaussian modes (IGMs). In addition, Ince-Gaussian modes constitute the continuous transition modes between HGMs and LGMs. In this study, various single-frequency Ince-Gaussian mode oscillations have been achieved in laser-diode-pumped microchip solid-state lasers, including LiNdP4O12 (LNP) and Nd:GdVO4, by adjusting the azimuthal symmetry of the laser resonator. The astigmatic pumping mechanism of InceGaussian modes formation is explored by numerical simulation. The experimental setup to force IGM operations in a LiNdP4O12 (LNP) laser-diode-pumped microchip solid-state laser is shown in Fig. 1(a). An elliptical LD beam was transformed into a circular one and focused onto the LNP crystal by using a microscope objective lens with a numerical aperture of 0.25 to obtain a tight focus giving a minimum spot size of approximately 75 m at the crystal. The LNP crystal was placed within a semiconfocal laser cavity, in which the sample was attached to a plane mirror M1 (99.8% reflective at 1064 nm and >95% transmissive at 808 nm) and a concave mirror M2 (99% reflective at 1064 nm, radius of curvature: 10 mm) was placed 5 mm away from the plane mirror. The two mirrors and the LNP crystal were assembled into one body. By adjusting the azimuthal symmetry, we achieved a variety of higher-order IG mode oscillations, IGp,m, where the central axis of the resonator was tilted with respect to the pump-beam axis, as shown in Fig. 1(a), in which the tilt angle was changed in the range of 0 < < 30 [mrad]. Such a tilt of the integrated laser resonator is considered to introduce an effect equivalent to off-axis pumping with a lateral shift of 0 < d < 150 m. Examples of far-field lasing patterns observed for different azimuthal symmetries at a constant pump power of 293 mW are shown in Fig. 1(b).

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Fig. 1. (a) Experimental setup for selective excitations of IG modes. (b) Examples of IG modes in a LiNdP4O12 laser. Pump power P = 293 mW. The second experiment was performed by replacing the LNP crystal by a 1-mm-thick a-cut Nd:GdVO4 crystal with 3 at.% Nd in the same resonator. Various IG modes were observed by changing the tilt of the laser cavity. In all cases, single-frequency, linearly -polarized emissions along the tetragonal c-axis were observed. Example patterns are shown in Fig. 2(a). A successive structural change to IG modes was also observed by changing the cavity position in the pump direction (z-axis), as shown in Fig. 2(b), where the pump beam diameter inside the Nd:GdVO4 crystal was changed. In the case of Fig. 2(b), higher-order IG modes were formed with decreasing pump beam diameter, i.e., tight pump focus.

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Fig. 2. Example IG modes observed in a Nd:GdVO4 laser for (a) different tilts and (b) different crystal positions in the pump direction (z-axis). The pump spot size decreased with z. We numerically demonstrate how a single IG mode pattern was chosen by using azimuthal tight focus pumping. The changes in lasing pattern with pump power and tight pump focus position were checked by numerical simulation of IG mode formation in a microchip resonator subjected to astigmatic pumping. The simulation code we used was created based on Endos simulation method, which could simulate a single-wavelength, single/multi-mode oscillation in an unstable/stable laser cavity. We summarize the method as follows. The method simulates the initial stimulated field with a partially coherent field in the space-frequency domain to avoid the dependence between the initial field selection and the conversion field in a stable laser cavity. The stimulated initial field propagating back and forth in the resonator is mimicked by Fresnel-Kirchhoff integration. Optical fields that are changed by laser mirrors and the gain medium are introduced by modifying the optical field at each position.

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Fig. 3 (a).Cavity configuration for azimuthal pumping and gain region used for simulations. (b) Simulated formation of IG mode lasing pattern starting from a random pattern. The laser cavity configuration assumed for simulations is illustrated in Fig. 3 (a), where the gain region was assumed to be localized near the pumped crystal surface corresponding to short absorption lengths for the LD pump beam in both laser crystals used in the experiment. When the gain region was located at the central axis of the cavity, the HG mode was realized as a stationary lasing pattern after iterations. When the gain region corresponding to the pumped focus was shifted laterally by d = 60 m, as depicted in the inset of Fig. 3 (b), a single IGe2,2 mode oscillation was numerically found to be always established as a stationary lasing pattern, starting from random initial patterns with different internal power variations.

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Fig. 4. (a) Numerical pattern changes leading to a stationary IG33 mode for an increased shift of the gain region from the lasing axis. (b) IG22 mode formation for an increased pump power. In addition to the selection of the relative gain position in the laser cavity after adjustment of the azimuthal symmetry of the microchip cavity, a change in the pumping power is another important factor in selecting the lasing IG mode. With increasing pumping power, both the effective gain region and the small signal gain increase. Assuming an increased gain region with a radius/thickness of 70/150 m and a doubled small signal gain, we can see a structural change in the lasing pattern from the IGe3,3 mode back to the IGe2,2, mode, which is most effectively amplified by the increased gain region, as shown in Fig. 4. This implies that the lasing mode is sensitive not only to the relative gain region with respect to the lasing axis, but also to the pump power, as demonstrated in the experiment. In summary, we report demonstrating a variety of single-frequency IG mode operations in several microchip solid-state lasers with a short cavity configuration. Forced IG mode operations were achieved easily by tilting the central axis of the resonator with respect to the axis of the laser diode (LD) pump beam. The changes in lasing pattern with pump power and tight pump focus position were demonstrated experimentally and reproduced by numerical simulation of IG mode formation in a microchip shortcavity resonator subjected to astigmatic pumping. The discovering of IGM formation in a real laser system is important, and will be beneficial to further study on the properties and potential applications of Ince-Gaussian modes.

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Analytical analysis of modulated signal in apertureless scanning near-field optical microscopyC. H. Chuang and Y. L. Lo*Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan*Email:

[email protected]

Optics Express, Vol. 15, No. 24, pp.15782-15796 (2007)

1

. Introduction

For many years, the resolution of optical microscopy was limited to the order of approximately 1/2 as a result of the far-field diffraction effect. Most popular equipment to break down the diffraction limit is scanning near-field optical microscopy (SNOM), a metallic aperture is used to confine the near-field light emanating from or entering the probe tip. However, the resolution is limited to approximately 50 nm since the tapered glass fiber tip causes a waveguide cut-off effect. Accordingly, an alternative SNOM configuration was proposed in which the optical fiber was replaced with small scatter, yielding an enhanced resolution of approximately 10 nm depending on the tip diameter. In this configuration, the incident light illuminates the small scatter and induces an enhanced electric field between the tip and the sample whose magnitude depends on the dipole effect. Measuring the near-field interaction electric field is the operating principle. This device is conventionally referred to as the apertureless scanning near-field optical microscope (ASNOM). However, in A-SNOM, the near-field electric field is seriously affected by a background interference electric field and therefore it is necessary to develop techniques for eliminating the background-scattering noise from the detected signal in order to improve the imaging resolution. This paper develops a detailed analytical model of the detected A-SNOM signal and investigates the variation in the signal contrast and intensity as a function of the phase modulation depth, the wavelength and angle of the incident light, and the amplitude of the AFM tip vibration. The analytical results are intended to clarify the factors determining the detection signal contrast such that the imaging capabilities of A-SNOM can be further improved. As comparison with the fore-experimental results, the authors adopted higher order harmonic radian frequency in order to improve signal contrast, and it consists with ones of our major findings.

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Fig.1 Model of A-SNOM near-field region. 2. Analytical model of A-SNOM Fig. 1 presents a schematic illustration of the A-SNOM near-field region. Note that to simplify the analytical model used in this study, an assumption is made that both the incident light and the detection is light pass through the same objective lens. As shown, the incident angle of the electric field denoted by . The most important, yet the weakest, is the interaction signal between the AFM tip and the sample. This interaction (or enhancement) effect can be described using the general model of quasielectrostatic theory. = where is the interaction electric field, eff is the effective polarizability, Ei is the amplitude of the are the frequency and initial phase of the interaction electric field, (1)

incident electric field, and and

respectively. Of these parameters, is a critically important factor in A-SNOM since it contains all the necessaries to predict the relative contrasts observable in A-SNOM. The value of eff is determined by the tip radius, the dielectric constants of the AFM tip and the sample, respectively, and the distance between them. During the imaging process, the AFM drives the probe with a vertical cosine displacement around a mean position Z0, with an amplitude A and radian frequency of 0, respectively. Therefore, the position of the probe at any time t can be written as Z(t)=Z0+Acos(0t) (2)

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An assumption is made that the AFM tip does not perturb the near-field region. As a result, the scattering electric field from the tip (See Fig. 1) can be expressed as = where ET and (3)

are the amplitude and initial phase of the scattering electric field, respectively, is the

radian frequency, and K is the wave number of the incident light (given by 2/). The third electric field in the near-field region is the scattering light from the sample surface. Since this light is not modulated by the AFM tip, it can be described simply as (4) where ES and are the amplitude and initial phase of the scattering light from the sample surface.

3. Modulation signals of A-SNOM As described above, the incident electric field

with an incident angle generates three major electric

fields in the near-field and far-field detection regions, namely the electromagnetic interaction electric field between the AFM tip and the sample, the scattering electric field from the AFM tip, and the scattering electric field from the sample. Thus, the total electric field coupled into the objective lens is equal to the sum of the three individual electric fields, i.e. (5) Applying the Fourier-Bessel series expansion and assuming that 1= +2Ksin()Z0, 2= -

+2Ksin()Z0 and 3=2Ksin()A I(t) can then be decomposed into the following major terms:

+ High Order Modulation Frequency Terms

(6)

Although Eq. (6) still appears complicated, it provides some indications as to how to deal with the three

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different electric fields,

,

, and

, within the detected signal.

4. Wavelength and incident angle effects in A-SNOM In optical systems, the longer wavelength results in the poorer resolution, and thus electron microscopes provide a far better resolution. In previous A-SNOM investigations, researchers have claimed that the resolution is independent of the wavelength of the incident light and have suggested that the key factors determining the resolution are actually the aperture and tip size in SNOM and A-SNOM, respectively. The qualitative observation that using the longer wavelength improves the signal contrast for a constant order of modulation radian frequency is found. In our study, a quantitative model is presented and given for more detail discussion in wavelength influence. To test this claim, this study examines the effect of the wavelength on the contrast and intensity of the various components in the A-SNOM lock-in detected signal. The relationships 3=2Ksin()A and K=2/ are substituted into Eq. (6) in order to get the relation between wavelength and signal contrast. Note that in accordance with the experimental results, the incident angle is specified as /4 and the amplitude A of the tip vibration is set to 60 nm. Therefore, Fig. 2 plots the results obtained for the variation of the signal contrast (S1/S2) with the wavelength of the incident light. From the results in Fig. 3, it is clear that the angle of the incident light is not as influential as the wavelength in the lock-in detection technique, i.e. since I20, I30, and I40 have very similar signal contrasts. However, the incident angle is known to have a key effect on the tip enhancement in A-SNOM Overall, combining the results presented here with those reported in the literature it can be inferred that a smaller incident angle provides both a better tip enhancement and an improved intensity contrast.

Fig.2 Variation of signal contrast with incident wavelength in A-SNOM.

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Fig. 3 Variation of signal contrast in different order of modulated frequency with incident angle. 5. Conclusions This study has presented a comprehensive modulation analysis of the detected signal in A-SNOM. A mathematical model has been constructed to describe the interference among the electromagnetic interaction field between the AFM tip and the sample, the scattering electric field from the AFM tip, and the scattering electric field from the sample surface. In conclusion, the analytical formulae and results presented in this study provide new insights into the complex phenomena of A-SNOM and indicate potential techniques for improving the signal resolution of A-SNOM measurement systems.

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Growth of SWNTs Film and Pattern as FET Device with IC Compatible ProcessS H Shiau1, C W Liu2, C Gau1* and B. T. Dai2of Aeronautics and Astronautics, and Center for Micro/Nano Science and Technology, National Cheng Kung University2National *Email: 1Institute

Nano Device Laboratories, Hsin-shi, Tainan

[email protected]

Nanotechnology, Vol. 19, 105303 (7pp), Feb 2008

S

ince the discovery of single walled carbon nanotubes (SWNTs),

synthesis methods, property measurements and applications of SWNTs has been widely studied. The carbon nanotubes (CNTs) grown can be used as either bundle or single tube in a micro or nanodevice. The individual SWNT could be manipulated as a channel of electron passed in the field effect transistors (FETs) devices due to excellent electrical and mechanical properties [1]. Snow et al. [2] have proposed as-grown carbon nanotubes networks (CNN) with a very low density of 1m-2 between source-drain electrodes and use them as FET. The devices were shown to be a p-type FET with very high field-effect mobility. However, the device has the disadvantages with very large channel dimension and very poor on-to-off ratio. This has severely limited practical application of this FET device. The growth of single semiconducting SWNTs in p-type was also shown in other reports [1]. Here we grow a SWNT network with a very high density on a silicon substrate, which can be treated essentially as a thin film, and show that the quality of this SWNTs film is very uniform and has high purity. The source gas used for growing this SWNTs film is alcohol. This thin film can be readily patterned to fabricate a SWNTs film transistor with IC compatible processes, and the FET device fabricated is shown to have very good electronic properties. To grow SWNTs one needs to make very small nanoparticles as catalysts on a substrate. The is done by reduction of chemical compounds in a solvent, such as, Co acetate ((CH3CO2)2Co-4H2O) and Mo acetate ((CH3COOH)2Mo) dissolved in ethanol. The key point to be able grow CNT film over the entire surface is the capability to spread the nanosize metal catalysts uniformly over the substrate. Therefore, the chemical compounds dissolved in the solvent have to be coated uniformly on the surface of the substrate. Since the ethanol solvent is highly hydrophilic, the surface of the substrate has to be hydrophilic in order that during calcinations and drying process of the solution, the droplets containing nanoparticles can be readily spread over the entire surface of the substrate. Therefore, substrates with hydrophilic surface or substrate treated to be hydrophilic can be selected for making nanoparticles and growth of SWNTs. To be compatible with IC fabrication process, the Si substrate grown with a thin layer of oxide is selected. Since the surface is still very hydrophobic, the surface of the substrate was treated to be hydrophilic by using the standard cleaning 1 (SC-1) solution (NH4OH:H2O2:H2O=1:1:5) for 5min at 75C and the standard cleaning 2 (SC-2) solution (HCL: H2O2:H2O=1:1:6) for 5min at 75C. Since after hydrophilic treatment, it is expected that the nanoparticles can be spread uniformly on the surface.

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Next, the Co acetate (CH3CO2)2Co-4H2O and Mo acetate (CH3COOH)2Mo (Co:Mo=0.01:0.01wt%) were premixed and dissolved in ethanol with sonication for 1 hour and then Co-Mo catalyst was dip-coated on a 75 thick dry oxide layer and p-type (100) Si substrate respectively. After drying at the room temperature, the samples were put into CVD quartz chamber, as shown in figure 1, and were under calcination at 400C for 15 min. When calcinations of Co-Mo nano-particles were finished, the following process is reduction of the catalysts. The chamber temperature was risen from 400C to 750C for 15 min with a mixed Ar/NH3 gases flow of 200 sccm/30 sccm and the tube pressure is kept at 20 torr. It is necessary to flow appropriate amount of ammonia gas to cause reduction of the oxidative Co-Mo catalysts. When the temperature in the CVD chamber reaches 750C, both Ar and NH3 gas flows were closed and the heated alcohol vapor was supplied into the quartz tube, with a process pressure of 10 torr, to grow SWNTs for 15 min. All the SWNT samples were analyzed by Micro Raman Spectrometry (Jobin Yvon/Labram HR) with a light source of Laser at wavelength of 633 nm and energy of 1.96eV. Raman shifts appear, as shown in Fig. 1, at both low (from 200cm-1 to 300 cm-1) and high frequency region (close to 1550 cm-1). The low frequency mode is called the radial breathing mode (RBM), and appearance of RBM indicates that the CNTs made is single wall. The diameters of the nanotubes can be estimated at the Raman shift from the empirical equation of d = 248/ and is from 1.27 nm to 0.85 nm. The strong band in the high frequency region is called G band with a weak mode - at 1571 cm-1 and a strong mode + at 1597cm-1[3]. In addition, the relatively weak D band on the left of G band represents the defect and impurity in the current SWNTs film. The strong G band and relatively weak D band indicates that the current SWNTs film has a very high purity in graphitization.

Figure 1. Raman spectra of the current SWNTs networks. In order to test and verify usefulness of the SWNTs film and the techniques developed for selective growth of SWNTs film, which is a worse technique, the top gated SWNTs film FETs [1] were selected to2 of 6


make as shown in figure 2. Figure 2. A schematic of top-gated SWNTs film field effect transistor. The channel length (L) and width (W) of the FETs fabricated were 50m and 10m, respectively. The channel length and width designed for this FET were 50m and 10m, respectively. Firstly, the SWNTs film was grown selectively on a silicon substrate pre-grown with a 150nm-thick oxide and treated with the SC-1 and SC-2 cleaning process. The SWNTs film is grown for 15 min and is about 250 nm thick. Then, a 30nm thick HfOxNy layer as a gate oxide was deposited by thermal evaporator onto the SWNTs film. Next, the contact holes on both the source and the drain regions were defined and patterned by photolithography and reactive ion etching (RIE) process. Finally, a 150nm-thick Ti layer used as the source-drain and gate electrodes was deposited by a thermal coater and patterned by a liftoff process. Figure 3 shows the source-drain current versus gate voltage for the SWNTs thin film transistor. The IdsVds characteristic of the SWNTs channel shown in the figure is an n-type semi-conductive channel, and the device is an n-type FET. Especially, the observation is quite different from the other studies [4-6]. Martel et al. [4] have presented that the individual SWNT FET had a p-type behavior due to the oxygen absorption. In addition, Snow. et al. [2] have also found that the carbon nanotube networks transistor is also a p-type. However, in our case, the electrical conductive property of the SWNTs film FET device made indicates that the FET is an n-type semiconductor.

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Figure 3. The Ids-Vds characteristic of SWNTs film transistors.

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Figure 4. The Ids-Vgs characteristic of SWNTs film transistors with applied Vds=0.1V. With applied the Vds of 0.1 V, the Ids-Vgs characteristic of the SWNTs film transistors is measured and is shown in figure 4 where the threshold voltage is found to be 2.4 V. The transconductance of the device can be calculated by dIds/dVgs, and is 1.2S. The effective mobility of the device can then be calculated by the equation (dIds/dVgs)/(VdsW/LoxLdS) [6], where is the dielectric constant of gate-oxide (and = ro where r is the dielectric constant of HfOxNy film and is

19.8, Figure 5. Log scale of the source-drain current vs gate voltage for SWNTs film transistors with applied Vds=0.1V. o is the dielectric constant at vacuum condition and is 8.8510-14 F/cm), W is the gate width (10m) and LdS is the length between source-drain electrodes (50m), and Lox is thickness of gate oxide. The mobility of the device is found to be 102.73 cm2/Vs. This mobility is much higher and superior than the thin film transistor made with n+ doped amorphous Si as channel which has a mobility of approximately 1 cm2/Vs. The good electronic quality of the current SWNT film is due to a combination of the low resistance of inter-SWNT contacts and the high mobility of the individual SWNTs. Figure 5 shows the log scale of Ids-Vgs curve where the on-to-off ratio of the device is approximately 104. The on-to-off ratio of the current device is still relatively low as compared with other thin film transistor with on-to-off ratio of 106. It appears that there is still room to improve the on-to-off ratio by improving the fabrication process and optimizing the dimension of the device.

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In summary, this study presents synthesis of a dense SWNTs network on a silicon substrate, which can be viewed as a thin film. The quality of this thin film has very high purity and is very uniform. Fabrication and patterning of this thin film with IC compatible process is demonstrated. The fabrication process can be readily applied to fabricate a SWNTs film transistor which is shown to have a superior electronic property than other kind of thin film transistors although there is still room to improve the onto-off ratio. The good electronic quality of the current SWNT film is due to a combination of the low resistance of inter-SWNT contacts and the high mobility of the individual SWNTs. Fabrication of this thin film can be widely applied to various other semiconductor devices. References Wind S J, Appenzeller J, Martel R, Derycke V and Avouris Ph 2002 Vertical scaling of carbon nanotube field-effect transistors using top gate electrodes Applied Physics Letter 80(20) 38173819.

Snow E S, Novak J P, Campbell P M and Park D 2003 Random networks of carbon nanotubes as an electronic material Applied Physics Letter 82(13) 2145-2147. Saito R, Jorio A, Hafner J H, Lieber C M, Hunter M, McClure T, Dresselhaus G and Dresselhaus M S 2001 Chirality-dependent G-band Raman intensity of carbon nanotubes Physical Review B, 64 085312. Martel R, Derycke V, Lavoie C, Appenzeller J, Chan K K, Tersoff J and Avouris Ph 2001 Ambipolar electrical transport in semiconducting single-wall carbon nanotubes Physics Review Letters 87 256805-256809. Heinze S, Tersoff J, Martel R, Derycke V, Appenzeller J and Avouris Ph 2002 Carbon nanotubes as Schottky barrier transistors Physics Review Letters, 89 106801-106805. Novak J P, Lay M D, Perkins F K and Snow E S 2004 Macroelectronic applications of carbon nanotube networks Solid-State Electronics, 48 1753-1756.

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Soft X-Ray Photoreactions of CF3Cl Adsorbed on Si(111)-77 Studied by Continuous-Time Photon-Stimulated Desorption Spectroscopy Near F(1s) EdgeChing-Rong Wen*, C.-Y. Jang, L.-C. Chou, J. Chen, Y.-H. Wu, S.-C. Chang, W.-C. Tsai, C.-C. Liu, S.-K. Wang, and Y. ShaiDepartment of Physics, National Cheng Kung University*Email:

[email protected]

J. Chem. Phys. 127, 114704 (2007)

T

he monochromatic soft x-ray-induced reactions of molecules adsorbed on

solid surfaces has become a subject of considerable interest. It is well known that the excitations of the valence-level or core-level electrons by the soft x-ray photons can cause the dissociation of adsorbed species. Because high-intensity soft x-ray synchrotron radiation (SR) can be used to cause site-specific chemical reactions on semiconductor surfaces by core-electron excitations of adsorbates or substrates, and its short wavelength nature may allow control of the photoninduced surface reactions to be done with extremely high spatial resolution, soft xray SR is considered to be a suitable photon source. Understanding the basic mechanisms responsible for the photochemical reactions of adsorbates on a semiconductor surface has become a very important research work. The site-specific chemical bond scission of the adsorbate or condensed layer systems on the solid surfaces will result in the selective ionic dissociation or desorption. The desorbed ion yield versus incident photon energy can be measured, and a photon-stimulated desorption (PSD) spectrum is obtained. In a PSD spectrum the spectral threshold and shape can provide the information on the basic excitation initiating the dissociation and desorption processes. It is generally assumed that the photon flux density is so small that only negligible beam damage of the adsorbate is caused by PSD during the time of measuring a PSD spectrum. Therefore, the PSD spectrum is reproducible in further repeated PSD scans. However, for high-intensity soft x rays, especially produced by third-generation synchrotron radiation sources, and/or the molecules with high photolysis cross sectionsfor example, some fluorinecontaining moleculesthe decay of the adsorbate concentration by PSD itself is not negligible. As a result, a dramatic change in a series of PSD spectra, which are measured one by one via repeating the incident photon energy scan, will be observed. This series of PSD spectra could be called continuoustime PSD spectra, and we named the method to obtain these spectra continuous-time PSD spectroscopy. Since a PSD spectrum can provide information on the local bonding and electronic structure of the surface, continuous-time PSD spectroscopy can be employed to monitor the variation of the surface chemical bonding structurethe disappearance of a specific state and the formation of a new bonding structure during irradiation of incident photons. In order to gain insight into the formation of the fluorination states of the bonding surface Si atom via

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the photon-induced dissociation of adsorbed CF3Cl molecules on a Si(111)-77 surface using monochromatic soft x-ray SR near the F(1s) edge, continuous-time core-level PSD spectroscopy was employed to study the F+ desorption yields. A series of F+ PSD spectra was measured. Figure 1 shows a series of F+ PSD spectra of CF3Cl adsorbed on Si(111)-77 at 30 K for various photon exposures using 681 704 eV photons. The CF3Cl dose of the surface is 0.31015 molecules/cm2 (~0.75 monolayer). The total photon exposure for each spectrum is given in units of 1016 photons/cm2 and shown on the right of each curve. Another series of F+ PSD spectra which extends the series of Fig. 1 is shown in Fig. 2. As mentioned earlier, each F+ PSD spectrum was measured one by one via repeating the incident photon energy scan, the variation of the PSD spectrum shape is due to the damage of the adsorbed CF3Cl molecules.

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FIG. 1. Continuous-time F+ PSD spectra of CF3Cl adsorbed on a Si(111)-77 surface at 30 K as a function of photon exposure using 681704 eV photons. The CF3Cl dose of the surface is 0.31015 molecules/cm2 (~0.75 monolayer). The total photon exposure for each spectrum is given in units of 1016 photons/cm2 and shown on the right of the figure. The 690.2 and 692.6 eV features in spectrum (a) are due to excitations of F(1s) core electron of adsorbed CF3Cl molecule to 11a1[(C-Cl)*] and (8 e+12a1) [(C-F) *] antibonding orbitals, respectively. The peak at the energy position of 687.0 eV in spectrum (b) is attributed to a transition from the F(1s) core level of surface SiF species to an unoccupied state whose symmetry is perpendicular to the surface.

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FIG 2. Continuous-time F+ PSD spectra (which extends the series of Fig. 1 of CF3Cl adsorbed on a Si(111)77 surface at 30 K as a function of photon exposure using 681704 eV photons. (a) is the same as Fig. 1 (f), and (b) to (f) are F+ PSD spectra for further photon exposures. The peaks at the energy positions of 687.0 and 689.6 eV in spectrum (f) are attributed to a transition from the F(1s) core level of surface SiF species to an unoccupied state whose symmetry is perpendicular to the surface and a transition from the F(1s) core level to a final state which is localized completely on the F atom, respectively. The F+ PSD and total electron yield (TEY) spectra of molecular solid CF3Cl near the F(1s) edge were also measured. Both F+ PSD and TEY spectra show two features at the energy positions of 690.2 and 692.6 eV, and are attributed to the excitations of F(1s) to 11a1[(C-Cl)*] and (8e+12a1)[(C-F)*] antibonding orbitals, respectively. Following Auger decay, two holes are created in the F(2p) lone pair and/or C-F bonding orbitals forming the 2h1e final state which leads to the F+ desorption. This PSD mechanism, responsible for the F+ PSD of solid CF3Cl, is employed to interpret the first F+ PSD spectrum in the sequential F+ PSD spectra. The variation of spectrum shapes in the sequential F+ PSD spectra indicates the dissipation of adsorbed CF3Cl molecules and the formation of surface SiF species as a function of photon exposure. From the sequential F+ PSD spectra the photolysis cross section of the adsorbed CF3Cl molecules by photons with varying energy is determined to be ~1.010-17 cm2.

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Fault-Tolerant Topology with Variable-Range Transmission Power Control in Wireless Sensor NetworksKuo-Feng Ssu*, Chiu-Wen Chen, Chun-Hao YangDepartment of Electrical Engineering, College of Electrical Engineering and Computer Science, National Cheng Kung University*Email:

[email protected]

IEEE TMC, vol. 6, no. 10, pp. 1199, Oct. 2007. IEEE PRDC, pp. 131-138, Dec. 2007.

I

n recent years, sensor networks have been pervasive in a wide range of

applications such as location tracking, earthquake report, habitats monitoring, forest surveillance, and healthcare [1]. A wireless sensor network is composed of a sink node and a large number of tiny sensor nodes that can perform computation and wireless communication. In multi-hop wireless sensor networks, sink node would flood the querying data to all sensor nodes or collect the replying information from sensor nodes. The capacity of its battery is very limited so energy consumption becomes an important issue that directly affects the network lifetime. Topology control algorithms were proposed to maintain network connectivity with lower energy consumption. To support the multi-hop communication, a connected dominating set (CDS) is selected for establishing a virtual backbone [24]. The CDS includes a subset of sensor nodes that enable communication in the network. Sensor nodes may fail during operation. A failed node in the backbone would break the connection of the network. Recent study suggested that the backbone should maintain a certain degree of redundancy for fault tolerance [5,6]. Multiple disjoint paths are thus needed for connecting every pair of nodes. The design requires more nodes in the backbone so the power efficiency is affected. A topology control protocol with both power saving and fault tolerance, named P-CDS (Power-CDS), is developed. P-CDS classifies the backbone nodes in two types, primary coordinator and backup coordinator. For failure free execution, the coordinators are responsible for data transmission while the backup coordinators remain in sleeping mode. After failed transmission is detected, some backup coordinators will be activated. The topology can be recovered by adjusting the transmission ranges of the coordinators and backup coordinators. Compared to the previous approaches, P-CDS reduces the number of active coordinators and also saves energy. In addition, the network topology built by P-CDS can survive consecutive failures with appropriate settings.

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Figure 1: An example for P-CDS: (a) failure-free execution. (b) node 2 fails. Two techniques are integrated in our mechanism, including scheduling the state of the coordinator and adjusting the transmission range of sensor nodes. For example, the ideal topology is shown as Figure 1 (a). Based on our mechanism, node 1, 2, 3, and 4 are primary-coordinators; node b1, b2, and b3 are backup-coordinators and they switch to the sleep state after being selected. When node 2 fails, node b1 and b2 can be awakened for helping transmission (see Figure 1(b)). Node b1 uses the full transmission range and node b2 uses the half. In addition, node 1 can enter sleep mode. In this way, redundant active sensor nodes are not necessary during failure-free execution. When failure occurs, the network can be configured automatically and remain connected. In experiments, P-CDS was compared with Span protocol [2] and k-Gossip protocol [5]. The simulations were implemented with the network simulator 2 (ns-2) [7]. To generate a network, n nodes were randomly placed in a 100m 100m field to form a connected graph. The number of n in the region was varied from 100 to 500. The sensor nodes initial energy was set to 50J. The maximum communication range Rmax was set to 24m. Any two nodes with distance less than Rmax were considered neighbors. The traffic model utilized the Constant Bit Rate (CBR) and the source originated three packets per second. In 2-Gossip, pk = 0.48 was set for a high success ratio. To guarantee to construct a fault-tolerant CDS, P-CDS were evaluated with p = 1 and 2. In the experiments, the energy model followed the specifications of the TR 1000 radio transceiver from RF Monolithics [8,9]. The size of selected coordinators is shown in Figure 2. The size of P-CDS is about 57% smaller than kGossip; Span is about 12% better than P-CDS in sparse networks (n 200). In dense networks (such as 500 nodes in the region), the size of P-CDS is about 16.6% compared to k-Gossip. k-Gossip typically produces the larger size of CDS that spends more energy. Span and P-CDS have relatively small backbone sizes, which increases slightly as n increases. Though Span produces the smaller average size than P-CDS, Span could not survive from sensor failures.

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Figure 2: Size of CDS.

Figure 3: Comparison for broadcast cost.3 of 4


Figure 3 illustrates the broadcast cost of the three protocols in terms of the transmission energy. The simulation result is quite similar to the case of CDS size. k-Gossip is still significantly higher than the other approaches because of the relatively larger size of CDS. When the number of nodes is 100, P-CDS saves about 10% of energy than k-Gossip. Moreover, P-CDS saves about 28% energy when there are 500 nodes in the region. Based on the study and simulation results, changing transmission power on sensor nodes not only save energy consumption dramatically but improves fault tolerance with appropriate arrangements. The approach improves both network lifetime and reliability successfully. References

I.F. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cayirci, A Survey on Sensor Networks, IEEE Communications Magazine, vol. 40, no. 8, pp. 102114, Aug. 2002. B. Chen, K. Jamieson, H. Balakrishnan, and R. Morris, Span: An Energy-Efficient Coordination Algorithm for Topology Maintenance in Ad Hoc Wireless Networks, Proceedings of the ACM International Conference on Mobile Computing and Networking (MobiCom), pp. 8596, July 2001.

R. Ramanathan and R. Hain, Topology Control of Multi-hop Radio Networks Using Transmit Power Adjustment, Proceedings of the IEEE Conference on Computer Communications (INFOCOM), pp. 404413, Mar. 2000. T.-C. Hou and V. Li, Transmission Range Control in Multihop Packet Radio Networks, IEEE Transactions on Communications, vol. 34, no. 1, pp. 3844, Jan. 1986. X. Hou and D. Tipper, Gossip-Based Sleep Protocol (GSP) for Energy Efficient Routing in Wireless Ad Hoc Networks, Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC), pp. 2125, Mar. 2004. N. Li and J.C. Hou, FLSS: A Fault-Tolerant Topology Control Algorithm for Wireless Networks, Proceedings of the ACM International Conference on Mobile Computing and Networking (MobiCom), pp. 275286, Sept. 2004. The Network Simulator - NS-2, 2006. http://www.isi.edu/nsnam/ns/ . ASH Transceiver Designers Guide, 2007. http://www.rfm.com . L.M. Feeney, An Energy Consumption Model for Performance Analysis of Routing Protocols for Mobile Ad Hoc Networks, Mobile Networks and Applications, vol. 6, no. 3, pp. 239249, June 2001.

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