E N J O Y T H E G E N U I N E 1. DNA molecule imaged by AFM. - - PDF document

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• 1. DNA molecule imaged by AFM. Uncoiled regions are marked by arrows.
• 2. Live cell image by AFM in physiological conditions. Two points of surface differ in stiffness.
• 3. Bundle of carbon nanotubes imaged by tip-enhanced Raman microscopy.

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SLIDE 3

Nano-photonic wire waveguides

Hamza Kurt,1 * 2 and Kadir Üstün1

1Department of Electrical and Electronics Engineering, TOBB University of Economics and Technology, Ankara 06560, Turkey 2Department of Electrical and Electronics Engineering, Middle East Technical University, Ankara 06531, Turkey

Abstract-We study the light propagation in nano-photonic wire silicon waveguides with right angle corners. The low transmission efficiency of 55% can be increased up to 99% by careful manipulation of the corner area. The study yields compact size structures and it may assist the implementation of optical interconnects to distribute effectively optical clock signals through the chip.

The dielectric waveguides are commonly occupied in photonic integrated circuits because compact and low-loss waveguide structures are the ingredient component of diverse photonic applications ranging from dense wavelength division multiplexing devices to optical interconnects in the H-tree branches [1]. The high density integration requires reduction

• f the device dimensions towards micro-meter to even nano-

meter domain. To fulfill this demand, photonic wire waveguide concept has been exploited by implementing the width of the structure in the sub-micron region [2-3]. The low loss nature of straight waveguide under the absence of scattering and leakage losses is disturbed by the inherit necessity of sharp corners which is an essential part to distribute optical signal through any location on the chip. Due to the huge bending losses, small bending angles should be

• implemented. High-index dielectric contrast confines light

strongly in the core section of the waveguide. However, the similar high-loss due to sharp corners in photonic wires makes the engineering of junction area a mandatory task. In this work, by integrating photonic wire waveguide corner region with photonic crystals we inhibit the light escaping mechanism by means of photonic band gap

• confinement. There is no cavity type modification at the

corner region. As a result of this, the solution is broadband.

Figure 1. The figure demonstrates a regular photonic wire waveguide surrounded by photonic crystal at the corner region. The width of the dielectric slab is 0.20a and the relative permittivity of the material is 12.

The photonic wire structure consists of a Si-core ) 45 . 3 ( 1 n surrounded with air ) . 1 ( 2 n

• cladding. The

waveguide width was selected considering single mode

• peration. Relying on the huge advancement in the

manufacturing capability of the sub-wavelength dimensional devices with CMOS technology, we selected the width of waveguide to be approximately 100 nm. Such values will be ultimately required when higher levels of integration are targeted [3]. Fig. 1 shows the manipulations performed in the proposed structures. We inserted two additional rods and fixed their locations. Another rod of the same radius is moved along the diagonal direction. The amount of applied shift for the rod location is traced by symbol x

. The improvement on the

transmission efficiency depends on the two step modifications. First, the addition of two rods improves the bending efficiency up to 80%. On top of that, the second improvement comes from the location search of one rod which is shown as dotted line in the figure. The transmission coefficient increases as the rod is shifted toward the corner region. The efficiency increases as the shift amount increases and reaches monotonically up to 99% value within the normalized frequency range of 0.30-0.36. It can be concluded from the analysis that the parameter tuning of position of the rod and addition of two rods dramatically change the transmission efficiency.

Figure 2. The time snap shots of continuous pulse propagation for a regular photonic wire right angle bend in left and same source propagation for the optimum structure designed in the present work in right.

To verify the high transmission through the 900 In conclusion, the huge bend losses of nano-photonic wire waveguide are replaced by the wideband transmission efficiency of approximately 99% which is achieved by embedding photonic crystal structure with appropriate parameters around the corner area. The study may facilitate the design of Si-based nano-photonic waveguide branches for future optical interconnects that may eventually lead to optical clock distribution by means

• f

H-tree waveguide configuration. bend in real-time, we study a continuous wave propagation in Fig. 2. The time snap shot of the movie confirms the almost perfect transmission without any radiation losses that occurs in the absence of PC. When there is no PC around the right angle region, huge amount of light escapes by coupling into radiation mode. However, the bending loss is negligible with the implementation of the proposed design in the present work. This work was partially supported by TUBITAK under Grant No. TBAG-108T717. *Corresponding author: hkurt@etu.edu.tr

[1] L. Pavesi and G. Guillot, Optical Interconnects: The Silicon Approach, Springer, 2006. [2] C. Manolatou, et. al., J. Lightwave Technol. 17, 1682- 1692 (1999). [3] G. T. Reed, Nature 427, 595–596 (2004). [4] S. K. Selvaraja, et. al., J. Lightwave Technol. 27, 4076-4083 (2010).

Oral Presentation, Theme G : Nano-Optics, Nano-Optoelectronics, Nano-Photonics 6th Nanoscience and Nanotechnology Conference, zmir, 2010 133

SLIDE 4

Frequency Domain Solvers to Design Functional Nano Structures

Ergun Simsek1 and Dilek Bolat1

1

Department of Electrical and Electronics Engineering, Bahcesehir University, Istanbul 34349, Turkey Abstract-This work deals with efficient frequency domain solvers specifically developed to design optical and plasmonic devices. Homogeneous and inhomogeneous objects embedded in multilayered media are analyzed using Method of Moment (MoM) and hybrid MoM- Finite Element Method (FEM), respectively. The capability of working with materials of complex permittivity makes these algorithms valid and useful both for microwave and optical regimes. Based on the good agreement between numerical results obtained with these algorithms and the

• nes found in the literature, we proposed an optical antenna optimum for a semiconductor laser diode operating at a wavelength of 830 nm and

an infrared sensor compatible with present silicon technology based optical devices.

Over the last two decades, there has been enormous progress in nanoscale fabrication and characterization techniques, which motivate many researchers around the globe to develop novel optical nano devices and apparatus. Plasmonic waveguides [1, 2], power dividers, ring resonators, directional couplers [3], optical antennas [4] and filters [5] are some of those devices under intensive investigations. In the pre- fabrication stage, numerical solvers are quite helpful for the researchers to select design parameters (such as material composition, dimensions, placement, etc.) for their specific problem of interest. For this purpose, time domain solvers are commonly used. However, it is very well known that time- domain methods might not be able to provide very accurate results for high-Q structures. One way to overcome this problem is using a frequency domain method. Frequency domain solvers discretize the solution domain, build a matrix, and invert that matrix to obtain the solution. Smaller solution domain means smaller matrix, and smaller matrix means less memory to store and less CPU time to

• invert. This is why every single step decreasing the size of the

solution domain may greatly reduce CPU time and memory

• requirements. In this direction, this work deals first with

developing a surface integral equation (SIE) frequency domain solver based on Method of Moments (MoM) to calculate electromagnetic scattered field from homogeneous objects embedded in a layered medium. Then, this SIE solver is adopted as a radiation boundary, where the volume enclosed by that boundary is meshed and solved with a finite element method (FEM) frequency domain solver. For the implementation of 3D radiation boundary condition, an artificial boundary, , is applied to truncate the arbitrarily 3D shaped inhomogenous scatterer(s) from the layered medium. The FEM is applied in the interior region to calculate the field, while the method of moments is applied on the outer boundary, , to relate the field and the induced current. This algorithm stores the sparse and symmetric FEM matrix by using a row-indexed scheme to reach its the non-zero elements quickly for the sake of computational efficiency. In addition, the CPU time for the evaluation of layered medium Green's functions is reduced by a simple interpolation technique [6]. One of the very important features of the implementation is the use of wavelength dependent complex permittivity to describe metals [7], which is extremely crucial for the design and analyses of plasmonic structures and optical antennas. Let us briefly describe the theory behind MoM, FEM, and hybrid MoM-FEM solver. Method of Moments: Assume that there is an arbitrarily shaped homogeneous object with the surface S , electrical permittivity

s

, and permeability

s

. The object is located in a multilayered background. Layer- i is described by its own permittivity, permeability, and height (

i

,

s

, and

i

h ), where

L

N i , 1,2, =

• and

L

N is the number of layers. In order to calculate the scattered EM field from the object, one can solve for the electric field integral equations (EFIE) for the exterior and interior problems. The former can be written as follows

• M

G J J K E ; , 1 ; =

EM EJ i J i

G j j

• (1)

where J and M are induced electric and magnetic currents, respectively, due to incident fields; is the angular frequency;

J

K ,

EJ

G ,

EM

G are different forms of dyadic layered medium Green's functions [6]. For the numerical solution, the unknown currents, J and M , are expended in terms of the basis functions, n f and

n

b , as ), ( = ) ( ), ( = ) (

1 = 1 =

r b r M r f r J

n n b N n n n b N n

m j

• (2)

where

b

N is the number of interior edges on the surface of the

• bject,

n

j and

n

m are the unknown coefficients for electric and magnetic current densities, respectively. When we apply the Galerkin type MoM, with the same type of functions for the testing m f and

m

b , we obtain

(3) 1 = (2) (1) 1 =

] [ =

mn n s N n mn mn n b N n m

Z m Z Z j S

• (3)

where , = ds E f S

inc m s m

• (4)

, =

(1)

ds s d f K f jk Z

n J m s s i i mn

• (5)

, =

(2)

ds s d f G f k j Z

n EJ m s s i i mn

• (6)

, 2 1 . . =

(3)

ds f f ds s d f G f V P Z

n m s n EM m s s mn

• (7)

where

i

k and

i

• wavenumber and intrinsic impedance of

layer- i , respectively. EFIE for the interior problem is not provided for the sake of brevity. The surface of the object is modeled using planar triangular patches and RWG (Rao, Wilton, Glisson) basis

Oral Presentation, Theme G : Nano-Optics, Nano-Optoelectronics, Nano-Photonics 6th Nanoscience and Nanotechnology Conference, zmir, 2010 134

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Figure 1: (a) Schematic presentation of the assertion and the excitonic band splitting (b) Optical response of the core/shell complex. The embedding medium is water of dielectric constant 1.678. Core is a prolate Au spheroid of spherical radius 25 nm, aspect ratio of π, bulk plasmon frequency of 9 eV/, relaxation constant of 67 meV/, and high-frequency dielectric constant of 9.84. With these parameters the LSPR within Drude model6 is at 693 nm. Shell is uniform, of thickness 5 nm. In solution Lorentzian excitonic transition has oscillator strength of 0.06, relaxation constant of 52 meV/, resonance frequency of 693 nm and high- frequency dielectric constant of 1.678. Optical response in classical electrodynamics (symbols), coherent plasmonic response of symmetric/ antisymmetric exciton-plasmon hybrid state (red/blue) and total coherent plasmonic response (solid) obtained using quantum mechanics of two- state systems. The exciton-plasmon coupling strength is 66 meV and the unperturbed exciton2 and plasmon states are at 685 nm and 693 nm.

Functionality of Excitons in Tuning Plasmonic Response of Core/Shell Hybrid Nanostructures

Demet Gülen,1*

1Department of Physics, Middle East Technical University, Ankara 06531, Turkey.

Abstract— We address the optical response of hybrid complexes consisting of a plasmonic metal core and an excitonic molecular shell. We first provide an investigation on the effects of effective dielectric medium provided by the metal core and the embedding medium on the optical response of the excitonic molecular shell. The effects predicted are then used to correlate the optical response of core/shell complexes with the

• ptical responses of subsystems and with the coupling between their corresponding localized excitations, exciton and surface plasmon.

Core/shell complexes composed of metallic nanoparticle cores coated with molecular shells in resonance with the localized surface plasmon resonance (LSPR) provide very flexible inorganic-organic hybrids for controllable design of

• ptical response at nanoscale. Understanding of the optical

response of core/shell complexes has great potential in the development of nanophotonic devices with functionalities such as molecular imaging, biological sensing, optical signal amplification, and plasmonic resonance energy transfer.1, 2 The motivation for understanding the optical response of core/shell complexes has increased greatly with the recent

• bservations of strong coupling between exciton(s) of the

molecular shell and LSPR of the metal core.3-5 The evidence for strong exciton-plasmon coupling is typically provided by measuring optical response of the hybrid complex. Strong coupling is characterized by the formation of a pair of peaks split around the nearly degenerate exciton and plasmon resonances in the optical response.3-5 This coupling is qualitatively attributed to an interaction between the polarizations of exciton and plasmon resonances.1-5 Potential applications mentioned above would greatly benefit from the controllable tuning of the exciton-plasmon coupling strength. The current emphasis has been on tuning of LSPR resonance to tailor the exciton-plasmon coupling and fine-tuning is achieved by exploiting the wealth of information existing on the control of LSPR of metal nanoparticles.1, 2 On the other hand the involvement of the intact optical response of the excitonic partner in fine-tuning has received less attention.5 Our assertion is that, in addition to the well-known EM field enhancement inside the shell provided by the plasmonic response of the core,1, 2 the intact excitonic response should also sense the effective dielectric medium (EDM) provided by the metal core and the embedding medium. After all, exciton is basically a localized polarized excitation just like the LSPR. In this study we will provide an investigation on the effects

• f EDM provided by the metal core and the embedding

medium on the optical response of the intact molecular shell. The effects predicted will then be used to understand the

• ptical response of a specific core/shell complex.

The investigation has been carried out through simulations

• f optical response of the intact molecular shell and the

core/shell hybrid complex in classical electrodynamics,6 and by a subsequent analysis of the hybrid optical response in terms of the coherent coupling between the excitonic states of the intact molecular shell and the intact surface plasmon state in the framework of quantum mechanics.7 We consider an Au nanocore coated uniformly by a J-aggregate nanoshell embedded in aqueous medium (see figure caption for further details). Excitonic resonance is shown to experience an EDM- induced band splitting. Despite the existence of two excitonic resonances, plasmonic splitting is shown to be due to coupling between only one of the exciton resonances and LSPR. Further investigation of dependencies of excitonic band splitting and exciton-plasmon coupling strength on the shell thickness and excitonic oscillator strength yields important insight into the mechanism of exciton-plasmon coupling. In summary, we detailed the functionality of the excitonic state(s) in fine-tuning the plasmonic response of Au nanorods functionalized by a molecular shell. In addition to contributing to an increased control over tuning the plasmonic response of core/shell complexes for their potential applications, our results can stimulate interest for analytical approaches aiming at a more fundamentally sound understanding of the exciton- plasmon coupling in these important hybrid nanosystems.

*Corresponding author: dgul@metu.edu.tr [1] J. Zhao et al., IEEE J. Sel. Top. in Quantum Electronics 14, 1418 (2008). [2] A. M. Schwartzberg and J. Z. Zhang, J. Phys. Chem. C 112, 10323 (2008). [3] N. T. Fofang et al., Nano Lett. 8, 3481 (2008). [4] A. Yoshida, Y. Yonezawa and N. Kometani, Langmuir 25, 6683 (2009). [5] W. Ni et al, Nano Lett. 10, 77 (2010). [6] C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley VCH Verlag GmbH&Co. KGaA: Weinheim (2004). [7] C. Cohen-Tannoudji, B. Diu and F. Laloë, Quantum Mechanics, vol. 1, Hermann and John Wiley and Sons: France (1977).

plasmon-exciton plasmon-exciton1-exciton2

Au nanoparticle-molecular shell hybrid in water

EDM effect

(a)

650 700 750 λ (nm) 25 50 absorption (a. u.) (b) Oral Presentation, Theme G : Nano-Optics, Nano-Optoelectronics, Nano-Photonics 6th Nanoscience and Nanotechnology Conference, zmir, 2010 135

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A Theoretical Analysis of the Gain spectrum of Quantum Dot Doped Fiber Amplifier for Various Pump Wavelengths

• L. Rahimi1*, A.A. Askari1 and A.R. Bahrampour2

1Department of Physics, Shahid Bahonar University, Kerman, Iran 2Department of Physics, Sharif University of Technology, Tehran, Iran

Abstract— We study a quantum dot doped fiber amplifier (QDFA) as an inhomogeneous medium. The inhomogeneity is due to the quantum dots (QDs) size distribution. The gain spectrum of QDFAs can be changed as the pump wavelength and power

• change. Analysis of the obtained results, it indicates that for any QDFA with a specific QDs size distribution function, the gain

sensitivity degree to pumping wavelength is a function of pumping power and the concentration of QDs. Lead salt QDs, PbS and PbSe, emit in the near to mid- infrared wavelengths range important for telecommunication [1]. In addition emission and absorption cross-sections of these QDs are approximately 5 orders of magnitude larger than of Er+3 ions. These unique properties made PbS and PbSe QDs excellent candidates for using in QDFAs. Lead salt QDs synthesize with various developed and size controllable methods [2]. Nevertheless, it is difficult to synthesize a sample of QDs with a very sharp size distribution

• function. In addition, in some QD based devices such as

QDFA, it is better to use multi-size QDs [3]. In the previous work, we studied an inhomogeneous PbSe QDFA theoretically [4]. In the previous and current work s, the QDs have been modeled as a two level system. The light propagation equations and the rate equations for the inhomogeneous medium have been written in the steady state approximation. By solving the system of these coupled nonlinear differential equations, various properties of the gain spectrum of PbSe QDFAs have been studied. For the computational implementations, first of all we required to calculate the homogeneous emission and absorption cross sections of PbSe QDs. The homogeneous emission and the first peak of the homogeneous absorption spectra were calculated and used in [4]. The absorption spectrum of QDs has a very wide bandwidth with several

• peaks. Koole et al. [5] have identified a total of 11 distinct
• ptical transitions in the absorption spectra of PbSe nano-
• crystals. We used their results to calculate the whole

homogeneous absorption spectrum via deconvolution method [6]. Indeed, homogeneous absorption spectrum used in the previous work is an approximation of one used in the current

• work. The emission and the first peak of the absorption spectra

are the same, unless in a Stokes shift. Using only the first peak of the homogeneous absorption spectrum, bounds us to work with the pump wavelengths in a limited range. In current work, this limitation is removed by using the whole homogeneous absorption spectrum, and we could study the variations of the gain spectrum characteristics with the pump wavelength. Fig.1 shows the gain spectra for different pumping wavelengths in a PbSe QDFA with 1.36 m length, 2×1020 m-3 QD concentration and fifty signals ranging from 1570-1720 nm. The pump power and input signals power have been chosen 500 mw and 1µw respectively. The size distribution function of QDs was a Gaussian function at 5.3 nm diameter and a standard deviation equals to 0.3 nm.

1580 1600 1620 1640 1660 1680 1700 1720 20 25 30 35 40 45

Signal Wavelength (nm) Gain (dB)

λ λ λ λp=1066 nm λ λ λ λp=1310 nm λ λ λ λp=700 (nm) λ λ λ λp=822 (nm)

Figure 1: The signal gains versus the signal wavelengths for different pump wavelengths.

Fig.2 shows the variation of the gain peak value with the pump wavelength for two different QD concentrations.

800 900 1000 1100 1200 1300 30 32 34 36 38 40 42 44

Pump Wavelength (nm) Gain Peak (dB)

Figure 2: The gain peak variation with the pump wavelength for 2×1020 m-3 (solid curve) and 1.5×1020 m-3 (dashed curve) QD concentration.

As shown in Fig. 2 the gain value decreases by increasing the pump wavelength. This is due to the smaller absorption cross- section for shorter wavelengths [5].

• Fig. 2 shows that in lower QD concentration the gain peak

value is less sensitive to the pump wavelength, which is the result of the medium saturation. In fact, when the number of QDs is small a pump can excite all of them easily; even if it’s absorption cross section be low. We observed that in low pump powers the difference of gain peak value for different pump wavelengths is large even in low concentrations. In summary, in this paper we studied a QDFA as an inhomogeneous medium and the influence of the pumping wavelength on the gain characteristics of a QDFA for different QD concentrations investigated.

*Corresponding author: Rahimi.laleh@Gmail.com [1] H. Du, et al., Nano.Lett. 2, 1321 (2002). [2] C. B. Murray et al., IBM J. RES. & DEV. 45, 47 (2001). [3] C. Cheng, Lightwave Tech, 26 , No. 11, 1404 (2008). [4] A. Bahrampour, et al., Optics Communications, 282, 4449 (2009). [5] R. Koole, et al., Small, 4, No. 1, 127 (2008). Oral Presentation, Theme G : Nano-Optics, Nano-Optoelectronics, Nano-Photonics 6th Nanoscience and Nanotechnology Conference, zmir, 2010 136

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Femtosecond Laser-Induced TiO2 Nanostructures on Titanium

• F. Ö. Ilday1, B. Oktem2, H. Kalaycioglu1

1Department of Physics, Bilkent University, Ankara, 06800, Turkey 2Material Science and Nanotechnology Graduate Program, Bilkent University, Ankara, 06800, Turkey

Abstract— We report on processing of materials using novel high-power femtosecond fiber lasers that have been developed in

• ur laboratory. In addition to surface texturing for biomedical applications, non-thermal ablation of tissue, nanosurgery on
• rganels of an individual cell, we have achieved formation of polarization-dependent nanostructures, such as nanolines and
• nanocircles. The structures are created on titanium surfaces, converting titanium to titania (TiO2, a semiconductor) through an

entirely new mechanism. Remarkably, the nanostructures appear to be self-organized, and display crack-propagation-like behavior, leading to the important feature of self-stitching of the patterns. This allows creation of arbitrarily large-area patterns.

In recent years, femtosecond laser processing has been demonstrated to be a promising technology for surface nanostructuring

• f

metals and semiconductors [1–4]. Nanostructuring of a variety of materials is gaining widespread importance owing to ever increasing applications

• f nanostructures in numerous fields such as microelectronics

and microelectromechanical systems, patterning for field- emitting displays. Ultra-short-pulse laser radiation has been shown to be highly effective for precision material processing and surface micro-modification because of minimal thermal and mechanical damage in various materials. Here, we report the formation of regular nanoscale grating-like structures (nanolines) on Ti surfaces with polarized femtosecond laser radiation under regular

• atmosphere. These self-organized nanostructures, which

appear as protrusions on the Ti surface, are made of TiO2 (of rutile form, as determined from Raman spectroscopy). The structures appear only if the peak laser intensity exceeds a sharp threshold level.

• Fig. 1. Schematics of the experimental set-up for producing nanostructures on

Ti using a mode-locked fiber laser delivering femtosecond pulses.

A schematic of the experimental set-up is shown in Fig. 1. The laser source is a homemade, all-fiber-integrated Yb amplifier seeded by a wave-breaking free Yb fiber oscillator. The laser delivers 150-200 fs pulses with up to 1 J pulse energy, at 1 MHz repetition rate, centered at 1040 nm. The beam is focused onto a highly polished Ti surface sample with a spot size of ~11 m (half-width for 1/e2 intensity). The sample is placed on a 3D motorized stage. Fig. 2(a) shows the SEM of nanolines formed by scanning the beam at a rate of 3 m/s for an average incident power of 95 mW (95 nJ pulse energy). The scan rate corresponds to >1 million pulses being incident on any given spot, i.e., one can safely regard these structures as adiabatically formed. Any beam interference effect can be safely ruled out, since it would be washed over as the beam very slowly scans over the surface.

• Fig. 2. (a) SEM and (b) AFM images (2x2 m2 of scan area) of nanolines

formed with at 95 nJ of incident pulse energy.

The height of each nanoline is about 200 nm as indicated by the AFM view (Fig. 1(b)). The cross-sections of the nanolines appear to be parabolic, which suggests explosive growth under far-from-equilibrium conditions. The nanolines are always parallel to the polarization of the laser radiation. Interestingly, the nanoline width and periodicity is found to be independent of laser intensity and scan rate, i.e., fluence (Fig. 3(a)), with mean values of ~380 nm and ~870 nm,

• respectively. The width of the pattern, however, increases with

incident intensity and with scan rate, and minimum achievable pattern width is 6 m for 11-m spot size, as shown in Fig. 3(b). The minimum peak intensity is estimated to be ~0.1 TW/cm2.

Oral Presentation, Theme G : Nano-Optics, Nano-Optoelectronics, Nano-Photonics 6th Nanoscience and Nanotechnology Conference, zmir, 2010 137

SLIDE 8

Figure 3. IV Characterization using solar simulator (AM 1.5G) Figure 1.Transmission Electron Microscope (TEM) images of Ge films grown on Si by (a) direct heteroepitaxy (b) MHAH technique [2], [3] Figure 2. Transmission Electron Microscope (TEM) image of p-i-n structure with multi QWs grown by MHAH h i

Silicon Germanium Multi-Quantum-Wells for High Efficiency Near Infrared Photodetectors

11 and Ali K. Okyay1

1

Department of Electrical and Electronics Engineering, Bilkent University, Ankara 06800, Turkey Abstract-A recently developed heteroepitaxy technique was used to grow Ge-rich SiGe multi-quantum-well structures for infrared detection

• applications. Different barrier and well thicknesses were designed. The resulting films were characterized by TEM, XPS and photoluminescence

studies.

Infrared sensors that can operate at the near infrared (NIR) wavelengths (1.3-1.7 m) are sought after in automotive, telecommunications and defense applications. InGaAs based devices dominate these industries due their high efficiency in the NIR. The high materials cost of InGaAs however is

• prohibitive. In addition, the hybrid integration of compound

semiconductors with the mature Si based VLSI technology is difficult and expensive. Si has spectral limitations in the NIR detection due to its large indirect (1.12 eV) and direct band gap (3.4 eV) energies. Ge overcomes the spectral limitation of Si owing to its smaller optical bandgap at 0.66 eV. Furthermore, monolithic integration of Ge with the low-cost Si CMOS biasing and amplification circuitry makes Ge a promising candidate. Large lattice mismatch between Ge and Si (4.2%) hinders the growth of high quality Ge layers on Si due to large number

• f defects, misfit and threading dislocations. These defects

reduce carrier lifetime and increase generation-recombination (G-R) rates. These increased G-R rates and defects result in higher leakage currents and noise. Furthermore, the defects act as recombination sites and yield lower optical efficiency. To enable growth of high quality Ge layers on Si, a novel growth technique, “Multiple Hydrogen Annealing for Epitaxy (MHAH)” is proposed. Figure 1a) shows a TEM image of a Ge film grown by direct heteroepitaxy on Si. Defects and surface roughness can be

• bserved clearly. Fig

1b) corresponds to a TEM image of a Ge film grown on Si with MHAH technique. 50x reduction in dislocation density and a surface roughness around 2.9 nm is achieved [2]. The defects and dislocations are mainly observed in the vicinity of Si-Ge interface, while the top surface of the Ge film is low in defect density. After growth of high quality Ge layers on Si different optical detector structures are grown. Growth of high quality p-i-n devices are introduced with proposed Ge quantum wells (QWs) in the active region. Subsequently, fabrication and

• ptical and electrical characterization of proposed p-i-n optical

detector structure is performed. Figure 2 shows a TEM image of grown p-i-n structure with MHAH

• technique. p-type Ge is

grown on top of highly doped p-type Si

• substrate. Intrinsic region

consists of Ge-rich SiGe multi-QW structures. Subsequently, n-type Ge is grown as the top layer. An

• ptical

detector structure is fabricated following the growth. A grid-shaped top contact is introduced with standard photolithography process and followed by back contact metallization. Electrical and optical characterization

• f

fabricated structure is

• performed. Samples are

illuminated using solar simulator with AM 1.5G filter and

• perated as solar cell in

forward bias mode. Figure 3 shows the IV measurement under solar simulator

• illumination. An open

circuit voltage (Voc) of 240 mV and short circuit current (Isc ) of 1.7 mA is achieved. Resulting efficiency of the device is relatively low since the contact pad design is not optimized which turns out high dark and leakage current. Devices consist

• f large contact area. Also defects that were formed during the

growth are another reason for low efficiency since they act as recombination sites which cause higher leakage current. To find a solution to overcome this issue, fabrication of “MESA” p-i-n structure is going to be performed with relatively small contact and device area. Fabricated optical devices will be

• perated as photodetector, solar cell and modulator. Electrical

and optical characterizations will also be performed. *Corresponding author: aokyay@ee.bilkent.edu.tr

[1] A. K. Okyay, C. Onbasli, B. Ercan, “High Efficiency Monolithic Photodetectors for Integrated Optoelectronics in the Near Infrared” [2] Ali K. Okyay, “Si Ge Photodetection Technologies for Integrated Optoelectronics”, PhD Thesis, Stanford University, 2007. [3] Ammar M. Nayfeh, Heteroepitaxial Growth of Relaxed Germanium on Silicon, Ph.D Thesis, Stanford University, 2006.

Oral Presentation, Theme G : Nano-Optics, Nano-Optoelectronics, Nano-Photonics 6th Nanoscience and Nanotechnology Conference, zmir, 2010 138

SLIDE 9

Plas mon-integrated Photodiodes for Biomolecular Sensing in Microfluidic Systems

Burak Türker1,2,3*, Hasan Güner1, Sencer Ayas1, Okan Öner Ekiz1, Handan Acar1 and Aykutlu Dâna1

1UNAM-National Nanotechnology Research Center, Bilkent University, Ankara 06800, Turkey 2Department of Biomedical Engineering, Afyon Kocatepe University, Afyon 03200, Turkey 3

• Abstract-We demonstrate an integrated sensor that combines a grating-coupled surface plasmon resonance surface having a bi-harmonic

surface topography embedded over a planar photodiode structure and an integrated fluidic chamber upon the grating structure. At the tuned angle of incidence (AOI) of a collimated external light source where the sharp plasmon resonance condition enabled, readout through the integrated photodiode monitors the enhanced transmission of light which has been employed as a sensitive refractive index (RI) sensing

• mechanism. Small RI changes depending on different liquid concentrations that are driven into the fluidic chamber make the resonance

condition change and thus result in a change in the detected photocurrent. An equivalent RI noise of 2x10-6

Hz RIU /

is obtained using a low- power He-Ne laser, compared to a shot-noise limited theoretical sensitivity of 10-8

Hz RIU /

.

Surface plasmon resonance (SPR) sensors employed as sensitive refractive index (RI) sensing mechanisms represent

• ne of the fastest developing label-free biosensor technologies

with a vast variety of applications in biology, food safety, medical diagnostics, agricultural and environmental monitoring [1]. However, today the main concern in the development of SPR is geared toward the design of easy-to- fabricate, low-cost, compact and sensitive biosensors. SPR sensors mainly differ in the optical platforms they are based on. Prism couplers [2,3], though result in comparable higher sensitivity values [4], their bulky nature makes it unsuitable for applications in integrated devices. Waveguide couplers [5] employing fibers, though appear to present the highest degree of miniaturization of SPR sensors, require expensive higher index prisms for the resonant coupling between a surface plasmon and a waveguide mode and moreover have to suffer from instable sensitivity responses due to deformations of the fiber. Due to their compact size, planarity and stability, gratings as plasmon couplers [6] are superior to prism and waveguide couplers. Tuning optical storage disks as grating templates [7] and creating elastomeric grating molds [8,9] are the challenging techniques in terms of developing a very straightforward fabrication method. In this work we focused on eliminating one of the main integrity constraints of the design of compact SPR sensors and we replaced the traditional far field detecting (CCD/CMOS detector arrays) mechanism [10,11] used for detecting surface

• plasmons. By integrating a photo-diode substrate employed as

a near field SPR detector below the patterned transparent polymer, i.e. the grating structure, we conserved the planarity

• f the device as illustrated in the figure above.

Transmission of electromagnetic waves through thin metal films via plasmons [12] is a subject that has widely been

• studied. Plasmon enhanced transmission through sub-

wavelength holes, hole arrays or periodically corrugated metal surfaces have been investigated both theoretically and experimentally since the pioneering works of Ebbesen et al [13]. Another novelty of our approach is to employ a grating with a bi-harmonic surface topography to obtain a plasmonic enhancement of transmission through the silver-metallized grating structures. By employing a bi-harmonic grating structure we discovered the possibility of SPR detection at a fixed AOI for two different wavelengths of incident light.

Figure 1. Cross-sectional schematic view of the device structure.

We then integrated a microfluidic chamber and at the tuned AOI of a collimated external light source where the sharp plasmon resonance condition is enabled, we evaluated the readouts from the integrated photodiode which is monitoring the enhanced transmission of light and detecting the small RI changes caused by the varying concentrations of alternating

• solutions. We experimented simply with DI and 5% NaCl

solutions and observed the shifts in the sharp plasmon resonance peaks. Our real time recordings revealed the repeatability and stability of the proposed sensor mechanism by observing photocurrent readouts relevant to both the first and second order resonance peaks. Using a low-power He-Ne laser beam we finally obtained an equivalent RI noise of 2x10-

6

Hz RIU /

, compared to a shot-noise limited theoretical sensitivity of 10-8

Hz RIU /

.

*Corresponding author: tuerker_burak@gmx.net

[1] Hoa X. D., Kirk A. G., Tabrizian M., Biosensors and Bioelectronics 23, 151–160 (2007). [2] Otto, Z. Phys. 216, 398 (1968). [3] Kretschmann, Z. Phys. 241, 313 (1971). [4] Homola, J., Chemical Reviews 108 (2), 462-493, (2008). [5] Van Gent J., Lambeck P. V., Kreuwel H. J. M., Gerritsma G. J., Sudholter E. J. R., Reinhoudt D. N., Popma T. J. A., Appl. Opt., 29, 2843 (1990). [6] Dakss M.L., Kuhn L., Heidrich P.F. and Scott B.A., Appl. Phys.

• Lett. 16, 523, (1970).

[7] Kaplan B., Guner H., Senlik O., Gurel K., Bayindir M., Dana A., Plasmonics, DOI 10.1007/s11468-009-9099-x, (2009). [8] Kocabas A., Dana A., Aydinli A., Appl. Phys. Lett. 89, 041123 (2006). [9] Kocabas A., Ay F., Dana A., Aydinli A., J. Opt. A: Pure Appl.

• Opt. 8, 85–87 (2006).

[10] Piliarik M., Vala M., Tichy I., Homola J., Biosensors and Bioelectronics 24, 3430–3435 (2009). [11] Ouellet E., Lausted C.,, Lin T., Yang W. T., Lagally E. T., Lab Chip, 10, 581–588 (2010). [12] Gurel K., Kaplan B., Guner H., Bayindir M., Dana A., Appl.

• Phys. Lett. 94, 233102 (2009).

[13] Ebbesen T. W., Lezec H. J., Ghaemi H. F., Thio T., Wolff P. A., Nature (London) 391, 667 (1998).

Oral Presentation, Theme G : Nano-Optics, Nano-Optoelectronics, Nano-Photonics 6th Nanoscience and Nanotechnology Conference, zmir, 2010 139

SLIDE 10

Nanophotonic Devices Based on Nanomembranes

Ozgenc Ebil1*, Ahmed Sharkawy1, Mathew Zablocki2 and Dennis Prather2

1EM Photonics Inc., Newark, DE, USA 2

Department of Electrical and Computer Engineering, University of Delaware, Newark, DE, USA Abstract- Nanomembranes (NM) are crystalline semiconductor materials (Si, GaAs, SiGe, etc.) that have been released from their substrates and redeposited on foreign, flexible or flat substrates enabling the best features of both materials. Although they are in fact crystalline in nature and possess the electronic/photonic properties of bulk material, they are flexible, deformable, and conformable. Photonic devices originally structured in an SOI substrate can be transferred and stacked on new substrates, rigid and flexible, to fabricate photonic and electronic devices that are not possible with conventional technologies.

The four-decades-old boom in microelectronics epitomized by the now famous Moore’s law is currently facing critical challenges as the miniaturization of components and the interconnection fabric between them approaches fundamental limits at the nanometer scale. While the brick wall on the road to miniaturization and ever increasing processing speed is still years away, time is now to explore alternatives to electrical connections between electronic components on an integrated

• circuit. One such natural alternative is the use of photons

instead of electrons for communication and perhaps processing on a chip scale. In this case, it appears that a page can be taken from the playbook of long haul communication where the replacement of copper wires by optical fiber made the telecommunication boom of the Internet possible. However, in the case of long haul telecommunication, the nodes enabling efficient routing of information packages are relatively few and far in between, so the high cost of the equipment that typically requires exotic materials, processing, and unique approaches to seamlessly merge photonic and electronic technology can be absorbed in the cost of the infrastructure serving thousands or millions of customers. This is not the case for personal computing or targeted military applications where the end user may be very sensitive to the cost of a single unit as many of those units may be required for the use by a single person. As a result, in the latter case, it would be extremely advantageous to realize the photonic capabilities within the well-established and inexpensive silicon technology. Photonic processors for high-speed signal processing functions are attractive because of their very high time- bandwidth product capabilities. Such processors can remove the bottlenecks caused by limited sampling speeds in conventional electrical signal processors. Moreover, optical delay line structures allow direct processing of high frequency signals that are already in the optical domain. The unique functional advantages of photonic signal processor including its inherent speed, the ability to perform parallel signal processing, and the ability to generate true time delays, have led to a diverse range of operations. These include signal filtering with programmable capabilities, multi-Gbit/s A/D converters, frequency converters and mixers, signal correlators, and beam formers for phased arrays. Silicon nanomembranes are single crystals of Si that have been released from SOI substrates and re-deposited on foreign flexible or flat substrates enabling the best features of different

• materials. Although they are in fact single crystals and posses

the electronic properties of bulk silicon, they are flexible, deformable, and conformable. Fabrication of 3D structures is also possible by multiple transfers and stacking of nanomembranes opening a wide variety of possible device designs and applications.

Figure 1. A graphic representation of a patterned silicon nanomembrane being transferred to a substrate that is flexible with a contoured surface profile

We have transferred several Si nanomembranes with SU-8 layers used as mechanical support and optical passivation between device layers. Silicon nanomembranes from a 260 nm SOI device layer have been transferred as large area arrays of 10μm x 10μm rectangular membranes. These membranes could be patterned before or after the transfer process and serve as photonic modules that could be picked from the new host wafer at will of a consumer to build devices in a lego like

• manner. In addition to small area membranes, it is

demonstrated that large area membranes are able to be transferred to secondary substrates as one piece. The process we have developed is also flexible enough to transfer intricate photonic devices without compromising device features. We have transferred multiple photonic devices that include waveguides less than 300 nm and photonic crystals with silicon features less than 50 nm. We have transferred a single optical switch node patterned in a 260nm thick silicon nanomembrane with eight photonic crystals interconnected by waveguides 650 nm in width. With the aid of a SU-8 structural support, the device was transferred without flaws, including regions of silicon that are unattached to neighboring silicon structures. In conclusion, we discussed our solution to the limitation by the implementation of a nanomembranes technology to realize the mentioned structures on mechanically rigid substrates and

• n flexible substrates. The transfer process of nanomembranes

was shown by releasing the photonic structures from their

• riginal SOI substrates and transferring and stacking them

individually as Si nanomembranes. This research was carried in collaboration between EM Photonics and the University of Delaware and was funded by Air Force Office of Scientific Research (Dr. G. Pomrenke, Program Manager) with contract number FA955008C0019. *Corresponding author: ebil@emphotonics.com

Oral Presentation, Theme G : Nano-Optics, Nano-Optoelectronics, Nano-Photonics 6th Nanoscience and Nanotechnology Conference, zmir, 2010 140

SLIDE 11

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SLIDE 12

SLIDE 13

Nano cold coating on the fluid substrate in the precision of 100 nm

Hossein Masalehdan1,5* , Kazem Jamshidi-Ghaleh2,5 , , Amin Maghbouli3 and Erik S. Lotfi4,6

1 Department of Applied Physics, Smithsonian Institute, Washington, D.C. 20013-7012, USA 2 Department of Physics, Azarbaijan University of Tarbiat Moallem, Tabriz, Iran 3 Mechanics Department, Sahand University of Technology, Tabriz, Iran 4 Faculty of Bonab Engineering and Technology, Bonab, Iran 5 Department of Physics Engineering (Optics-Laser), Bonab IAU/YRC 6 Quantum Chemistary Laboratory, Chemistry Department, Rice University, 6100 Main street, Houston, TX 77005, USA

Processing of cermet such as WC–Co is not easy by cold spray deposition, although cold spray process by sputtering coating system can eliminate the degradation of the WC phase as compared to conventional high velocity oxygen fuel (HVOF) or plasma spraying process. In this study, WC–23%Co powders with nano-sized WC were deposited by cold spray process using Neon gas. Microstructural characterization and phase analysis of feedstock powders and as-deposited coatings were carried out by SEM and XRD. The results show, as expected, that there is no detrimental phase transformation. It is also observed that nano-sized WC in the feedstock powder is maintained in the cold sprayed coatings. It is demonstrated that it is possible to fabricate the nano-structured WC–Co coatings with low porosity and very high hardness (

• 2070 HV) by cold spray

deposition with reasonable powder preheating.

The cold gas dynamic spray, or cold spray, is a relatively new coating process in which coatings of ductile materials can be produced without significant heating of the sprayed powders[1]. The kinetic energy of the impinging particles is sufficient to produce considerable epoxy deformation and high interfacial pressures and temperatures, which appear to produce a solid state bond. The particle kinetic energy at impact is significantly lower than the energy required to melt the particle suggesting that the deposition mechanism is primarily, or perhaps entirely, a solid state process. Therefore, cold spray is regarded as solid state coating process although impact induced surface melting of the coated substrate may be possible. The most important parameter for cold spray process is its critical particle velocity prior to impact on substrate. For a given material and substrate(as epoxy seem here), there exists a specific critical particle velocity. The particle with a velocity lower than the critical velocity will lead to the erosion

• f the substrate. Only the particles reached to a velocity larger

than the critical velocity can be deposited to produce the

• coating. The critical velocity may be influenced by the size

and size distribution of the particles as well as particle and substrate material properties. The cold spray process is suggested as a new potential alternative method for spraying not only WC–Co powders but also nanostructured materials due to its low temperature during processing [2]. It is also expected that all the nanostructure is kept intact during the process. Successful deposition of pure metals, alloys, and composites by the cold spray process is reported numerously. Individual nanoparticles can not be successfully sprayed because of their low mass. Thus, the agglomeration of nanoparticles into microscopic particles can also allow the use

• f conventional powder feeders for thermal spraying-

dynamical spaering. Powder after agglomeration is relatively spherical and compact, and the size of WC is in the range of 100–200 nm .

• Fig. 1 shows the typical microstructure of the WC–23%Co

coating produced by cold spraying [3] and it contains of high magnification view of the cross section of the coating

• specimen. Coating surface shows some cracks formed due to

brittleness of the coating and some tiny pores. In general, the microstructure of the coating is very dense and exhibits no lamellar structure, which is typical to thermal spinning dynamical sprayed coatings. The crosssectional microstructure shows that the coating is very dense and uniform. Large cracks and pores are not observed. The average hardness of the coating is above 2600HV, which is approaching the intrinsic hardness of WC (2600–2850 HV).

• Fig. 1. Typical cross-sectional scanning electron micrographs of the spinning

epoxy substrate coating specimen by cold spraying(Spaering). (a) Surface side

• f the coating, (b) middle of the coating and (c) substrate side of the coating.

* Masalehdan@NASA.gov/H.Masalehdan@Gmail.com

[1] Barreiros, F.M, Vieiraand, M.T, Castanho, J.M. 2009,Fine tuning injection feedstock by nano coating SS powder, Metal Powder Report, 64:18-21. [2] Meng, F., Cao, L., Song, X. , Z.. 2009, Photocatalytic degradation of methyl orange by nano-TiO2 thin films prepared by RF magnetron sputtering, Chinese Optics Letters, 7:956-9. [3] Maghbouli, A., Masalehdan, H., 2008. Optical and mechanical fundamentals of spinning cool coating, Developments

• f Mechanics Conference, OSA Series, 327-9, USA.

Oral Presentation, Theme G : Nano-Optics, Nano-Optoelectronics, Nano-Photonics 6th Nanoscience and Nanotechnology Conference, zmir, 2010 141