Research Projects in Nano-Technology

Transcription

1 Research Projects in Nano-Technology Ibrahim Abdulhalim Our research is multidisciplinary combining nanostructures, liquid crystals, devices and methods for biosensing and biomedical optical imaging applications. Here is a short summary of the main projects. Plasmonic and photonic nanostructures for biosensing: In this activity we use surface Plasmon resonance in Kretschmann configuration both in the angular and the spectral modes, Fluorescence microscopy and Raman scattering from metallic nanosculptured thin films, and grating based structure, see figure below: SPR in Kretschmann Enhanced Transmission via nanoslits metal film incident light reflected light Field distribution x λ,θ θ in Nano dielectric Other Grating Structures Analyte analyte R λ λ, θ Metallic sculptured thin films SEF, SERS, SPR Waveguide Substrate T λ d α h We have recently demonstrated surface Plasmon resonance (SPR) sensor with enhanced sensitivity by adding a nm-thick dielectric with high dielectric constant to the metal (see Amit Lahav et.al., Optics Letter 28). Electric Field Intensity 5 Si incident reflected light light 2nd Si 4 interface metal Field distribution metal 3 film Nano dielectric analyte 2 2nd metal NGWSPR 1 interface SPR Distance from the prism-metal interface (nm)

2 ρ ρ ( a) x 1 4 (b ) θ ( c) x 1 4 (d ) θ ρ y y θ x y 2 x In the spectral mode we found recently that the figure of merit of these sensor is enhanced particularly in the infrared. This fact is being used for detecting large biological entities in water (Shalabney and Abdulhalim, Optics Letters 212). See figure below: Spectrometer Metal Dielectric R 1.8 NGWSPR Optical fiber.6 Focusing objective θ i Analyte.4 P-polarized beam Collimated beam Collimation objective White light Optical fiber source SF11 prism SF11 glass Index matching fluid.2 SPR Wavelength (nm) A special SPR imaging approach was developed using a diverging beam and the fast Radon transform for line detection. This approach improves the precision of the sensor and allows multichannel sensing (Alina Karabchevsky et.al. Sensors and Actuators B 211, J. Nanophotonics 211). Small concentration of BPA in water was detected (~1ng/L). See figure below. 1 9 (a) 8 Multiple channel sensing Reflection % Theoretical Experimental y x θ

3 Another type of nano-photonic structures being investigated in our group is the metallic sculptured thin films (STFs) which are made of nanocolumns of metal with controlled porosity and orientation. IOM-Leipzig As nano-rods like structures the surface Plasmon wave is localized near their tips and hence the electromagnetic wave density is enhanced. As a result we have demonstrated recently the enhancement of fluorescence signal from fluorophores near these structures (see Abdulhalim et.al., Appl.Phys.Lett., 29) and used it for detecting bacteria in water as shown in the figure below. Ag nanorod STF immersed in aqueous solution of luminescent E. coli (fluorescence image) 5 Ag-STF 2nm 4 Ag dense film immersed in aqueous solution of luminescent E. coli (fluorescence image) Signal (counts) Ag Ag-Ref Al Al-Ref Wavelength (nm) Ag dense film Ag nanorod STF dilution: 1/1 (antibody to PBS) Antibody: Anti-Rabbit IgG (whole molecule)- FITC Catalog number from Sigma: F ul PBS + 1ul antibody Raman signal enhancement is another promising technique we are working on for sensing because it provides specificity, that is it can tell also the type of the pollutant not only its concentration. A SERS

4 enhancement factor of 18 was estimated (Shalabney et.al., J. Nanophotonics 212). See figure below: 1 3 Counts (subtract background) Counts x After dipping for 24h in solution of 1% aminothiophenol in ethanol 1nm Ag slanted columns on Si(1) substrate Reference Raman shift (1/cm) Since the STFs are porous then when used as SPR sensors in the Kretschmann configuration they exhibit enhanced sensitivity due to the increase of the surface to volume ratio (see Atef Shalabney et.al., J. Photonics and Nanostructures 29, Sensors and Actuators B 211). Sensitivity (deg/riu) Incident light Reflected light P1= prism P1=.1 P1=.2 STF P1=.3 analyte P1=P2= Porosity of 2nd Another promising structure for sensing is the use of resonant enhanced transmission through nano-holes in metals. Presently we are optimizing one-dimensional array of metal nanoslits for Biosensing in water (see

5 Alina Karabchevsky et.al., J. Photonics and Nanostructures, 29). Our main goal in these studies is to come up with a highly sensitive and reliable sensor that can be easily integrated in water to monitor small quantities of pollutants. In parallel to the fundamental studies that we are performing, we also design and build prototype sensors that will be used in a true water purifying system. Transmission % TM Transmission % TM 8 (a) DI 6 1% Ethanol 2% Ethanol 4 3% Ethanol 4% Ethanol 2 5% Ethanol λ(nm) 4 69 Recently (Olga Krasnykov et.al., Optics Communications 21) we have increased the penetration depth inside the analyte by using infrared wavelengths in order to increase the sensitivity and be able to detect large bioentities such as cells λ(nm) Incident TM polarization λ(nm) slope=435.3nm/riu (c) n 2 nm 24 nm (a) 118 nm 5 nm 95 PMMA Ag 95 Ag 95 Ag PMMA kˆ Reflected beam Analyte z PMMA x SiO 2 Transmitted beam Sensitivity (µm/riu) Sensitivity (µm/riu) λ - first resonance (µm) Wavelength (µm) Pitch (µm)

6 This later sensor is being developed further in fiber form for real time blood analytes sensing within a catheter. Nanophotonic liquid crystal devices for biomedical imaging: In this activity liquid crystals (LCs) are combined with nanostructures in order to come up with miniature devices to control the optical properties of the light such as producing tunable filters and polarizations controllers. These devices can be controlled with a small voltage in a high speed. Recently we have combined such novel LC devices into spectropolarimetric skin imaging system being evaluated at Soroka hospital for skin cancer diagnosis. In the figure below a wide dynamic range filter is demonstrate using a novel concept (O. Aharon et.al., Optics Express 29, Optics Letters 29, J. Biomedical Optics 211). Polarization control devices were also developed and integrated into the same system. The idea is by grabbing images at many polarization states and many wavelengths the information content on the tissue structure is enhanced and the reliability of the devices increases (Avner Safrany et.al., Optics Letters 29, J. Biomedical Optics 21).

7 Lab setup Prototype in action at Soroka University Hospital In combining LC with nano-porous Si photonic crystal structure we are trying to create a narrow band tunable filter. Recently we have discovered that the molecules get ordered within the cylindrical nanopores in a special way. Applying a voltage to the composite caused the filter peak to split into two polarization dependent peaks, thus indicating that the composite is biaxial (Shahar Mor et.al., Applied Physics Letters 21). In the figure below: (a) Atomic force microscope image of the top of the P-Si 1D structure used, schematic cross section view of the ed structure where AL stands for alignment ; (c) and (d) are 2x polarized microscope images of the two samples, showing their corresponding filter color of green and red; (e)-(h) schematic drawings to illustrate some of the LC configurations inside the pores showing the UA, homeotropic, PR and ER configurations respectively. (a) (c) (e) (f) 25 nm (g) Glass substrate AL (d) LC (h) PSi 1D PC Si substrate

8 Recently we have discovered the possibility of photoalignment of liquid crystals on nano dimensional chalcogenide glass films (Miri Gelbaor, Appl.Phys.Lett. 211) and we have shown that it is a result of the photoinduced anisotropy in these materials even for such thin films as thin as 2nm (I. Abdulhalim et.al., Optical Materials Express, 211). Polarization microscope images at different voltages V 1.6V 1.9V Transmission spectra at different voltages for 5µm cell at 45 o between 2 crossed polarizers (a) x7, SEM micrograph of a typical nonirradiated chalcogenide AS 2 S 3 thin film prepared for this study. (b-d) x1 microscope images of LC device oriented at 45º between two cross polarizers: shows the transition between irradiated and non irradiated areas, (c) irradiated area alone, (d) nonirradiated area alone. (e-f) x1 micrograph images of LC device oriented at extinction position between two crossed polarizers in irradiated (e) and in nonirradiated (f) areas. LC cell response at 537nm near the Friedericksz threshold showing Vth~.99V in agreement with the theoretical value: Chalcogenide glasses act also as semiconductors and a photocurrent is generated when light is shined on them. Using this property we have demonstrated writing an image in the blue and reading it with the red in transmission (Miri Gelbaor et.al., to be published 213). Another important activity involves the incorporation of liquid crystal devices into full field optical coherence tomography system. With the assistance of a single retarder incorporated into FF-OCT system based on the Linnik microscope, Ph.D student Avner Safrani has succeeded to obtain high resolution 3D images of cell nuclei both in the axial and the lateral directions (A. Safrani and I. Abdulhalim, Appl.Optics 211, Optics Letters, 212).

9 λ 71 nm; λ = 1nm; NA = 1.5 = eff OCT image 2 µm below top surface of Onion cell nuclea 2lp/mm A new project just started involves developing OASLMs for night vision. The figure below shows the principle of such device as well as preliminary results on image conversion from the blue to the red using chalcogenide glass nano (<1nm thick) (Miri Gelbaor et.al., to be published 213). This optically addressed spatial light modulator (OASLM) has many applications in image processing, conversion, correlation, and optical computing. Modulated red beam Glass substrate with Transparent electrode and alignment LC Alignment Photosensor Substrate with top electrode Blue writing beam Red Reading beam