Meriç Özcan

Method to calculate the far field of three-dimensional objects for computer-generated holography

Here, a new method for calculating the computer-generated holograms of three-dimensional (3D) objects is presented along with a review of current techniques. The method, the planar layers method (PLM), is established on the idea of representing 3D objects in discrete planar layers perpendicular to the observation plane, then calculating the total far field pattern by summing up the far field patterns of each layer. Simulation results, computational complexity, and error comparisons reveal that this new method can be used to calculate far field patterns--hence, the holograms--of computer-synthesized objects very efficiently.

Real-time, auto-focusing digital holographic microscope using graphics processors

The most significant advantage of holographic imaging is that one does not need to do focusing alignment for the scene or objects while capturing their images. To focus on a particular object recorded in a digital hologram, a post-processing on the recorded image must be performed. This post-processing, so called the reconstruction, is essentially the calculation of wave propagation in free space. If the objects optical distance to the recording plane is not known a priori, focusing methods are used to estimate this distance. However, these operations can be quite time consuming as the hologram sizes increase. When there is a time constraint on these procedures and the image resolution is high, traditional central processing units (CPUs) can no longer satisfy the desired reconstruction speeds. Then, especially for real-time operations, additional hardware accelerators are required for reconstructing high resolution holograms. To this extend, todays commercial graphic cards offer a viable solution, as the holograms can be reconstructed tens of times faster with a graphics processing unit than with the state-of-the-art CPUs. Here we present an auto-focusing megapixel-resolution digital holographic microscope (DHM) that uses a graphics processing unit (GPU) as the calculation engine. The computational power of the GPU allows the DHM to work in real-time such that the reconstruction distance is estimated unsupervised, and the post-processing of the holograms are made completely transparent to the user. We compare DHM with GPU and CPU and present experimental results showing a maximum of 70 focused reconstructions per second (frps) with 1024 × 1024 pixel holograms.

Self-induced Crystallinity in RF Magnetron Sputtered ZnO Thin Films

ZnO films were coated on the order of micrometer thickness on various substrates using RF magnetron sputtering. Glass, mica and Si were used as amorphous and crystalline substrates to study film growth. X-ray diffraction measurements revealed a self-induced, (002) oriented texture on all substrates. Effects of residual stresses on growth behavior and possible mechanisms of textured crystallization on crystalline and amorphous substrates are discussed.

Digital holographic microscopy and focusing methods based on image sharpness

Digital holographic microscope allows imaging of opaque and transparent specimens without staining. A digitally recorded hologram must be reconstructed numerically at the actual depth of the object to obtain a focused image. We have developed a high‐resolution digital holographic microscope for imaging amplitude and phase objects with autofocusing capability. If the actual depth of an object is not known a priori, it is estimated by comparing the sharpness of several reconstructions at different distances, which is very demanding in means of computational power when the recorded hologram is large. In this paper, we present 11 different sharpness metrics for estimating the actual focus depths of objects. The speed performance of focusing is discussed, and a scaling technique is introduced where the speed of autofocusing increases on the order of square of the scale ratio. We measured the performance of scaling on computer‐generated holograms and on recorded holograms of a biological sample. We show that simulations are in good agreement with the experimental results.

Digital holography image reconstruction methods

In recent years the field of digital holography became an attractive research area following the developments of CCD-arrays and an ever increasing computational power of computers. Here we investigate digital holography reconstruction methods and compare them for the accuracy and the computational speed. In addition, possible discrepancies in the calculation of the diffraction integral via fourier transform is clarified and it is compared to convolution methods. The proper evaluation of discrete Fresnel diffraction equation is demonstrated by creating artificial holograms and numerically reconstructing them. Simulation results and experimental work is presented.

A compact, automated and long working distance optical tweezer system

We demonstrate a compact, automated, long working distance optical tweezer system using a novel mechanism for controlling the position of the optical trap. Our system uses a single focusing lens with a working distance of 4.5 mm and the trapping beam is steered by moving the lens with a miniature coil-magnet assembly. The sample is imaged through a 100× microscope objective and a CCD camera captures the magnified image. A custom image processing software detects the position of the laser beam and identifies the sample objects. This information is used to generate appropriate electrical signals to drive the coils which move the focusing lens along the desired path. The system is fairly simple and power efficient due to minimal usage of optical elements in the laser path; hence our setup is simple, low-cost and requires low optical power. Computer-generated arbitrary trapping paths and time-shared trapping patterns are successfully demonstrated. Efficient trapping of micron size spheres with laser powers as low as 1.5 mW is observed.

Three-dimensional image reconstruction of macroscopic objects from a single digital hologram using stereo disparity

We present depth extraction of macroscopic three-dimensional (3D) objects from a single digital hologram using stereo disparity. The method does not require the phase information of the hologram but two perspectives of the scene, which are easily obtained by dividing the hologram into two parts (two apertures) before the reconstruction. Variation of the hologram division is countless since each piece of a single hologram contains all the information regarding the scene; therefore, stereo disparity can be calculated along any arbitrary direction. We investigated the effects of gradual and sharp divisions of the holograms for the disparity map calculations, specifically for divisions in the vertical, horizontal, and diagonal directions. After obtaining the depth map from the stereo images, a regular two-dimensional image of the object is merged with the depth information to form 3D visualization of the object. Holograms were recorded with a rigid endoscope, and experimentally obtained depth profiles of the objects are in very good agreement with the actual profiles.

High sensitivity displacement sensing with surface plasmon resonance

A surface plasmon resonance (SPR) sensor for sensing displacement of a thin membrane is described. We assume a thin membrane is located in close proximity of a metal film in the usual SPR configuration. A displacement of the membrane changes the plasmon resonance condition and by processing the reflectance we can deduce the deflection amount. In particular, we analyze the reflectance of this system using Fresnel’s formulas for multilayer films and we discuss the angular scanning, differential phase measurement and wavelength scanning methods to obtain the amount of displacement. We propose that such a system can be used as a pressure sensor or an optical microphone. If one uses a cantilever instead of a membrane, same system might have a use in atomic force microscopy applications. The minimum resolvable displacement can be as low as 10−4Å/ √ Hz limited by the laser phase noise and the shot noise of the detection system.A surface plasmon resonance (SPR) sensor for sensing displacement of a thin membrane is described. We assume a thin membrane is located in close proximity of a metal film in the usual SPR configuration. A displacement of the membrane changes the plasmon resonance condition and by processing the reflectance we can deduce the deflection amount. In particular, we analyze the reflectance of this system using Fresnels formulas for multilayer films and we discuss the angular scanning, differential phase measurement and wavelength scanning methods to obtain the amount of displacement. We propose that such a system can be used as a pressure sensor or an optical microphone. If one uses a cantilever instead of a membrane, same system might have a use in atomic force microscopy applications. The minimum resolvable displacement can be as low as 10 -4 a/√Hz limited by the laser phase noise and the shot noise of the detection system.

Concentric toroids as a wideband and efficient substrate for surface-enhanced Raman spectroscopy applications

ABSTRACT Designing a structure having high local field enhancement, wideband resonance, and large hot spot area is the key element to obtain a large enhancement factor for surface-enhanced Raman spectroscopy applications. Here, the concentric toroid structures in dimer configuration is proposed, which shows a large local field intensity in a wide spectral range and the region that leads to a high-surface-enhanced Raman scattering signal intensity. Calculations show that the average surface-enhanced Raman scattering enhancement is up to 60 times more compared to the conventional dimer toroid structures with similar size.

Real-time and tunable substrate for surface enhanced Raman spectroscopy by synthesis of copper oxide nanoparticles via electrolysis

Here we show the capability of copper oxide (CuO) nanoparticles formed on copper (Cu) electrodes by the electrolysis as a real time active substrate for surface enhanced Raman scattering (SERS). We have experimentally found that using just the ultra pure water as the electrolyte and the Cu electrodes, ions are extracted from the copper anode form copper oxide nanoparticles on the anode surface in matter of minutes. Average particle size on the anode reaches to 100 nm in ninety seconds and grows to about 300 nm in five minutes. This anode is used in Raman experiments in real time as the nanoparticles were forming and the maximum enhancement factor (EF) of Raman signals were over five orders of magnitude. Other metal electrodes made of brass, zinc (Zn), silver (Ag) and aluminum (Al) were also tried for the anode material for a possible real-time substrate for SERS applications. Experimentally obtained enhancement factors were above five orders of magnitude for brass electrodes like the copper but for the other metals no enhancement is observed. Electron microscope images show the cubic nanoparticle formation on copper and brass electrodes but none in the other metals studied.