Browsing by Author "Tokel, Onur"

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Open Access
1.06μm-1.35μm coherent pulse generation by a synchronously-pumped phosphosilicate Raman fiber laser
(OSA, 2017) Elahi, Parviz; Makey, Ghaith; Turnalı, Ahmet; Tokel, Onur; İlday, Fatih Ömer
Summary form only given. Rare-earth-doped fiber lasers are attractive for microscopy and imaging applications and have developed over the past decades rapidly. They are unable to cover near-infra-red region entirely and therefore Raman and parametric process are promising for producing new wavelengths which are out of emission band of the current fiber lasers. Here, we demonstrate a synchronously-pumped Raman laser system for producing coherent signals spanning from 1.06 μm to 1.35 μm. The laser system comprises a passively-mode-locked oscillator, two stages of amplifier and a phosphosilicate Raman oscillator. The schematic of experimental setup is shown in Fig. 1(a). A mode locked oscillator operating at 37 MHz is using as a seed source. The output pulse duration and central wavelength are 6 ps and 1065 nm, respectively. 6 mW output from oscillator is launched to pre amplifier comprises 85-cm long Yb 401-PM pumped by a single mode diode through a PM wavelength division multiplexer (WDM). The power amplifier consists of a 3.5-m long Yb 1200-DC-PM with 6 μm core diameter and 125 μm cladding diameter pumped by a temperature stabilized, high power multimode diode laser via a multimode pump-signal combiner (MPC). A 30/70 coupler is employed for delivering pump signal at 1060 nm to the Raman oscillator comprises 4.2-m long ph-doped fiber. To synchronize pump and Raman and achieve coherent pulses, we adjust the length of cavity by a precise translation stage. By using proper filter inside the Raman cavity, different wavelengths are achieved.
Open Access
Advances in plasmonic technologies for point of care applications
(American Chemical Society, 2014) Tokel, Onur; İnci, Fatih; Demirci, Utkan
Infectious diseases have considerable economic and societal impact on developing settings. For instance, malaria is observed more commonly in sub-Saharan Africa and India. The societal impact of acquired immune deficiency syndrome (AIDS) and tuberculosis is high, through targeting adults in villages and leaving behind declining populations. Highly sensitive and specific lab assays such as cell culture methods, polymerase chain reaction (PCR), and enzyme-linked immunosorbent assay (ELISA) are available for diagnosis of infectious diseases in the developed world. They require sample transportation, manual preparation steps, and skilled and well-trained technicians. These clinical conventional methods provide results in several hours to days, precluding rapid detection and response at the primary care settings. Another diagnostic challenge is identifying multiple pathogens.
Open Access
Applying the principle of orthogonality of high dimensional random vectors to obtain high-density, large-volume 3D holographic display
(OSA, 2018) Makey, Ghaith; Yavuz, Özgün; Kesim, Denizhan Koray; Turnalı, Ahmet; Elahi, Parviz; Toumi, J.; El-Daher, M. S.; Ilday, Serim; Tokel, Onur; İlday, F. Ömer
The efforts toward truly 3D displays are far from exploiting the full potential of holography. Here, we apply the principle of orthogonality of high dimensional random vectors to obtain unprecedented dense, large volume holograms.
Open Access
Breaking crosstalk limits to dynamic holography using orthogonality of high-dimensional random vectors
(Nature Publishing Group, 2019) Makey, Ghaith; Yavuz, Özgün; Kesim, Denizhan K.; Turnalı, Ahmet; Elahi, Parviz; İlday, Serim; Tokel, Onur; İlday, F. Ömer
Holography is the most promising route to true-to-life three-dimensional (3D) projections, but the incorporation of complex images with full depth control remains elusive. Digitally synthesized holograms1,2,3,4,5,6,7, which do not require real objects to create a hologram, offer the possibility of dynamic projection of 3D video8,9. Despite extensive efforts aimed at 3D holographic projection10,11,12,13,14,15,16,17, however, the available methods remain limited to creating images on a few planes10,11,12, over a narrow depth of field13,14 or with low resolution15,16,17. Truly 3D holography also requires full depth control and dynamic projection capabilities, which are hampered by high crosstalk9,18. The fundamental difficulty is in storing all the information necessary to depict a complex 3D image in the 2D form of a hologram without letting projections at different depths contaminate each other. Here, we solve this problem by pre-shaping the wavefronts to locally reduce Fresnel diffraction to Fourier holography, which allows the inclusion of random phase for each depth without altering the image projection at that particular depth, but eliminates crosstalk due to the near-orthogonality of large-dimensional random vectors. We demonstrate Fresnel holograms that form on-axis with full depth control without any crosstalk, producing large-volume, high-density, dynamic 3D projections with 1,000 image planes simultaneously, improving the state of the art12,17 for the number of simultaneously created planes by two orders of magnitude. Although our proof-of-principle experiments use spatial light modulators, our solution is applicable to all types of holographic media.
Open Access
Buried waveguides written deep inside silicon
(OSA, 2017) Turnalı, Ahmet; Tokel, Onur; Kesim, Denizhan Koray; Makey, Ghaith; Elahi, Parviz; İlday, Fatih Ömer
Summary form only given. Silicon waveguides are widely used as optical interconnects and they are particularly important for Si-photonics. Si-based devices, along with other optical elements, are entirely fabricated on the top surface of Si wafers. However, further integration of photonic and electronic devices in the same chip requires a new approach. One alternative is to utilize the bulk of the wafer for fabricating photonic elements. Recently, we reported a direct-laser-writing method that exploits nonlinear interactions and can generate subsurface modifications inside silicon without damaging the surface. Using this method, we fabricated several functional optical elements including gratings, lenses, and holograms. In this work, we demonstrate optical waveguides entirely embedded in Si.
Open Access
Computer-generated holograms embedded in bulk silicon with nonlinear laser lithography
(IEEE, 2016) Turnalı, Ahmet; Tokel, Onur; Makey, Ghaith; Pavlov, Ihor; İlday, Fatih Ömer
Recently, we have showed a direct laser writing method to form subsurface structures inside silicon by exploiting nonlinear interactions. Here, we demonstrate utilization of this phenomenon to create computer-generated holograms buried in silicon.
Open Access
Controlling laser-induced self-organized patterns via engineered defects
(IEEE, 2015) Ergecen, E.; Yavuz, Özgün; Tokel, Onur; Pavlov, Ihor; Rızaoğlu, Anıl; İlday, F. Ömer
Nanoscale periodic surface structures are of paramount importance in material science [1]. The recently demonstrated Nonlinear Laser Lithography (NLL) technique allows creating nanostructure arrays over indefinitely large surfaces with remarkable periodicity, not attainable by conventional laser induced periodic surface structure (LIPSS) methods [2]. Using NLL with linearly polarized femtosecond pulses, nanolines parallel to polarization emerge from initial surface roughness, and propagate on the surface as the laser beam is scanned. Here, we demonstrate that the final surface patterns depend not only on the polarization, but also on the surface morphology, which is controllable by introducing artificial defects. We use line defects as a control parameter to select a surface tiling from a set of available ones.
Open Access
Direct laser writing of volume fresnel zone plates in silicon
(IEEE, 2015) Turnalı, Ahmet; Tokel, Onur; Pavlov, Ihor; İlday, F. Ömer
Functional optical elements fabricated on silicon (Si) constitute fundamental building blocks of Si photonics [1]. For the fabrication of these elements, conventional lithography and etching techniques are used. In spite of the success of these techniques, a functional optical element embedded inside silicon simply does not exist. Here, we present a maskless, one-step laser writing technique for creating phase-type Fresnel zone plates in the bulk of Si. Due to their effectiveness over a broad spectra, Fresnel zone plates (FZPs) are widely used in various micro-imaging applications [2]. Similar lenses have been fabricated inside silica [3,4], but are limited to the transparency window of silica. The silicon counterpart of these elements have been impossible to fabricate so far. By exploiting nonlinear absorption within the focal volume of a tightly focused laser, we generated permanent refractive index changes in Si. The imprinted high-index contrast was then used to fabricate a FZP inside Si. This three dimensional (3D) method can allow for alignment-free multilens systems. Moreover, using silicon as the lens material is fully CMOS compatible and applicable to silicon integrated optics, including single and array detectors.
Open Access
Doppler effect on nanopatterning with nonlinear laser lithography
(OSA, 2017) Yavuz, Özgün; Kara, Semih; Tokel, Onur; Pavlov, Ihor; İlday, Fatih Ömer
Summary form only given. Just five years after invention of the laser, laser induced periodic structures (LIPSS) had been reported. However, the structure period is not very uniform in LIPSS. Recently, with nonlinear laser lithography (NLL), long range ordered periodic surface structures had been maintained by exploiting various feedback mechanisms and nonlinearities. Albeit, fine tuning of structure period remains challenging. Here, we present an analogy between Doppler effect and structure period of the NLL which adds a capability of changing the structure period.
Open Access
High-efficiency multilevel volume diffraction gratings inside silicon
(American Chemical Society, 2023-11-08) Bütün, Mehmet; Saylan, Sueda; Sabet, Rana Asgari; Tokel, Onur
Silicon (Si)-based integrated photonics is considered to play a pivotal role in multiple emerging technologies, including telecommunications, quantum computing, and lab-chip systems. Diverse functionalities are either implemented on the wafer surface (“on-chip”) or recently within the wafer (“in-chip”) using laser lithography. However, the emerging depth degree of freedom has been exploited only for single-level devices in Si. Thus, monolithic and multilevel discrete functionality is missing within the bulk. Here, we report the creation of multilevel, high-efficiency diffraction gratings in Si using three-dimensional (3D) nonlinear laser lithography. To boost device performance within a given volume, we introduce the concept of effective field enhancement at half the Talbot distance, which exploits self-imaging onto discrete levels over an optical lattice. The novel approach enables multilevel gratings in Si with a record efficiency of 53%, measured at 1550 nm. Furthermore, we predict a diffraction efficiency approaching 100%, simply by increasing the number of levels. Such volumetric Si-photonic devices represent a significant advance toward 3D-integrated monolithic photonic chips.
Open Access
Highly stable periodic structures using nonlinear laser lithography
(IEEE, 2016) Yavuz, Oğuzhan; Pavlov, Ihor; Tokel, Onur; Ergeçen, E.; Rızaoğlu, Anıl; İlday, Fatih Ömer
Nonlinear laser lithography (NLL) emerged as a novel surface structuring method allowing long-range periodic order. We present mathematical formalism for NLL, analysis of structure stability to perturbations and a way to control final tiling patterns.
Open Access
Holograms deep inside Silicon
(Optical Society of America, 2016) Makey, Ghaith; Tokel, Onur; Turnalı, Ahmet; Pavlov, Ihor; Elahi, Parviz; Yavuz, Ozg ¨ un; İlday, F. Ömer
Through the Nonlinear Laser Lithography method, we demonstrate the first computer generated holograms fabricated deep inside Silicon. Fourier and Fresnel holograms are fabricated buried inside Si wafers, and a generation algorithm is developed for hologram fabrication. © OSA 2016.
Open Access
Investigating the potential of laser-written one-dimensional photonic crystals inside silicon
(TUBITAK, 2022-08-31) Tokel, Onur
The field of silicon photonics is based on introducing and exploiting advanced optical functionality. Current efforts in the field are based on conventional micro/nanofabrication methods, leading to optical functionality over wafer surfaces. A complementary and emerging field is introducing analogous optics directly within the wafer using lasers. Here we investigate the theoretical feasibility of a subclass of such optics, photonic crystals. Our efforts will guide future experimental efforts towards in-chip spectral control.
Open Access
Laser nanofabrication deep inside silicon wafers
(IEEE, 2021-09-30) Sabet, Rana Asgari; Ishraq, Aqiq; Tokel, Onur
Here, we introduce the first controlled nanofabrication capability in the bulk of silicon wafers. We exploit smart use of Bessel beams and demonstrate "in-chip" nano-structuring with features lower than 250 nm.
Open Access
Laser writing deep inside silicon for 3D information Processing
(IEEE, 2015) Tokel, Onur; Turnalı, Ahmet; Pavlov, Ihor; İlday, F. Ömer
Micromachining of silicon (Si) with lasers is being investigated since the 1970s [1]. So far, generation of controlled subsurface modification in the bulk of Si with high precision has not been achieved. This is highly desirable since successful integration of Si photonics and data transfer elements with conventional Si integrated circuits is proposed to lead to new generations of microprocessors [2,3]. Available techniques fabricate these optical and electronic elements on the top layer of the silicon-on-insulator platform. Despite the remarkable successes of conventional techniques, none of the available methods make use of the bulk of Si for positioning functional elements. Here, we report a method for photo-inducing deeply buried (down 1 mm) structures in Si wafers with pulsed infrared lasers. We demonstrate large aspect-ratio structures with 1-µm widths and long range order over millimetre scales. We further demonstrate multilevel spatial information encoding capabilities in subsurface barcodes.
Open Access
Laser-slicing of silicon with 3D nonlinear laser lithography
(OSA, 2017) Tokel, Onur; Turnalı, Ahmet; Çolakoğlu, T.; İlday, Serim; Borra, M. Z.; Pavlov, Ihor; Bek, A.; Turan, R.; İlday, Fatih Ömer
Recently, we have showed a direct laser writing method that exploits nonlinear interactions to form subsurface modifications in silicon. Here, we use the technique to demonstrate laser-slicing of silicon and its applications.
Open Access
Laser-written depressed-cladding waveguides deep inside bulk silicon
(OSA - The Optical Society, 2019-03) Turnalı, Ahmet; Han, Mertcan; Tokel, Onur
Laser-written waveguides created inside transparent materials are important components for integrated optics. Here, we demonstrate that subsurface modifications induced by nanosecond pulses can be used to fabricate tubular-shaped “in-chip” or buried waveguides inside silicon. We first demonstrate single-line modifications, which are characterized to yield a refractive index depression of ≈2×10−4≈2×10−4 compared to that of the unmodified crystal. Combining these in a circular geometry, we realized 2.9-mm-long, 30-μm core-diameter waveguides inside the wafer. The waveguides operate in a single-mode regime at a wavelength of 1300 nm. We use near- and far-field imaging to confirm waveguiding and for optical index characterization. The waveguide loss is estimated from scattering experiments as 2.2 dB/cm.
Open Access
Mathematical model of nonlinear laser lithography
(IEEE, 2015) Yavuz, Özgün; Ergecen, E.; Tokel, Onur; Pavlov, Ihor; İlday, F. Ömer
Laser induced periodic surface structures (LIPSS) had been observed just five years after the invention of laser [1]. Among the numerous LIPSS techniques none of them could maintain long-range order [2]. However, it has recently been demonstrated that long order periodic surface structures can be produced using nonlinear laser lithography (NLL) [3]. Here, we present a mathematical foundation for NLL.
Open Access
Optical waveguides written deep inside silicon by femtosecond laser
(OSA, 2017) Pavlov, Ihor; Tokel, Onur; Pavlova, S.; Kadan, V.; Makey, Ghaith; Turnalı, Ahmet; Çolakoğlu, T.; Yavuz, O.; İlday, Fatih Ömer
Summary form only given. Photonic devices that can guide, transfer or modulate light are highly desired in electronics and integrated silicon photonics. Through the nonlinear processes taking place during ultrafast laser-material interaction, laser light can impart permanent refractive index change in the bulk of materials, and thus enables the fabrication of different optical elements inside the material. However, due to strong multi-photon absorption of Si resulting delocalization of the light by free carriers induced plasma defocusing, the subsurface Si modification with femtosecond laser was not realized so far [1, 2]. Here, we demonstrate optical waveguides written deep inside silicon with a 1.5-μm high repetition rate femtosecond laser. Due to pulse-to-pulse heat accumulation for high repetition rate laser, additional thermal lensing prevents delocalization of the light around focal point, allowing the modification. The laser with 2-μJ pulse energy, 350-fs pulse width, operating at 250 kHz focused in Si produces permanent modifications. The position of the focal point inside of the sample is accurately controlled with pumpprobe imaging during processing. Optical waveguides of ~20-μm diameter, and up to 5.5-mm elongation are fabricated by translating the beam focal position along the optical axis. The waveguides are characterized with a 1.5-μm continuous-wave laser, through optical shadow-graphy (Fig. 1 a-b, e) and direct light coupling (Fig.1 c-d, f). The measured refractive index change obtained by quantitative shadow-graphy is ~6×10 -4 . The numerical aperture of the waveguide measured from decoupled light is 0.05.
Open Access
Physical model for subsurface silicon writing
(IEEE, 2016) Tokel, Onur; Turnalı, Ahmet; Pavlov, Ihor; İlday, Fatih Ömer
We have recently reported a direct laser writing method enabling buried structures deep inside silicon. Here we study the formation of these subsurface structures. We take advantage of Nonlinearity Engineering to understand this new phenomenon.