Browsing by Subject "Ultrafast lasers"

Now showing 1 - 15 of 15

Open Access
Development of a rapid-scan fiber-integrated terahertz spectrometer
(Springer New York LLC, 2014) Keskin, H.; Altan, H.; Yavas, S.; Ilday, F. O.; Eken, K.; Sahin, A. B.
Scientists in terahertz (THz) wave technologies have benefited from the recent developments in ultrafast laser technologies and RF technologies and applied these new gained techniques into characterizing a wide variety of phenomena. Undoubtedly, the most successful of these applications has been in the development of time-domain terahertz spectroscopic and imaging systems which has been utilized in the characterization of dielectrics and semiconductors. This pulsed technique has allowed users to characterize dynamical behavior inside materials under illumination with picosecond resolution. Typically pump/probe or similar dynamical measurements require the use of amplified pulses derived from free-space solid state lasers in the μJ-mJ range and since interferometric techniques are typically used in pulsed measurements the measurement time of a THz spectrum can last at least tens of minutes. Better systems can be realized based on fiber laser technologies. Here we discuss the advantages of a THz spectrometer driven by an ultrafast Ytterbium doped fiber laser whose repetition rate can be tuned rapidly allowing for rapid dynamical measurements. The efficient gain medium, robust operation and compact design of the system opens up the possibility of exploring rapid detection of various materials as well as studying dynamical behavior using the high brightness source.
Open Access
Discrete similariton and dissipative soliton modelocking for energy scaling ultrafast thin-disk laser oscillators
(Institute of Electrical and Electronics Engineers, 2018) İlday, Fatih Ömer; Kesim, Denizhan Koray; Hoffmann, M.; Saraceno, C. J.
Since their first demonstration, modelocked thin-disk lasers have consistently surpassed other modelocked oscillator technologies in terms of achievable pulse energy and average power by several orders of magnitude. Surprisingly, state-of-the-art results using this technology have so far only been achieved in modelocking regimes where soliton pulse shaping is dominant (i.e., soliton modelocking with semiconductor saturable absorber mirrors or Kerr lens modelocking), in which only small nonlinear phase shifts are tolerable, ultimately limiting pulse energy scaling. Inspired by the staggering success of novel modelocking regimes applied to overcome these limitations in modelocked fiber lasers, namely the similariton (self-similarly evolving pulses) and dissipative soliton regimes, here, we explore these nonlinearity-resistant regimes for the next generation of high-energy modelocked thin-disk lasers, whereby millijoule pulse energies appear to be realistic targets. In this goal, we propose two possible implementations. The first is based on a passive multipass cell and designed to support dissipative solitons in an all-normal dispersion cavity. The second incorporates an active multipass cell and is designed to support similaritons. Our numerical investigations indicate that this is a very promising path to increase the pulse energy achievable directly from modelocked oscillators toward the millijoule level while additionally simplifying their implementation by eliminating the need for operation in cumbersome vacuum chambers.
Open Access
Femtosecond laser crystallization of amorphous Ge
(American Institute of Physics, 2011) Salihoglu, O.; Kürüm, U.; Yaglıoglu, G. H.; Elmali, A.; Aydınlı, Atilla
Ultrafast crystallization of amorphous germanium (a-Ge) in ambient has been studied. Plasma enhanced chemical vapor deposition grown a-Ge was irradiated with single femtosecond laser pulses of various durations with a range of fluences from below melting to above ablation threshold. Extensive use of Raman scattering has been employed to determine post solidification features aided by scanning electron microscopy and atomic force microscopy measurements. Linewidth of the Ge optic phonon at 300 cm -1 as a function of laser fluence provides a signature for the crystallization of a-Ge. Various crystallization regimes including nanostructures in the form of nanospheres have been identified.
Open Access
Femtosecond laser written waveguides deep inside silicon
(Optical Society of America, 2017) Pavlov, I.; Tokel, O.; Pavlova, S.; Kadan, V.; Makey, G.; Turnalı, A.; Yavuz, Ö.; Ilday, F. Ö.
Photonic devices that can guide, transfer, or modulate light are highly desired in electronics and integrated silicon (Si) photonics. Here, we demonstrate for the first time, to the best of our knowledge, the creation of optical waveguides deep inside Si using femtosecond pulses at a central wavelength of 1.5 μm. To this end, we use 350 fs long, 2 μJ pulses with a repetition rate of 250 kHz from an Er-doped fiber laser, which we focused inside Si to create permanent modifications of the crystal. The position of the beam is accurately controlled with pump-probe imaging during fabrication. Waveguides that were 5.5 mm in length and 20 μm in diameter were created by scanning the focal position along the beam propagation axis. The fabricated waveguides were characterized with a continuous-wave laser operating at 1.5 μm. The refractive index change inside the waveguide was measured with optical shadowgraphy, yielding a value of 6 × 10−4, and by direct light coupling and far-field imaging, yielding a value of 3.5 × 10−4. The formation mechanism of the modification is discussed.
Open Access
Femtosecond laser-induced TiO2 nano structures on titanium
(Optical Society of America, 2010) Öktem, Bülent; Kalaycıoğlu, Hamit; İlday, F. Ömer
We report formation of polarization-dependent nanostructures (nanolines, nanocircles) by high repetition-rate femtosecond laser pulses on titanium surface through a novel mechanism, converting Ti to TiO2. Arbitrarily large-area patterns are created by self-stitching of these patterns. © 2010 Optical Society of America.
Open Access
Geometric parametric instability of femtosecond pulses in graded-index multimode fiber
(Optical Society of America, 2017) Teğin, Ugur; Ortaç, Bülend
We numerically and experimentally study the spatio-temporal femtosecond pulse evolution in graded-index multimode fiber at normal dispersion regime. We report the first demonstration of geometric parametric instability sidebands generation with ultrashort pulses in the literature.
Open Access
Long-term repetition-frequency stabilization of all-normal-dispersion Yb-doped fiber laser to the cesium standard
(OSA, 2010) Ülgüdür, Coşkun; İlday, Ömer Fatih; Hamid, R.
Repetition-frequency stabilization of a Yb-doped fiber laser to the Cesium standard is reported. Laser amplitude and phase noise is characterized. Performance is limited to 2x10-14 at 100000 averaging time by intrinsic stability of the Cs-standard.
Open Access
Multi-GHz burst-mode fiber lasers
(Institute of Electrical and Electronics Engineers, 2020) İlday, Fatih Ömer
Ultrafast lasers with GHz repetition rates is necessary to access the highly efficient ablation-cooled laser-material processing regime. Burst-mode operation allows access to such high repetition rates at microjoule-level pulse energies without needing kW-level average powers. This talk will review the development of the first femtosecond burst-mode fiber lasers to date.
Open Access
Nanosecond pulsed laser ablated sub-10 nm silicon nanoparticles for improving photovoltaic conversion efficiency of commercial solar cells
(Institute of Physics Publishing Ltd., 2017) Rasouli, H. R.; Ghobadi, A.; Ghobadi, T. G. U.; Ates, H.; Topalli, K.; Okyay, Ali Kemal
In this paper, we demonstrate the enhancement of photovoltaic (PV) solar cell efficiency using luminescent silicon nanoparticles (Si-NPs). Sub-10 nm Si-NPs are synthesized via pulsed laser ablation technique. These ultra-small Si nanoparticles exhibit photoluminescence (PL) character tics at 425 and 517 nm upon excitation by ultra-violet (UV) light. Therefore, they can act as secondary light sources that convert high energetic photons to ones at visible range. This down-shifting property can be a promising approach to enhance PV performance of the solar cell, regardless of its type. As proof-of-concept, polycrystalline commercial solar cells with an efficiency of ca 10% are coated with these luminescent Si-NPs. The nanoparticle-decorated solar cells exhibit up to 1.64% increase in the external quantum efficiency with respect to the uncoated reference cells. According to spectral photo-responsivity characterizations, the efficiency enhancement is stronger in wavelengths below 550 nm. As expected, this is attributed to down-shifting via Si-NPs, which is verified by their PL characteristics. The results presented here can serve as a beacon for future performance enhanced devices in a wide range of applications based on Si-NPs including PVs and LED applications.
Open Access
Nonlinear laser lithography for indefinitely large-area nanostructuring with femtosecond pulses
(Nature publishing group, 2013) Öktem, B.; Pavlov, I.; Ilday, S.; Kalaycıoǧlu, H.; Rybak, A.; Yavaş, S.; Erdoǧan, M.; Ilday F. Ö.
Dynamical systems based on the interplay of nonlinear feedback mechanisms are ubiquitous in nature. Well-understood examples from photonics include mode locking and a broad class of fractal optics, including self-similarity. In addition to the fundamental interest in such systems, fascinating technical functionalities that are difficult or even impossible to achieve with linear systems can emerge naturally from them if the right control tools can be applied. Here, we demonstrate a method that exploits positive nonlocal feedback to initiate, and negative local feedback to regulate, the growth of ultrafast laser-induced metal-oxide nanostructures with unprecedented uniformity, at high speed, low cost and on non-planar or flexible surfaces. The nonlocal nature of the feedback allows us to stitch the nanostructures seamlessly, enabling coverage of indefinitely large areas with subnanometre uniformity in periodicity. We demonstrate our approach through the fabrication of titanium dioxide and tungsten oxide nanostructures, but it can also be extended to a large variety of other materials.
Open Access
Spatiotemporal instability of femtosecond pulses in graded-index multimode fibers
(Institute of Electrical and Electronics Engineers Inc., 2017) Teǧin, U.; Ortaç, B.
We study the spatiotemporal instability generated by a universal unstable attractor in normal dispersion graded-index multimode fiber for femtosecond pulses for the first time. Experimentally observed spatiotemporal instability sidebands are 91-THz detuned from the pump wavelength of 800 nm. Detailed analysis carried out numerically by employing coupled-mode pulse propagation model. Numerically obtained results are well-aligned with experimental observations. Spatial evolution of the total field and spatiotemporal instability sidebands is calculated numerically, and for the input pulses of 200-fs duration, formation and evolution of spatiotemporal instability are shown in both spatial and temporal domains. Our results present the unique features of spatiotemporal instability, such as remarkable frequency shift with inherited beam shape of instability sidebands.
Open Access
Surface texturing of dental implant surfaces with an ultrafast fiber laser
(Optical Society of America, 2010) Öktem, Bülent; Kalaycıoğlu, Hamit; Erdoǧan, M.; Yavaş, S.; Mukhopadhyay P.; Tazebay, Uygar Halis; Aykaç, Y.; Eken, K.; İlday, F. Ömer
Controlled modification of implant surfaces using femtosecond, picosecond and nanosecond pulses from home-built all-fiber-integrated lasers is demonstrated. Picosecond and femtosecond pulses offer superior control over the surface texture. Increasing cell attachment to surface is discussed. ©2010 Optical Society of America.
Open Access
Synthesis of blue-shifted luminescent colloidal GaN nanocrystals through femtosecond pulsed laser ablation in organic solution
(Springer Netherlands, 2016-05) Demirel, A.; Öztaş T.; Kurşungöz, C.; Yılmaz, İ.; Ortaç, B.
We demonstrate the synthesis of GaN nanocrystals (NCs) with the sizes of less than the doubled exciton Bohr radius leading quantum confinement effects via a single-step technique. The generation of colloidal GaN nanoparticles (NPs) in organic solution through nanosecond (ns) and femtosecond (fs) pulsed laser ablation (PLA) of GaN powder was carried out. Ns PLA in ethanol and polymer matrix resulted in amorphous GaN-NPs with the size distribution of 12.4 ± 7.0 and 6.4 ± 2.3 nm, respectively, whereas fs PLA in ethanol produced colloidal GaN-NCs with spherical shape within 4.2 ± 1.9 nm particle size distribution. XRD and selected area electron diffraction analysis of the product via fs PLA revealed that GaN-NCs are in wurtzite structure. Moreover, X-ray photoelectron spectroscopy measurements also confirm the presence of GaN nanomaterials. The colloidal GaN-NCs solution exhibits strong blue shift in the absorption spectrum compared to that of the GaN-NPs via ns PLA in ethanol. Furthermore, the photoluminescence emission behavior of fs PLA-generated GaN-NCs in the 295–400 nm wavelength range is observed with a peak position located at 305 nm showing a strong blue shift with respect to the bulk GaN.
Open Access
Terahertz time-domain study of silver nanoparticles synthesized by laser ablation in organic liquid
(IEEE Microwave Theory and Techniques Society, 2016-07) Koral, C.; Ortaç, B.; Altan, H.
We report the investigation of laser-synthesized Ag nanoparticles (Ag-NPs) in an organic liquid environment by using terahertz time-domain spectroscopy (THz-TDS) technique. Colloidal Ag-NPs with an average diameter of 10 nm in two-propanol solution through nanosecond pulsed laser ablation were synthesized. THz-TDS measurements were performed on different volumetric concentration of Ag-NPs suspensions placed in 2-mm path length quartz cuvette. Due to the dispersive and highly absorptive nature of the nano liquids, an approach based on extracting the optical properties through the changes in amplitude and phase solely around the main peak of THz waveform is developed. This approach allowed for an accurate estimation of the complex refractive index of the Metallic-NPs suspension for the different prepared volumetric concentrations. In addition, using Maxwell-Garnett theory, the NP concentration is also extracted. This method shows that the time-domain nature of the THz pulse measurement technique is extremely useful in instances where slight variations in highly dispersive samples need to be investigated.
Open Access
The universality of self-organisation: a path to an atom printer?
(Springer Science and Business Media Deutschland GmbH, 2023-04-07) Ilday, S.; Ilday, F. Ömer
More than 30 years ago, Donald Eigler and Erhard Schweizer spelt the letters IBM by positioning 35 individual xenon atoms at 4 K temperature using a scanning tunnelling microscope. The arrangement took approximately 22 h. This was an outstanding demonstration of control over individual atoms. Since then, 3D printers developed into a near-ubiquitous technology. Nevertheless, with typical resolutions in the micrometres, they are far from the atomic scale of control that the IBM demonstration seemed to herald. Even the highest resolution achieved with ultrafast lasers driving two-photon polymerisation barely reaches 100 nm, three orders of magnitude distant from the atomic scale. Here, we adopt a long-term view when we ask about the possibility of a 3D atom printer, which can build an arbitrarily shaped object of macroscopic dimensions with control over its atomic structure at room temperature and within a reasonable amount of time. After discussing the state-of-the-art technology based on direct laser writing, we identify three fundamental challenges to overcome. The first is the fat fingers problem, which refers to laser wavelengths being much larger than the size of the atoms. The second one is complexity explosion, namely, the number of processing step scales with the inverse cube of the resolution, leading to prohibitively long processing times. The third challenge is the increasing strength of random fluctuations as the size of the smallest volume element to be printed approaches the atomic scale. This requires control over the fluctuations, which we call mischief of fluctuations. Although direct-writing techniques offer sufficient resolution, speed, and excellent flexibility for the mesoscopic scale, each of the three fundamental problems above appears enough to render the atomic scale unreachable. Each of these arise out of a need to control each atom individually and with precision. In contrast, the three challenges of direct writing are not fundamental limitations to self-organisation, this chapter proposes a potential path to a 3D atom printer, where laser-driven self-organisation can complement direct-writing techniques by bridging the atomic and mesoscopic scales.