Theses supervised by Prof. Dr. Alphan Sennaroğlu
17 theses · Koç University
Wavelength dependent photothermal conversion efficiency of photosensitizers for photothermal therapy
Today, cancer treatment is one of the most important research fields since it is the second most lethal disease worldwide. Among the current treatment methods that have strong potential for future cancer therapies is photothermal therapy (PTT). Nanoparticles have become popular photosensitizers for photothermal therapy (PTT), as they can be targeted to specific cancer tissues and deliver a chemotherapeutic drug, providing a multimodal therapeutic approach. Photothermal conversion efficiency of nanoparticles is critical in the assessment of their therapeutic use in PTT. In this thesis, we describe an accurate calorimetric method for the determination of the photothermal conversion efficiency of nanoparticles in solution. A tightly focused continuous wave laser beam was used to irradiate a cuvette containing a solution of silver sulfide-glutathione quantum dots (Ag2S-GSH QDs), and the maximum steady-state temperature rise was measured with an infrared camera. The data were analyzed using two different photothermal conversion efficiencies, the intrinsic and external conversion efficiencies, to relate the induced heating power of the nanoparticles to the absorbed and incident optical powers, respectively. Measurements with a tunable Ti3+:sapphire laser showed that the intrinsic photothermal conversion efficiency of Ag2SGSH QDs exceeded 91% over the 720−810 nm wavelength range. The method was also used to analyze poly (acrylic acid)-coated superparamagnetic iron oxide nanoparticles (PAA/SPIONs), and the intrinsic photothermal conversion efficiency was determined to be 83.4% at 810 nm. At 640nm, PAA/SPIONs with 600 µg/ml of iron had intrinsic and extrinsic photothermal conversion efficiency of 76% and 63%, respectively. This approach is useful for the evaluation of various potential nanoparticles for photothermal therapy applications. In additional in vivo experiments, Ag2S-GSH Herceptin QDs were injected into mice grown with tumor SKBR3. Cisplatin was used as chemotherapeutic drug with the QDs. Both Cisplatin and Ag2S-GSH Herceptin QDs were injected subcutaneously into the mouse and investigated for different drug dozes of 10 mg/kg and 20 mg/kg. The mice were then irradiated with a fiber-coupled diode laser at 793 nm. 10 mg/kg of QDs gave 6.2℃ temperature rise whilst 20 mg/kg of QDs raised the surface temperature by almost 9 ℃ at 1.49 W/cm2 of laser intensity. When the laser intensity was increased to 1.59 W/cm2 and applied for 10 minutes, the temperature rise was recorded as 15℃. The experimental methods described in this thesis work should be useful in the characterization of new potentially important nanoparticles for photothermal therapy applications.
Femtosecond laser fabrication and optical characterization of low-loss diamond waveguides
In addition to its excellent mechanical and thermal properties, diamond also possesses favorable linear and nonlinear optical properties, including a wide spectral transparency window from the ultraviolet to infrared, a high refractive index, and a reasonably high nonlinear refractive index, among others. In recent years, diamond has also been demonstrated as a promising platform for quantum information and sensing applications due to the presence of spin-active nitrogen vacancy centers (NV) with long coherence times at room temperature. These NV centers generate spin-dependent fluorescence when excited at 532 nm and enable the measurement of numerous physical quantities such as strain, magnetic field, and temperature with high sensitivity. In such quantum sensing applications, optical waveguides play a crucial role, since they can be used for addressing and spatially linking NV centers or for guiding fluorescence between different locations inside the diamond crystal. In this thesis, fabrication and optical characterization of femtosecond (fs) laser written waveguides with varying geometries and parameters in a single crystal CVD grown diamond has been demonstrated. Depressed circular cladding, half-ring, and double-line waveguides were fabricated. Design parameters such as core size and number of written tracks were varied to minimize propagation losses. Characterization of the waveguides was performed at 633 nm, which is close to the peak fluorescence wavelength of the nitrogen NV centers in diamond. Important experimental results revealing the dependence of the propagation loss and refractive index contrast on the design parameters were obtained. The maximum refractive index contrast was estimated as 22.7x10-5 for the fabricated waveguides. The measured propagation loss values of 2.05 dB/cm and 1.20 dB/cm, obtained with circular depressed cladding and half-ring waveguides, respectively, are, to the best of our knowledge, the lowest propagation loss values reported so far among fs laser written diamond waveguides. It is forseen that the photonic devices based on the diamond waveguides examined in this thesis can find applications in quantum communication.
Development and characterization of advanced mid-infrared solid state lasers
This thesis investigates the experimental development of advanced solid-state lasers operatingin the 2-?m region which have potential applications in medical surgery, remote sensing, andpumping of other mid-infrared solid-state lasers. In particular, lasing characteristics andspectroscopic properties of thulium-doped crystals and glasses in the near- and mid-infraredhave been studied.The first part of the thesis focuses on two experiments aimed at improving thecontinuous-wave (cw) operation of the thulium-doped YAlO3 (Tm:YAP or Tm:YAlO) lasers.In the first experiment, the effect of doping (thulium or Tm3+) concentration on diode-endpumpedlaser operation was investigated. Three crystals with different Tm3+ concentrations of1.5, 3 and 4 % were tested and best power efficiency was obtained with the 1.5% Tm:YAPcrystal. Main conclusion of this work is that the cross relaxation effect, which enhances twomicronlaser emission and weakens the competing 1.6-micron emission, is already effectivewith 1.5 % doping level based on the fluorescence data and rate equation analysis. In thesecond experiment, low-threshold operation of a cw Tm:YAP laser was experimentallydemonstrated under Ti:sapphire pumping. In the experiments, a 4% Tm:YAP crystal was usedinside a resonator with tight focusing geometry and lasing threshold powers as low as 10 mWhave been obtained when double pumping was employed. As an application of the cwTm:YAP laser, fiber-coupled version of the diode-pumped system was further built on amobile platform and used in collaboration with a biomedical group from Bogazici Universityto evaluate its potential in tissue welding. The preliminary results investigating the interactionof laser radiation with tissues indicate that lower intensities (around 15 W/cm2) are suitablefor welding while higher intensities lead to coagulation and ablation.Next, spectroscopic studies of two thulium-doped hosts were performed to investigatetheir lasing potential in the 2-?m region. These were Tm3+:LuAG, a laser-active thuliumdoped garnet and Tm2O3:(0.85)TeO2-(0.15)WO3, a thulium-doped telluride-based glass whichwas prepared at İstanbul Technical University. For the Tm3+:LuAG crystal, it was observedthat the 1470-nm (3H4?3F4) emission vanished for 5% Tm3+ concentration due to crossrelaxation while its strength was 60% of the 1800-nm (3F4?3H6) emission for the 0.5% Tm3+concentration. We determined an emission cross section of 1.2±0.2 x 10-21 cm2 at 2023 nm,the free running wavelength for this crystal. For the Tm2O3:(0.85)TeO2-(0.15)WO3 glass, thefluorescence spectra of the 0.25 mol% and 1.0 mol% samples and the calculated emissioncross sections (3.73 ± 0.1 x 10-21 cm2 at 1460 nm and 6.57 ± 0.07 x 10-21 cm2 at 1808 nm)indicate that this glass is a potentially important candidate for mid-infrared emission systems,with the 0.25 mol% Tm2O3 doping concentration being suitable for fiber-optic amplifiersoperating around 1.5 ?m and 1.0 mol% suitable for laser systems near 2.0 ?m. However, todate, lasing trials in thulium-doped samples have not been successful possibly due toexcessive passive losses or reabsorption in these samples.To further test the lasing potential of the TeO2-WO3 glass as a laser host, weexperimented with neodymium-doped samples and successfully demonstrated lasing at 1065nm. This was the first demonstration of lasing in this novel telluride glass host. By using the0.5 mol% sample, we obtained 11 ?J of output energy with a slope efficiency of 12% by using114 ?J of pump energy. Hence, our studies have shown that TeO2-WO3 is a new, telluridebasedlaser glass host which can be used in the development of efficient bulk glass or fiberlasers in the infrared.
Genişletilmiş darbeli yükselticiler için femtosaniye Cr:ZnSe laserinin geliştirilmesi
In this thesis work, we describe the three main components of a chirped pulse amplifier (CPA). It is a technique in which nano-joule pulse energies of femtosecond lasers are amplified to micro-joule or milli-joule levels. In particular, the seed laser was experimentally constructed and the numerical design of the stretcher and compressor was completed. The seed laser is a continuous-wave, Kerr-lens mode-locked femtosecond Cr2+:ZnSe laser pumped by a 1800-nm thulium fiber laser. The astigmatically compensated, asymmetric x-cavity contained a 2.4-mm-long Cr2+:ZnSe sample with a pump absorption coefficient of 11.6 cm-1, and it was terminated with a 1% output coupler. Dispersion compensation was achieved by using a MgF2 prism pair. During Kerr-lens mode-locked operation, the laser generated 95-fs pulses at a pulse repetition rate of 94.3 MHz and with 40 mW of output power. The center wavelength of the pulses was 2.42 ?m. The pulses had a spectral width of 69 nm and a time-bandwidth product of 0.335, which is close to the transform limit for hyperbolic secant shaped pulses. In addition, by using the soliton area theorem, the nonlinear refractive index was estimated as 1.4x10-14 cm2/W for Cr2+:ZnSe. The calculated value agrees well with what has been reported in the literature and it is almost 100 times larger than the nonlinear refractive index of sapphire. In the case of the stretcher and compressor design, based on our numerical calculations, the stretcher allows us to stretch 100fs pulses to 100ps by using a diffraction grating pair with a groove density of 300 grooves/mm and separation of 76 cm.
Çok yansımalı kovuk kip kilitli femtosaniye laserlerinde darbe enerjisi optimizasyonu
We performed a general analysis of all loss mechanisms to determine the design parameters of a Multipass-Cavity (MPC) mode-locked femtosecond Ti: Sapphire laser having the maximum output pulse energy. Advantage of the MPC design is that it allows the generation of higher output pulse energies by increasing the cavity length and consequently reducing the repetition rate. The idea of the present work is to take into account the beam clipping losses at the notches and surface of the MPC mirrors as well as the well-known reflection losses. Since losses increase with number of bounces, an optimum exists where the output pulse energy is maximized. Implementing this idea in simulations in principle can give an optimum MPC configuration with much higher pulse energy. Experimental implementation of the optimum configuration confirmed the simulation results completely. In the standard MPC configuration, maximum measured pulse energy was around 14.8nJ. In the optimum configuration, we measured 32.7nJ of pulse energy. This corresponds to an increase in the pulse energy of about 101%.
Malzeme karakterizasyonu için yakın ve orta kızılaltı darbeli lazerlerin geliştirilmesi
This thesis investigates the experimental development of novel advanced pulsed lasers in the near and mid infrared and use of pulsed lasers in the optical characterization of quantum dots. In particular, the development of solitary and chirped-pulse Cr4+:Forsterite laser, Nd doped tellurite glass laser and continuous-wave injection-seeded gain-switched Cr:ZnSe laser was investigated. In the second part, pulsed Ti:sapphire lasers operated in the nanosecond and femtosecond regimes were used to measure the two-photon absorption characteristics of CdTe-CdS quantum dots.The first part of the thesis focuses on the experimental demonstration of a mode-locked, low-threshold, room-temperature, multipass-cavity femtosecond Cr4+:Forsterite laser. The repetition rate was lowered to 11.7 MHz by introducing a q-preserving multipass cavity. Pulse formation could be achieved by using the Kerr-lens mode-locking technique. Nearly transform-limited 5.1-nJ pulses with a duration of 68 fs were generated at an output power of only 60 mW. Furthermore, as short as 41-fs pulses could be obtained with a pulse energy of 3.4 nJ at an absorbed pump power of only 1.1 W.In the second part of the thesis, the Cr4+:Forsterite laser was operated in the positive dispersion regime. We obtained record 81-nJ pulses directly from the Cr4+:Forsterite oscillator at a pulse repetition rate of 4.9 MHz. In that case, pulse duration at the output was 5.5 ps. By using an external grating compressor , the pulses were then compressed to 607 fs. High-energy mode-locked oscillators operating in this wavelength range should be important in various biomedical applications including deep-tissue multi-photon microscopy as well optical coherence tomography.In the third part, lasing action in a novel, neodymium-doped tellurite-based glass with the host composition (0.8)TeO2-(0.2)WO3 was demonstrated. To our best knowledge, this is the first time laser operation from Nd:Tellurite glass with this composition was reported in the literature. During gain switched operation, the glass laser produced 11 µJ of output energy at 1065 nm with 114 µJ of pump energy. The slope efficiency with respect to the incident energy was 12%. Then, the laser was operated at 1370 nm by using the other laser transition of the Nd3+ ion . To our best knowledge, lasing action at 1370 nm was demonstrated for the first time in a bulk tellurite glass host for the energy transition 4F3/2?4I13/2 of the Nd3+ ion. The laser was operated in gain-switched regime at 1 kHz with a threshold pulse energy of 59 ? J. The slope efficiency was further determined to be 5.5 %. As high as 6 ? J-pulses with a duration of 1.74 ? s were obtained. From the threshold analysis, the emission cross section came to 1.57 x 10-20 cm2 in the presence of excited-state absorption. These results indicate that doped tellurite glasses have the potential of being used in the development of efficient bulk glass lasers and fiber lasers in the near infrared.In the fourth part of the thesis, we reported a narrow-linewidth, tunable, gain-switched Cr:ZnSe laser operating between 2255 and 2455 nm. The spectral width of the laser was reduced from 125 nm to 0.65 nm by using injection seeding. Seeding was achieved with another tunable, continuous-wave Cr:ZnSe laser. The wavelength of the laser was controlled by tuning the wavelength of the seed laser. The seeded gain-switched oscillator produced as high as 157 µJ-pulses with 598 µJ incident pump pulse energy at a repetition rate of 1 kHz.Finally, we demonstrated an application of pulsed Ti:sapphire lasers in the optical characterization of quantum dots. By using pulse lasers, we analyzed the two-photon absorption and resulting emission of core-shell CdTe-CdS q-dots. We showed that emission strength of the sample was independent of the wavelength of the pump laser in the range of 740-860 nm. Moreover, we found out the luminescence efficiency to be 60.7 % with respect to Rodamin 6G. Furthermore, the two-photon absorption cross-section was determined to be 4.1x106 GM.
Grafen ve karbon nanotüp doyabilen soğurucular ile çok yansımalı kovuklu femtosaniye kip-kilitli katı-hal lazerinin enerjisinin ölçeklendirilmesi
In mode-locking applications, single-walled carbon nanotube saturable absorbers (SWCNT-SAs) and graphene saturable absorbers (graphene-SAs) have emerged as important alternatives to semiconductor saturable absorber mirrors (SESAMs) due to their favorable optical characteristics, low cost, and relatively simple fabrication scheme. Therefore, it is of great interest to explore the limits of energy scaling in lasers mode-locked with SWCNT-SAs and graphene-SAs.In this thesis work, due to their unique wavelength range for biomedical applications, a room-temperature Cr4+: forsterite laser operating near 1.3 ?m was used in the mode-locking experiments. By incorporating a q-preserving multi-pass cavity (MPC) extension to the short x-cavity, an effective optical path length of ~ 60 m was obtained and the repetition rate of the pulses was reduced to 4.51 MHz from 144 MHz. In addition, by using double chirped (DCM) and Gires-Tournois interferometer (GTI) mirrors, nonlinear phase distortions due to self phase modulation were compensated to generate solitary femtosecond pulses. Furthermore, the power efficiency of different laser resonator architectures containing the graphene-SA or SWCNT-SA was investigated.The Cr4+: forsterite laser, mode-locked with a SWCNT-SA produced 121-fs pulses with 10-nJ pulse energy and 84-kW peak power at the wavelength of 1247 nm. The mode-locked spectrum had a width of 16 nm, corresponding to a time-bandwidth product of 0.37. Hence, the pulses were nearly transform-limited with a temporal profile of sech2.For graphene-SA mode-locking experiments, graphene samples with different number of layers were fabricated and characterized. The Cr4+: forsterite laser mode-locked with a single layer graphene-SA produced 96-fs pulses with 5.3-nJ pulse energy and 55-kW peak power at 1252 nm. In this case, the mode-locked spectrum had a width of 18.2 nm and the measured time-bandwidth product came to 0.33, also indicating that nearly transform limited solitary pulses were generated.To our knowledge, both of the results report the highest peak powers obtained to date from a femtosecond solid-state bulk laser mode locked by using SWCNT-SA and graphene-SA. Limitations to energy scaling were further explored during the experiments.
Numerical modeling of dispersive and nonlinear pulse distortions in a cr2+:znse regenerative amplifier
Cr2+:ZnSe is an attractive gain medium for ultra-short pulse generation and amplification in the mid IR spectrum at room temperature. It shows a high and broadband stimulated emission cross section coupled with a quantum yield of almost unity. A drawback is the exceptionally high nonlinear refractive index of 1.7 10-18 m2/W, which makes it susceptible to self-phase modulation effects. In amplifiers the pulse shaping effects should be calculated to make an ideal recompression possible. In the thesis work presented here, we demonstrate a numerical approach to determine both the pulse power and the dispersive broadening of a pulse, while it is propagating in a regenerative amplifier. The Frantz- Nodvik equations were extended by accounting for distortion effects from self-phase modulation and media dispersion for a Gaussian pulse. The result is a numerical code that determines the ideal extraction time and the ideal compressor dispersion for a Cr2+:ZnSe regenerative amplifier. The code has been successfully used with experimental results to estimate the stimulated emission cross section of the amplifier gain medium and the amount of chirp in the pulse. The best-fit value of the stimulated emission cross section for the Cr2+:ZnSe medium was determined to be 2.055 10-24 m2.
Kerr-odaklamalı kip-kilitli Cr:LiSAF lazerinden kazanç-anahtarlamalı çıkış aynası yardımıyla 15 femtosaniye altı darbe üretimi
Generation of ultrashort laser pulses with long?term stability and robustness is very important for many applications such as material processing, biomedical imaging, and spectroscopy. Kerr?lens mode?locking (KLM) is one of the well?known techniques used to generate ultrashort pulses. One drawback of the KLM method is that the modulation depth resulting from the Kerr nonlinearities is typically weak especially in lasers with low nonlinear refractive index. This makes it very difficult to apply the KLM technique to colquiriite type gain media such as Cr:LiCAF and Cr:LiSAF which have low nonlinear refractive indices. Recently, a novel approach has been proposed to overcome this drawback by using what can be referred to as gain?matched output couplers (GMOC). The spectral transmission characteristics of a GMOC are designed in such a way as to provide a higher effective gain for a broadband pulse as opposed to a continuous wave beam, which favors the mode?locked operation of a laser oscillator with much higher temporal stability. In this thesis work, this approach which utilizes the GMOC technology has been applied to a low?cost, directly diode pumped Cr:LiSAF laser around 850 nm. In the experiments, we used a low?cost red diode around 660 nm which could pump the Cr:LiSAF laser without any cooling requirement. An astigmatically compensated x?fold cavity was constructed to house a 6 mm long, 1.5 % Cr doped LiSAF crystal. The GMOC mirror used in the experiments had a transmission of 0.7 % near the center wavelength of the emission band. To obtain the shortest pulses, dispersion compensation was used with chirped mirrors and fused silica prism pairs. In this case, the net cavity dispersion was around ~?50fs2. When KLM operation was initiated with the GMOC mirror, this configuration resulted in the generation of nearly transform?limited pulses as short as 13 fs, with 25 mW of average output power, at a repetition rate of 126 MHz near 855 nm. Other cases with different levels of cavity dispersion and the tunability of the femtosecond laser were also investigated. The experimental results clearly demonstrated that the use of a GMOC mirror enables a very robust and stable operation of a Cr:LiSAF laser despite the fact that the nonlinear refractive index of the medium is low.
Yüksek tekrarlama frekanslı femtosaniye Cr:LiCAF lazerlerinin geliştirilmesi ve çok yansımalı Ti:Safir lazerinin THz radyasyonu üretimindeki uygulaması
There is a great deal of interest for developing low-cost, robust, compact and efficient femtosecond lasers for time and frequency domain applications. In recent years, Cr^(3+)-doped colquiriites (Cr:LiSAF, Cr:LiCAF, Cr:LiSGaF) have emerged as promising alternatives to Ti:Sapphire lasers near 800 nm because of numerous favorable laser characteristics. These include the possibility of direct diode pumping with low cost diodes, broad emission bands to generate fs pulses and high quantum efficiency near room temperature. The first part of this thesis work focuses on the design and experimental demonstration of a directly diode pumped, low cost, and highly efficient 800-nm Cr:LiCAF laser which can be operated above 100 MHz with the goal of developing high repetition rate oscillators for frequency comb applications. In the experiments, a four mirror x-cavity laser containing a 7% doped Cr:LiCAF crystal was endpumped with a recently developed high brightness, 1-W tapered diode at 675 nm. By using 915 mW of pump power, the Cr:LiCAF laser produced 340 fs pulses around 800 nm with an output power of 51 mW, at a repetition rate of 120 MHz. A semiconductor Bragg reflector (SBR) was added into the resonator to initiate the mode- locked operation of the laser. We expect that, with the higher pump powers available from the tapered diode, it should be possible to reach GHz level repetition rates with this resonator for frequency comb applications. The second part of the thesis work utilizes a home-built multi-pass cavity femtosecond Ti:Sapphire laser, generating 98 fs pulses with a repetition rate of 5.53 MHz at 776 nm, to obtain THz radiation from a photoconductive antenna. The output of the Ti:Sapphire laser with an average power of 60-70 mW power was focused on a low-temperature grown GaAs (Lt-GaAs) structured photoconductive antenna which generated THz waves with a signal strength of 5 mV detected with a second photoconductive antenna. The generated THz waves were in the frequency range of 0.1-0.7 THz. With improved antenna and detection design, we expect to further improve the bandwidth and efficiency of THz generation.
Doğrusal olmayan yöntemler kullanılarak kızılaltı bölgesinde çalışan laser kaynaklarının geliştirilmesi
This thesis investigates the development of an efficient and robust femtosecond laser which produces mid infrared pulses by employing different mode locking techniques. In particular, Kerr lens mode locking and graphene saturable absorber mode locking techniques were successfully applied to the Cr:ZnSe laser which operates around 2.45 μm. In the second part of the thesis, linear and non-linear optical properties of a gated graphene based supercapacitor were experimentally characterized, and voltage dependent optical and ultrafast properties were discussed. The first part of the thesis is the demonstration of a Kerr lens mode-locked Cr:ZnSe laser and evaluation of the performance of four different cost-effective and practical dispersion compensation methods. The highest pulse energy was obtained with a 6% output coupler and a CaF2 prism pair, where as high as 1.81 nJ was obtained with 2 W of pump power. When a 1% output coupler was used with the CaF2 prism pair, higher intracavity energy led to the generation of pulses as short as 92 fs. Based on the mode locking parameters of the laser, the nonlinear refractive index (n2) of ZnSe was estimated as (1.2±0.2)×10−18 m2/W by using the soliton area theorem. The second part of the thesis involves the demonstration of graphene saturable absorber mode-locked operation of a stable femtosecond Cr:ZnSe laser. The laser produced 176-fs pulses at a repetition rate of 78 MHz with an average power of 80 mW. The measured time-bandwidth product was 0.39 with a 3% output coupler. With a 6% output coupler, 2.4-nJ, 200-fs pulses were generated near 2400 nm at a pulse repetition frequency of 78 MHz. The average mode-locked output power was 185 mW. This work was the first demonstration of a femtosecond Cr:ZnSe laser passively mode locked with a graphene saturable absorber. The third part of the thesis focuses on the optical characterization of graphene samples grown on quartz, YAG and CaF2. We determined the saturation fluence and modulation depth of the samples as 34 μJ/cm2 and 0.6%, 60 μJ/cm2 and 0.8%, 39 μJ/cm2 and 0.6% for graphene on quartz, YAG and CaF2, respectively. The final part is the linear and non-linear characterization of the optical properties of a voltage controlled saturable absorber based on graphene super capacitors. Pump-probe measurements revealed that the ultrafast response of the graphene has similar dynamics with an average fast and slow time constants of 283 fs and 1.9 ps up to 1V of applied voltage. The saturation fluence at 1250 nm monotonically increases with the applied bias, becoming 456 μJ/cm2 at 1V and the modulation depth decreases from 1.09% at 0V to 0.69% at 1V. Hence, the modulation depth of the device was adjusted in the range of 1.1-0.7%.Our results indicate that, utilization of cost effective dispersion techniques as well as novel saturable absorbers such as graphene will lead to the development of practical sources of mid-infrared femtosecond pulse sources suitable for spectroscopic, medical and nonlinear optical applications.
Grafen-tabanlı hızlı doyabilen soğurucular ve katıhal lazerlerinin kip-kilitlenmesindeki uygulamaları
Graphene and other carbon-based nanomaterials have emerged as alternative saturable absorbers to other semiconductor saturable absorbers due to their favorable optical characteristics and relatively simple growth schemes. It has been demonstrated that the graphene saturable absorbers (GSAs), can be used as effective modulators to generate femtosecond pulses from lasers, operating in the near and mid infrared. Therefore, it is of great interest to explore the mode locking performance of the GSAs, grown on different substrates, in power-scaled lasers. However, one possible drawback of a monolayer graphene saturable absorber stems from its small-signal optical insertion loss of 5%, which leads to challenges in operating low gain lasers. Such challenges can be obviated by using graphene-based supercapacitor structures so that when a voltage bias is applied, the Fermi level of graphene can be shifted to reduce the absorption via Pauli blocking. The first part of this thesis focuses on the linear and nonlinear optical characterization of the GSAs grown on different substrates (quartz and YAG) and their application to mode locking of an energy-scaled multipass-cavity (MPC) Cr4+:forsterite laser operating near 1250 nm. The Cr4+:forsterite resonator, mode-locked with the GSA on quartz or the GSA on YAG, produced nearly transform-limited pulses with sub-100 fs durations and 53-kW pulse peak powers. To the best of our knowledge, both results report the highest peak powers obtained from a GSA mode-locked femtosecond solid-state laser. The remaining parts of this thesis investigate the optical characterizations and mode locking applications of the graphene-based supercapacitors as fast saturable absorbers with voltage-controllable optical insertion losses. The first supercapacitor design (voltage-controlled graphene saturable absorber, VCG-SA) consisted of a high dielectric electrolyte, sandwiched between two graphene electrodes. Linear and non-linear optical characterizations of the device were performed to explore its voltage-dependent optical properties and the results indicated that the supercapacitor architecture enabled the modulation of absorption up to the visible region (2.4 eV) even with few volts (0-3V) of applied bias. The VCG-SA was further employed in the MPC Cr4+:forsterite laser to investigate the voltage-dependent mode-locked operation. The MPC Cr4+: forsterite laser produced, 84-fs pulses with a time-bandwidth product of 0.32 at 1V of applied bias. To further reduce the insertion losses at zero bias, a second graphene-based supercapacitor architecture with the same electrolyte sandwiched between a graphene and a notched gold electrode was fabricated. Mode locking performance of the voltage-controlled graphene-gold saturable absorber (VCG-gold-SA) was investigated again in the MPC Cr4+:forsterite laser, operating near 1250 nm. Since one of the graphene electrodes of the previous design was replaced with a notched gold electrode, the optical insertion loss of the VCG-gold-SA was reduced at zero bias with respect to the previous design involving two graphene electrodes. In this case, the resonator generated 80-fs nearly transform-limited pulses with a pulse peak power of 42 kW at 0.8V of applied bias. Finally, to push the mode locking capability of these supercapacitor devices to as low a wavelength as possible, a Ti3+:sapphire laser, operating near 800 nm was used. The Ti3+:sapphire resonator successfully produced femtosecond pulses at wavelengths as short as 795 nm. To our knowledge, this is the shortest wavelength, where a graphene-based supercapacitor was used as a fast saturable absorber to generate femtosecond pulses. The novel graphene supercapacitor devices, first demonstrated as fast saturable absorbers in this thesis, have the potential to be used for femtosecond pulse generation from lasers operating over a broad wavelength range and with different levels of gain.
Alexandrite lazerinin 760 nm'de ve Tm3+:YLF lazerinin 2300 nm'de sürekli dalga diyot pompalı ve darbeli rejimlerde çalışması
Alexandrite (Cr3+:BeAl2O4) and Tm3+:YLF lasers are important, emerging solid-state lasers which generate coherent radiation in the near-infrared (700-820 nm) and mid-infrared (1800-2100 nm and 2200-2400 nm) regions of the electromagnetic spectrum, respectively. Both lasers have potential applications in diverse fields including biomedical imaging, surgery, ranging and spectroscopy. The experimental studies presented in this thesis investigate both the continuous-wave diode pumping and pulsed operations of Alexandrite lasers near 760 nm and Tm3+:YLF lasers near 2300 nm. In both cases, low threshold continuous-wave diode-pumped operation was demonstrated. In the case of the Alexandrite laser, pulsed operation was achieved by using the method of self-Q-switching. Passive Q-switching was employed in the case of the Tm3+:YLF laser to generate pulses by using a Cr2+:ZnSe saturable absorber. The first part of the thesis focuses on the investigation of the temperature-dependent spectroscopic properties of the Alexandrite crystal and diode-pumped operation of the Alexandrite laser. First, the emission intensity and the fluorescence lifetime of Alexandrite were shown to decrease with increasing crystal temperature. Second, the diode-pumped laser performance was investigated in detail. In the laser experiments, the maximum output power of 48 mW was obtained with a slope efficiency of 36% at the input diode pump power of 170 mW. The laser slope efficiency decreased from 36% to 12% as the temperature of the gain medium was increased from room temperature to 200°C. Self Q-switching with pulse widths in the range of 5-15 µs and repetition rates in the range of 10-35 kHz was further observed by slightly changing the curved mirror separation of the cavity. In the second part of this thesis, the continuous-wave operation of a 2.3-µm Tm3+:YLF laser was investigated. First, the excitation spectrum of the Tm3+:YLF was measured by using a tunable, narrow-linewidth Ti3+:sapphire laser. Also, the average absorption cross-section of the 1.5 at. % Tm3+:YLF was determined to be 0.77×10-20 cm2 by using power-dependent and position-dependent absorption saturation data. A single-mode 120-mW diode laser was then used for pumping the Tm3+:YLF laser cavity at 792 nm. In this configuration, low threshold lasing could be achieved with as low as 25 mW of input pump power by using a 1% output coupler. The maximum output power of 10.5 mW was obtained at 2305 nm with a slope efficiency of 11.4%. Second, by using a 250-mW diode laser, the threshold pump power and slope efficiency were measured as a function of effective output coupling. The minimum threshold pump power of 4 mW was measured at 0% output coupling. Power efficiency measurements showed that the highest slope efficiency of 10% was obtained around 0.7% output coupling and that the slope efficiencies beyond this output coupling decreased monotonically. The stimulated emission cross-section at 2305 nm was determined from the laser threshold data as 0.55×10-20 cm2. The third part of this thesis focuses on the pulsed operation of the Tm3+:YLF laser at 2.3 µm. To the best of our knowledge, passive Q-switching of a 2.3-µm Tm3+:YLF laser was demonstrated for the first time by using a Cr2+:ZnSe saturable absorber. The pulse durations and repetition frequencies of the passively Q-switched pulses were in the ranges of 1.2-1.4 µs and 0.3-2.1 kHz, respectively. By using the power-dependent repetition frequency data, the small-signal loss of the Cr2+:ZnSe saturable absorber was further determined. In addition to passive Q-switching of the 2.3-µm Tm3+:YLF laser, preliminary data on pulsing generated by using a semiconductor saturable absorber and a graphene saturable absorber are also presented. We foresee that both Alexandrite and 2.3-µm Tm3+:YLF lasers operated in continuous-wave or pulsed regimes will find numerous scientific and technological applications.
Grafen ile kip-kilitleme ve Kerr-odaklı kip-kilitleme yöntemleri kullanılarak darbe üretilen 850 nm civarinda çalişan Cr3+:LiSAF ve 2300 nm civarinda çalişan Tm3+:YLF laserleri
Femtosecond lasers operating in the near-infrared and mid-infrared regions can be used in many scientific and technological applications such as pumping of optical parametric oscillators to reach longer wavelengths, electronic/vibrational spectroscopy, and biomedical imaging. This thesis work particularly focuses on Cr3+:LiSAF and Tm3+:YLF lasers, which provide broadly tunable coherent emission near 850 nm and 2.3 µm. The broad tuning range of these lasers further makes it possible to generate femtosecond pulses. In addition, both gain media can be used to construct low-cost lasers by using low-power, widely available diode pump lasers. In recent years, graphene has attracted a great deal of attention as a passive mode locker for the generation of femtosecond pulses, due to its fast and broadband saturable absorption. In the experiments described below, both graphene as well as Kerr-lens mode locking were employed to produce ultrashort pulses with record performance from Cr3+:LiSAF and Tm3+:YLF lasers operating at 850 nm and 2.3 µm. In the first part of the thesis, we report a femtosecond Cr3+:LiSAF laser, mode locked by using a monolayer graphene saturable absorber (GSA) for the first time. The tight-focusing resonator architecture made it possible to operate the Cr3+:LiSAF laser with only two 135-mW, 660-nm low-cost single-mode diode lasers. At a pump power of 270 mW, the laser produced nearly transform-limited 68-fs pulses at 850 nm with a pulse repetition rate of 135 MHz and an average output power of 11.5 mW. By carefully optimizing the resonator group delay dispersion, we further demonstrated the shortest pulses directly generated to date from a GSA mode-locked laser. In particular, with a pump power of 275 mW, the Cr3+:LiSAF laser produced as short as 19-fs, nearly transform-limited pulses with a repetition rate of 107 MHz and average output power of 8.5 mW. Once mode locking was initiated with the GSA, stable femtosecond pulses could be obtained. In addition, the femtosecond output of the laser could be tuned from 836 nm to 897 nm with pulse durations in the range of 80-190 fs. We further performed detailed mode locking initiation tests across the full cavity stability range of the laser to verify that pulse generation was indeed initiated by the GSA and not by Kerr lens mode locking. In the next series of experiments, we developed a new source of mid-infrared femtosecond pulses, based on a Tm3+:YLF laser at 2303 nm. Two different techniques were employed to generate femtosecond pulses with the Tm3+:YLF laser. In the first experiment, an undoped ZnSe substrate was included in the resonator to provide enhanced nonlinear phase modulation during KLM operation. The Tm3+:YLF laser was end-pumped with a continuous-wave Ti3+:sapphire laser at 780 nm. With 880 mW of pump power, the KLM Tm3+:YLF laser generated 514-fs pulses at a pulse repetition rate of 41.5 MHz with an average power of 14.4 mW. In the second experiment, we developed, for the first time to our knowledge, a GSA mode-locked Tm3+:YLF laser operating near 2.3 µm. Since graphene introduces a constant loss of 2.3% per transit, the resonator was extended and double pumped to obtain sufficient intracavity pulse energy. GSA mode-locked Tm3+:YLF laser produced 1-ps pulses at a repetition rate of 17.2 MHz with 42 mW of average output power at 2303 nm. The major contributions of this thesis work may be summarized as follows. We demonstrated, for the first time to our knowledge, graphene mode-locked operation in a Cr3+:LiSAF laser at 850 nm. We further generated the shortest pulses (19 fs) from a GSA mode-locked solid-state laser. A systematic method was proposed and demonstrated to clearly identify the initiation mechanism of mode locking. Finally, Kerr-lens mode locked and GSA mode locked operations of a mid-infrared Tm3+:YLF laser were demonstrated for the first time. We foresee that the experiments described in this thesis can lead to the development of novel femtosecond laser sources in the near- and mid-infrared regions.
Alexandrite ve Cr:LiSAF kazanç ortamı tabanlı yakın-kızılaltı femtosaniye lazer kaynakları
Alexandrite (Cr3+:BeAl2O4) ve Cr:LiSAF (Cr3+:LiSrAlF6 ) are promising near-infrared gain media. They have tuning ranges extending from 700 nm to 820 nm and from 780 nm to 1110 nm, broad enough to support sub-10 fs pulse generation. They can be directly pumped with low-cost red diodes. Furthermore, since the emision cross section-fluorescence lifetime products of these media are greater than that of Ti:sapphire (Ti3+:Al2O3), they have low laser thresholds. Due to their high quantum efficiency and high figure of merit, these gain media can be utilised as low-cost, robust, compact and efficient alternatives to Ti:sapphire lasers near 800 nm. In particular, Alexandrite has superior thermo-mechanical properties which make it suitable for the development of high power laser applications. Cr:LiSAF has low passive losses, making it advantageous in the development of ultralow-jitter sources. However, the main drawback of these media is their low Kerr-nonlinearity. In this thesis, we present experiments aimed at the investigation of various emerging mode locking techniques in order to generate ultrashort pulses from Cr:LiSAF and Alexandrite lasers. The first part of the thesis focuses on the use of a gain matched output coupler (GMOC) for robust and efficient Kerr-lens mode locking (KLM) of Cr:LiSAF lasers. As a pump source, a tapered diode (TD) laser with a power of 1W was utilized at 685 nm. In the experiments, it was shown that GMOC provided a significant improvement in the efficiency and robustness of the KLM operation by reducing the gain-filtering effect. The tapered diode pumped Cr:LiSAF oscillator generated pulses as short as 14.5 fs with 106 mW of average output power and 60kW of peak power at an incident pump power of 940 mW. A femtosecond tuning range extending from 807 nm to 919 nm was also achieved with sub-50-fs long pulses. In the second part of this thesis, we present the femtosecond pulse generation experiments from a multipass-cavity Alexandrite laser. First, to the best of our knowledge, we demonstrated the generation of the shortest femtosecond pulses reported to date from a Kerr-lens mode-locked Alexandrite laser operating near 750 nm. A continuous-wave, 532-nm laser was used as a pump source and the performance of both the short and extended resonators was investigated. We increased the optical path length of the short-cavity Alexandrite resonator with a multipass-cavity extension to scale up the pulse energy and to eliminate the multi-wavelength spectral instabilities observed during continuous-wave operation. The resulting increase in the intracavity pulse energy provided enhanced Kerr-nonlinearity and eliminated the Q-switching instabilities during mode-locked operation. The Kerr-lens mode-locked MPC Alexandrite laser produced nearly transform-limited, 70-fs pulses at a pulse repetition rate of 5.6 MHz with only 1W of pump power. The time-bandwidth product was further measured to be 0.331. We then present the graphene mode-locked operation of a femtosecond Alexandrite laser at 750 nm. We adopted a resonator configuration similar to what was previously used for Kerr-lens mode locking experiments. We used a monolayer graphene saturable absorber to obtain mode locking with a pump power of 5W at 532 nm. The resonator generated nearly transform-limited, 65-fs pulses with a time-bandwidth product of 0.319 at a pulse repetition rate of 5.56 MHz. During mode-locked operation of the cavity, the output power was around 8 mW, corresponding to a pulse energy and peak power of 1.4 nJ and 22 kW. These experiments further show that graphene can be used to initiate mode locking at wavelengths as low as 750 nm. We believe that the results described in this thesis will pave the way for the development of low cost and efficient alternative femtosecond laser sources with a wide tunability around 800 nm, a wavelength range which is important for various scientific and technological applications.
Tek duvarlı karbon nanotüp doyabilen soğurucuları ile kip kilitlenmiş Cr:LiSAF lazerinden 21 fs darbe üretimi
Single-walled carbon nanotube saturable absorbers (SWCNT-SAs) possess many favourable nonlinear optical properties over a wide range of wavelengths, such as strong absorption saturation, low non-saturable losses, short recovery times, and low-cost fabrication schemes. These properties have made SWCNT-SAs viable alternatives to semiconductor saturable absorber mirrors (SESAMs) for the mode locking of solid state and fiber lasers over a very broad wavelength range. In this thesis, the aim was to demonstrate the ultrashort pulse generating capability of SWCNT-SAs. First, the characterization of the SWCNT-SA used in the experiments was conducted. The absorption of the SWCNT-SA was measured to be around 2% at 850 nm, which is the free-running wavelength of Cr:LiSAF lasers. After performing pump-probe spectroscopy, the fast and slow decay times of the SWCNT-SA were measured to be 0.4 and 2.7 ps. The saturation fluence and the single pass modulation depth of the SWCNT-SA were further determined to be 45 μJ/cm2 and 0.3%, respectively. To test the ultrashort pulse generating capability of the SWCNT-SA, a Cr:LiSAF laser was chosen since it has a broad emission band, enabling the generation of ultrashort pulses with femtosecond duration. In the experiments, the Cr:LiSAF laser was pumped with 210 mW of pump power with two low-cost, low-power diode lasers at 660 nm, and produced around 9 mW of mode-locked output power centered around 875 nm. The pulse duration of 21 fs, which was achieved during the experiments, represents the shortest pulses directly generated from a SWCNT-SA mode-locked solid-state laser. The pulse repetition rate was 47.9 MHz with a pulse energy of 0.18 nJ and peak pulse power of 9 kW. The time-bandwidth product of the produced pulses was 0.56. Kerr lens mode locking iv experiments were further conducted to test if the SWCNT-SA puts any limitation on the obtainable pulse duration. The experiments clearly showed that the obtainable pulse duration was not limited by the SWCNT-SA. Furthermore, the continuous-wave operation of the Cr:LiSAF laser at 850 nm was experimentally investigated in detail. Accurate measurements of the transmissions of the output couplers used in the experiments were conducted. Then for the Cr:LiSAF laser constructed with different output couplers, the threshold pump power and power efficiency data were measured while the laser was tuned to operate exactly at 850 nm by using an intracavity tuning prism. The data taken enabled a complete experimental characterization of the continuous-wave Cr:LiSAF laser and provides useful guidelines for the construction of similar laser systems.
Green diode pumping of femtosecond Ti3+:sapphire lasers and the first demonstration of 2.3-µm laser operation in trivalent thulium-doped KY3F10 and BaY2F8 fluoride crystals
This thesis aims at the development of new femtosecond laser architectures in the near infrared and mid infrared regions of the electromagnetic spectrum. The first part focuses on direct green diode pumping of a Ti3+:sapphire laser by using recently developed 520-nm laser diodes. The second part of the thesis explores 2.3 µm laser operation of new fluoride hosts doped with Tm3+ ion. Tunable laser operation near 2.3 µm was demonstrated for the first time in the fluoride hosts KY3F10 and BaY2F8 doped with trivalent Tm ion. In the particular case of Tm3+ doped KY3F10 laser, graphene saturable absorber mode locking was further demonstrated yielding sub-picosecond pulses at 2.3 µm. The final part of the thesis describes the construction of a homemade experimental setup which employs 808 nm and 640 nm lasers for the evaluation of photothermal therapy and photodynamic therapy potential of various nanoparticles. In the first part of the thesis, direct green diode pumping of a Ti3+:sapphire laser was investigated near 800 nm by using a short x-cavity and an extended multipass cavity (MPC) configuration. The continuous wave (cw) power performance of the short cavity and MPC was first optimized. A paraxial beam propagation algorithm based on the ABCD method was used to obtain the best mode matching between the 520-nm diode and Ti3+:sapphire cavity. The optimized short cavity produced 90 mW of output power at 780 nm with 880 mW of diode pump power with the 3% output coupler. The short x-cavity was then extended with an MPC to lower the pulse repetition rate of the composite laser to 5.8 MHz during Kerr lens mode-locked operation. With 860 mW of diode pump power, the mode-locked MPC Ti3+:sapphire laser generated 95-fs pulses with a pulse energy of 5.1 nJ. These results represent the highest pulse energy obtained to date with a single diode pumped Ti3+:sapphire laser. In the second part of the thesis, laser action at 2.3 µm was demonstrated for the first time in the fluoride host KY3F10 doped with Tm3+ ion. In the experiments 8 at.% Tm3+:KY3F10 crystal was pumped with a homemade, tunable Ti3+:sapphire laser. Excitation spectrum, tunability and laser power performance of the Tm3+.KY3F10 laser was investigated in the cw regime. With a 1% output coupler, the Tm3+:KY3F10 laser produced 31 and 122 mW of output power in single-end and double-end pumping configurations, respectively. In addition, the output wavelength of the laser was continuously tuned over a range of 125 nm between 2260 and 2385 nm. Furthermore, the lifetime of the upper laser level (3H4) was measured to be 16 µs. By using the lasing threshold data the stimulated emission cross section of the 2.3 µm laser transition was determined to be (1.62±0.09) × 10−24 m2. After the continuous wave laser characterization, mode-locked operation was obtained by extending the cavity length and by adding a graphene saturable absorber. The graphene mode-locked Tm3+:KY3F10 laser produced 739-fs pulses at 2340 nm at a pulse repetition frequency of 54 MHz. These results represent the first experimental demonstration of continuous wave laser operation and graphene mode-locked operation of the Tm3+:KY3F10 gain medium around 2.3 µm. The third part of the thesis focuses on experiments where 2.3 µm lasing was demonstrated for the first time in Tm3+ doped BaY2F8 crystal. Since the BaY2F8 crystal was anisotropic, two different pumping polarizations (E//x and E//y) were investigated and lasing was obtained at 2290 nm for both cases. Power efficiency and excitation spectrum measurements were then performed each pumping configuration. For E//x pumping configuration, 42 mW (100 mW) of output power was obtained in single-end (double-end) pumping scheme. For E//y pumping, 43 mW of output power was obtained in double-end pumping scheme. The remaining power characterization was performed for E//x pumping. Tunable operation was obtained over a range of 152 nm between 2233 and 2385 nm. Absorption saturation measurements were further carried out to determine the saturation intensity and the absorption cross section of the gain medium. The lifetime of the upper laser level was measured to be 292 µs. The emission cross section for the Tm3+:BaY2F8 gain medium was found to be smaller ((0.66±0.06)⨯10-24 m2) than that of the Tm3+:KY3F10 medium ((1.62±0.09) × 10−24 m2). The final part of the thesis describes an emerging application of near infrared lasers in photothermal therapy (PTT) and photodynamic therapy (PDT). In the experiments, a homemade setup was constructed for the evaluation of the PTT and PDT potential of different nanoparticles irradiated with 640 nm and/or 808 nm lasers. A dichroic mirror was used to combine the 640 and 808 nm beams so that they can be applied simultaneously or individually. During the PTT measurements, the temperature increase of the nanoparticle solutions was recorded as a function of time to determine the PTT efficiency. A tunable Ti3+:sapphire laser was also used to explore the wavelength dependence of the PTT efficiency. PTT measurements were further carried out by irradiating nanoparticles injected into cancer cell cultures, bacteria cultures and bacteria biofilms in 96-well plates. The samples were placed inside a container whose temperature was maintained at 37°C. Experiments showed that incident fluence levels of 3 W/cm2 provided effective PTT and PDT effect, demonstrating their potential in next generation medical therapy systems.