Yasemin Pepe

Aggregation-enhanced plasmonic coupling improves nonlinear absorption and optical limiting in PMMA/AgNP thin films compared with nanofibers.

Nikel oksit (NiO) kompozit filmlerin doğrusal olmayan soğurma (NA) ve optik sınırlama davranışına NiO konsantrasyonun ve giriş ışık şiddetinin etkisi araştırılmıştır. Doğrusal optik ölçüm sonuçları NiO konsantrasyonu arttıkça band aralığı içerisindeki yerelleşmiş kusur seviyelerinin arttığını ortaya çıkarmıştır. Kompozit filmlerin NA ve optik sınırlama davranışlarını ortaya çıkarmak için açık yarık Z-tarama deneyleri 532 nm dalga boylu lazer ışığı kullanılarak farklı giriş ışık şiddetleri altında gerçekleştirilmiştir. Yüksek NiO konsantrasyonuna sahip olan kompozit film en güçlü NA davranışı göstermekle birlikte artan giriş ışık şiddeti ile NA davranışı daha da güçlenmiştir. En güçlü NA davranışının en yüksek katkılama konsantrasyonunda elde edilmesi katkılamanın artmasıyla artan kusur seviyelerinin NA’ya katkıda bulunan NA mekanizmalarını desteklemesine atfedilmiştir. En küçük optik sınırlama eşik şiddet değerli en güçlü optik sınırlama davranışı en yüksek NiO konsantrasyonlu kompozit film için elde edilmiştir. Yüksek doğrusal geçirgenlik, güçlü NA ve düşük optik sınırlama eşik şiddet değerine sahip olan NiO_20% kompozit filmler görünür bölgede etkin optik sınırlayıcı olarak kullanılmaya aday olabilirler.

Optical limiter materials are essential for protecting people’s health due to its higher penetrating capabilities. In this study, Ga_0.85In_0.15Se and Ga_0.90In_0.10Se crystals were synthesized using Bridgemann-Stochbarger method to investigate their optical limiting performance in near infrared region. Linear optical results revealed that although the crystals had approximately the same band gap energy, the Ga_0.90In_0.10Se crystal had more defects due to its higher thickness. Open aperture Z-scan measurements were conducted with femtosecond pulsed laser at 1200 nm excitation wavelength. The examined crystals’ nonlinear absorption (NA) behavior was shown, with the Ga_0.90In_0.10Se crystal exhibiting the strongest NA behavior. Considering to their band gap energy and Urbach energies, the main NA mechanism was the two-photon absorption (2PA) and its contribution to NA increased with increase of the input intensity. Among the studied ternary crystals Ga_0.90In_0.10Se crystal showed high limiting performance with 1.11 mJ/cm² optical limiting threshold. They are attractive candidates for efficient optical limiting applications at 1200 nm due to their strong NA behavior.

Identification of photonic materials with high infrared transmittance and high nonlinear optical coefficients is one of the main emphases in material science as a result of the rapid advancement in infrared photonics. In this study, undoped and B (boron) -doped InSe single crystals were grown by using the modified vertical Bridgman method, and their nonlinear optical properties were investigated to reveal their usability as an optical limiter in the near-infrared region. The decreasing band gap energies and increasing defect states were determined with increasing B concentration in InSe single crystals. The effect of the B concentration on the nonlinear absorption (NA) and optical limiting properties of the InSe single crystals was investigated via open aperture (OA) Z-scan experiments under ultrafast laser excitation at 1200 nm wavelength with 100 femtosecond pulse duration. Two-photon absorption (TPA) was the dominant NA mechanism at 1200 nm excitation wavelength in the femtosecond domain. The results revealed that the NA became stronger with increasing input intensity and increasing amount of B dopant atoms in the InSe single crystal. The observed enhanced NA can be attributed to two possible events (i) increasing input intensity induced more excited electrons which led to more contribution to NA through TPA and (ii) increasing B dopant atoms in InSe single crystal induced more defect states. The NA may be more enhanced with the contribution of these defect states related NA mechanisms. The high transparency and strong NA behavior at the near-infrared region make these single crystals exceptional potential candidates for developing various optoelectronics and filters at the near-infrared spectral region.

In this study, the effect of the dopant element on the linear, nonlinear absorption and optical limiting properties of ultrasonically sprayed MoO₃ thin films is presented. The linear optical results showed an increase with the density of the defect states in the bandgap with doping, in conjunction with a decrease in bandgap energy and an increase in Urbach energy. Broad photoluminescence emissions are detected in the range of 350 and 600 nm, decreasing in intensity by doping. To reveal the defect states effects’ on the nonlinear absorption (NA) behavior, OA Z-scan data were analyzed with two theoretical models considering only two photon absorption (2PA) (model 1), and one photon absorption (OPA), 2PA and free carrier absorption (model 2). The NA behavior is observed and found to get enhanced by increasing input intensity and doping atoms due to generation of new oxygen vacancies and formation of further defect states. The NA coefficient values of the thin films in model 2 are 100 times higher than that of 2PA coefficient values in model 1. This result revealed the strong effect of defect states on the NA behavior. Among the investigated dopant atoms, Cu resulted in enhanced NA due to the higher density of defect states. While the genuine 2PA is the dominant NA mechanism for V and Fe doped MoO₃ thin films, OPA and 2PA are the dominant NA mechanisms for the Ni, Zn and Cu doped MoO₃ thin films due to their higher concentration of defect states. Cu-doped MoO₃ thin film has a lower optical limiting threshold of 0.026 mJ/cm² due to its enhanced NA behavior. Considering the results obtained, this study opens the door to the potential of doped MoO₃ thin films to be used as optical limiters in the visible wavelength region.

In this study, 3-amino-4-hydroxybenzenesulfonic acid-based imine compounds; 3-(2,5-dihydroxybenzylideneamino)-4-hydroxybenzenesulfonic acid (1), 3-(2,4-dihydroxybenzylideneamino)-4-hydroxybenzenesulfonic acid (2) and 4-hydroxy-3-(2-hydroxy-3-methoxybenzylideneamino)-4-hydroxybenzenesulfonic acid (3) was synthesized. The compounds were analyzed using various spectroscopy methods, and the experimental UV–vis data matched the theoretical predictions. The compound1displayed lower stability, higher reactivity, and easier photoexcitation due to a smaller HOMO-LUMO energy gap. The investigated compounds1-3showed promise as chemosensors for anions, providing visible detection in daylight conditions. The compound3exhibited selective fluorescence at specific wavelengths. The compounds1-3interacted with DNA through electrostatic interactions. Also, compounds1-3showed higher antioxidant activity than BHT. However, fluorescence measurements indicated that the emission signals were strongly influenced by the position and strength of the electron-donating group. Adding a hydroxy or methoxy moiety near the -OH group on the phenyl ring decreased the fluorescence signal due to intersystem crossing and intramolecular charge transfer mechanisms, respectively. These findings were supported by femtosecond transient absorption spectroscopy measurements. The results emphasize the significance of substituents in imines derived from 3-amino-4-hydroxybenzenesulfonic acid in determining their biological activities, as well as their optical and sensor properties.

The compositional effect of Mo/W ratio on linear, nonlinear absorption and optical limiting behavior of the NaBi(Mo_xW_1−xO₄)₂ single crystals grown by Czochralski technique was investigated. X-ray diffraction patterns of the studied crystals presented well-defined peaks associated with the tetragonal crystalline structure. The nonlinear absorption performance and optical limiting threshold were determined using an open-aperture Z-scan technique. A theoretical model including one photon absorption (OPA), two photon absorption (TPA) and free carrier absorption was used to determine the nonlinear absorption parameters. All of the results showed that defect states, which strongly affect nonlinear absorption (NA) and optical limiting behaviors, can be tuned with the Mo/W ratio, enabling NaBi(Mo_xW_1−xO₄)₂ single crystals to be used in desired optoelectronic applications. Linear optical absorption analysis revealed that bandgap energy and defect states can be tuned by changing the Mo/W ratio in the crystal structure. The obtained results showed that all the studied crystals had NA behavior and the nonlinear absorption coefficient decreased with increasing Mo/W ratio. Sequential TPA is the main NA mechanism for these crystals due to the fact that the incident light energy is lower than the bandgap energies and the existence of the real intermediate state around 2.32 eV.

Polyvinylpyrrolidone/hexagonal boron nitride (PVP/hBN) composite nanofibers were produced by the electrospinning method at 4, 6, and 8 wt% concentration of hBN in an effort to examine the effects of defect distribution and density on the nonlinear absorption and optical limiting behavior. Open aperture (OA) Z-scan experiments were performed at 1064, 532 and 355 nm under various input intensities. While scanning electron microscopy revealed homogeneous and cylindrical nanofibers for each composition, the average diameter of the cylindrical nanofibers decreased from 349 nm to 296 nm with increasing hBN concentration. The linear absorption results showed an increase in the defect states and distribution in the energy band gap with the addition of hBN. All composite nanofibers have a positive nonlinear absorption (NA) signal and decrease with increasing input density at 1064 and 532 nm due to the filling effect of defect states with one photon absorption (OPA). While an increase in β_eff and I_SAT values is obtained with increasing hBN concentration at all excitation wavelengths, a more pronounced NA behavior is observed at 1064 nm. OPA is the dominant mechanism due to the defect distribution inside the band gap of PVP at 1064 nm with significant contribution from sequential and simultaneous two photon absorption. Although similar NA mechanisms occurred at 532 and 355 nm, excited state absorption from defect states of PVP to defect states of hBN in the band gap led to higher nonlinear absorption behavior compared to 532 nm. The lower onset optical limiting threshold was obtained to be 0.58 mJ cm⁻² at 1064 nm for PVP nanofibers with 8 wt% of hBN. The broad absorption spectrum, strong NA behavior and low limiting threshold properties make PVP/hBN composite nanofibers good candidates as effective optical limiters in a wide spectral region.