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A polaron approach to photorefractivity in Fe : LiNbO<sub>3</sub>
摘要： The thermally activated, incoherent hopping of small electron polarons generated by continuous illumination in iron-doped lithium niobate is simulated by a Marcus-Holstein model for which all the input parameters are known from literature. The results of the calculations are compared with a comprehensive set of data obtained from photorefractive, photogalvanic and photoconductive measurements under green light excitation on samples with different doping levels and stoichiometries and composition dependence of the photorefractive observables can be interpreted by a change in the abundance of the different hop types that a polaron performs before being captured by a deep Fe trap. Moreover, by a comparison between experimental and numerical data we obtain new insights on the initial photo-excitation part of the photorefractive process. In particular all results are consistent if a single value of the photogalvanic length LP G = (1.44 ± 0.05) ? is assumed for all the samples and all the temperatures. The photo-generation efficiency φ under green light excitation (somewhere denoted as quantum efficiency) is also estimated. It appears to decrease from 10-15% at room temperature to about 5% at 150K. This behavior is qualitatively interpreted in terms of a temperature-dependent re-trapping probability of the light-emitted particles from the initial Fe donor center.
关键词： photo-generation efficiency,polaron,photogalvanic length,Fe:LiNbO3,Marcus-Holstein model,photorefractivity
更新于2025-09-09 09:28:46
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Effect of Hf <sup>4+</sup> doping on structure and enhancement of upconversion luminescence in Yb:Tm:LiNbO <sub/>3</sub> crystals
摘要： A series of Yb:Tm:LiNbO3 crystals doped with x mol% Hf4+ ions (x = 2, 4, and 6) were grown by the Czochralski method. The dopant occupancy and defect structure of Hf:Yb:Tm:LiNbO3 crystals were investigated by x-ray diffraction and infrared transmission spectra. The influence of Hf4+ ions concentration on UV–VIS–NIR absorption spectra of Hf:Yb:Tm:LiNbO3 crystals was discussed. The upconversion luminescence of Hf:Yb:Tm:LiNbO3 crystals was obtained under 980 nm excitation. Strong emissions were observed at 475 nm in the blue wavelength range and 651 nm in the red wavelength range. Remarkably, enhancement of the red and blue upconversion luminescence was achieved by tridoping Hf4+ ions.
关键词： crystal structure,Hf:Yb:Tm:LiNbO3 crystals,upconversion luminescence
更新于2025-09-09 09:28:46
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High extinction ratio integrated optical modulator for quantum telecommunication systems
摘要： A method for increasing the extinction ratio of integrated optical Mach-Zehnder modulators based on LiNbO3 via the photorefractive effect is proposed. The influence of the photorefractive effect on the X- and Y-splitters of intensity modulators is experimentally studied. An increase in the modulator extinction ratio by 17 dB (from 30 to 47 dB) is obtained. It is shown that fabricated modulators with a high extinction ratio are important for quantum key distribution systems.
关键词： LiNbO3,extinction ratio,photorefractive effect,quantum telecommunication systems,integrated optical modulator
更新于2025-09-09 09:28:46
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Nano-domain nucleation in front of moving domain wall during tip-induced polarization reversal in ion-sliced LiNbO<sub>3</sub> thin films
摘要： We study the domain formation during tip-induced polarization reversal in ion-sliced single crystal LiNbO3 thin films by a piezoresponse force microscope. Usually only one central domain would be formed under the conductive tip when voltage pulses are applied to it. However, in ion-sliced LiNbO3 thin films, a ring of nano-domains was observed in front of the central domain wall during domain expansion, which are subsequently absorbed into the central domain and result in a rough domain profile. This unique formation process can be attributed to local maximum of electric field beyond the wall and polarization pinning at the interface, which is experimentally confirmed by erasing the switched domain back with lower biases and finding these nano-domains were difficult to remove.
关键词： LiNbO3 thin films,piezoresponse force microscope,polarization reversal,nano-domains
更新于2025-09-04 15:30:14
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[IEEE 2018 15th European Radar Conference (EuRAD) - Madrid, Spain (2018.9.26-2018.9.28)] 2018 15th European Radar Conference (EuRAD) - A DC to 40 GHz, High Linearity Monolithic GaAs Distributed Amplifier with Low DC Power Consumption as a High Bit-Rate Pre-Driver
摘要： This paper presents the design and the performance of a six stage GaAs MMIC distributed amplifier (DA) for optical driver applications. The DA is fabricated in a commercially available 0.15 μm GaAs p-HEMT technology. The amplifier exhibits 13 dB of small gain over 40 GHz of 3 dB bandwidth with a power consumption equal to 550 mW. Group delay time variation up to 30 GHz is only ±7 ps. The output power is higher than 16 dBm (4 Vpp), which makes the circuit suitable as a preamplifier for lithium-niobate (LiNbO3) optical modulator driver. The DA is demonstrated as a part of the modulator driver in a 12.5 GBps PAM-4 (25Gbps) optical system by using the eye diagram as a figure of merit.
关键词： Optical Drives,LiNbO3 Optical modulator,electro-optical modulation,Wideband amplifier,Optical amplifier,modulator,Traveling-wave amplifier,pre-driver,MMIC,Distributed Amplifier
更新于2025-09-04 15:30:14