RESUMEN
A parity-time (PT) symmetric optoelectronic oscillator (OEO) with optical injection locking is proposed and experimentally demonstrated to generate single-mode microwave signals. A distributed feedback laser is injection locked, which functions as a frequency multiplier for the coarse mode selection of the PT-symmetric OEO. The PT symmetry in the OEO is implemented by using a polarization-dependent Sagnac loop to form two identical loops for coupling. By tuning the two polarization controllers in the Sagnac loop, the gain/loss and coupling coefficient of the two loops can be controlled, and single-mode oscillation in the OEO could be achieved at the broken PT-symmetry condition. Microwave signal generation at the frequency of 6.427 GHz is obtained from the proposed OEO. The phase noise is about -105 dBc/Hz at an offset frequency of 10 kHz.
RESUMEN
The influence of doping on the ultrafast carrier dynamics in InSb has been studied by time-resolved terahertz spectroscopy with photogenerated carrier densities from 1.5×1018 to 9.5×1019cm-3 at 800 nm. The photoinduced absorption and carrier recovery process show doping type dependence. The carrier recovery time of intrinsic InSb is greater than that of p-doped InSb but less than that of n-doped InSb at low carrier densities. At high carrier densities, compared with intrinsic InSb, the doped InSb is more prone to transient Auger recombination, which indicates that the appearance of the fast decay process depends on the carrier densities. The photoinduced absorption of terahertz probe pulse of n-doped InSb is significantly less than that of p-doped and intrinsic InSb; however, that of p-doped InSb is close to that of intrinsic InSb, which demonstrates that the high concentration of electrons can accelerate the efficiency of transient Auger recombination. Our analysis provides assistance to the design, manufacture, and improvement of photovoltaic detectors.
RESUMEN
Ultrafast carrier dynamics in intrinsic and n-doped InSb crystals were studied by time-resolved terahertz spectroscopy using an optical pump-terahertz probe setup with pump fluence from 32 µJ/cm2 to 1910 µJ/cm2. With photoexcitation at 800 nm, the ultrafast photoinduced absorption and carrier recovery process of intrinsic and n-doped InSb showed strong pump fluence dependence. It was found that the magnitude of photoinduced absorption first increased and then decreased with pump fluence. The carrier recovery process could be well fitted with a single exponential curve at low pump fluence, but could be well fitted with a biexponential curve at high pump fluence when a fast photocarrier relaxation appeared. The magnitude of photoinduced absorption increased gradually at low pump fluence due to the increase of the carrier at the bottom of the conduction band by impact ionization. The magnitude of photoinduced absorption decreased gradually at high pump fluence, possibly due to the efficiency of transient Auger recombination greater than the rate of carriers generated in the impact ionization process. The fast decay process appearing at high pump fluence was thought to be dominated by transient Auger recombination.
RESUMEN
Size dependence of spin dynamics in colloidal CdSe quantum dots (QDs) are investigated with circularly polarized pump-probe transmission spectroscopy at room temperature. The excitation energy is tuned to resonance with the lowest exciton (1S(h)1S(e)) energy of the CdSe QDs. The exciton spin dynamics of CdSe QD with the diameter of 5.2 nm shows monoexponential decay with a typical time constant of about 1-3 ps depending on the excitation energy. For the cases of CdSe QDs with smaller size (with the diameter of 4.0 and 2.4 nm), the exciton spin relaxation shows biexponential decay, a fast component with time constant of several ps and a slow one with time constant of hundreds of ps to nanosecond time scale. The fast spin relaxation arises from the bright-dark transition, i.e., J = ±1 â -/+2 transition. This process is dominated by the hole spin flips, while the electron spin conserves. The slow spin relaxation is attributed to the intralevel exciton transitions (J = ±1 â -/+1 transition), which is relevant to the electron spin flip. Our results indicate that the exciton spin relaxation pathways in CdSe QD are controllable by monitoring the particle size, and polarized pump-probe spectroscopy is proved to be a sensitive method to probe the exciton transition among the fine structures.
RESUMEN
By employing femtosecond pump-probe configuration, we successfully realized narrowband terahertz wave generation and detection in both photorefractive periodically poled lithium niobate (PPLN) and periodically poled Mg:LiNb(3) (PP-Mg:LN) crystal. Using an applied magnetic field, we achieved modulation of the terahertz wave in a photorefractive PPLN crystal. The terahertz wave depends strongly on the magnitude of the applied magnetic field in the photorefractive PPLN crystal. Terahertz wave independence of the magnetic field in PP-Mg:LN crystal was also identified. The interaction of the magnetic field and photorefractive PPLN crystal is believed to occur due to the Lorentz force, which results in the buildup of a space-charge field in a photorefractive PPLN crystal.