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Optimized uni-traveling carrier photodiode and mushroom-mesa structure for high-power and sub-terahertz bandwidth under zero- and low-bias operation
摘要： In this paper, physically-based simulations are carried out to investigate and design uni-traveling carrier photodiode (UTC-PD) for high-power sub-terahertz wave generation at zero- and low-bias operation. The reliability of the physically-based simulation is demonstrated by comparing with our experimental result. Both the bandwidth and RF output power of the proposed UTC-PD is significantly improved by careful design the built-in electric field distribution under high-power input. For the optimized UTC-PD with the mesa diameter of 5 μm, its 3dB bandwidth large than 100 GHz even if the photocurrent reaches 6 mA under zero-bias operation. The device can reach a high bandwidth of 92.4 GHz, 105 GHz, and 119.5 GHz under the reverse bias of 0.5 V, 1 V, and 2 V, respectively, even the input photocurrent as high as 18.2 mA. The peak output-power of the device has enhanced at least 7 dB even at 170 GHz and zero- or low-bias operation. Besides, a novel design of mushroom-mesa UTC-PD (MM-UTC-PD) is proposed which with 4.3% improved high-speed performance. The MM-UTC-PD can trade-off between the external quantum-efficiency and bandwidth when miniaturized junction size is required.
关键词： ultrafast optics,sub-terahertz,optical communications,high-power photodiodes,optical receivers
更新于2025-09-11 14:15:04
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Ultrashort pulse reconstruction using a deterministic phase retrieval method with Gaussian-envelope gates
摘要： A novel scheme of ultrashort pulse characterization is proposed that is capable of reconstructing the complex-valued envelope of a pulse in the time domain by use of a second harmonic generation system with two Gaussian gate pulses. In this scheme, we apply an analytic (noniterative) phase-retrieval method for coherent diffractive imaging in the space domain to the pulse characterization, and so the spectral phase of an unknown pulse can be retrieved from only two spectra of the unknown pulse modulated with a Gaussian gate and its time-shifted Gaussian one. Computer-simulated examples demonstrate that the pulse retrieval is robust. In particular, it is shown that the reconstruction of an unknown pulse using approximate Gaussian gates derived from replicas of the pulse itself is also effective, provided the spectrum of the unknown pulse has a gentle slope within the spectral bandwidth of the gate pulse.
关键词： Ultrafast measurements,Pulse characterization,Phase retrieval,Ultrafast optics
更新于2025-09-09 09:28:46
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Ultrafast Nonlinear Optical Excitation Behaviors of Mono- and Few-Layer Two Dimensional MoS2
摘要： The layered MoS2 has recently attracted significant attention for its excellent nonlinear optical properties. Here, the ultrafast nonlinear optical (NLO) absorption and excited carrier dynamics of layered MoS2 (monolayer, 3?4 layers, and 6?8 layers) are investigated via Z-scan and transient absorption spectra. Our experimental results reveal that NLO absorption coefficients of these MoS2 increase from ?27 × 103 cm/GW with more layers at 400-nm laser excitation, while the values decrease from 2.0 × 103 cm/GW at 800 nm. In addition, at high pump fluence, when the NLO response occurs, the results show that not only the reformation of the excitonic bands, but also the recovery time of NLO response decreases from 150 ps to 100 ps with an increasing number of layers, while the reductive energy of A excitonic band decreases from 191.7 meV to 51.1 meV. The intriguing NLO response of MoS2 provides excellent potentials for the next-generation optoelectronic and photonic devices.
关键词： ultrafast photonic devices,Ultrafast optics,two-dimensional materials
更新于2025-09-04 15:30:14
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Design Study of Time-Preserving Grating Monochromators for Ultrashort Pulses in the Extreme-Ultraviolet and Soft X-Rays
摘要： The design of grating-based instruments to handle and condition coherent ultrafast pulses in the extreme-ultraviolet is discussed. The main application of such instruments is the monochromatization of high-order laser harmonics and free-electron-laser pulses in the femtosecond time scale. Broad-band monochromators require the use of diffraction gratings at grazing incidence. A grating can be used for the spectral selection of ultrashort pulses without altering the pulse duration in a significant way, provided that the number of illuminated grooves is equal to the resolution. We discuss here the design conditions to be fulfilled by a grating monochromator that does not increase the pulse duration significantly longer than the Fourier limit.
关键词： monochromators,diffraction gratings,ultrafast optics
更新于2025-09-04 15:30:14