arXiv subjects

Lei Zhai

Publications and source records attributed to Lei Zhai.

8 recordsLinked to original sources

arXiv2026

Hybrid integrated narrow linewidth semiconductor laser based on the distributed feedback from an external deformed microcavity

Optical microcavities with rotational symmetry have been widely used for narrowing linewidth and reducing frequency noise, however, the narrow but wavelength dependent optical feedback restricts the narrow linewidth laser works only at some discrete wavelength matching the resonance of the microcavity. Here, we demonstrate a narrow linewidth semiconductor laser with continuous wavelength tunability by hybrid integrating a DFB laser chip with a deformed microcavity fabricated on a 220 nm SOI wafer. The deformed microcavity with vortex radius demonstrates the unique characteristics of unidirectional energy storage, wavelength self-adaptivity, and self-focusing of the Rayleigh scattering based distributed feedback. In addition, the strength of Rayleigh scattering is also significantly enhanced by the high numerical aperture silicon waveguide. The optical feedback signal measured by the optical frequency domain reflectometry (OFDR) shows that the deformed microcavity can effectively lengthen the equivalent propagation distance without wavelength dependence. With the wavelength self-adaptive optical feedback from the deformed microcavity, the intrinsic linewidth of a DFB laser diode is narrowed to 525 Hz and the side mode suppression ratio (SMSR) is improved to 76 dB in a maximum allowable continuous wavelength tuning range of 2.25 nm. The frequency noise and relative intensity noise (RIN) are reduced to 2.98 Hz2 /Hz and -148.74 dB/Hz at the offset frequency of 1 MHz, respectively. The work demonstrated here paves a new way for integrated tunable narrow linewidth lasers, which are of crucial importance in high-speed communication and high-precision spectroscopy

physics.optics↗

arXiv2026

Hybrid integrated narrow linewidth laser with external distributed optical feedback from a silicon strip waveguide

External optical feedback via Rayleigh scattering from an integrated microresonator or an optical fiber has been demonstrated to significantly narrow the intrinsic linewidth of semiconductor lasers. Wavelength matching between the lasing cavity and the external high-Q microresonator is required to accumulate Rayleigh scattering based optical feedback. Optical fiber can provide Rayleigh scattering based optical feedback for any lasing wavelength. However, optical fibers hundreds of meters or even kilometers long are required for the accumulation of Rayleigh scattering based optical feedback, hindering the integration of narrow linewidth lasers. Here, we present an integrated scheme that collects distributed feedback signal with weak wavelength dependence by exploiting surface radiation in a silicon waveguide. The effects of waveguide width on the intensities of the surface radiation and distributed optical feedback signal are first numerically analyzed by introducing a collection coefficient. Numerical calculations show that a 1 {\mu}m-wide strip waveguide yields optimal performance for excitation and collection of distributed optical feedback, which is also experimentally verified by measuring the feedback signal with an optical frequency-domain reflectometry. Benefitting from the enhanced distributed optical feedback that is 34.72 dB higher than that in a single-mode fiber, the hybrid integrated laser demonstrates an intrinsic linewidth of 1.52 kHz, a side-mode suppression ratio (SMSR) of 74.71 dB, and a frequency noise of 24.44 Hz2/Hz. Furthermore, within a maximum allowable wavelength tuning range of 2.342 nm, the linewidth narrowing ratio depends little on the wavelength for all the waveguides with different widths.

physics.optics↗

arXiv2019

New insight into the diffusion of hydrogen and helium atoms in tungsten

Based on the in-detail tracking of the movements of atoms in a large number of molecular dynamics simulation boxes, we find that the diffusion of H and He atoms in single-crystal W is composed of non-Markovian jumps. The waiting time distribution of the triggering of jumps is not the usually recognized exponential distribution, but a temperature-dependent power-law distribution. The power-law distribution of the waiting time may lead to clear ergodicity-breaking diffusion, a phenomenon that has formerly been reported to only occur in complex systems such as living biological cells or soft matter. The present finding provides an insightful new view for the analysis and simulation of the puzzling H/He behaviors in W that are observed in experiments. Our findings will inspire reconsiderations on how to bridge the multiscale theoretical predictions with experimental observations, not only for H/He in W but also for H/He in other materials.

cond-mat.mtrl-sci↗

arXiv2011

Coulomb Blockade and Hopping Conduction in Graphene Quantum Dots Array

We show that the low temperature electron transport properties of chemically functionalized graphene can be explained as sequential tunneling of charges through a two dimensional array of graphene quantum dots (GQD). Below 15 K, a total suppression of current due to Coulomb blockade through GQD array was observed. Temperature dependent current-gate voltage characteristics show Coulomb oscillations with energy scales of 6.2-10 meV corresponding to GQD sizes of 5-8 nm while resistance data exhibit an Efros-Shklovskii variable range hopping arising from structural and size induced disorder.

cond-mat.mes-hall↗

arXiv2010

Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets

We elucidate on the low mobility and charge traps of the chemically reduced graphene oxide (RGO) sheets by measuring and analyzing temperature dependent current-voltage characteristics. The RGO sheets were assembled between source and drain electrodes via dielectrophoresis. At low bias voltage the conduction is Ohmic while at high bias voltage and low temperatures the conduction becomes space charge limited with an exponential distribution of traps. We estimate an average trap density of 1.75x10^16 cm^-3. Quantitative information about charge traps will help develop optimization strategies of passivating defects in order to fabricate high quality solution processed graphene devices.

cond-mat.mes-hall↗

arXiv2010

Position dependent photodetector from large area reduced graphene oxide thin films

We fabricated large area infrared photodetector devices from thin film of chemically reduced graphene oxide (RGO) sheets and studied their photoresponse as a function of laser position. We found that the photocurrent either increases, decreases or remain almost zero depending upon the position of the laser spot with respect to the electrodes. The position sensitive photoresponse is explained by Schottky barrier modulation at the RGO film-electrode interface. The time response of the photocurrent is dramatically slower than single sheet of graphene possibly due to disorder from the chemically synthesis and interconnecting sheets.

cond-mat.mes-hall↗

arXiv2010

High yield fabrication of chemically reduced graphene oxide field effect transistors by dielectrophoresis

We demonstrate high yield fabrication of field effect transistors (FET) using chemically reduced graphene oxide (RGO) sheets suspended in water assembled via dielectrophoresis. The two terminal resistances of the devices were improved by an order of magnitude upon mild annealing at 200 0C in Ar/H2 environment for 1 hour. With the application of a backgate voltage, all of the devices showed FET behavior with maximum hole and electron mobilities of 4.0 and 1.5 cm2/Vs respectively. This study shows promise for scaled up fabrication of graphene based nanoelectronic devices.

cond-mat.mes-hall↗

arXiv2008

Photoresponse in large area multi-walled carbon nanotube/polymer nanocomposite films

We present a near IR photoresponse study of large area multi-walled carbon nanotube/poly(3-hexylthiophene)-block-polystyrene polymer (MWNT/P3HT-b-PS) nanocomposite films for different loading ratio of MWNT into the polymer matrix. We show that the photocurrent strongly depends on the position of the laser spot with maxiumum photocurrent occurring at the metal - film interface. In addition, compared to the pure MWNT film, the photoresponse is much larger in the MWNT/polymer composite films. The time constant for the photoresponse is slow and varies between 0.6 and 1.2 seconds. We explain the photoresponse by Schottky barrier modulation at the metal - film interface.

cond-mat.mtrl-sci↗