Huiyang Deng

Dielectric laser acceleration of sub-100 keV electrons with silicon dual-pillar grating structures

We present the demonstration of high gradient (370 MeV/m) laser acceleration and deflection of sub-relativistic electrons with silicon dual pillar grating structures using both evanescent inverse Smith-Purcell modes and coupled cosh-like modes.

Optical gating and streaking of free electrons with sub-optical cycle precision

The temporal resolution of ultrafast electron diffraction and microscopy experiments is currently limited by the available experimental techniques for the generation and characterization of electron bunches with single femtosecond or attosecond durations. Here, we present proof of principle experiments of an optical gating concept for free electrons via direct time-domain visualization of the sub-optical cycle energy and transverse momentum structure imprinted on the electron beam. We demonstrate a temporal resolution of 1.2±0.3 fs. The scheme is based on the synchronous interaction between electrons and the near-field mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams.

Bias-dependence of luminescent coupling efficiency in multijunction solar cells

In this work we describe the dependence of luminescent coupling efficiency on the bias-voltage in multijunction solar cells. We combine a theoretical derivation and Sentaurus simulations to show that the luminescent coupling efficiency has a significant dependence on the bias voltage, and such dependence is mainly due to the change in the ratio between radiative and non-radiative recombination currents at different bias voltage. We further show that such change is due to the variation in the recombination rate distribution in the cell. In addition to showing the necessity of including bias-dependence in luminescent coupling modeling, this work also demonstrates the importance of including a bias-dependent luminescent coupling efficiency to accurately model multijunction solar cells.

Acceleration of sub-relativistic electrons with an evanescent optical wave at a planar interface

We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration elements surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.

Transverse and longitudinal characterization of electron beams using interaction with optical near-fields

We demonstrate an experimental technique for both transverse and longitudinal characterization of bunched femtosecond free electron beams. The operation principle is based on monitoring of the current of electrons that obtained an energy gain during the interaction with the synchronized optical near-field wave excited by femtosecond laser pulses. The synchronous accelerating/decelerating fields confined to the surface of a silicon nanostructure are characterized using a highly focused sub-relativistic electron beam. Here the transverse spatial resolution of 450 nm and femtosecond temporal resolution of 480 fs (sub-optical-cycle temporal regime is briefly discussed) achievable by this technique are demonstrated.

Ultra-thin film nanostructured gallium arsenide solar cells

State-of-the-art III-V cells have reached the highest energy conversion efficiency among all types of solar cells. However, these cells are not applicable to widespread terrestrial solar energy system yet due to the high cost of epitaxial growth. Ultra-thin film absorbers with advanced light management is one of the most promising solutions to drive down the cost. In this paper, we present an ultra-thin film nano-window gallium arsenide (GaAs) solar cell design. This ultrathin cell consists of a nano-structured Al0.8Ga0.2As window layer on the front side to reduce the reflection and to trap the light, and a metal reflector on the back side to further increase the light path. The 300 nm thick GaAs cell with Al0.8Ga0.2As nano-window shows a broad band absorption enhancement from visible to near infrared (NIR), achieving a spectrally averaged absorption of 94% under normal incidence. In addition, this cell shows excellent angular absorption properties, achieving over 85% spectral averaged absorption at up to 60 degree off normal incidence. Meanwhile, this structure with planar junction and nano-window has solved the issue of low fill factor and low open-circuit voltage in nano-structured GaAs solar cell. A nano-window cell with a 3 μm thick GaAs junction demonstrated an open circuit voltage of 0.9V.

Titanium oxide electron-selective layers for contact passivation of thin-film crystalline silicon solar cells

In crystalline silicon (c-Si) solar cells, carrier selective contacts are among the remaining issues to be addressed in order to reach the theoretical efficiency limit. Especially in ultra-thin-film c-Si solar cells with small volumes and higher carrier concentrations, contact recombination is more critical to the overall performance. In this paper, the advantages of using TiOX as electron-selective layers for contact passivation in c-Si solar cells are analyzed. We characterize the metal/TiOX/n-Si electron-selective contact with the contact recombination factor J0c and the contact resistivity ρc for the first time. Experimental results show that both J0c and ρc decrease after the insertion of TiOX. In addition, the effect of post-deposition rapid-thermal-annealing (RTA) at different temperatures is also evaluated. The best J0c of 5.5 pA/cm2 and the lowest ρc of 13.6 mΩ·cm2 are achieved after the RTA process. This work reveals the potential of TiOX as an electron-selective layer for contact passivation to enable high-efficiency ultra-thin c-Si solar cells with a low cost.

Sub-optical-cycle control of free electrons by optical near-fields

In this contribution we report on research of the interaction between optical near-fields of periodic nanostructures and free-electron beams with potential application in future miniaturized laser-based accelerating devices [1, 2], in ultrafast electron microscopy or diffraction experiments [3, 4] or in photon-induced near-field electron microscopy [5]. Here we experimentally demonstrate a technique allowing sub-optical cycle temporal gating and streaking of electrons at sub-relativistic energies (25–30 keV). A focused electron beam interacts with the near-field mode induced by infrared femtosecond laser pulses on the surface of a silicon nanograting. The field pattern above the surface of a periodic structure can be decomposed to its spatial Fourier components, which propagate along the surface with different phase velocities. Synchronization of the phase velocity of a particular spatial harmonic with the velocity of the co-propagating electrons leads to efficient energy transfer between the laser field and electrons [1, 2]. As this interaction is linear in electric field, the temporal structure of the oscillating electromagnetic field of the femtosecond laser pulse is imprinted to the electron beam energy and/or transverse momentum with sub-cycle precision (200 as in this experiment [6]).

The bias-dependence of luminescent coupling efficiency in multijunction solar cells

In this work we describe the dependence of luminescent coupling efficiency on the bias-voltage in multijunction solar cells. We combine a theoretical derivation and Sentaurus simulations to show that the luminescent coupling efficiency has a significant dependence on the bias voltage, and such dependence is mainly due to the change in the ratio between radiative and non-radiative recombination currents at different bias voltage. We further show that such change is due to the variation in the recombination rate distribution in the cell. In addition to showing the necessity of including bias-dependence in luminescent coupling modeling, this work also demonstrates the importance of including a bias-dependent luminescent coupling efficiency to accurately model multijunction solar cells.