Charge carriers in graphene behave like massless Dirac fermions (MDFs) with linear energy-momentum dispersion , providing a condensed-matter platform for studying quasiparticles with relativistic-like features. Artificial graphene (AG)-a structure with an artificial honeycomb lattice-exhibits novel phenomena due to the tunable interplay between topology and quasiparticle interactions . So far, the emergence of a Dirac band structure supporting MDFs has been observed in AG using molecular , atomic and photonic systems , including those with semiconductor microcavities . Here, we report the realization of an AG that has a band structure with vanishing density of states consistent with the presence of MDFs. This observation is enabled by a very small lattice constant (a = 50 nm) of the nanofabricated AG patterns superimposed on a two-dimensional electron gas hosted by a high-quality GaAs quantum well. Resonant inelastic light-scattering spectra reveal low-lying transitions that are not present in the unpatterned GaAs quantum well. These excitations reveal the energy dependence of the joint density of states for AG band transitions. Fermi level tuning through the Dirac point results in a collapse of the density of states at low transition energy, suggesting the emergence of the MDF linear dispersion in the AG.

Optical control of exciton fluxes is realized for indirect excitons in a crossed-ramp excitonic device. The device demonstrates experimental proof of principle for all-optical excitonic transistors with a high ratio between the excitonic signal at the optical drain and the excitonic signal due to the optical gate. The device also demonstrates experimental proof of principle for all-optical excitonic routers.

We report on observation of the spin transport of spatially indirect excitons in GaAs/AlGaAs coupled quantum wells (CQW). Exciton spin transport over substantial distances, up to several micrometers in the present work, is achieved due to orders of magnitude enhancement of the exciton spin relaxation time in CQW with respect to conventional quantum wells.Spin physics in semiconductors includes a number of interesting phenomena in electron transport, such as currentinduced spin orientation (the spin Hall effect), 1-3 spininduced contribution to the current, 4 spin injection, 5 and spin diffusion and drag. [6][7][8][9][10][11] Besides the fundamental spin physics, there is also considerable interest in developing semiconductor electronic devices based on the spin transport, which may offer advantages in dissipation, size, and speed over chargebased devices; see ref 12 and references therein.Optical methods have been used as a tool for precise injection, probe, and control of electron spin via photon polarization in semiconductors. A major role in the optical properties of semiconductors near the fundamental absorption edge is played by excitons. The spin dynamics of excitons in GaAs single quantum wells (QW) was extensively studied in the past; see refs 13-15 and references therein. It was found that the spin relaxation time of excitons in single QW is of the order of a few tens of picoseconds. Because of the short spin relaxation time, no spin transport of excitons was observed until this work.Here, we report on the observation of the spin transport of spatially indirect excitons in GaAs coupled quantum wells (CQW). The spin relaxation time of indirect excitons is orders of magnitude longer than one of regular direct excitons. In combination with a long lifetime of indirect excitons, this makes possible spin transport of indirect excitons over substantial distances.The spin dynamics of excitons can be probed by the polarization resolved spectroscopy. In GaAs QW structures, the σ + (σ -) polarized light propagating along the z axis creates a heavy hole exciton with the electron spin state s z ) -1/2(s z ) +1/2) and hole spin state m h ) +3/ 2(m h ) -3/2). In turn, heavy hole excitons with S z ) +1 (-1) emit σ + (σ -) polarized light. Excitons with S z ) (2 are optically inactive. The polarization of the exciton emission P ) (I + -I -)/(I + + I -) is determined by the recombination and spin relaxation processes. For an optically active heavy hole exciton, an electron or hole spin-flip transforms the exciton to an optically inactive state ( Figure 1a) causing no decay of emission polarization. The polarization of emission decays only when both the electron and hole flip their spins. This can occur in the two-step process due to the separate electron and hole spin flips and the singlestep process due to the exciton spin flip. [13][14][15] The rate equations describing these processes 14,15 yield for the case when the splitting between S z ) (1 and (2 states ∆ is smaller than k B T the polarization of the exc...

We demonstrate experimental proof of principle for all-optical excitonic transistors where light controls light by using excitons as an intermediate medium. The principle of operation of all-optical excitonic transistors is based on the control of exciton fluxes by light.