Electromagnetically Induced Transparency via Spin Coherences in Semiconductors

We theoretically consider coherence times for spins in two quantum computer architectures, where the qubit is the spin of an electron bound to a P donor impurity in Si or within a GaAs quantum dot. We show that low-temperature decoherence is dominated by spin-spin interactions, through spectral diffusion and dipolar flip-flop mechanisms. These contributions lead to $1--100\ensuremath{-}\ensuremath{\mu}\mathrm{s}$ calculated spin coherence times for a wide range of parameters, much higher than former estimates based on ${T}_{2}^{*}$ measurements.

Since discrete multilevel transitions of quantum-dot molecules driven by external electromagnetic fields can exhibit quantum coherence effects, such an optical characteristic can be utilized to control propagation of electromagnetic wave through a quantum-dot molecule dielectric film. Since inner-dot tunneling in quantum-dot molecules can be controlled by a gate voltage, destructive quantum coherence among multilevel transitions in quantum-dot molecule would give rise to EIT (electromagnetically induced transparency). In this report, we shall investigate controllable on- and off-resonance tunneling effects of an incident electromagnetic wave through such a quan-tum-dot-molecule dielectric film, of which the optical response is tuned by the switchable gate voltage. We have found from the theoretical mechanism that a high gate voltage can cause the EIT phenomenon of quan-tum-dot-molecule systems, and under the condition of on-resonance light tunneling through the thin film, the probe field will propagation without loss if the probe frequency detuning is zero. By taking advantage of these effects sensitive to the tunable gate voltage, such quantum coherence would be inte-grated in certain photonic structures, and some devices such as photonic switching and transistors can be designed. Transient evolution of optical characteristics in the quantum-dot-molecule dielectric film (once the tunable gate voltage is turned on or off) is also considered in this report.

Summary form only given. Coherent spin information can flow across interfaces of dissimilar materials in engineered structures. The interfaces appear surprisingly permeable over a broad range of temperatures, and the transport of spin information can be controlled with both electric and magnetic fields. Even in materials where momentum scattering is strongly enhanced by defects, spin coherence persists to room temperature. In order to manipulate these states, we have developed additional techniques with the potential for coherent all-optical control over electron spins in nanostructures on femtosecond time scales. Spin splittings corresponding to effective magnetic field strengths of 20 T have been produced using circularly-polarized, below-bandgap pulses with the optical Stark effect. Changes in the magnetization of resonantly tipped nuclei demonstrate the ability to perform all-optical nuclear magnetic resonance (NMR). This sensitive and spatially selective NMR technique may serve as a basis for the coherent manipulation of nuclear moments at the atomic level.

Coherent optical processes on Cs D2 line magnetically induced transitions

: With support from AFOSR, ONR, and NSF (through CUOS), the Alaska Meeting on Fundamental Optical Processes in Semiconductors (AMFOPS) was held in Girdwood Alaska, Aug. 5-10, 2001. The meeting was highly successful (many of the attendees have informed me that this was the best conference they have attended in years). The goal of the conference was to bring together the leading international groups working in semiconductor optics in a workshop format to present the most recent exciting results, and to discuss the future directions of the field. A major emphasis was place on having a strong US presence (since most of the significant conferences in the field have been held in Europe or Asia in recent years), and almost all the active U.S. groups were represented.

Quantum coherence or phaseonium medium with electromagnetic-induced transparency (EIT) may have been widely explored, but the incorporation of boundaries into finite structures like thin films and photonic crystals introduce additional resonant features. A narrow transmission peak exists in resonant medium due to multiple reflections and interference. The corresponding analytical formulas for absorptive and EIT media are derived. A double dip feature is found only for transverse magnetic polarized light, due to longitudinal electric field component in a Fabry-Perot thin film. We study these resonant features in a finite superlattice and discuss potential applications of the features. For phaseonium medium with laser-driven gain, transmission and reflection peaks beyond unity appear between the two EIT resonances. Realizations using solid-state materials such as doped crystals and quantum dots with potential applications are discussed.

An all-optical realization scheme of electromagnetically induced transparency (EIT) in a single silicon optomechanical microring resonator is proposed and demonstrated. Due to the strong mechanical Kerr effect and well-designed microring resonator, two modes with a resonant frequency separation of 292 GHz (2.35 nm) can be tuned into resonance when the control power is about 4.3 µW, and the EIT spectrum is achieved. Our work provides a constructive solution for realizing EIT in a single microcavity with a low mode density. Furthermore, this device is fully integrated on-chip and compatible with current complementary metal-oxide semiconductor (CMOS) processing and has great potential in applications such as light storage, optical sensing, and quantum optics.

We demonstrate a spin manipulation scheme that controls the amplitude as well as the phase of the quantum beats from electron spin coherence by exploiting the relative phase between relevant Larmor precessions and by tipping the precessing spin at a specified time. Theoretical analyses using a phenomenological $N$-exciton eigenstate model indicate that the spin manipulation scheme can take advantage of the underlying exciton-exciton interactions inherent in a semiconductor and can be more effective in an excitonic system than in a corresponding atomiclike system.

A range of semiconductors can host both spin and valley polarizations. Optical experiments on single layers of transition metal dichalcogenides now show that inter-valley scattering can accelerate spin relaxation. The recently discovered monolayer transition metal dichalcogenides (TMDCs) provide a fertile playground to explore new coupled spin–valley physics1,2,3. Although robust spin and valley degrees of freedom are inferred from polarized photoluminescence (PL) experiments4,5,6,7,8, PL timescales are necessarily constrained by short-lived (3–100 ps) electron–hole recombination9,10. Direct probes of spin/valley polarization dynamics of resident carriers in electron (or hole)-doped TMDCs, which may persist long after recombination ceases, are at an early stage11,12,13. Here we directly measure the coupled spin–valley dynamics in electron-doped MoS2 and WS2 monolayers using optical Kerr spectroscopy, and reveal very long electron spin lifetimes, exceeding 3 ns at 5 K (two to three orders of magnitude longer than typical exciton recombination times). In contrast with conventional III–V or II–VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. Supported by a model of coupled spin–valley dynamics, these results indicate a novel mechanism of itinerant electron spin dephasing in the rapidly fluctuating internal spin–orbit field in TMDCs, driven by fast inter-valley scattering. Additionally, a long-lived spin coherence is observed at lower energies, commensurate with localized states. These studies provide insight into the physics underpinning spin and valley dynamics of resident electrons in atomically thin TMDCs.

In this paper, we review our recent experimental studies on electromagnetically induced transparency (EIT) from electron spin coherences in semiconductor quantum wells. Coherent Raman resonances, manifestations of EIT from electron spin coherences at relatively low pump intensities, were demonstrated in both V-type and Λ-type three-level systems via heavy-hole exciton and trion transitions in undoped and doped quantum wells, respectively. Coherent Raman resonances from electron spin coherences via light-hole transitions were also demonstrated in a waveguide geometry that enables a long optical interaction length as well as a large absorption coefficient. Experimental approaches that can avoid or reduce detrimental many-body effects in quantum wells are suggested for the realization of nearly ideal EIT processes.

Optical Processes in Semiconductors: Optical Spectroscopy

The Einstein relation between spontaneous emission and absorption was originally derived for a system consists of a two-state subsystem representing matter and harmonic fields representing radiation. The derivation is based on the detailed balance between these two subsystems under thermal equilibrium. The relationship was later investigated in connection with the interactions between radiation field and solids or semiconductors. The simple derivation dose not hold for semiconductors in general. In certain limiting cases, simple relation was obtained. The validity of this relation is important not only because of its fundamental role connecting two of the most fundamental optical processes in semiconductors, but mostly also because of its wide use as a practical method to measure the optical gain of a semiconductor. The validity of this relation for semiconductors has been an issue of controversial for some time. In this paper we numerically examine the validity of this relationship for several different lineshapes including Lorentzian, Gaussian, Sech, and a convoluted double Lorentzians (CDL). We find out that at relatively low density above transparency level, all first three lineshapes violate the Einstein relation. The relation is approximately valid at high density. At very high density, the validity of the Einstein relation holds well for all three lineshapes. The reason behind this observation is explained. The CDL lineshape has been shown analytically to obey the Einstein relationship previously. We show that for a 2D semiconductor with parabolic bands, the CDL lineshape can be integrated analytically. This analytic lineshape is compared with a simple Lorentzian lineshape.

This chapter contains sections titled: Introduction Basics of Photoluminescence Band-to-Band Transitions Optical Transitions in GaN Group-II Element Related Transitions Blue Luminescence Band in Undoped GaN Surface-Related Blue Luminescence in Etched GaN Optical Properties of GaN Doped with Rare Earths Optical Properties of Alloys Optical Properties of Nitride Heterostructures Quantum Dots Intraband or Intersubband Transitions in GaN/AlGaN Quantum Wells Nonlinear Optical Properties of III-Nitrides References

We present a fully quantum mechanical theory for an interacting system of photons and Coulomb correlated electrons and holes in a semiconductor, using a pertubation series in the exciting laser field. The theory provides microscopic descriptions of nonlinear optical processes in semiconductors related to the electromagnetic field quantization. As an application of the theory we show that it is possible to transfer the exciton-exciton Coulomb correlation to photons, thus producing pairs of near-gap photons with a high degree of quantum entanglement. The photon pairs emerge from the spontaneous optical decay of biexcitons into two polaritons. The pair intensity correlations, calculated in the low density limit for a CuCl slab and for semiconductor microcavities, exhibit quantum features which can be observed by coincidence detection.

With the advent of nanophotonics, there has been a considerable interest in studying the optical processes in semiconductors and their nanostructures. It appears from the literature that the investigations have been carried out on the assumption that the carrier energy spectra are invariant quantities in the presence of intense light waves, which is not fundamentally true. The physical properties of semiconductors in the presence of light waves which change the basic dispersion relation are relatively less investigated in the literature. In this chapter we shall study the EEM in III–V, ternary and quaternary semiconductors and their nanostructure on the basis of newly formulated electron dispersion law under external photo excitation under different physical conditions.