In this letter we report on a metal–semiconductor–metal photodetector based on thick relaxed Ge layers, epitaxially grown on silicon after insertion of a low-temperature-grown Ge buffer layer. The detector shows a good responsivity at normal incidence at both 1.3 and 1.55 μm, with a maximum responsivity of 0.24 A/W at 1.3 μm under a 1 V bias. A response time of about 2 ns has been measured.

We consider a sub-wavelength periodic layered medium whose slabs are filled by arbitrary linear metamaterials and standard nonlinear Kerr media and we show that the homogenized medium behaves as a Kerr medium whose parameters can assume values not available in standard materials. Exploiting such a parameter availability, we focus on the situation where the linear relative dielectric permittivity is very small thus allowing the observation of the extreme nonlinear regime where the nonlinear polarization is comparable with or even greater than the linear part of the overall dielectric response. The behavior of the electromagnetic field in the extreme nonlinear regime is very peculiar and characterized by novel features as, for example, the transverse power flow reversing. In order to probe the novel regime, we consider a class of fields (transverse magnetic nonlinear guided waves) admitting full analytical description and we show that these waves are allowed to propagate even in media with ǫ < 0 and µ > 0 since the nonlinear polarization produces a positive overall effective permittivity. The considered nonlinear waves exhibit, in addition to the mentioned features, a number of interesting properties like hyper-focusing induced by the phase difference between the field components.

Resonant optical second-harmonic and sum-frequency generation are applied to probe electronic transitions at the Ca-terminated epitaxial CaF2/Si(l 11) interface. A band gap of 2.4 eV is established for the interface states, a value twice as large as that in bulk Si, but only j of the band gap in CaF2. The experimental three-wave-mixing spectra can be modeled by a two-dimensional band gap and a narrow resonance 150 meV below the band edge, the latter being tentatively assigned to a transition to a bound two-dimensional exciton.PACS 42.65.Ma, Understanding the nature of solid/solid interfaces is an area of great fundamental and practical importance. The characteristics of interfaces strongly influence the behavior of electronic devices with small dimensions and are critical in determining the morphology of thin-film growth, particularly in the case of epitaxial structures. Fully developed buried interfaces present difficulties for analysis: The unique features of the interface are present only in a few atomic layers of material, but the interfacial region will generally be covered by an overlayer of many times this thickness. Consequently, many of the sensitive and highly developed techniques appropriate for surfaces may not be suitable for this interesting class of problems. In this Letter, we present the first application of three-wave-mixing spectroscopy to the problem of solid/solid interfaces. The method is purely optical and can, because of its large probing depth, be used to investigate buried interfaces. It relies on the second-order nonlinear optical processes of second-harmonic and sum-frequency generation. These effects are forbidden (within the electric-dipole approximation) in centrosymmetric media and, hence, exhibit a high degree of sensitivity to interfaces, where the inversion symmetry must be broken. l The technique has been previously employed in studies of the electronic 2,3 and vibrational 4,5 spectra of molecular monolayers. Here we report for three-wave-mixing spectroscopy of the epitaxial interface of CaF2/Si(l 11). This material system 6 " 12 has recently attracted considerable attention as a prototype of a well-controlled semiconductor/insulator interface, as well as for its potential technological importance. The resonant second-harmonic and sum-frequency spectra presented in this work permit a value for the previously unknown interface band gap to be established directly. The observed energy difference of 2.4 eV between the filled and empty interface states stands in marked contrast to the band gaps of the bulk Si (1.1 eV) and CaF2 (12.1 eV), indicating the distinctive nature of the interfacial region.The three-wave-mixing spectra for the CaF2/Si(lll) interface were obtained by exposing the sample to laser radiation from a tunable source. The intensity of the reflected light at the second-harmonic (SH) frequency was measured as a function of wavelength. Sumfrequency (SF) data were collected for mixing of the tunable source with the light from a laser operating at a fixed frequency. Tuna...

We investigate propagation of a transverse magnetic field through a nonlinear metamaterial slab of sub-wavelength thickness and with a very small and negative linear dielectric permittivity. We prove that, for a given input intensity, the output intensity is a multi-valued function of the field incidence angle so that the transmissivity exhibits angular multi-stability and a pronounced directional hysteresis behavior. The predicted directional hysteresis is a consequence of the fact that the linear and nonlinear contributions to the overall dielectric response can be comparable so that the electromagnetic matching conditions at the output slab boundary allow more than one field configurations within the slab to be compatible with the transmitted field.Optical hysteresis behavior and bistability are fascinating nonlinear phenomena which have attracted a large research interest in the last decades [1] mainly for their potential photonic applications as optical memories, logic gates and optical computing devices [2]. The possibility of engineering the dielectric permittivity and the magnetic permeability offered by metamaterials [3] has recently allowed to prove that the feedback mechanism supporting bistability is facilitated by the the opposite directionality of the phase velocity and the energy flow in the negative index metamaterial [4]. Nonlinear propagation of light through alternating slabs of positive and negative refractive index materials characterized by a vanishing average refractive index has been considered and bistable switching [5] and gap soliton formation [6] have been predicted. Metal-dielectric multilayer structures have also proved to be media exhibiting interesting bistable behavior [7] since the small average dielectric permittivity combined with the nonlinearity allows the sign of the effective overall dielectric constant to be dependent on the optical intensity [8,9].In this Letter, we investigate electromagnetic transmission through a metamaterial slab of sub-wavelength thickness, characterized by a very small and negative linear dielectric permittivity and exhibiting focusing Kerr nonlinearity. We numerically solve Maxwell equations for the problem of reflection and transmission of an inclined incident transverse magnetic (TM) plane wave and we show that the slab transmissivity is, for a given input intensity, a multi-valued function of the field incidence angle. The novel directional hysteresis behavior is due to the fact that the slab can host the extreme nonlinear regime where the linear and nonlinear contributions to the overall dielectric response can be comparable [8] so that the electromagnetic matching conditions at the output slab boundary allows more than one possible field configurations compatible with the transmitted field.Consider a transverse magnetic (TM) monochromatic field (with time dependence exp(−iωt)) E z (x, z)ê z and H = H y (x, z)ê y propagating through the metamaterial slab, embedded in vacuum, reported in Fig.1(a) of sub-wavelength thickness (along the zaxis) L ...

No abstract