Semiconductor Gratings Research Articles

Low-frequency active surface plasmon optics on semiconductors

A major challenge in the development of surface plasmon optics or plasmonics is the active control of the propagation of surface plasmon polaritons (SPPs). Here, we demonstrate the feasibility of low-frequency active plasmonics using semiconductors. We show experimentally that the Bragg scattering of terahertz SPPs on a semiconductor grating can be modified by thermal excitation of free carriers. The transmission of SPPs through the grating at certain frequencies can be switched completely by changing the temperature less than 100°C. This semiconductor switch provides a basis for the development of low-frequency surface-plasmon optical devices.

All-optical switching of the transmission of electromagnetic radiation through subwavelength apertures

Unprecedented optical control of the surface plasmon polariton assisted transmission of terahertz radiation through subwavelength apertures is rendered possible by carrier-induced changes to the dielectric properties of a semiconductor grating. Although the study presented is static, the extension of our approach to dynamic switching and tuning is deemed straightforward, opening the way for the realization of ultrafast surface plasmon based devices.

Propagation of Surface Plasmon Polaritons on Semiconductor Gratings

We present time-domain measurements of terahertz surface plasmon polaritons (SPPs) propagating on gratings structured on silicon surfaces. Using single-cycle pulses of terahertz radiation to excite SPPs in a broad frequency range, we observe that the efficient SPPs scattering on the semiconductor periodic structure introduces significant dispersion and modifies the SPPs propagation. A stop gap, or a frequency range where SPPs are Bragg reflected, is formed by the structure. This gap depends strongly on the Si doping density and type. The resonant scattering at the edge of the gap reduces the group velocity by more than a factor of 2. The measurements show a good agreement with our numerical calculations based on the reduced Rayleigh equation.

Thermal switching of the enhanced transmission of terahertz radiation through subwavelength apertures.

We demonstrate that the extraordinary transmission of terahertz radiation through semiconductor gratings of subwavelength apertures can be switched completely by varying the temperature. The enhanced transmission, which is due to the resonant tunneling of surface plasmon polaritons that can be excited in semiconductors at terahertz frequencies, is controlled by thermally modifying the density of free carriers. The transmission through metal gratings cannot be switched in the same way since the carrier density is temperature independent. Thus semiconductors offer an interesting alternative to metals in enhancing the transmission of electromagnetic radiation.

Efficient light absorption in metal–semiconductor–metal nanostructures

A nanoscale metal–semiconductor grating is proposed for efficient and ultrafast photodetection. Theoretical and experimental results of efficient absorption in nanoscopic semiconductor wires are presented. The strong confinement of light in subwavelength metal–semiconductor gratings is achieved by Fabry–Pérot resonances involving vertical transverse magnetic surface-plasmon waves and transverse electric guided waves. Photodetectors have been fabricated with 40×100nm cross sections of Ag and GaAs wires. The reflectivity and photocurrent mesurements are in good agreement with theoretical estimates.

Pure spin current gratings in semiconductors generated by quantum interference

We demonstrate that the quantum mechanical interference between the probability amplitudes for the two-photon absorption of a fundamental (1.55 μm)∼150 fs pulse and for the one-photon absorption of a noncollinearly propagating second-harmonic (775 nm) pulse can create transient, ballistic, purely spin-polarized current gratings in bulk GaAs at room temperature. For fundamental and second-harmonic pulses having orthogonal linear polarizations, two periodically modulated ballistic spin-polarized current gratings are injected that have opposite spins and opposite propagation directions at each point along the grating. Consequently, there is no initial modulation of the charge current, carrier population, or net spin. Before the carrier momentum relaxes, the transport associated with these spin currents forms two oppositely spin-polarized population gratings that are exactly out of phase spatially and that decay by electronic spin diffusion in a time of 3.2 ps. In addition, charge density gratings are directly produced by the quantum interference process, and they decay by ambipolar diffusion and recombination (∼17.6 ps). The polarization selection rules and sample orientation are used to separate the contributions of the current and density gratings.

Characterization of optical diffraction gratings by use of a neural method

Optical scatterometry by use of a neural network is now recognized as an efficient method for retrieving dimensions of gratings in semiconductors or glasses. For an on-line control, a small number of measurements and a rapid data treatment are needed. We demonstrate that these requirements can be met by combining data preprocessing and a proper neural learning method. A good accuracy is attainable with the measurement of only a few orders, even in the presence of experimental errors, with a reduction in learning and computing time.

Influence of two-photon absorption on modulational instability

The instability of a plane wave in an optical medium with two-photon absorption is studied. The analysis is based on the modified nonlinear Schrödinger equation. The linearized equation for the modulation is shown to have an exact solution in terms of confluent hypergeometric functions. It is found that the gain spectrum varies with position. This may result in a change of the wave dynamics and in a decrease of the repetition rate of the pulse train developed from the plane wave. The application of the results to the optical pulse propagation in semiconductor gratings and fiber gratings is discussed.

NONLINEAR ANALYSIS OF DISPERSION FROM DOUBLE PERIODIC SEMICONDUCTOR GRATING

We have derived the dispersion relation for double GaAs semiconductor grating, using nonlinear boundary condition approach. The grating medium is assumed to non-magnetic and it has sinusoidal undulation of angular frequency ω on both side of the film. In the limit when plasma wavevector k p is greater than k‖ wavevector of the wave, we obtained two branches in the dispersion curve which corresponds to electrostatic charge disposition on the two sides of grating. In the limiting case k p < k‖, the grating radiate the electromagnetic field in the vicinity area.

The nonlinear-boundary-condition approach for the study of surface-plasmon dispersion in semiconductor gratings

A new nonlinear-boundary-condition approach for deriving the dispersion relation on the surface of GaAs grating structures is presented. We have used the linear set of wave equations and introduced the grating nonlinearities through the boundary conditions only. We have observed that the presence of a grating splits the surface-plasmon modes into two parts and generates a band gap. This approach is simple and, in the limiting case, reduces to well known theoretical calculations. The dispersion relation is solved for thin-film and thick-film gratings, in the limit in which the dielectric constant is less than zero. The results thus obtained may be used for millimetre waves and are also applicable to the optical region.

3-Dimensional Photonic Crystals at Optical Wavelengths

Different experimental strategies towards the 3-dimensional photonic crystals operating at optical wavelength are classified. The detailed discussion is devoted to the recent progress in photonic crystals fabricated by template method — the photonic band gap materials on the base of opal. The control of photonic properties of opal-based gratings is achieved through impregnating the opal with high refractive index semiconductors and dielectrics. Experimental study demonstrated the dependence of the stop band behaviour upon the type of impregnation (complete or partial) and showed a way for approaching complete photonic band gap. The photoluminescence from opal- semiconductor gratings revealed suppression of spontaneous emission in the gap region with following enhancement of the emission efficiency at the low-energy edge of the gap.

Index gratings in semimagnetic semiconductors

Index gratings in semimagnetic semiconductors

Index gratings in semimagnetic semiconductors

Efficient free-carrier-induced refractive-index gratings are recorded in Cd 0.7Mn 0.3Te semimagnetic semiconductors owing to the trapping of electrons. Such a process also generates a space-charge field and a subsequent photorefractive grating with a smaller efficiency. These gratings are studied both experimentally and theoretically as well as the polarization modulation of diffraction efficiency due to residual birefringence.

An optical heterodyne experiment for sensitive detection of laser induced photocarrier gratings in semiconductors

We measure the diffraction of a cw probe laser beam from an absorption index grating set up in a thin film semiconductor by illumination with a moving optical interference fringe pattern. Taking advantage of the heterodyne detection principle whereby the weak diffracted laser beam is brought to interfere with the probe beam we are able to work with diffraction efficiencies as low as 10−14. When applied to amorphous hydrogenated silicon the diffraction efficiency depends in a characteristic way on an applied external electric field. We identify two physical mechanisms for the diffraction, namely a periodic temperature profile related to the absorption of the laser grating and the quadratic electro-optical effect.

Metastable optical gratings in compound semiconductors

Spatially nonuniform optical illumination of semiconductors containing metastable defects, such as GaAs:EL2 and DX-related defects in III–V and II–VI semiconductors, induces metastable photorefractive and free-carrier gratings. Detailed calculations of the space-charge gratings in these materials are made as functions of fringe spacing and optical exposure. In both defect cases a unique nonequilibrium Fermi level, obtained from a charge balance approximation, can be defined that describes the spatial charge transfer and the resulting modulation of the optical properties of the semiconductor, without resorting to multiple quasi-Fermi levels. The process of writing holographic gratings with low intensities is analogous to adiabatic modulation doping. The transfer of charge from photoquenched regions to nonphotoquenched regions determines the highest spatial resolution that can be supported in the materials. In the case of EL2, the modulation doping produces a p-i-p-i doped superlattice. The DX defects support smaller Fermi level modulation, but have larger optical effects based on persistent photoconductivity and free-carrier gratings.

Characterization of semiconductor sub-micron gratings: is there an alternative to scanning electron microscopy?

The characterization of sub-micron gratings is usually performed on scanning electron microscopes, although the dimensions of the features often make the observations difficult. We report here on a new method applicable to III - V and II - VI semiconductors: we use transmission electron microscopy on wedge-shaped samples oriented along a [111] zone axis. Grating parameters can be obtained with a precision of 1 nm, along with information on the shape and the regularity of the ridge profile, the surface quality as well as the local chemical composition.

Transient Gratings in Semiconductors

Transient Gratings in Semiconductors

Diffraction of ballistic electrons by semiconductor gratings: Rigorous analysis, approximate analyses, and device design

A rigorous coupled-wave analysis is developed to model ballistic electron diffraction by semiconductor gratings with periodic effective mass and for potential energy variations. This analysis includes expressions for diffracted angles, evanescent and propagating orders, the Bragg condition, and diffraction efficiencies. Two approximate diffraction regimes, Bragg and Raman-Nath, are defined in which the rigorous coupled-wave equations (RCWEs) can be solved analytically, and the approximations required, the approximate solutions, and the restrictions placed on the grating parameters for each regime are given. It is shown that both the Bragg regime and the Raman-Nath regime are achievable with physically fabricated semiconductor grating structures. In addition, it is shown that both narrow and broad angular and energy selectivities can be achieved through control of the effective thickness of the grating. These results are used in the design of a two-dimensional electron gas (2-DEG) switch and a 2-DEG broadcast device. >

Introduction to the Theory of Space-Charge Grating Formation in Photoconducting Polymers

ABSTRACTPhotoconducting electrooptic polymers are a new class of photorefractive material. Most applications of photorefractive materials require the formation of a space-charge grating. The formation of space-charge gratings in photorefractive semiconductors has been studied extensively. We consider the formation of photorefractive gratings in a polymer.

Dry etching process for fabrication of optoelectronic gratings in III-V substrates

A novel, three-step dry etching process for the fabrication of first-order diffraction gratings in III-V semiconductors is reported. The process takes advantage of the etching of thin films of SiO2 and of a sensitive, organosilicon electron-beam resist by the H2/CH4 plasma. It reduces wafer handling and eliminates wet stripping of the resist. The gratings were imaged with a scanning tunnelling microscope.