Near and far field

About: Near and far field is a research topic. Over the lifetime, 15922 publications have been published within this topic receiving 220571 citations.

Surface‐emitting second order distributed Bragg reflector laser with dynamic wavelength stabilization and far‐field angle of 0.25°

Abstract: Operation of a dynamically stabilized surface‐emitting AlGaAs channeled‐substrate‐planar large‐optical‐cavity distributed Bragg reflector laser with a 0.25°×8° far‐field pattern is reported. With 50% duty cycle and 50 ns–2 ms pulse length operation at room temperature, thresholds as low as 150 mA were obtained. No side modes were observed to 40 dB below the principal wavelength intensity under 100% modulation depth operation.

Generalized spectral method for near-field optical microscopy

Abstract: Electromagnetic interaction between a sub-wavelength particle (the “probe”) and a material surface (the “sample”) is studied theoretically. The interaction is shown to be governed by a series of resonances corresponding to surface polariton modes localized near the probe. The resonance parameters depend on the dielectric function and geometry of the probe as well as on the surface reflectivity of the material. Calculation of such resonances is carried out for several types of axisymmetric probes: spherical, spheroidal, and pear-shaped. For spheroids, an efficient numerical method is developed, capable of handling cases of large or strongly momentum-dependent surface reflectivity. Application of the method to highly resonant materials, such as aluminum oxide (by itself or covered with graphene), reveals a rich structure of multi-peak spectra and nonmonotonic approach curves, i.e., the probe-sample distance dependence. These features also strongly depend on the probe shape and optical constants of the model. For less resonant materials such as silicon oxide, the dependence is weak, so that the spheroidal model is reliable. The calculations are done within the quasistatic approximation with radiative damping included perturbatively.

The determination of the surface conductivity of a partially coated dielectric

Abstract: A variational method is given for determining the essential supremum of the surface conductivity of a partially coated anisotropic dielectric medium from a knowledge of the far field pattern of the time-harmonic electric field at fixed frequency corresponding to an incident plane wave. It is assumed that the shape of the scatterer has been determined (e.g., by solving the far field equation and using the linear sampling method). Numerical examples are given for the scalar case with constant surface conductivity.

Terahertz Radiation from Semiconductors

Abstract: We provide a review of the different schemes for generating terahertz (THz) radiation using photoconductive emitters excited by femtosecond lasers. The discussion is concerned principally with large-area emitters because of their relative simplicity, both in fabrication and analysis, and their capabilities for high-power THz generation. In addition to discussing the principal characteristics of these emit- ters, we present a simple, but unified description of their behavior. The principal ingredients in this description are a Drude-Lorentz model for the carrier dynamics combined with a suitable solution of the radiation problem for a current sheet. This formalism permits one to examine the nature of the generated THz waveforms in the near and far field, as well as to consider the effect of the material properties and excitation conditions on the THz emission process. Within this picture, we describe the origin of the strong enhancement in THz emission that is observed upon appli- cation of an external magnetic field to emitters relying on transient photocurrents flowing perpendicular to the surface. Saturation processes that limit the efficiency of THz emission are also an important feature of these devices. These effects are dis- cussed in terms of perturbation of the bias field induced by space-charge screening and the THz radiation field.

Theoretical treatment for scattering scanning near-field optical microscopy

Abstract: A scanning probe interacts with the optical near field near a sample surface. The whole system radiates, and the radiation is collected in the far-field domain. The collected far field contains subwavelength information about the sample surface. A microscopic description of the device is presented. A self-consistent procedure is employed that takes into account the optical coupling effects between the microscopic objects in the probe–surface system to calculate the induced field at the probe and at the surface microscopic features. The radiation stemming from the system can then be described in the half-space. The intensity of the field radiation at the entrance of the far-field detector determines the received signals. Two incident fields, total internal reflection and external reflection, are examined numerically. Subwavelength resolutions of the device are clearly shown. The results are used to discuss other relevant aspects: the resolution of the device, the position of the far-field detector, and the probe material.