Total Internal Reflection Regime Research Articles

Object-dependent spatial resolution of the reflection-mode terahertz solid immersion microscopy.

Terahertz (THz) solid immersion microscopy is a novel promising THz imaging modality that overcomes the Abbe diffraction limit. In our prior work, an original reflection-mode THz solid immersion microscope system with the resolution of 0.15λ (in free space) was demonstrated and used for imaging of soft biological tissues. In this paper, a numerical analysis, using the finite-difference time-domain technique, and an experimental study, using a set of objects with distinct refractive indexes, were performed in order to uncover, for the first time, the object-dependent spatial resolution of the THz solid immersion microscopy. Our findings revealed that the system resolution remains strongly sub-wavelength 0.15-0.4λ for the wide range of sample refractive indices n = 1.0-5.0 and absorption coefficients α = 0-400 cm-1 (by power). Considering these findings, two distinct regimes of the THz solid immersion microscopy were identified. First is the total internal reflection regime that takes place when the sample refractive index is relatively low, while the sub-wavelength resolution is enabled by both the evanescent and ordinary reflected waves at the interface between a high-refractive-index material and an imaged object. Second is the ordinary reflection regime that occurs when the sample refractive index is high enough, so that there is no more total internal reflection at the interface, while only the ordinary reflected waves inside a high-refractive-index material are responsible for the sub-wavelength resolution. The resultant conclusions are general and can be applied for analysis of solid immersion lenses operating in other spectral ranges, such as visible and infrared, given linear nature of the Maxwell's equations.

Integrated Gires-Tournois interferometers based on evanescently coupled ridge resonators.

We propose integrated Gires-Tournois interferometers (GTIs) for guided modes of dielectric slab waveguides. The proposed GTIs consist of one or several dielectric ridge resonators separated by subwavelength-width grooves patterned into an abruptly terminated slab waveguide and operate at oblique incidence of the fundamental transverse-electric-polarized mode. The grooves act as partially reflective mirrors, whereas the end facet of the last ridge works in the total internal reflection regime and reflects all the incident radiation. We show that the single-ridge structure provides a nonlinear staircase-like phase response characteristic for GTIs. By using several properly arranged ridges, one can engineer group delay or group delay dispersion in a required spectral range. As an example, we design a three-ridge GTI providing an almost constant group delay dispersion in a 50-nm-wide wavelength range. The proposed planar GTIs may find application in integrated optical circuits for introducing or compensating for chromatic dispersion.

Some remarks on enhanced total internal reflection

Abstract The regime of total internal reflection of plane wave on the boundary between a lossless dielectrics and the lossy, or gainy, medium is analysed. It is shown that in order to achieve enhanced total internal reflection (TIR), ie magnitude of the reflection factor larger than one, the existence of the second boundary, virtually maybe infinitely far away, is inevitable. Some contradictory results published in literature are discussed.

Use of electrophoretic light scattering for investigation the parameters of macromolecules

We present a new method for investigation of the electrophoretic mobility and radius of macromolecules based on electrophoresis in the regime of total internal reflection. The theory of dynamic light scattering and its application for the analysis of data obtained by the technique of capillary electrophoresis in the regime of total internal reflection is considered. Experimental setup for simultaneously separation and registration of macromolecules under study was developed. The results of studies of suspensions with latex microspheres are discussed.

Light scattering in albumin solution under the influence of electric field in the regime of total internal reflection

In the paper the application of light scattering from biological macromolecules under the influence of electric fields in the regime of total internal reflection is discussed. An experimental setup on prism for the regime of total internal reflection was developed. The water solution of albumin with the concentration 4 % and pH of the medium is equal to 9.3 was used as an object of study. The obtained results demonstrate the applicability of method for study of biological macromolecules by light scattering in various fields of science.

Optimizing power oscillations in an ellipsometric system

Ellipsometry is a powerful and well-established optical technique used in the characterisation of materials. It works by combining the components of elliptically polarized light in order to draw information about the optical system. We propose an ellipsometric experimental set up to study polarization interference in the total internal reflection regime for Gaussian laser beams. The relative phase between orthogonal states can be measured as a power oscillation of the optical beam transmitted through a dielectric block and whose orthogonal components are then mixed by a polarizer. We show under which conditions the plane wave analysis is valid and when the power oscillation can be optimized to reproduce a full pattern of oscillation and to simulate quarter and half wave plates.

Dual-band enhanced optical absorptance of graphene with resonant tunneling in total internal reflection regime

We propose a stratified structure, where the monolayer graphene is sandwiched between two thin film stacks with optimized period number. A band-pass filter based on the stratified structure is realized. The dual-band enhanced optical absorptance of graphene with resonant tunneling in total internal reflection (TIR) regime is investigated theoretically. Both absorptance peaks are enhanced up to almost 100% in the pass-band filter based on resonant tunneling through a graphene sheet over the communication wavelength range due to the strong light field confinement. With the increasing of the bottom mirror layers, the values of the dual-band absorptance peaks improve accordingly while the peak locations are unchanged. The variation of the angle and wavelength can be exploited to realize tunable filters with good rejection.

Wide-aperture total absorption of a terahertz wave in a nanoperiodic graphene-based plasmon structure

The terahertz absorption spectrum in a periodic array of graphene nanoribbons located on the surface of a dielectric substrate with a high refractive index (terahertz prism) is studied theoretically. The total absorption of terahertz radiation is shown to occur in the regime of total internal reflection of the terahertz wave from the periodic array of graphene nanoribbons, at the frequencies of plasma oscillations in graphene, in a wide range of incidence angles of the external terahertz wave even at room temperature.

Giant cross-polarization conversion of terahertz radiation by plasmons in an active graphene metasurface

Results of theoretical investigation of the cross-polarization conversion of terahertz (THz) radiation by the graphene metasurface formed by a periodic array of graphene nanoribbons located at the surface of a high-refractive-index dielectric substrate are presented. Giant polarization conversion at the plasmon resonance frequencies takes place without applying external DC magnetic field. Pumping graphene by its direct optical illumination or diffusion pumping allows for compensating the Drude losses in graphene and leads to further enhancement of the polarization conversion. It is shown that the total polarization conversion can be achieved in the total internal reflection regime of THz wave from the graphene metasurface at room temperature.

Air gap resonant tunneling bandpass filter and polarizer.

We describe a bandpass filter based on resonant tunneling through an air layer in the frustrated total internal reflection regime, and show that the concept of induced transmission can be applied to the design of thin film matching stacks. Experimental results are reported for Si/SiO2-based devices exhibiting a polarization-dependent passband, with bandwidth on the order of 10 nm in the 1550 nm wavelength range, peak transmittance on the order of 80%, and optical density greater than 5 over most of the near infrared region.

Universal Long-Range Nanometric Bending of Water by Light.

Resolving mechanical effects of light on fluids has fundamental importance with wide applications. Most experiments to date on optofluidic interface deformation exploited radiation forces exerted by normally incident lasers. However, the intriguing effects of photon momentum for any configuration, including the unique total internal reflection regime, where an evanescent wave leaks above the interface, remain largely unexplored. A major difficulty in resolving nanomechanical effects has been the lack of a sensitive detection technique. Here, we devise a simple setup whereby a probe laser produces high-contrast Newton-ring-like fringes from a sessile water drop. The mechanical action of the photon momentum of a pump beam modulates the fringes, thus allowing us to perform a direct noninvasive measurement of a nanometric bulge with sub-5-nm precision. Remarkably, a <10 nm difference in the height of the bulge due to different laser polarizations and nonlinear enhancement in the bulge near total internal reflection is isolated. In addition, the nanometric bulge is shown to extend far longer, 100 times beyond the pump spot. Our high precision data validate the century-old Minkowski theory for a general angle and offer potential for novel optofluidic devices and noncontact nanomanipulation strategies.

Empirical model of total internal reflection from highly turbid media

We demonstrate, to the best of our knowledge, a first accurate empirical model for reflectance measurements from highly turbid media over the full range of incident angles, i.e., for reflectivity values going from unity in the total internal reflection regime to nearly zero when almost all the light is transmitted. Evidence that our model is accurate is provided by extraction of the particle size, followed by independent verification with dynamic light scattering. Our methodology is in direct contrast with the prevalent approach in turbid media of focusing on only the critical angle region, which is just a small subset of the entire reflectance data.

Pulse measurements by randomly quasi phase matched second harmonic generation in the regime of total internal reflection

We demonstrate experimentally the frequency doubling in disordered quadratic nonlinear media in the regime of total internal reflection of the fundamental beam. We show how the transversely emitted second harmonic can be employed for an efficient characterization of short laser pulses.

Surface plasmon resonance from metallic columnar thin films

Surface plasmon (SP) waves on the interface of a dielectric (such as water) and a metallic columnar thin film (CTF) of porosity as high as 0.55 were experimentally and theoretically investigated. The CTFs were made of Al, Au, Ag, or Cr. As the porosity increases, the SP resonance (SPR) dip was found to widen, shift to higher wave numbers, and become asymmetric due to increasing scattering losses. With further increase of porosity, the SPR dip was found to disappear, leaving behind only a peak near the onset to the total internal reflection regime. The shape of the nanoislands constituting the CTF is better described as ellipsoidal than as spherical or spheroidal, indicating thereby the existence of orientational biaxial anisotropy even for CTFs thinner than 60 nm. For a best fit between the theoretical calculations and the experimental data, the CTF was divided into two layers having different porosity and nanoisland shape, particularly for the Ag- and Au-CTFs. The sensitivity of the CTF-based SPR signal to refractive index variations of an analyte infiltrating the nanopores of and in the region adjoining the metallic CTF was found to be doubly enhanced compared to that for the SPR signal from a nonporous metallic film.

Reflection of radiation from moving inhomogeneities of a medium: Inhomogeneous wave regimes and the ambiguity problem of doppler frequency shifts

For the problem of the reflection and transmission of monochromatic radiation incident on a moving inhomogeneity of medium characteristics, the regimes for transforming a real frequency into a complex one are discovered. This makes it possible to develop spectroscopy of complex frequencies for absorbing media or near the regime of total internal reflection. It is also shown that, taking into account the complex frequencies, Doppler frequency shifts are found unambiguously if initial conditions for radiation and medium are specified.

Ray tracing and reflectivity measurements in nematic hybrid cells

We propose an experimental procedure to show that an optical beam may propagate in a planar-homeotropic hybrid nematic crystal cell for incidence angles i larger than the critical one. Its effect on the reflectance coefficient R as a function of i is accounted for by using total internal reflection techniques. We model the propagation of a linearly polarized Gaussian beam of finite diameter through the cell and the reflectance curves R vs. i are calculated theoretically in the total internal reflection regime by using an analytical description. The experimental procedure, cell assembling and methodology are designed so that there is reasonable consistency with the model assumptions. In the optical limit approximation this model allows us to calculate analytically the phase shift, the trajectory and the reflectivity curves of a beam traveling with an extraordinary polarization (P-polarization) inside the cell. Since only some parts of the reflected beams will interfere, Berreman’s method is not suitable for the physical situation considered. The possibility of leakage through the tunneling effect across the cell is also considered and estimated quantitatively. The comparison between the theoretical results and the experimental data shows good agreement between our experimental results and the theoretical curves R vs. i. We compare our results with other measurements reported in the literature and find that the behavior of the reflectance curve is highly affected by the cell thickness-beam waist ratio. The values of this parameter can produce different behaviors of the reflectivity measurements. The analysis of the model predictions and experimental results suggest a possible interpretation of the physical origin of the reflectivity curves and give a qualitative insight into the phenomena behind the total internal reflection regime. Finally, we emphasize the limitations and advantages of the model and of the experimental results presented.

Phase measurements of surface electromagnetic waves on silver with excitation through the substrate

Surface electromagnetic waves are excited in the visible and near-IR regions of the spectrum, and interference measurements are performed. Their excitation is effected by a helium-neon laser (3.39, 1.15, and 0.63 µm) on the interface between air and a silver film of thickness 100 µm deposited on a substrate in the form of a prism of fused quartz. The exciting radiation is supplied from the substrate side in a regime of total internal reflection in the prism on the edge of the silver film. The wave vector of the surface electromagnetic wave investigated is determined from the results of phase measurements. The dependence of the efficiency of the excitation of surface electromagnetic waves on the angle of incidence of the exciting radiation onto the substrate is investigated. The real part of the dielectric function of the silver film is calculated.

Optical sensor based on coated light guides

A new type of intensity-modulated sensors based on a coated light guide is presented. These sensors operate preferentially in the regime of total internal reflection (TIR), rather than relying on violation of TIR as in conventional intensity-modulated sensors. Using a physical model of the new sensor structure, the output light intensity is computed as a function of the complex reflection and transmission coefficients at the substrate-film and film-ambient interfaces. Based on the theoretical results, an experimental setup has been constructed and evaluated for measurements on various liquids. The results confirm the predicted dependence of the output signal on the characteristics of the substrate-film-ambient system. Of particular importance is the novel feature that the output signal increases as the refractive index of the ambient medium increases, at certain incidence angles.