It is well known that atmospheric turbulence severely limits the applications based on the laser propagation though the
atmosphere. The most common disturbances occurring due to the atmospheric turbulence are beam spreading, beam
wandering, and scintillation. These effects are continuously changing in response to atmospheric conditions. In this
study, we create a Non-Kolmogorov turbulence model which is based on the geometrical optics approximation and the
property of Gamma function and integrate with in Gaussian beam analytically. This approach helps us to understand the
propagation of the laser beam at different wavelengths in the atmospheric turbulence.
Improving sensitivity in the infrared spectrum is a challenging task. Detecting infrared light over a wide bandwidth and at low power consumption is very important. Novel solutions can be acquired by mimicking biological eyes such as compound eye with many individual lenses inspired from the nature. The nature provides many ingenious approaches of sensing and detecting the surrounding environment. Even though compound eye consists of small optical units, it can detect wide-angle electromagnetic waves and it has high transmission and low reflection loss. Insects have eyes that are superior compared to human eyes (single-aperture eyes) in terms of compactness, robustness, wider field of view, higher sensitivity of light intensity and being cheap vision systems. All these desired properties are accompanied by an important drawback: lower spatial resolution. The first step to investigate the feasibility of bio-inspired optics in photodetectors is to perform light interaction with the optical system that gather light and detect it. The most common method used in natural vision systems is the ray analysis. Light wave characteristics are not taken into consideration in such analyses, such as the amount of energy at the focal point or photoreceptor site, the losses caused by reflection at the interfaces and absorption cannot be investigated. In this study, we present a bio-inspired optical detection system investigated by wave analysis. We numerically model the wave analysis based on Maxwell equations from the viewpoint of efficient light detection and revealing the light propagation after intercepting the first interface of the eye towards the photoreceptor site.
The reported "stopped rainbow" concept in tapered metamaterial1 and plasmonic2 guiding microstructures has revealed the possibility to obtain local wave enhancement together with spatial chromatic resolution. Recently, this phenomenon has also been demonstrated in graded defect waveguides in photonic crystals3. As the wave is stopped in such single mode defect waveguides, the energy of the stopped wave will be restricted due to the limited volume of the mode, which seriously limits the "brightness" (i.e. its local intensity) of the trapped rainbow. For many applications more desirable would be to stop the light in a bulk of a structure, and to harvest the energy of the stopped wave across all the structure, without any principal restrictions imposed by the mode volume. Such stopping of waves in bulk of a structure has been shown for acoustic waves in sonic crystals recently4 and also for electromagnetic waves in multilayer dielectric slabs5. However high radiation losses in the latter case are inevitable due to the weak index confinement. Here we present a first experimental demonstration of stopped microwave in a chirped 3D photonic crystals. We show that the complete 3D photonic bandgap may significantly reduce the external losses and we also show that the local intensity can be enhanced up to two order of magnitudes. This allows an important increase of absorption/photodetection of microwave radiation. We further demonstrate that the different microwave components stop and reflect at different depths of the chirped structure, which offers a frequency-resolved microwave detection.
In this work we explore a possibility to apply ultrafast 3D laser nanolithography in conjunction with pyrolysis to acquire glass-ceramic 3D structures in micro- and nano-scale. Laser fabrication allows for production of initial 3D structures with relatively small (hundreds nm - μm) feature sizes out of SZ2080 hybrid material. Then, postfabrication heating at 600°C in Ar atmosphere decomposes organic part of the material leaving the glass-ceramic component of the hybrid. Resulting structures are uniformly shrunk by 40%. This brings us one step closer to fabricating highly efficient slow-light absorbers.
We propose a high sensitivity photodetection tool at near-infrared frequencies, based on a principle of slowed- and stopped-light in chirped photonic micro/nano-structures. The main goal is to substantially increase the efficiency of photodetection and provide chromatic resolution in infrared photodetection. In particular we concentrate on the design of the chirped photonic micro/nano-structures providing a maximum field enhancement, and frequency dependence of stopped light distribution.
In this study, we propose a drop-out mechanism based on the enhanced interaction between a defect waveguide and defect microcavities in three-dimensional chirped woodpile photonic crystals (WPCs). We first show that light can be gradually slowed down in the defect waveguide (WG), which is obtained by gradually changing the period of the surrounding WPC along the propagation direction. In result, the waveguide mode gradually approaches the band edge region, while this phenomenon has three consequences. First, the Fourier components of propagating wave will be spatially separated as each frequency will reach its zero velocity at different positions. Second, as the wave slows down, it will penetrate deeper into the surrounding cladding, thus increasing the coupling efficiency between the WG and a nearby placed resonator. Third, the high density of states near the band edge result in highly efficient light scattering of a nearby placed resonator, which in turn increases the quality factor of the interaction. Following this idea, the acceptor type cavities, which are tuned to the localized frequencies, are side-coupled to the WG at respective wave localization areas. Furthermore, drop channels have been introduced to read-out the trapped spectra, showing that the targeted frequencies can be detected selectively. Compared to previous studies, our approach has the advantages of low radiation losses, the absence of any reflection feedback and both enhanced quality factor and transmission of the captured light.
Nanobeam cavity waveguides have drawn great attention of the researchers due to being a useful optical platform for
several applications, e. g. optical switching and filtering.1 Almost all of the past studies investigated high quality (Q)
factors without considering polarization independency. In the literature Zhang et al. proposed a device that enables high
Q for both transverse electric (TE) and transverse magnetic (TM) modes for a specific frequency.2 In our study we
demonstrate a three-dimensional study of polarization independent nanobeam cavity waveguide that consists of annular
photonic crystals (PCs) showing similar optical properties for both TE and TM modes.3 Besides, a detailed analysis of
the shift of the overlapped frequency is investigated with respect to height and width variation of the nanobeam structure.
The designed waveguide is composed of 12 air holes and 4 annular PCs located in the Silicon (nSi=3.46). The radii of all
air holes in the structure are 0.36a. The annular PCs at the interior section have inner dielectric radii of 0.18a and the
outer ones have inner dielectric radii of 0.20a. Silica (nSilica=1.52) material is used as a substrate. The width and height of
the waveguide may be tuned in order to obtain high Q factors at the desired frequency for both polarizations. In our
analysis, we investigated the relation between widths, height and cavity frequency for both TE and TM cases. Obtained
frequencies are fitted to cubic polynomials of the structural parameters width and height. Overlapped frequency curve is
revealed by an equalization of the polynomials of TE and TM resonant frequencies. The findings elucidate the effect of
the parameters on the overlap mechanism of resonant mode matching for both polarizations.
Since the first proposal of the idea of optical cloaking, huge research effort has been spent to implement hiding objects. We propose a broad band all-dielectric partial (unidirectional) cloaking device that hides arbitrary shaped objects. The cloaking structure is designed utilizing graded index (GRIN) photonic crystals. Refractive index distribution of the structure is chosen as a hyperbolic secant profile. In order to generate desired index profiles, both low and high dielectric backgrounds are chosen. The main principle of the cloaking in the study is separating the beam into two main parts while propagating through the composite device. Each part of the separated beam is strongly focused at the center of the stacked GRIN devices. Then these beams diverge and converge repeatedly without deteriorating the planar input field profile. This mechanism dramatically reduces the intensity at the center of the device. Therefore, existence of an object at the cloaked region almost does not affect wave front of the exiting beam due to this special light manipulation mechanism. In this manner, an observer cannot detect the hidden object. GRIN medium is a special type of inhomogeneous environment and light propagation is greatly affected by the presence of GRIN. Any partial cloaking solution as long as being practical and broadband in nature can be preferred. In this case, material selection and easy transferring the design to other electromagnetic spectrum regions become crucial. Therefore, the proposed idea in this work collects these desirable features.
We present a two-dimensional electromagnetic analysis of light propagation through the human eye to examine the eye’s optical properties. The electromagnetic approach has intriguing advantages over the conventional and frequently implemented ray optics analysis. The chromatic, spherical, and coma aberrations and the intensity of the focused light at the retina are computed in this work via full-wave analysis. We also investigate the effects of the cornea’s and lens’s curved structures on the focusing mechanism. The focal length and chromatic and spherical aberrations are observed to change owing to age-related refractive index variation in the lens. In addition, the effects of the lens and curvatures of the human eye on focusing are analyzed. Consequently, for both young and old human eye lenses, the differences due to the aberration variations, curvature surfaces, and gradient index are explored by the wave approach. The intensity distributions on the retina for both on- and off-axis illumination are calculated. A strong correlation between the locations of the nerve fibers and the intensity distribution is confirmed. On the basis of the findings, we can conclude that visual impairment due to deterioration of the human eye structure is more dramatic than that due to aging.
Electromagnetic beams are subject to spatial spreading as they propagate. We have investigated the light propagation
passing through a finite-aperture which is obtained by the two-dimensional square-lattice photonic crystals. It is
observed that the beam that is coupled to the free-space by exiting the axicon-shape photonic crystal resists considerably
against the diffraction. The inspection of the beam profile in the transverse to the propagation direction reveals the
appearance of the side-lobes and we attributed the limited-diffraction beam propagation to these artificially created
lobes. We show that an order of magnitude improvement for beating the diffraction length is achievable with axiconshape
photonic crystal. The advantages of the presented photonic crystal based axicon over the bulk refractive axicons
are the compactness and the integrated nature of the former one in addition to the flexibility of engineering individual
unit cells of photonic crystal structure.
We carry out the study of two-dimensional photonic-crystal waveguide arrays (PCWA) composed of N waveguides
coupled evanescently with each other. The coupling properties of the waveguide modes are investigated using coupledmode
theory. One straightforward application of such an analysis is to channel input power from a central waveguide to
side waveguides. As a result, the appropriate designs of PCWAs may permit the realization of efficient, compact and
novel power dividers. For instance, we show that power dividers, switchers, and Mach-Zehnder interferometers can be
feasible using N=3 channels. On the other hand, N=5 waveguides can split the input power by 1/4 at a certain length.
We present two-dimensional photonic-crystal waveguides for fluid-sensing applications in the sub-terahertz range. The
structures are produced using a standard machining processes and are characterized in the frequency range from 67 to
110 GHz using a vector network analyzer. The photonic crystal consists of an air-hole array drilled into a high-density
polyethylene block. A waveguide is introduced by reducing the diameter of the holes in one row. The holes can be
loaded with liquid samples. For all structures we observe photonic band gaps between 97 and 109 GHz. While the pure
photonic crystal shows the deepest stop band (28 dB), its depth is reduced by 5 dB when inserting a waveguiding
structure. The depth of the photonic band gap is further reduced by several decibels depending on the refractive index of
the liquid that is inserted. With this type of fluid sensor we can clearly distinguish between cyclohexane and
tetrachloromethane with refractive indices of 1.42 and 1.51, respectively. The results are in good agreement with
theoretical calculations based on the 2D finite-difference time-domain (FDTD) method.
Time-Resolved Laser-Induced Fluorescence Spectroscopy (TR-LIFS) represents a potential tool for the in-situ characterization of bioengineered tissues. In this study, we evaluate the application of TR-LIFS to non-intrusive monitoring of matrix composition during osteogenetic differentiation. Human adipose-derived stem cells, harvested from 3 patients, were induced in osteogenic media for 3, 5, and 7 weeks. Samples were subsequently collected and probed for time-resolved fluorescence emission with a pulsed nitrogen laser. Fluorescence parameters, derived from both spectral- and time-domain, were used for sample characterization. The samples were further analyzed using Western blot analysis and computer-based densitometry. A significant change in the fluorescence parameters was detected for samples beyond 3 weeks of osteogenic differentiation. The spectroscopic observations: 1) show increase of collagen I when contrasted against the time-resolved fluorescence spectra of commercially available collagens; and 2) are in agreement with Western blot analysis that demonstrated significant increase in collagen I content between 3- vs. 5-weeks and 3- vs. 7-weeks and no changes for collagens III, IV, and V. Our results suggest that TR-LIFS can be used as a non-invasive means for the detection of specific collagens in maturing connective tissues.
The design of fiber-optic probes plays an important role in optical spectroscopic studies, including fluorescence spectroscopy of biological tissues. It can affect the light delivery and propagation into the tissue, the collection efficiency (total number of photons collected vs. total number of photons launched) and the origin of collected light. This in turn affects the signal to noise ratio (SNR) and the extend of tissue interrogation, thus influencing the diagnostic value of such techniques. Three specific fiber-optic probe designs were tested both experimentally and computationally
via Monte Carlo simulations. In particular, the effects of probe architecture (single-fiber vs. two bifurcated multifiber probes), probe-to-target distance (PTD), and source-to-detector separation (SDS) were investigated on the collected diffuse reflectance of a Lambertian target and an agar-based tissue phantom. This study demonstrated that probe architecture, PTD, and SDS are closely intertwined and considerably affect the light collection efficiency, the extend of target illumination, and the origin of the collected reflected light. Our findings can be applied towards optimization of
fiber-optic probe designs for quantitative fluorescence spectroscopy of diseased tissues.