UltrabroadbandOptical Diffraction Tomography

In our previous work, a highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes in the refractive index of a surrounding liquid was developed (1). We proposed a technique for creating a temperature insensitive refractometer that utilizes core and cladding modes in an open-top ridge waveguide architecture in order to discriminate between Bragg wavelength changes in temperature and refractive index (2). In this work, a technique for creating a temperature insensitive refractometer that utilizes TE and TM modes in an open-top ridge waveguide design is presented. By using the TE mode resonance as a temperature reference, the relative shift of the TM mode can be monitored in order to measure the refractive index of liquids under test. Specifically, the device fabricated here produces a relative resonance shift of 1 pm for every 1×10 -4 of measured index change, with a temperature sensitivity For temperature insensitive sensors based on fiber Bragg gratings, several techniques have been proposed to discriminate between Bragg resonance spectral shifts associated with refractive index measurements and those induced by fluctuations in temperature. These techniques are implemented by using: a second Bragg grating in a side-polished fiber Bragg grating refractometer (3-4), higher order modes in an etched-core of a fiber Bragg grating sensor (5-6), and higher order modes in a tilted fiber Bragg grating sensor (7-12). We proposed a technique for creating a temperature insensitive refractometer that utilizes core and cladding modes in an open-top ridge waveguide architecture in order to discriminate between Bragg wavelength changes in temperature and refractive index. The relative shift of the core mode resonance to cladding mode resonance is used to measure the refractive index of substances under test. The device fabricated produced a relative resonance shift of 1 pm for every 5×10 -4 of measured index change, with a temperature sensitivity ~ 0.5 pm/°C (2). Taking a similar approach, here, we reported another technique for creating a temperature insensitive refractometer that utilizes TE and TM modes in an open-top ridge waveguide architecture in order to discriminate between changes in temperature and refractive index. In our previous work (1), a highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes in the refractive index of a surrounding liquid was developed. The structure of the open-top ridge waveguide is as shown in Fig.1. The center ridge waveguide with the Bragg grating is tested as a refractometer by coupling the light source into the end of the waveguide. The function of the two adjacent waveguides is to act as a barrier and to partially prevent the liquid from flowing away from the waveguide containing the grating. The guided light of the center waveguide couples evanescently into the surrounding liquid through the top and sides of the waveguide. When a Bragg grating is induced in the core of an open-top ridge waveguide with a larger birefringence, TE and TM resonances are observed when the light guided by the core is phase matched by the grating structure. Both TE and TM resonances are sensitive to the liquid refractive index on the top layer of the open-top ridge waveguide. The TE and TM sensitivities to temperature fluctuations however, are more closely matched. These characteristics can be used to decouple fluctuations of the Bragg resonance of the core mode due to temperature from those changes that are due to variation in the refractive index of the analyte liquid. In the experiments presented here, the variation of TE and TM resonances are investigated as a function of temperature and the external refractive index nt. A theoretical model is developed to investigate the performance of some potential waveguide structures. Relationships between the waveguide core size, refractive index distribution, as

Wave theory and geometrical optics are used to investigate the effect of small changes in size and index of refraction on the resonance wavelength of spherical microresonators. It is shown that changes in the index of refraction have two effects: These changes affect the phase jump on the surface and the optical path length in the resonator. Under certain conditions the effect of the external or internal index of refraction becomes negligible. The influence of the order number of the resonance modes is investigated. Finally, the results of the theoretical analyses are applied to calculate the effect of temperature on the resonance wavelength.

The Xenopus spindle is as dense as the surrounding cytoplasm.

Limited angle optical diffraction tomography (ODT) is a 3D quantitative phase imaging method that allows to retrieve information about 3D refractive index (RI) distribution of live, unlabeled biosamples. The main limitation of this method is that its common transmission configuration results in very low axial resolution. On the other hand, optical coherence tomography (OCT), working in its most popular reflection configuration retrieves information about the gradient of the RI of investigated samples. However, the results are of qualitative nature. Moreover, due to low numerical aperture of the objective lens typically used in OCT systems, the resolution is high in the axial direction and relatively low in transverse direction. From the point of view of K-space filling, these two imaging modalities are complementary. Here we present a method of combining ODT projections with OCT scans. The combined technique, called optical coherence diffraction tomography (OCDT) operates in transflective mode, where ODT is captured in transmission and OCT in reflection. Theory behind conversion of OCT scans into ODT projections is given. With the use of numerical simulations we show what enhancement can be obtained when OCT and ODT data are combined directly. Also, experimental verification is presented.

: The optical distortion in neodynium doped glass which is induced by pump radiation is discribed. Optical distortion was observed at 6328A and the optical path length was found to be dependent on four primary effects: (1) change in physical length; (2) Change in refractive index due to temperature rise; (3) Change in index resulting from stress; (4) Change in index associated with an excited state population of neodymium ions. The experimental techniques used and the results obtained are presented. Included are measurements of optical path length variations, pump-induced birefringence, change in physical length, change in refractive index, bulk temperature rise, and the deflection of a light beam. The theory of thermal optic distortion was developed for the first time to include Fermat's principle. This approach leads to equations defining both the slope and trajectory of rays through the material. The resulting equations are employed to predict ray refraction, beam divergence, and the optical path length through the material as a function of radius, time, and polarization. Good agreement between theory and experiment is achieved provided a new term is added to the expression for the change in refractive index. This term arises from the fact that the polarizability of the neodymium ion in its excited 4F(3/2) level is different from its value in the 4I(9/2) ground level. The inclusion of this new term in the expression for the change in refractive index implies that large optical distortions can exist in 'athermalized' glass.

GeO<SUB>2</SUB>-B<SUB>2</SUB>-O<SUB>3</SUB>-SiO<SUB>2</SUB> thin films were fabricated by plasma enhanced chemical vapor deposition method. Boron codoping into a GeO<SUB>2</SUB>-SiO<SUB>2</SUB> thin film induced large absorption in the vicinity of 240nm, and OH absorption decreased compared to GeO<SUB>2</SUB>-SiO<SUB>2</SUB> films. These films of 5 micrometers in thickness exhibited large positive refractive index change without hydrogen loading by irradiation with ArF (193nm) excimer laser pulses. Induced refractive index change was approximately 0.002 which was measured by the prism coupling method. A waveguide was written in this high photosensitive glass film by UV irradiation. The guided mode of the waveguide seems to be single and estimated refractive index change was approximately from 0.003 to 0.004. Three unique phenomena were found in 0.2micrometers thick films on Si substrate. First, these films exhibited large negative refractive index and positive thickness changes by irradiation with ArF laser pulses. Induced negative index change was larger than 0.02 and thickness change was more than 1%. Silica films doped only boron or germanium didn't exhibit such negative index changes. Second, the annealing before laser irradiation decreased the photosensitivity of these films remarkably. Third, these induce refractive index and thickness changes were decreased with time rapidly. These mechanisms were under investigation.

Holographic tomography (HT) or optical diffraction tomography provides slice-by-slice information about the refractive index (RI) of three-dimensional (3D) samples and is emerging as an important label-free imaging modality for life sciences. HT systems go beyond digital holographic microscopes (DHM) that provide a two-dimensional representation of the total accumulated phase acquired by a plane beam on transmission through a 3D sample. While the early HT systems used a direct reconstruction methodology based on the Fourier diffraction theorem, in recent years, there is an increasing shift towards iterative optimization frameworks for solving the 3D RI reconstruction problem. Iterative frameworks naturally offer several advantages for addressing the data incompleteness issues (e.g. missing illumination angles) and have superior noise handling capability, since they employ suitable constraint functions. Despite this algorithmic framework shift, the HT system hardware still largely uses the multi-angle illumination geometries that were suitable for reconstructions based on the Fourier diffraction theorem. The present work examines the possibility of HT reconstruction through the use of on-axis structured illumination(s) that are nominally incident on the 3D sample only along the direction of the optical axis of the system. Through a simulation study, it is shown that a cross-talk free slice-by-slice 3D RI reconstruction of the sample is possible in this case via the use of sparsity penalties if the slice-to-slice distance obeys a design curve based on the notion of effective depth of focus. The simulation results for two-, three- and four-slice 3D objects with laterally overlapping features clearly outline the separate roles played by the slice-to-slice de-correlation of the field propagating through the 3D sample (modeled via multi-slice beam propagation) and that of the sparsity penalty used to guide the iterative solution. Our results suggest the possibility of realizing an axial structured illumination tomography (ASIT) system configuration that avoids the use of hardware-intensive multi-angle illumination geometry. An ASIT system can, for example, be realized by minimal modification of a traditional DHM system.

Nanodiamonds have acknowledged growing attention due to their facile functionalization, stable fluorescence, low toxicity, and decent biocompatibility. However, despite advances applications of nanodiamonds in drug delivery, catalysis and bio-sensing, laser irradiation still limits long-time tracking of nanodiamonds in living cells due to phototoxicity. Here, using optical diffraction tomography, we performed quantitative morphological and biophysical analysis of living cells via endocytosis or electroporation of nanodiamonds (5 nm, 35 nm, and 100 nm) without the need for a fluorescence label. Optical diffraction tomography is an inexpensive and noninvasive microscopy technique, which images cells and subcellular structures as a function of their refractive index. The laser excitation power is much weaker than in the case of fluorescence microscopy, which reduces phototoxicity. Thanks to the very high refractive index of diamond, nanodiamonds in HeLa cells can be clearly discriminated from cellular structures using optical diffraction tomography. As an application, we show that aggregation and deaggregation of internalized nanodiamonds can be detected via changes in the refractive index distribution of the entire cell. Optical diffraction tomography successively images prevention of in-cell particle aggregation through polyglycerol coating of nanodiamonds. In the case of endocytosis, optical diffraction tomography shows deaggregation of nanodiamonds after a prolonged incubation time.Together, our findings implicate that refractive index measurements are a favorable tool to track nanodiamonds, without a fluorescent label, inside living cells. This could be useful to study real-time therapeutic or metabolic activities in living cells using very weak laser irradiation. Finally, the elaborate creation of fluorescent defects in nanodiamonds becomes redundant.

A detail study was done on the sensitivities of 1-D photonic crystal (PC) and 2-D PC coupled cavity sensors with changing sensing layer parameters of thickness and refractive index (RI). Though both refractive index and thickness are interrelated they have significant individual affects on device response. In 1-D PC shifts in normal transmission peak due to surface change in thickness and RI and in 2-D PC coupled cavity shifts in transmission dip due to surface changes are observed. Here sensitivity analysis in change in thickness and RI on these devices was done for four cases; case 1: change in thickness from 2nm-10nm on PC sensors, case 2: change in thickness from 75nm-175nm on PC sensors, case 3: change in RI in thin film (6nm) on surface and case 4: change in RI in thick film (100nm) on sensors surface. Sensitivities due to change in thickness (S_t) of 1-D PC and 2-D PC coupled cavity were calculated from the slope of the sensitivity curves and found to be (for RI of 1.4) 1.423nm/nm and 2.285nm/nm for case 1 and 0.455nm/nm and 0.801nm/nm for case 2. Sensitivities due to change in RI (S_r) of 1-D PC and 2-D PC coupled cavity were obtained from the transmission peak and dip shifts due to change in RI from 1(air) to 2. Sr for 1-D PC and 2-D PC coupled cavity were found to be 70nm/RIU and 103nm/RIU for case 3 and 143nm/RIU and 213nm/RIU for case 4. The results are based on FDTD simulations.

Thermal sensitivity of photonic crystal fibers in opto-electronic oscillators

We measure the absorption coefficient and refractive index changes produced in photorefractive quantum wells of GaAs by using a high-sensitivity periodically phase-modulated two-wave mixing technique. The technique allows us to measure both the amplitude and phase of the refractive and absorption index changes simultaneously. Thus, there is no necessity to use the Kramers–Kronig relation when either the absorption or refractive index is known. We also measure the response time of the grating formation in the frequency domain, yielding at I0 = 11 mW cm−2, which shows that quantum wells of GaAs are one of the fastest materials for dynamic holography.

This paper quantitatively describes the optical distortion in neodymium-doped glass which is induced by pump radiation. We have found the optical path length at 6328 Å dependent on four primary effects: (1) change in physical length; (2) change in refractive index due to temperature rise; (3) change in index resulting from stress; (4) change in index associated with an excited-state population of neodymium ions. Section I presents the experimental techniques used and the results obtained. Included in this section are measurements of optical-path-length variations, pump-induced birefringence, change in physical length, change in refractive index, bulk temperature rise, and the deflection of a pencil of rays. Section II compares the results of Section I with the theory set forth in the previous paper. Good agreement between theory and experiment is achieved provided a term which takes account of the index change associated with an excited-state population of neodymium ions is included. This term arises from the fact that the polarizability of the neodymium ion in its excited 4F3/2 level is different from its value in the 4I9/2 ground level. The inclusion of this new term in the expression for the change in refractive index implies that large optical distortions can exist in ``athermalized'' glass.

Absorption coefficient and refractive index changes of a quantum ring in the presence of spin-orbit couplings: Temperature and Zeeman effects

Effect of intense high-frequency laser field on the linear and nonlinear intersubband optical absorption coefficients and refractive index changes in a parabolic quantum well under the applied electric field

Due to the limitations of measurement equipment and the influence of factors such as the environment and target, measurement errors may occur during the data acquisition process of airborne LiDAR bathymetry (ALB). The refractive index of water is defined as the propagation ratio of the speed of light waves in a vacuum to that in water; this ratio influences not only the propagation speed of the laser pulse in water but also the propagation direction of the laser pulse entering water. Therefore, the influence of refractive index changes in water on the ALB errors needs to be analyzed. To this end, the principle of ALB is first briefly introduced. Then, the calculation method for the refractive index of water is described with Snell’s law and an empirical formula. Finally, the influence of refractive index changes on ALB errors is analyzed using the derived formula at the water–air interface and in the water column. The experimental results showed that in a constant elevation of 50 m for a bathymetric floor, the refractive index changes in water caused by temperature, salinity, and depth are less than 0.001. The maximum bathymetric error and maximum planimetric error caused by the refractive index changes at the water–air interface are 0.036 m and 0.015 m, respectively. The ALB errors caused by refractive index changes in the water column are relatively low, and the water column does not need to be layered to calculate the ALB errors. The influence of refractive index changes in water on the ALB error is minimal, accounting for only a small proportion of all bathymetric errors. Thus, it is necessary to determine whether the effect of the ALB error due to refractive index changes in water needs to be corrected based on the accuracy requirements of the data acquisition. This study and analysis can provide a reference basis for correcting ALB errors.