Variation Of Complex Refractive Index Research Articles

Tunable IR perfect absorbers enabled by tungsten doped VO2 thin films

The temperature tunability of complex dielectric constants of vanadium dioxide (VO2) makes it a promising phase-change material for use in active, dynamic, tunable photonics applications. Specifically, the semiconductor-to-metal phase transition in VO2 enables reversible, broadband, and large complex refractive index variation and paves the way for a plethora of applications. Although the critical temperature for phase-transition is 68 °C for VO2 films, its transition temperature can be reduced to room temperature by tungsten-doping of vanadium dioxide. Such a degree of freedom in controlling the critical temperature through tungsten doping provides further tunability of the thermochromic behavior. In this work, we investigate a variety of W-doped VO2 thin films deposited by laser ablation of targets with increasing W doping content and report detailed infrared characterization together with numerical simulations. Our experimental results indicate that the perfect absorption can be achieved at different temperatures, within the VO2 insulator-to-metal phase transition process, as a function of W doping content. Tunable subwavelength layers allow perfect absorption under different temperature conditions around λ = 12 µm. We show that a high dynamic range of reflectivity can be achieved when the temperature is increased above the phase transition temperature. Furthermore, we observe perfect absorption at 11.8 µm at room temperature for a W content of 0.75%. We believe that W-doped VO2 thin films with tunable and controllable perfect absorption will open the way for a class of promising thermo-optical devices including thermos-photovoltaics, infrared filters, radiative cooling devices, and thermal emitters.

VO2 nanophotonics

The intriguing physics of vanadium dioxide (VO2) makes it not only a fascinating object of study for fundamental research on solid-state physics but also an attractive means to actively modify the properties of integrated devices. In particular, the exceptionally large complex refractive index variation produced by the insulator-to-metal transition of this material opens up interesting opportunities to dynamically tune optical systems. This Perspective reviews some of the exciting work done on VO2 for nanophotonics in the last decade and suggests promising directions to explore for this burgeoning field.

Generation of regular optical patterns and photonic structures by a single Gaussian beam in a photorefractive LiNbO3:Fe crystal

We report 2D micrometric scale regular optical patterns formation induced by a laser beam propagating through a Fe doped lithium niobate (LN:Fe) crystal. A single mode CW laser radiation at 632.8 nm wavelength and beam powers from 0.5 to 10 mW and LN:Fe samples with 0.03 wt% Fe concentration and thicknesses of 4 mm and 10 mm were used in the experiments. The observed phenomenon of optical patterns formation in photorefractive crystal is explained with the light-induced complex refractive index variation with a central minimum (negative lens) and symmetric maxima (positive lenses) on each side due to photovoltaic effect in the crystal. Light-induced complex lenses produce beam defocusing from the beam center and focusing on the periphery. Mutual interference of the beams leads to a regular optical pattern formation. Regular optical pattern, in its turn, leads to the formation of quasi-crystalline photonic structure in LN:Fe crystal. Experimental results are supported by numerical simulations.

Simulation and performance evaluation of fiber optic sensor for detection of hepatic malignancies in human liver tissues

A fiber optic sensor is proposed for the identification of healthy and cancerous liver tissues through determination of their corresponding refractive index values. Existing experimental results describing variation of complex refractive index of liver tissues in near infrared (NIR) spectral region are considered for theoretical calculations. The intensity interrogation method with chalcogenide fiber is considered. The sensor’s performance is closely analyzed in terms of its sensitivity at multiple operating wavelengths falling in NIR region. Operating at shorter NIR wavelengths leads to greater sensitivity. The effect of design parameters (sensing region length and fiber core diameter), different launching conditions, and fiber glass materials on sensor’s performance is examined. The proposed sensor has the potential to provide high sensitivity of liver tissue detection.

Complex refractive index variation in proton-damaged diamond

An accurate control of the optical properties of single crystal diamond during microfabrication processes such as ion implantation plays a crucial role in the engineering of integrated photonic devices. In this work we present a systematic study of the variation of both real and imaginary parts of the refractive index of single crystal diamond, when damaged with 2 and 3 MeV protons at low-medium fluences (range: 10(15) - 10(17) cm(-2)). After implanting in 125 × 125 μm(2) areas with a scanning ion microbeam, the variation of optical pathlength of the implanted regions was measured with laser interferometric microscopy, while their optical transmission was studied using a spectrometric set-up with micrometric spatial resolution. On the basis of a model taking into account the strongly non-uniform damage profile in the bulk sample, the variation of the complex refractive index as a function of damage density was evaluated.

Simultaneous identification of the low-field-induced tiny variation of complex refractive index for anisotropic and opaque magnetic-fluid thin film by a stable heterodyne Mach-Zehnder interferometer

We use a heterodyne Mach-Zehnder interferometer to simultaneously and simply measure the complex refractive index by only normal incidence on the specimen, instead of using a complicated measurement procedure or instrument that only measures the real or imaginary part of the complex refractive index. To study the tiny variation of the complex refractive index, the small complex refractive-index variation of a rare-concentration magnetic-fluid thin film, due to a weak field of less than 200 Oe, was processed by this interferometer. We also present the wavelength trend of the complex refractive index of magnetic fluids to verify the appearance of the slight change in a small wavelength range.

Femtosecond pulse laser interactions with thin silicon films and crater formation considering optical phonons and wave interference

The extensive numerical study is conducted to examine interaction of femtosecond pulse laser with thin silicon film structure by using the two-temperature model (2TM) and the three-temperature model (3TM), on the basis of the relaxation time approximation to Boltzmann’s transport equation. The present study investigates mainly transient behaviors for carrier number density, electron temperatures, and phonon temperatures, and it compares the results of 3TM with those of 2TM for energy transfer characteristics. In particular, the one-dimensional model is extended to two-dimensional model for the direct estimation of laser-induced crater formation. In addition, the thin film optics and the electromagnetic theory are used to simulate the propagation of laser energy absorption in stratified media with variation of complex refractive index. It is found that quite different tendencies are observed for femtosecond pulse lasers between 2TM and 3TM. For the 3TM, there is clearly nonequilibrium between optical phonons and acoustic phonons when the femtosecond lasers are irradiated, whereas as the laser pulse becomes longer, two different phonons are thermally in equilibrium state. The crater depths and shapes are significantly affected by film thickness, laser fluence, and pulse duration because of the wave interference effect as well as the difference in energy relaxation times. Moreover, the optical properties are found to depend on laser absorption and laser pulses. This tendency which is successfully estimated by the two-dimensional 3TM takes place in thin film structures differently unlike the bulk silicon.

VA-3 Index-guided complementary self-aligned laser arrays by MOCVD

Summary form only given. The self-aligned laser utilizes a built-in complex refractive index variation along the plane of the junction in order to stabilize the lateral optical mode. This index variation results from modal radiation loss into a low-bandgap high-refractive-index GaAs:n layer placed close to the active layer outside the stripe. Conventional self-aligned lasers are difficult to fabricate because they require a regrowth on a high composition AlGaAs layer within the stripe region. We have developed a complementary self-aligned structure which eliminates the difficulty associated with the regrowth interface by placing it outside the stripe region. The self-aligned laser lends itself nicely to incorporation into multi-element arrays because of its simple fabrication and excellent mode control characteristics. Here, we present results on single-stripe and six-stripe complementary self-aligned laser arrays fabricated using a conventional double heterostructure active region. These data indicate that a considerable reduction in threshold current as well as improved optical coupling can be expected in self-aligned arrays fabricated with a graded barrier quantum well active layer.