Tunable Nano-photonic Devices

For high speed photonic systems and networks, encoding electronic signal onto optical carrier requires electro-optic modulators in which electromagnetic fields of the optical carrier can be manipulated electronically. The central focus of this research is twofold. First, tunable properties and tuning mechanisms of different optical materials like Graphene, Vanadium di-oxide, and Indium Tin Oxide are characterized systematically in telecommunication wavelength region. Then, these materials are implemented to design novel nano-photonic devices such as electrooptic modulators and tunable couplers with high efficiency and miniature footprint suitable for photonic integration. Specifically, we experimentally investigated the complex index of graphene in near infrared wavelength through the reflectivity measurement on a SiO2/Si substrate. The measured change of reflectivity as the function of applied gate voltage is highly correlated with theoretical modeling based on the Kubo formula. Based on a fiber-optic pump-probe setup we demonstrated that short optical pulses can be translated from pump wavelength to probe wavelength through dielectric-to-metal phase transition of vanadium di-oxide. In this process, pump leading edge induced optical phase modulation on the probe is converted into an intensity modulation through an optical frequency discriminator. We also theoretically modeled the permittivity of Indium Tin-Oxide with different level of doping concentration in near infrared region. We proposed an ultra-compact electro-optic modulator based on switching plasmonic resonance “ON” and “OFF” of ITO-on-graphene via tuning of graphene chemical potential through electrical gating. The plasmonic resonance of ITO-on-graphene significantly enhances the electromagnetic field interaction with graphene which allows the reduction of modulator size compare to graphene based modulators without ITO. We presented a scheme of modeiv multiplexed near infrared modulator by tuning of ITO permittivity as the function of carrier density through externally applied voltage. The wisely patterned ITO on top of an SOI ridge waveguide portrayed the independent modulation of two orthogonal modes simultaneously, which enhances functionality per area for on-chip photonic applications. We proposed a theoretical model of tunable anisotropic metamaterial composed of periodic layers of graphene and Hafnium Oxide where transversal permittivity can be tuned via changing the chemical potential of graphene. A novel metamaterial assisted tunable photonic coupler is designed by inserting the proposed artificial tunable metamaterial in the coupling region of a parallel waveguide coupler. The coupling efficiency can be tuned by changing the permittivity of metamaterial through external electrical gating

Comparison between thermoelastic and ablative induced elastic waves in soft media using ultra-fast line-field low coherent holography

Laser induced elastic waves in soft media have great potential to characterize tissue biomechanical properties. The instantaneous increase in local temperature caused by absorption of laser energy leads to a mechanical perturbation in the sample, which can then propagate as a pressure (or an elastic) wave. The generation of the elastic wave can be via thermoelastic or ablative processes depending on the absorption coefficient of the sample and incident laser fluence. It is critical to differentiate between these regimes because only the thermoelastic regime is useful for nondestructive analysis of tissues. To investigate the transition point between these two different regimes, we induced elastic waves in tissue mimicking agar phantoms mixed with different concentrations of graphite powder. The elastic waves were excited by a 532nm pulsed laser with a pulse duration of 6 ns. The fluence of the pulsed laser was tuned from 0.08 J/cm2 to 3.19 J/cm2 , and the elastic wave was captured by ultra-fast line-field low coherent holography system capable of single-shot elastic wave imaging with nanometer-scale displacement sensitivity. Different concentrations of graphite powder enabled excitation in sample with controlled and variable attenuation coefficient, enabling measurement of the transition between the thermoelastic and ablative regimes. The results show that the transition from thermoelastic to ablative generated waves was 0.75 J/cm2 and 1.84 J/cm2 for phantoms with optical attenuation coefficients of 6.64±0.32 mm-1 and 26.19±1.70 mm-1, respectively. Our results show promise for all optical biomechanical characterization of tissues

Direct observation of bulk second-harmonic generation inside a glass slide with tightly focused optical fields

Bulk second-harmonic generation (SHG) inside glass slides is directly detected unambiguously without interference from surface contributions. This is enabled by tightly focused and highly localized ultrashort laser pulses. The theoretical calculations based on vector diffraction theory and the phenomenological model of SHG inside centrosymmetric materials agree well with the measured far-field SHG radiation patterns for different polarization states of the fundamental beam. The results indicate that the observed bulk SHG is predominantly related to the bulk parameter δ′ and originates from the three-dimensional field gradient in the focal region

Proučavanje raspodjele mnogostrukosti u cijelom faznom prostoru pri relativističkim nuklearnim sudarima

A comparative study has been made among three distributions - the negative binomial distribution (NBD), the generalised multiplicity distribution (GMD) and the distribution followed by the two mechanism model (TMM), in order to describe the multiplicity distribution of charged secondary particles produced in 200 200 A GeV 32S – AgBr interaction.Načinili smo usporedbu triju raspodjela: negativne binomijalne raspodjele, poopćene raspodjele mnogostrukosti i raspodjele prema modelu dvaju mehanizama, radi opisa raspodjela mnogostrukosti nabijenih čestica koje su bile proizvedene u sudarima iona 32S energije 200 A GeV u AgBr emulziji

Complex refractive index tunability of graphene at 1550 nm wavelength

This is the published version. Copyright 2015 American Institute of PhysicsThe complex refractive index of graphene fabricated using chemical vapor deposition is characterized at 1550 nm wavelength through the reflectivity measurement on a SiO2/Si substrate. The observed tunability of the complex reflective index as the function of gate electric voltage is in agreement with the prediction based on the Kubo formula

Proučavanje raspodjele mnogostrukosti u cijelom faznom prostoru pri relativističkim nuklearnim sudarima

A comparative study has been made among three distributions - the negative binomial distribution (NBD), the generalised multiplicity distribution (GMD) and the distribution followed by the two mechanism model (TMM), in order to describe the multiplicity distribution of charged secondary particles produced in 200 200 A GeV 32S – AgBr interaction.Načinili smo usporedbu triju raspodjela: negativne binomijalne raspodjele, poopćene raspodjele mnogostrukosti i raspodjele prema modelu dvaju mehanizama, radi opisa raspodjela mnogostrukosti nabijenih čestica koje su bile proizvedene u sudarima iona 32S energije 200 A GeV u AgBr emulziji

Study of multiplicity distribution in full phase space in ultrarelativistic nuclear collision

This paper was published in Fizika B 6 (1997) page 37 with a typing error in the byline. The name of the fourth author was misspelled. Above is the corrected byline

Differentiation of murine colon pathology by optical and mechanical contrast using optical coherence tomography and elastography

Colon pathologies including colon cancer and ulcerative colitis afflict hundreds of thousands of people in the United States. Clinical detection of colon diseases is generally performed through colonoscopy. However, these methods usually lack the sensitivity or resolution to detect diseased tissue at early stages. Even high resolution optical techniques such as confocal microscopy and optical coherence tomography (OCT) rely on structural features to detect anomalies in tissue, which may not be sufficient for early disease detection. If changes in tissue biomechanical properties precede morphological changes in tissue physiology, then mechanical contrast would enable earlier detection of disease. In this work, we utilized optical coherence elastography (OCE) to assess the biomechanical properties of healthy, cancerous, and colitis tissue. Additionally, the optical properties of each sample were also assessed as a secondary feature to distinguish tissue types. The Young’s modulus, as measured by the propagation of an elastic wave, of the healthy, cancerous, and colitis tissue was 10.8 ± 1.0 kPa, 7.12 ± 1.0 kPa, and 5.1 ± 0.1 kPa, respectively. The variations in the OCT signal intensity over depth, as measured by the slope-removed standard deviation of each A-scan was 5.8 ±.0.3 dB, 5.1 ± 0.4 dB, and 5.5 ± 0.2 dB for healthy, cancerous, and colitis tissue, respectively. This work shows OCT structural imaging combined with OCE can detect minute changes in colon tissue optical scattering and elastic properties, which may be useful for detection various colon diseases, such as colitis and colon cancer

Study of multiplicity distribution in full phase space in ultrarelativistic nuclear collision

This paper was published in Fizika B 6 (1997) page 37 with a typing error in the byline. The name of the fourth author was misspelled. Above is the corrected byline