Ritwick Das

High-Performance Bimetallic SPR Sensor Based on Periodic-Multilayer-Waveguides

We propose a high-performance bimetallic surface plasmon resonance (SPR) sensor based on periodic-multilayer waveguide and analyze its performance with respect to SPR active metals, such as gold (Au) and aluminum (Al). We found that an ultra thin (~ 3 nm) layer of gold (Au) over aluminum (Al) protects aluminum (Al) from oxidation and exhibits better sensor performance. Using couple-mode theory, we show that the sensitivity and detection accuracy could be appropriately tailored for bimetallic (Au+Al) configuration.

Continuous-wave optical parametric oscillator pumped by a fiber laser green source at 532 nm.

We report a high-power, cw, singly resonant optical parametric oscillator (SRO) using a simple, compact fiber pump laser architecture in the green. The SRO, based on MgO:sPPLT, is pumped by 9.6 W of single-frequency cw radiation at 532 nm obtained by single-pass second-harmonic generation (SHG) of a 30 W Yb fiber laser, also in MgO:sPPLT. Using two identical crystals of 30 mm length for SHG and SRO, we generate cw idler powers of up to 2 W over 855-1408 nm, with a peak-to-peak power stability <11.7% over 40 min, in a TEM(00) spatial mode with M(2)<1.26. Using finite output coupling of the resonant wave, we extract 800 mW of signal power with peak-to-peak power stability <10.7% over 40 min, and a frequency stability <75 MHz over 15 min. The signal and idler output have TEM(00) beam profile with M(2)<1.52 across the tuning range.

Broadband, high-power, continuous-wave, mid-infrared source using extended phase-matching bandwidth in MgO:PPLN

We report a compact and viable source of broadband, high-power, cw, mid-IR radiation based on a singly resonant optical parametric oscillator (SRO) pumped by a wide-bandwidth cw Yb fiber laser centered at 1060 nm. By exploiting the extended phase-matching bandwidth in a 50 mm crystal of MgO:PPLN and a ring SRO cavity, we obtain 5.3 W of broadband idler output for 25.5 W of pump at >80% depletion, transferring a pump bandwidth of 73.9 cm(-1) to an idler spectrum spread across an equal bandwidth centered at 3454 nm. By deploying output coupling of the signal, we generate 11.2 W of total power at 44% extraction efficiency with a pump depletion of >73% at the maximum available pump power. Measurements of transverse modal power confirm Gaussian distribution of signal and idler beams.

Tamm-plasmon and surface-plasmon hybrid-mode based refractometry in photonic bandgap structures

The transverse magnetic (TM) polarized hybrid modes formed as a consequence of coupling between Tamm plasmon polariton (TM-TPP) mode and surface plasmon polariton (SPP) mode exhibit interesting dispersive features for realizing a highly sensitive and accurate surface plasmon resonance (SPR) sensor. We found that the TM-TPP modes, formed at the interface of distributed Bragg reflector and metal, are strongly dispersive as compared to SPP modes at optical frequencies. This causes an appreciably narrow interaction bandwidth between TM-TPP and SPP modes, which leads to highly accurate sensing. In addition, appropriate tailoring of dispersion characteristics of TM-TPP as well as SPP modes could ensure high sensitivity of a novel SPR platform. By suitably designing the Au/TiO₂/SiO₂-based geometry, we propose a TM-TPP/SPP hybrid-mode sensor and achieve a sensitivity ≥900  nm/RIU with high detection accuracy (≥30  μm⁻¹) for analyte refractive indices varying between 1.330 and 1.345 in 600-700 nm wavelength range. The possibility to achieve desired dispersive behavior in any spectral band makes the sensing configuration an extremely attractive candidate to design sensors depending on the availability of optical sources.

Defect-assisted saturable absorption characteristics in Mn doped ZnO nano-rods

We have investigated the effect that manganese (Mn)-doping in ZnO sub-wavelength rods (or nanorods) has on nonlinear optical properties, namely two-photon absorption (TPA) and nonlinear refraction using the single-beam Z-scan technique. Mn-doped ZnO nanorods (NRs) were prepared by a low temperature aqueous growth technique. The results show that the Mn-doping concentration primarily determines whether ZnO NRs will exhibit saturable absorption (SA) or two-photon-absorption (TPA) characteristics in an open-aperture experiment. At high Mn-doping concentrations, ZnO NRs exhibit SA behaviour which can be attributed to a high occupation probability of defect states as well as the saturation of linear absorption of sub-wavelength rod aggregates at high optical fluence. In contrast to high Mn-doping concentration in ZnO NRs, we observed TPA features in 0.5% Mn-doped ZnO NRs. The employability of such structures in the area of optical limiting and switching is essentially derived from the possibility to tune the nonlinear optical absorption which could be realized by appropriate Mn-doping in ZnO NR architecture.

Design considerations and propagation characteristics of channel Bragg-plasmon-coupled-waveguides

We present a detailed design principle and propagation characteristics of channel Bragg-plasmon-coupled-waveguide. We have found that there is a significant change in the slope of the phase-velocity dispersion curve leading to an ultranarrow interaction bandwidth (∼765 pm) and group-velocity dispersion (GVD∼±4.5×104 ps/km-nm) that is an appreciably large GVD using a waveguide mode-coupling geometry in any region of the optical spectrum. The effect of waveguide parameters such as channel width, number of bilayers, etc. on a mode-coupling mechanism is also studied with significant emphasis on the propagation loss suffered by the supermodes of the structure. The proposed waveguide exhibits sensitivity as high as 7500 nm/RIU, thereby opening a route for biochemical sensing.

On the modal characteristics of surface plasmon polaritons at a metal-Bragg interface at optical frequencies

A detailed mathematical analysis along with a theoretical model for the modes supported at the interface of a metal and periodically stratified medium (Bragg structure) is presented. The modes that are supported at the interface of a plasmon active metal (such as gold) and a Bragg structure are commonly known as surface plasmon-Bragg modes. We found that these modes have effective indices lower than any of the material indices of the layers comprising the Bragg structure, and they are highly dispersive when compared to the conventional surface plasmon modes that are supported at the metal and dielectric interface. The plausible physical explanation behind the strong dispersive behavior of the surface plasmon-Bragg mode is provided. Finally, the comparison of dissipation loss for the surface plasmon-Bragg modes is investigated and it has been shown that there is more than fivefold enhancement in the magnitude of propagation lengths as compared to the conventional surface plasmon mode.

Guided-Mode Analysis of Tamm-Plasmon Polariton at Metal–Heterostructure Dielectric Interface

We present a comprehensive analysis for transverse electric (TE) and transverse magnetic (TM) polarized guided Tamm-plasmon polariton (TPP) mode at metal-heterostructure media interface. We explicitly show that the quarter-wavelength stack condition will not be satisfied for TE or TM polarized TPP mode due to the existence of null-point at metal-heterostructure media boundary. Therefore, we propose an alternate route to design TPP waveguide by solving the mode-dispersion relation for different geometrical parameters in a TiO2/SiO2 bilayer system. The guided TPP-modes (TE and TM) exhibit interesting dispersion characteristics which can be tailored as per the desired application. The group index of TM polarized TPP mode remains constant over a significant wavelength range which results into zero group-velocity dispersion (GVD) at λ ≈ 630 nm wavelength. Also, the propagation length for TM-polarized TPP modes vary between 25 μm to 50 μm in a 630-650 nm wavelength range. However, the variation of GVD for TE-modes exhibit a monotonic variation with an exceptionally large GVD ≈ -3 × 104 ps/km·nm around λ = 632.8 nm.

A comparative study of optical nonlinearities of trans-A2B-corroles in solution and in aggregated state

A series of novel A3-corrole and trans-A2B-corroles have been synthesized with the aim of developing organic materials with improved nonlinear optical (NLO) properties. All three newly synthesized corroles have been characterized by various spectroscopic techniques including single crystal X-ray structural analysis of the representative one. The crystal structure analysis of 10-(4-hydroxyphenyl)-5,15-bis(2-bromo-5-fluorophenyl) corrole shows several O–H⋯N interactions. The self aggregates of all three corroles were prepared on a silicon wafer as well as on a quartz substrate by using a drop-casting method in a dichloromethane and methanol (1 : 2) solvent mixture. In all three free base corroles, well defined and nicely organized three-dimensional objects with diameter of ca. 320 nm (nanospheres), 450 nm (nanobulbs), and 120 nm (nanodiscs) were obtained. The NLO properties (nonlinear refractive index, n2 and two-photon absorption coefficient, β) of all of the corrole derivatives in toluene solution and in aggregated form were measured by the Z-scan technique. The nonlinear refractive indices, n2 of the free base corroles (in toluene solution) were found out to be −16.8 × 10−18 m2 W−1, −7.8 × 10−18 m2 W−1 and −25.9 × 10−18 m2 W−1 respectively and for the corresponding aggregates (nanoparticles of the free base corroles), it was found out to be −1.1 × 10−15 m2 W−1, −1.9 × 10−15 m2 W−1, and 71.8 × 10−15 m2 W−1 respectively. Similarly, the two-photon absorption coefficient, β of all the synthesized free base corroles (in toluene solution) were found out to be 5.7 × 10−15 m W−1, 1.9 × 10−15 m W−1 and 17.2 × 10−15 m W−1 respectively and for the corresponding aggregates (nanoparticles), the values were 4.0 × 10−13 m W−1, 2.0 × 10−13 m W−1, and 444.0 × 10−13 m W−1 respectively. These NLO properties of the free base corrole derivatives (in solution and in aggregates) have been explored with a specific aim to identify the possibility of their applications in ultrafast switching devices for use in high-speed fiber-optic communications and photonic integrated circuits.

Yb-fiber laser pumped high-power, broadly tunable, single-frequency red source based on a singly resonant optical parametric oscillator

We present an efficient and tunable source generating multi-watt single-frequency red radiation by intra-cavity frequency doubling of the signal in a MgO-doped periodically poled LiNbO3 (MgO:PPLN)-based singly resonant optical parametric oscillator (SRO). By optimally designing the SRO cavity in a six-mirror configuration, we generate ≈276  nm tunable idler radiation in mid-infrared with a maximum power of Pi=2.05  W at a pump power of Pp=14.0  W. The resonant signal is frequency doubled using a 10 mm-long BiB3O6 (BiBO) crystal which resulted in tunability of a red beam from ≈753 to 780 nm band with maximum power Pr≈4.0  W recorded at λr≈756  nm. The deployment of a six-mirror SRO ensures single-frequency generation of red across the entire tuning range by inducing additional losses to Raman modes of LiNbO3 and, thus, inhibiting their oscillation. Using a scanning Fabry-Perot interferometer (FPI), nominal linewidth of the red beam is measured to ≈3  MHz which changes marginally over the entire tuning range. Long-term (over 1 h) peak-to-peak frequency fluctuation of the generated red beam is estimated to be about 3.3 GHz under free-running conditions at Pp=14.0  W. The generated red beam is delivered in a TEM00 mode profile with M2≤1.32 at maximum power in a red beam.