Sukanta Debbarma

Low-loss chalcogenide waveguides for chemical sensing in the mid-infrared.

We report the characteristics of low-loss chalcogenide waveguides for sensing in the mid-infrared (MIR). The waveguides consisted of a Ge₁₁.₅As₂₄Se₆₄.₅ rib waveguide core with a 10nm fluoropolymer coating on a Ge₁₁.₅As₂₄S₆₄.₅ bottom cladding and were fabricated by thermal evaporation, photolithography and ICP plasma etching. Over most of the functional group band from 1500 to 4000 cm⁻¹ the losses were < 1 dB/cm with a minimum of 0.3 dB/cm at 2000 cm⁻¹. The basic capabilities of these waveguides for spectroscopy were demonstrated by measuring the absorption spectrum of soluble Prussian blue in Dimethyl Sulphoxide.

Adaptive optics enhanced direct laser writing of high refractive index gyroid photonic crystals in chalcogenide glass

Chiral gyroid photonic crystals are fabricated in the high refractive index chalcogenide glass arsenic trisulfide with an adaptive optics enhanced direct laser writing system. The severe spherical aberration imparted when focusing into the arsenic trisulfide is mitigated with a defocus decoupled aberration compensation technique that reduces the level of aberration that must be compensated by over an order of magnitude. The fabricated gyroids are shown to have excellent uniformity after our adaptive optics method is employed, and the transmission spectra of the gyroids are shown to have good agreement with numerical simulations that are based on a uniform and diffraction limited fabrication resolution.

Low loss coupling to sub-micron thick rib and nanowire waveguides by vertical tapering

Highly nonlinear planar glass waveguides have been shown to be useful for all optical signal processing. However, the typical SMF-28 fiber to waveguide coupling loss of ~5dB/end remains a barrier to practical implementation. Low loss coupling to a fiber using vertical tapering of the waveguide film is analyzed for rib and nanowire waveguides and experimentally demonstrated for ribs showing polarization and wavelength independence over >300nm bandwidth. Tapers with essentially zero excess loss led to total losses from the waveguide to fiber core of 1.1dB per facet comprising only material absorption (0.75dB) and mode overlap loss (0.36dB), both of which can be eliminated with improvements to processing and materials.

Bragg-mirror-like circular dichroism in bio-inspired quadruple-gyroid 4srs nanostructures

The smooth and tailorable spectral response of Bragg mirrors has driven their pervasive use in optical systems requiring customizable spectral control of beam propagation. However, the simple nature of Bragg mirror reflection prevents their application to the control of important polarization states such as circular polarization. While helical and gyroid-based nanostructures exhibiting circular dichroism have been developed extensively to address this limitation, they are often restricted by the spectral inconsistency of their optical response. Here we present the fabrication and characterization of quadruple-gyroid 4srs nanostructures exhibiting bio-inspired Bragg-mirror-like circular dichroism: a smooth and uniform band of circular dichroism reminiscent of the spectrum of a simple multilayer Bragg-mirror. Furthermore, we demonstrate that the circular dichroism produced by 4srs nanostructures are robust to changes in incident angle and beam collimation, providing a new platform to create and engineer circular dichroism for functional circular polarization manipulation.

Emission properties of erbium-doped Ge-Ga-Se glasses, thin films and waveguides for laser amplifiers

We report, for the first time, Er-doped Ge-Ga-Se films and waveguides deposited using co-thermal evaporation and patterned with plasma etching. The emission properties of the bulk glasses were studied as a function of Erbium doping, showing for the first time that there is a clear concentration quenching effect in the Ge-Ga-Se glasses with a linear radiative lifetime degradation slope of −0.48 ms/mol% Er from a low concentration lifetime of 1.7 ms, even when sufficient Gallium is present to ensure homogeneous distribution of the Erbium. A region between approximately 0.5 and 0.75 mol% Erbium however is shown to provide sufficient doping, good photoluminescence and adequate lifetime to envisage practical planar waveguide amplifier devices. Film emission properties at 0.7 mol% doping were studied and compared with the bulk counterpart showing adequate lifetimes and photoluminescence. Erbium doped films with ~0.8 dB/cm propagation loss at 1550 nm limited by Mie scattering off small particles ejected from the evaporation crucible were fabricated. Planar hybrid Er-Ge-Ga-Se/As2S3 rib waveguides fabricated through photolithography and plasma etching demonstrated propagation losses of ~2 dB/cm at 1650 nm limited by particulate scattering.

Simultaneous compensation for aberration and axial elongation in three-dimensional laser nanofabrication by a high numerical-aperture objective

One of the challenges in laser direct writing with a high numerical-aperture objective is the severe axial focal elongation and the pronounced effect of the refractive-index mismatch aberration. We present the simultaneous compensation for the refractive-index mismatch aberration and the focal elongation in three-dimensional laser nanofabrication by a high numerical-aperture objective. By the use of circularly polarized beam illumination and a spatial light modulator, a complex and dynamic slit pupil aperture can be produced to engineer the focal spot. Such a beam shaping method can result in circularly symmetric fabrication along the lateral directions as well as the dynamic compensation for the refractive-index mismatch aberration even when the laser beam is focused into the material of a refractive index up to 2.35.

Chalcogenide glass planar MIR couplers for future chip based Bracewell interferometers

Photonic integrated circuits are established as the technique of choice for a number of astronomical processing functions due to their compactness, high level of integration, low losses, and stability. Temperature control, mechanical vibration and acoustic noise become controllable for such a device enabling much more complex processing than can realistically be considered with bulk optics. To date the benefits have mainly been at wavelengths around 1550 nm but in the important Mid-Infrared region, standard photonic chips absorb light strongly. Chalcogenide glasses are well known for their transparency to beyond 10000 nm, and the first results from coupler devices intended for use in an interferometric nuller for exoplanetary observation in the Mid-Infrared L’ band (3800-4200 nm) are presented here showing that suitable performance can be obtained both theoretically and experimentally for the first fabricated devices operating at 4000 nm.

Hybrid waveguide from As 2 S 3 and Er-doped TeO 2 for lossless nonlinear optics

The fabrication and characterization of loss-compensated dispersion-engineered nonlinear As(2)S(3) on Er:TeO2 waveguides is reported for the first time, to the best of our knowledge. The hybrid waveguide is a strip loaded structure made from an Er-doped TeO2 slab and an etched As(2)S(3) strip. Almost complete loss compensation is demonstrated with 1480 nm pumping and a fully lossless waveguide with high nonlinear coefficient can be achieved with higher 1480 nm pump power.

Hybrid waveguide from As₂S₃ and Er-doped TeO₂ for lossless nonlinear optics

The fabrication and characterization of loss-compensated dispersion-engineered nonlinear As(2)S(3) on Er:TeO2 waveguides is reported for the first time, to the best of our knowledge. The hybrid waveguide is a strip loaded structure made from an Er-doped TeO2 slab and an etched As(2)S(3) strip. Almost complete loss compensation is demonstrated with 1480 nm pumping and a fully lossless waveguide with high nonlinear coefficient can be achieved with higher 1480 nm pump power.

Hybrid waveguide from As_2S_3 and Er-doped TeO_2 for lossless nonlinear optics

The fabrication and characterization of loss-compensated dispersion-engineered nonlinear As(2)S(3) on Er:TeO2 waveguides is reported for the first time, to the best of our knowledge. The hybrid waveguide is a strip loaded structure made from an Er-doped TeO2 slab and an etched As(2)S(3) strip. Almost complete loss compensation is demonstrated with 1480 nm pumping and a fully lossless waveguide with high nonlinear coefficient can be achieved with higher 1480 nm pump power.