Anuradha M. Agarwal

Mid-infrared materials and devices on a Si platform for optical sensing

Abstract In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiNx waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.

Nonlinear Group IV photonics based on silicon and germanium: from near-infrared to mid-infrared

Abstract Group IV photonics hold great potential for nonlinear applications in the near- and mid-infrared (IR) wavelength ranges, exhibiting strong nonlinearities in bulk materials, high index contrast, CMOS compatibility, and cost-effectiveness. In this paper, we review our recent numerical work on various types of silicon and germanium waveguides for octave-spanning ultrafast nonlinear applications. We discuss the material properties of silicon, silicon nitride, silicon nano-crystals, silica, germanium, and chalcogenide glasses including arsenic sulfide and arsenic selenide to use them for waveguide core, cladding and slot layer. The waveguides are analyzed and improved for four spectrum ranges from visible, near-IR to mid-IR, with material dispersion given by Sellmeier equations and wavelength-dependent nonlinear Kerr index taken into account. Broadband dispersion engineering is emphasized as a critical approach to achieving on-chip octave-spanning nonlinear functions. These include octave-wide supercontinuum generation, ultrashort pulse compression to sub-cycle level, and mode-locked Kerr frequency comb generation based on few-cycle cavity solitons, which are potentially useful for next-generation optical communications, signal processing, imaging and sensing applications.

Generation of two-cycle pulses and octave-spanning frequency combs in a dispersion-flattened micro-resonator.

We show that octave-spanning Kerr frequency combs with improved spectral flatness of comb lines can be generated in dispersion-flattened microring resonators. The resonator is formed by a strip/slot hybrid waveguide, exhibiting a flat and low anomalous dispersion between two zero-dispersion wavelengths that are separated by one octave from near-infrared to mid-infrared. Such flattened dispersion profiles allow for the generation of mode-locked frequency combs, using relatively low pump power to obtain two-cycle cavity solitons on a chip, associated with the octave-spanning comb bandwidth. The wavelength dependence of the optical loss and of the coupling coefficient and thus wavelength dependent Q-factor are also considered.

Nonlinear conversion efficiency in Kerr frequency comb generation

We analytically and numerically investigate the nonlinear conversion efficiency in ring microresonator-based mode-locked frequency combs under different dispersion conditions. Efficiency is defined as the ratio of the average round trip energy values for the generated pulse(s) to the input pump light. We find that the efficiency degrades with growth of the comb spectral width and is inversely proportional to the number of comb lines. It depends on the cold-cavity properties of a microresonator only and can be improved by increasing the coupling coefficient. Also, it can be increased in the multi-soliton state.

Air-clad silicon pedestal structures for broadband mid-infrared microphotonics

Toward mid-infrared (mid-IR) silicon microphotonic circuits, we demonstrate broadband on-chip silicon structures, such as: (i) straight and bent waveguides and (ii) beam splitters, utilizing an air-clad pedestal configuration which eliminates the need for typical mid-IR-lossy oxide cladding. We illustrate a sophisticated fabrication process that can create high-quality pedestal structures in crystalline silicon, while preserving its mid-IR transparency. A fundamental waveguide mode is observed between λ=2.5 μm and λ=3.7 μm, and an optical loss of 2.7 dB/cm is obtained at λ=3.7 μm. Our pedestal silicon structures show 50:50 mid-IR power splitting enabling the further development of mid-IR silicon microphotonics.

Planar silicon nitride mid-infrared devices

Integrated mid-infrared devices including (i) straight/bent waveguides and (ii) directional couplers are demonstrated on silicon nitride (SiN) thin films prepared by optimized low-pressure chemical vapor deposition. The deposited SiN film has a broad spectral transparency from visible up to a wavelength of λu2009=u20098.5u2009μm (as seen from Fourier transform infrared spectroscopy). Our SiN waveguide shows a dominant fundamental mode with an optical loss of 2.1u2009dB/cm at λu2009=u20093.7u2009μm. In addition, we demonstrate an efficient SiN directional coupler between λu2009=u20093.55u2009μm to λu2009=u20093.75u2009μm where an 8u2009dB extinction ratio is achieved within each channel upon wavelength scanning. With the inherent advantage of complementary metal–oxide–semiconductor compatibility, our SiN platform paves the way to create sophisticated photonic circuits that are desired for mid-infrared nonlinear light generation and chip-scale biochemical sensors.

Increased bandwidth with flattened and low dispersion in a horizontal double-slot silicon waveguide

We propose a horizontal double-slot silicon waveguide to achieve flattened and low dispersion from −26u2009u2009ps/(km·nm) to 21u2009u2009ps/(km·nm) over an 878xa0nm wavelength range, which is much larger than that obtained from a single-slot waveguide. The mechanism of using this double-slot structure to increase the bandwidth is studied. Our study shows that the mode transition process from one strip mode to two slot modes helps extend the bandwidth of the flattened and low dispersion to longer wavelengths compared with a single-slot waveguide. Furthermore, we show by simulation the supercontinuum generation with a 3xa0dB bandwidth of 188xa0nm in an all-normal-dispersion profile with an input pulse of 50xa0fs.

Point defect states in Sb-doped germanium

Defect states in n-type Sb-doped germanium were investigated by deep-level transient spectroscopy. Cobalt-60 gamma rays were used to generate isolated vacancies and interstitials which diffuse and react with impurities in the material to form four defect states (E37, E30, E22, and E21) in the upper half of the bandgap. Irradiations at 77u2009K and 300u2009K as well as isothermal anneals were performed to characterize the relationships between the four observable defects. E37 is assigned to the Sb donor-vacancy associate (E-center) and is the only vacancy containing defect giving an estimate of 2u2009×u20091011u2009cm−3u2009Mrad−1 for the uncorrelated vacancy-interstitial pair introduction rate. The remaining three defect states are interstitial associates and transform among one another. Conversion ratios between E22, E21, and E30 indicate that E22 likely contains two interstitials.

Integrated Midinfrared Laser Based on an Er-Doped Chalcogenide Microresonator

On-chip midinfrared (mid-IR) optical sources play an increasingly important role in building a fully integrated photonic system for sensing, imaging, communications, and signal processing applications. In this paper, we present a practical room-temperature planar design for an Er-doped chalcogenide-based mid-IR laser, which features a multimode microresonator. The cavity is carefully designed to have a high Q-factor at both pump (0.66 μm) and lasing (3.6 μm) wavelengths, which are separated by more than two octaves. This is achieved by using a novel idea of operating the cavity as a microdisk at the pump wavelength and as a microring at the lasing wavelength. Detailed numerical analysis shows that the high-Q feature of the cavity greatly reduces the lasing threshold to 7.6 μW, with a laser slope efficiency of 10%.