Spectrometer based on a compact disordered multi-mode interferometer.

We present a compact, CMOS compatible, photonic integrated circuit (PIC) based spectrometer that combines a dispersive array element of SiO2-filled scattering holes within a multimode interferometer (MMI) fabricated on the silicon-on-insulator (SOI) platform. The spectrometer has a bandwidth of 67 nm, a lower bandwidth limit of 1 nm, and a peak-to-peak resolution of 3 nm for wavelengths around 1310 nm.

Non-diffracting beam generated from a photonic integrated circuit based axicon-like lens.

We demonstrate an on-chip silicon-on-insulator (SOI) device to generate a non-diffracting beam of ≈850 µm length from a diffractive axicon-like lens etched using a low resolution (200 nm feature size, 250 nm gap) deep-ultraviolet lithographic fabrication. The device consists of circular gratings with seven stages of 1x2 multimode interferometers. We present a technique to apodize the gratings azimuthally by breaking up the circles into arcs which successfully increased the penetration depth in the gratings from ≈5 µm to ≈60 µm. We characterize the device's performance by coupling 1300±50 nm swept source laser in to the chip from the axicon and measuring the out-coupled light from a grating coupler. Further, we also present the implementation of balanced homodyne detection method for the spectral characterization of the device and show that the position of the output lobe of the axicon does not change significantly with wavelength.

Efficiency of evanescent excitation and collection of spontaneous Raman scattering near high index contrast channel waveguides.

We develop and experimentally verify a theoretical model for the total efficiency η0 of evanescent excitation and subsequent collection of spontaneous Raman signals by the fundamental quasi-TE and quasi-TM modes of a generic photonic channel waveguide. Single-mode silicon nitride (Si3N4) slot and strip waveguides of different dimensions are used in the experimental study. Our theoretical model is validated by the correspondence between the experimental and theoretical absolute values within the experimental errors. We extend our theoretical model to silicon-on-insulator (SOI) and titanium dioxide (TiO2) channel waveguides and study η0 as a function of index contrast, polarization of the mode and the geometry of the waveguides. We report nearly 2.5 (4 and 5) times larger η0 for the fundamental quasi-TM mode when compared to η0 for the fundamental quasi-TE mode of a typical Si3N4 (TiO2 and SOI) strip waveguide. η0 for the fundamental quasi-TE mode of a typical Si3N4, (TiO2 and SOI) slot waveguide is about 7 (22 and 90) times larger when compared to η0 for the fundamental quasi-TE mode of a strip waveguide of the similar dimensions. We attribute the observed enhancement to the higher electric field discontinuity present in high index contrast waveguides.

Bright and dark plasmon resonances of nanoplasmonic antennas evanescently coupled with a silicon nitride waveguide.

In this work we investigate numerically and experimentally the resonance wavelength tuning of different nanoplasmonic antennas excited through the evanescent field of a single mode silicon nitride waveguide and study their interaction with this excitation field. Experimental interaction efficiencies up to 19% are reported and it is shown that the waveguide geometry can be tuned in order to optimize this interaction. Apart from the excitation of bright plasmon modes, an efficient coupling between the evanescent field and a dark plasmonic resonance is experimentally demonstrated and theoretically explained as a result of the propagation induced phase delay.

Visible-to-near-infrared octave spanning supercontinuum generation in a silicon nitride waveguide.

The generation of an octave spanning supercontinuum covering 488-978 nm (at -30 dB) is demonstrated for the first time on-chip. This result is achieved by dispersion engineering a 1-cm-long Si3N4 waveguide and pumping it with an 100-fs Ti:Sapphire laser emitting at 795 nm. This work offers a bright broadband source for biophotonic applications and frequency metrology.

Evanescent excitation and collection of spontaneous Raman spectra using silicon nitride nanophotonic waveguides.

We experimentally demonstrate the use of high contrast, CMOS-compatible integrated photonic waveguides for Raman spectroscopy. We also derive the dependence of collected Raman power with the waveguide parameters and experimentally verify the derived relations. Isopropyl alcohol (IPA) is evanescently excited and detected using single-mode silicon-nitride strip waveguides. We analyze the measured signal strength of pure IPA corresponding to an 819 cm⁻¹ Raman peak due to in-phase C-C-O stretch vibration for several waveguide lengths and deduce a pump power to Raman signal conversion efficiency on the waveguide to be at least 10⁻¹¹ per cm.

Silicon Nitride Background in Nanophotonic Waveguide Enhanced Raman Spectroscopy.

Recent studies have shown that evanescent Raman spectroscopy using a silicon nitride (SiN) nanophotonic waveguide platform has higher signal enhancement when compared to free-space systems. However, signal-to-noise ratio from the waveguide at a low analyte concentration is constrained by the shot-noise from the background light originating from the waveguide itself. Hence, understanding the origin and properties of this waveguide background luminescence (WGBL) is essential to developing mitigation strategies. Here, we identify the dominating component of the WGBL spectrum composed of a broad Raman scattering due to momentum selection-rule breaking in amorphous materials, and several peaks specific to molecules embedded in the core. We determine the maximum of the Raman scattering efficiency of the WGBL at room temperature for 785 nm excitation to be 4.5 ± 1 × 10-9 cm-1·sr-1, at a Stokes shift of 200 cm-1. This efficiency decreases monotonically for higher Stokes shifts. Additionally, we also demonstrate the use of slotted waveguides and quasi-transverse magnetic polarization as some mitigation strategies.

Single mode waveguide platform for spontaneous and surface-enhanced on-chip Raman spectroscopy.

We review an on-chip approach for spontaneous Raman spectroscopy and surface-enhanced Raman spectroscopy based on evanescent excitation of the analyte as well as evanescent collection of the Raman signal using complementary metal oxide semiconductor (CMOS)-compatible single mode waveguides. The signal is either directly collected from the analyte molecules or via plasmonic nanoantennas integrated on top of the waveguides. Flexibility in the design of the geometry of the waveguide, and/or the geometry of the antennas, enables optimization of the collection efficiency. Furthermore, the sensor can be integrated with additional functionality (sources, detectors, spectrometers) on the same chip. In this paper, the basic theoretical concepts are introduced to identify the key design parameters, and some proof-of-concept experimental results are reviewed.