Siddhartha Ghosh

Aluminum nitride grating couplers

Grating couplers in sputtered aluminum nitride, a piezoelectric material with low loss in the C band, are demonstrated. Gratings and a waveguide micromachined on a silicon wafer with 600 nm minimum feature size were defined in a single lithography step without partial etching. Silicon dioxide (SiO(2)) was used for cladding layers. Peak coupling efficiency of -6.6 dB and a 1 dB bandwidth of 60 nm have been measured. This demonstration of wire waveguides and wideband grating couplers in a material that also has piezoelectric and elasto-optic properties will enable new functions for integrated photonics and optomechanics.

Mn55 Nuclear Magnetic Resonance in Manganese Ferrite

The observation of the nuclear magnetic resonance of Mn55 in ferromagnetic MnFe2O4 is reported. The resonance frequency in a sphere is 586.0 Mc in an external field of 2730 Oe; and the linewidth is found to vary from 1.7 Mc at 1.65°K to 2.2 Mc at 4.2°K. The nuclei involved have been identified as associated with Mn2+ ions located on A sites in the spinel lattice. The double resonance technique used in these experiments allows the direct measurement of the nuclear spin‐lattice relaxation time T1. The magnitude of T1 at 1.65°K is found to be 1.5×10−3 sec.

Photonic microdisk resonators in aluminum nitride

We demonstrate integrated photonic microdisk resonators in sputtered c-axis oriented aluminum nitride (AlN) films. A 400u2009nm thick layer of AlN was patterned in a single lithography step with silicon dioxide used for the top and bottom cladding layers. AlN disks with a radius of 20u2009μm at various spacings to an 850u2009nm wide waveguide were tested. A loaded quality factor of 28 350 is shown in these microdisks, with an extracted peak shift over power ratio of 0.0495u2009pm/μW. The demonstration of photonic resonators in a piezoelectric material can lead to novel optomechanical functionalities.

Piezoelectric actuation of aluminum nitride contour mode optomechanical resonators

We present a fully-integrated monolithic aluminum nitride optomechanical device in which lateral vibrations generated by a piezoelectric contour mode acoustic ring resonator are used to produce amplitude modulation of an optical signal in a whispering gallery mode photonic ring resonator. Acoustic and optical resonances are independently characterized in this contour mode optomechanical resonator (CMOMR). Electrically driven mechanical modes are optically detected at 35MHz, 654MHz and 884MHz.

Lithium Niobate on Insulator (LNOI) grating couplers

One-dimensional grating couplers and waveguides are fabricated in 500 nm X-cut Lithium Niobate thin film making use of the Lithium Niobate on Insulator (LNOI) platform. Single coupler exhibit -12 dB peak insertion loss.

Nuclear and Ferromagnetic Relaxation in MnFe2O4

The Mn55 nuclear spin‐lattice relaxation time, and the ferromagnetic resonance linewidth have been studied in the same single crystal of ferrimagnetic MnFe2O4 as a function of the temperature. The low‐temperature linewidth is accurately described in terms of a ``slow relaxation theory based on the low‐lying levels of the Fe2+ impurities present. The NMR T1 results indicate that the dominant nuclear relaxation mechanism arises from the complex frequency pulling, brought about by ``slow relaxation in the ferromagnetic spin system.

Laterally vibrating resonator based elasto-optic modulation in aluminum nitride

An integrated strain-based optical modulator driven by a piezoelectric laterally vibrating resonator is demonstrated. The composite structure consists of an acoustic Lamb wave resonator, in which a photonic racetrack resonator is internally embedded to enable overlap of the guided optical mode with the induced strain field. Both types of resonators are defined in an aluminum nitride (AlN) thin film, which rests upon a layer of silicon dioxide in order to simultaneously define optical waveguides, and the structure is released from a silicon substrate. Lateral vibrations produced by the acoustic resonator are transferred through a partially etched layer of AlN, producing a change in the effective index of the guided wave through the interaction of the strain components with the AlN elasto-optic (p) coefficients. Optical modulation through the elasto-optic effect is demonstrated at electromechanically actuated frequencies of 173 MHz and 843 MHz. This device geometry further enables the development of MEMS-based optical modulators in addition to studying elasto-optic interactions in suspended piezoelectric thin films.

Integrated piezoelectrically driven acousto-optic modulator

This paper presents a new type of acousto-optic modulator based on the conjunction of a piezoelectric contour mode resonator (CMR) with a photonic whispering gallery mode resonator (WGMR). The monolithic device fabricated in aluminum nitride (AlN) exhibits the coupling of piezoelectrically-generated lateral vibrations into a traveling-wave photonic ring resonator in a fully-integrated platform with electrodes directly patterned on the CMR body. We demonstrate the optical sensing of a piezoelectrically actuated mechanical mode at 654 MHz, enabling new possibilities for MEMS-based RF-photonics applications or new degrees of control of phonon-photon interactions in the field of optomechanics.

Elasto-optic modulator integrated in high frequency piezoelectric MEMS resonator

This paper presents a new type of MEMS-based optical modulator, which operates on the elasto-optic effect. The device consists of a photonic racetrack resonator embedded into a laterally-vibrating piezoelectric resonator for strain to be transferred. This constitutes the first demonstration of a MEMS strain-based optomechanical device in which the operation is based on refractive index changes due to the elasto-optic effect rather than conventional physical displacements. Optical modulation is observed at the device resonance frequency of 843 MHz. The device could be used for signal modulation in RF-Photonic systems. As an example of such applications, an elasto-optic based electronic oscillator is also demonstrated.

Piezoelectrically-acutated opto-acoustic oscillator

This paper describes the implementation of an opto-acoustic oscillator with the use of an integrated piezoelectrically-driven acousto-optic intensity modulator in aluminum nitride. Optical output collected from on-chip grating couplers is converted into the electrical domain via an avalanche photodiode, and used to drive the modulator by providing an input to its RF probing pads. Oscillation is observed at 35.2 MHz and 652.8 MHz, corresponding to peaks in the forward transmission of the modulator. An initial phase noise of -72 dBc/Hz at 10 kHz offset from the 652.8 MHz carrier is recorded, with a large contribution from thermal noise in the amplifier chain. Some proposals for future improvements in phase noise performance are provided.