We have fabricated sub-micron opto-chemical probes for pH, oxygen and calcium monitoring and demonstrated their
application in intracellular and extracellular monitoring of neurons (cortical neuronal cultures and acute hippocampal
slices). Using these probes, we have measured extracellular pH in the stratum radiatum of the CA1 region of mouse
hippocampus upon stimulation of presynaptic Schaffer collateral axons. Synaptic transmission was monitored using
standard electrophysiological techniques. We find that the local pH transiently changes in response to synaptic
stimulation. In addition, the geometry of the functionalized region on the probe combined with high sensitivity imaging
enables simultaneous monitoring of spatially adjacent but distinct compartments. As proof of concept we impaled
cultured neurons with the probe measured calcium and pH inside as well as directly outside of neurons as we changed
the pH and calcium concentration in the physiological solution in the perfusion chamber. As such these probes can be
used to study the impact of the environment on both cellular and extra-cellular space. Additionally as the chemical
properties of the surrounding medium can be controlled and monitored with high precision, these probes enable
differential measurement of the target parameter referenced to a stable bath. This approach eliminates the uncertainties
associated with non-chemical fluctuations in the fluorescent emission and result in a self-calibrated opto-chemical probe.
We have also demonstrated multifunctional probes that are capable of measuring up to three parameters in the
extracellular space in brain slices.
High optical quality (high-Q) whispering-gallery mode (WGM) microresonators are key enablers for numerous highperformance photonic devices, including ultrasensitive molecular detectors and advanced light sources such as narrowlinewidth lasers and comb generators. For sensing applications, the unique characteristics of such WGM devices appear
to be particularly relevant in the mid-IR (MIR) spectral region because of the stronger molecular absorption bands in this
spectral region. However, most current WGM-based passive and active devices function in the near-IR (NIR) spectral
region. We propose the development of reproducible high-Q WGM microresonators for the MIR by using low phonon
energy glasses (such as fluorides, chalcogenides, and tellurides) along with an elegant and reproducible microsphere
fabrication technique based on the use of novel state-of-the-art microheaters. In this paper, we first review the current
state-of-the-art of WGM MIR microresonators and related optoelectronic devices, and then present recent results of our
work on fabrication and characterization of high-Q WGM optical microresonators with several fluoride (ZBLAN, InF3 and AlF3) glasses. Intrinsic quality factors in excess of ten million have been measured in the NIR regime in the fluoridebased microspheres fabricated in our lab with the proposed -- highly reliable and reproducible – microheater fabrication method, and similar or better performances are expected from similar microspheres at MIR wavelengths between 2 to 5 microns. We next discuss potential applications of these microresonators, notably for low-threshold and narrowlinewidth
MIR lasers and MIR comb applications.
The quest for low power and high frequency electro-optical modulator has been one of the important endeavors in
microwave photonics. The advent of microdisk electro-optic modulator created a new domain in optical modulator and
photonic microwave receiver design by exploiting the unique properties of high quality (high-Q) Whispering-Gallery
Mode (WGM) optical cavities. High-Q crystalline WG cavities were the first devices used as compact and low power
resonant electro-optical modulators and gradually semiconductor and polymer based microdisk and microring
modulators emerged from this core technology. Due to its small size, high sensitivity and limited bandwidth, originally
microdisk modulator was developed with the objective of replacing the conventional microwave wireless receiver frontend
with a sensitive photonic front-end. Later it was shown that the electro-optic microdisk modulator could also
function as a microwave frequency mixer in optical domain. Starting from fundamentals of resonant electro-optic
modulation in high-Q WGM cavities, in this paper we review the development of high sensitivity microdisk modulators
and the recent progress toward more efficient modulation at higher frequencies. Next related topics such as singlesideband
modulation, all-dielectric photonic receiver, and semiconductor microring modulators are briefly discussed.
Finally, photonic microwave receiver configurations that employ high-Q optical resonance for modulation, filtering and
mixing are presented. We will show that high-Q optical resonance is one of the promising routes toward the general idea
of an all-optical microwave receiver free of high frequency electronic transistors, mixers and filters.