Near-field imaging techniques at terahertz (THz) frequencies are severely restricted by diffraction. To date, different detection schemes have been developed, based either on sub-wavelength metallic apertures or on sharp metallic tips. However high-resolution THz imaging, so far, has been relying predominantly on detection techniques that require either an ultrafast laser or a cryogenically-cooled THz detector, at the expenses of a lack of sensitivity when high resolution levels are needed. Here, we demonstrate two novel near-field THz imaging techniques able to combine strongly sub-wavelength spatial resolution with highly sensitive amplitude and phase detection capability. The first technique exploits an interferometric optical setup based on a THz quantum cascade laser (QCL) and on a near-field probe nanodetector, operating at room temperature. By performing phase-sensitive imaging of THz intensity patterns we demonstrate the potential of our novel architecture for coherent imaging with sub-wavelength spatial resolution improved up to 17 μm. The second technique is a detector-less s-SNOM system, exploiting a THz QCL as source and detector simultaneously. This approach enables amplitude- and phase-sensitive imaging by self-mixing interferometry with spatial resolution of 60-70 nm.
We have studied the optical properties of a hybrid system consisting of cyanine dye J-aggregates (both PIC and TDBC)
attached to a spherical microcavity. Instead of the commonly accepted chemical bonding of dye molecules to the surface
of microspheres or deposition of dye-doped sol-gel film, in our experiments microspheres were coated with J-aggregate
shell utilizing the layer-by-layer assembly of the ultrathin films. In this approach we aimed to take advantage of light
confinement in the Whispering Gallery Modes (WGMs) microcavity by placing the emitter (shell of J-aggregates) just at
the rim of the microsphere, where the resonant electromagnetic field reaches its maximum. A periodic structure of
narrow peaks was observed in the photoluminescence spectrum of the J-aggregates, arising from the coupling between
the emission of J-aggregates and the WGMs of the microcavity. The most striking result of our study is the observation
of polarization sensitive mode damping caused by re-absorption of J-aggregate emission. This effect manifests itself in
dominating emission from the transverse magnetic modes in the spectral region of J-aggregates absorption band where
the transverse electric (TE) modes are strongly suppressed. Strong suppression of TE modes reflects preferential
tangential orientation of transition dipole moment of J-aggregates in deposited microcavity shell. Observed polarization
sensitive mode damping observed in the spectral region of high J-aggregate absorption can be used for suppression of
unwanted modes in high Q optical resonators. We also demonstrate that the emission intensity can be further enhanced
by depositing a hybrid layer of J-aggregates and Ag nanoparticles onto the spherical microcavity. Owing to the concerted
action of WGMs and plasmonic hot spots in the Ag aggregates, we observe strongly enhanced Raman signal from the Jaggregates.
Microcavities covered by J-aggregates and plasmonic nanoparticles could be thus useful for a variety of
photonic applications in basic science and technology.