We demonstrate a gas spectroscopy technique, using self-mixing in a 3.4 terahertz quantum-cascade laser (QCL). All previous QCL spectroscopy techniques have required additional terahertz instrumentation (detectors, mixers, or spectrometers) for system pre-calibration or spectral analysis. By contrast, our system self-calibrates the laser frequency (i.e., with no external instrumentation) to a precision of 630 MHz (0.02%) by analyzing QCL voltage perturbations in response to optical feedback within a 0-800 mm round-trip delay line. We demonstrate methanol spectroscopy by introducing a gas cell into the feedback path and show that a limiting absorption coefficient of ∼1×10 cm is resolvable.

No abstract

Near-field microscopy techniques operating in the terahertz (THz) frequency band offer the tantalizing possibility of visualizing with nanometric resolution the localized THz fields supported by individual resonators, micro-structured surfaces, and metamaterials. Such capabilities promise to underpin the future development and characterization of a wide range of devices, including THz emitters, detectors, optoelectronic modulators, sensors, and novel optical components. In this work, we report scattering-type scanning near-field optical microscopy using a THz-frequency quantum cascade laser (QCL) to probe coherently the localized field supported by individual microresonator structures. Our technique demonstrates deep sub-wavelength mapping of the field distribution associated with in-plane resonator modes in plasmonic dipole antennas and split ring resonator structures. By exploiting electronic tuning of the QCL in conjunction with the coherent self-mixing effect in these lasers, we are able to resolve both the magnitude and the phase of the out-of-plane field. We, furthermore, show that the elliptically polarized state of the QCL field can be exploited for the simultaneous excitation and measurement of plasmonic resonances in these structures while suppressing the otherwise dominant signal arising from the local material permittivity.

No abstract

We report all-electronic coherent scattering-type scanning near-field microscopy (s-SNOM) using a terahertzfrequency quantum cascade laser. By exploiting the coherent selfmixing effect in these lasers, in conjunction with electronic frequency tuning of the laser, we demonstrate spatial mapping of both the amplitude and the phase of the scattered field with deeply subwavelength resolution. We apply our technique for coherent microscopy of a phonon-resonant crystal. The extracted amplitude and phase parameters reveal clear contrast when compared to both metallic and nonresonant dielectric materials and show excellent agreement with those calculated using a finite-dipole model of the near-field interaction between the s-SNOM tip and the resonant sample in the Reststrahlen band. Our technique paves the way for fast nanoscale-resolved mapping of the dielectric function of solid state systems and optoelectronic nanodevices at terahertz frequencies.