Frequency chirped Fourier-Transform spectroscopy.

Fast (sub-second) spectroscopy with high spectral resolution is of vital importance for revealing quantum chemistry kinetics of complex chemical and biological reactions. Fourier transform (FT) spectrometers can achieve high spectral resolution and operate at hundreds of ms time scales in rapid-scan mode. However, the linear translation of a scanning mirror imposes stringent time-resolution limitations to these systems, which makes simultaneous high spectral and temporal resolution very difficult. Here, we demonstrate an FT spectrometer whose operational principle is based on continuous rotational motion of the scanning mirror, effectively decoupling the spectral resolution from the temporal one. Furthermore, we show that such rotational FT spectrometer can perform Mid-IR dual-comb spectroscopy with a single comb source, since the Doppler-shifted version of the comb serves as the second comb. In our realization, we combine the advantages of dual-comb and FT spectroscopy using a single quantum cascade laser frequency comb emitting at 8.2 µm as a light source. Our technique does not require any diffractive or dispersive optical elements and hence preserve the Jacquinot's-, Fellgett's-, and Connes'-advantages of FT spectrometers. By integrating mulitple broadband sources, such system could pave the way for applications where high speed, large optical bandwidth, and high spectral resolution are desired.

Mid-infrared quantum cascade laser frequency combs based on multi-section waveguides.

We present quantum cascade laser (QCL) frequency comb devices with engineered waveguides for managing the dispersion. The QCL waveguide consists of multiple sections with different waveguide widths. The narrow and wide sections of the waveguide are designed in a way to compensate the group velocity dispersion (GVD) of each other and thereby produce a flat and slightly negative GVD for the QCL. The QCL exhibits continuous comb operation over a large part of the dynamic range of the laser. Strong and narrow-linewidth intermode beatnotes are achieved in a more than 300 mA wide operation current range. The comb device also features considerably high output power (>380mW) and wide optical bandwidth (>55cm-1).

Frequency noise correlation between the offset frequency and the mode spacing in a mid-infrared quantum cascade laser frequency comb.

The generation of frequency combs in the mid-infrared (MIR) spectral range by quantum cascade lasers (QCLs) has the potential for revolutionizing dual-comb multi-heterodyne spectroscopy in the molecular fingerprint region. However, in contrast to frequency combs based on passively mode-locked ultrafast lasers, their operation relies on a completely different mechanism resulting from a four-wave mixing process occurring in the semiconductor gain medium that locks the modes together. As a result, these lasers do not emit pulses and no direct self-referencing of a QCL comb spectrum has been achieved so far. Here, we present a detailed frequency noise characterization of a MIR QCL frequency comb operating at a wavelength of 8 µm with a mode spacing of ∼7.4 GHz. Using a beat measurement with a narrow-linewidth single-mode QCL in combination with a dedicated electrical scheme, we measured the frequency noise properties of an optical mode of the QCL comb, and indirectly of its offset frequency for the first time, without detecting it by the standard approach of nonlinear interferometry applied to ultrafast mode-locked lasers. In addition, we also separately measured the noise of the comb mode spacing extracted electrically from the QCL. We observed a strong anti-correlation between the frequency fluctuations of the offset frequency and mode spacing, leading to optical modes with a linewidth slightly below 1 MHz in the free-running QCL comb (at 1-s integration time), which is narrower than the individual contributions of the offset frequency and mode spacing that are at least 2 MHz each.

Biexcitonic molecules survive excitons at the Mott transition.

When the carrier density is increased in a semiconductor, according to the predictions of Sir Nevil Mott, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma. This crossover, usually referred to as the Mott transition, is driven by the mutual effects of phase-space filling and Coulomb screening because of the presence of other charges nearby. It drastically affects the optical and electrical characteristics of semiconductors and may, for example, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. Usually, the possible existence of excitonic molecules (or biexcitons) is neglected in the understanding of the Mott transition because the biexciton is supposed to be less robust against screening effects. Here, against common beliefs, we observe that the biexciton in a GaN quantum well is more stable towards the Mott transition than the exciton.

Exciton drift in semiconductors under uniform strain gradients: application to bent ZnO microwires.

Optimizing the electronic structures and carrier dynamics in semiconductors at atomic scale is an essential issue for innovative device applications. Besides the traditional chemical doping and the use of homo/heterostructures, elastic strain has been proposed as a promising possibility. Here, we report on the direct observation of the dynamics of exciton transport in a ZnO microwire under pure elastic bending deformation, by using cathodoluminescence with high temporal, spatial, and energy resolutions. We demonstrate that excitons can be effectively drifted by the strain gradient in inhomogeneous strain fields. Our observations are well reproduced by a drift-diffusion model taking into account the strain gradient and allow us to deduce an exciton mobility of 1400 ± 100 cm(2)/(eV s) in the ZnO wire. These results propose a way to tune the exciton dynamics in semiconductors and imply the possible role of strain gradient in optoelectronic and sensing nano/microdevices.