Filippos Kapsalidis

Room-temperature nine-µm-wavelength photodetectors and GHz-frequency heterodyne receivers

Room-temperature operation is essential for any optoelectronics technology that aims to provide low-cost, compact systems for widespread applications. A recent technological advance in this direction is bolometric detection for thermal imaging, which has achieved relatively high sensitivity and video rates (about 60 hertz) at room temperature. However, owing to thermally induced dark current, room-temperature operation is still a great challenge for semiconductor photodetectors targeting the wavelength band between 8 and 12 micrometres, and all relevant applications, such as imaging, environmental remote sensing and laser-based free-space communication, have been realized at low temperatures. For these devices, high sensitivity and high speed have never been compatible with high-temperature operation. Here we show that a long-wavelength (nine micrometres) infrared quantum-well photodetector fabricated from a metamaterial made of sub-wavelength metallic resonators exhibits strongly enhanced performance with respect to the state of the art up to room temperature. This occurs because the photonic collection area of each resonator is much larger than its electrical area, thus substantially reducing the dark current of the device. Furthermore, we show that our photonic architecture overcomes intrinsic limitations of the material, such as the drop of the electronic drift velocity with temperature, which constrains conventional geometries at cryogenic operation. Finally, the reduced physical area of the device and its increased responsivity allow us to take advantage of the intrinsic high-frequency response of the quantum detector at room temperature. By mixing the frequencies of two quantum-cascade lasers on the detector, which acts as a heterodyne receiver, we have measured a high-frequency signal, above four gigahertz (GHz). Therefore, these wide-band uncooled detectors could benefit technologies such as high-speed (gigabits per second) multichannel coherent data transfer and high-precision molecular spectroscopy.

Dual-wavelength DFB quantum cascade lasers for NO and NO 2 trace gas analysis

Quantum cascade lasers (QCLs) are powerful semiconductor light sources, based on intersubband transitions, which can be tailored to operate in the mid-infrared (MIR) region of the electromagnetic spectrum [1]. This region is of particular interest for applying various gas absorption spectroscopy schemes, as many of the characteristic transitions of several gas molecules occur at energies that are situated in this region [2].

Recent advances of multispecies mid-IR spectroscopy for mobile applications

Novel designs and fabrication of multi-wavelength mid-IR quantum cascade lasers [1, 2] have triggered highly attractive progress in instrumental developments towards compactness and multispecies detection, addressing the demands for high-precision and selective measurements of a large variety of molecular species, as well as for compact, robust and field deployable gas analyzers. This work highlights our latest achievements in the field through various examples of instruments and their applications. Thereby, we also illustrate the importance of further developing the key elements of laser spectrometers, such as laser driving concept and electronics [3], multipass cell design [4] as well as optimized solutions for data acquisition [5]. It is only through the combination of all these components that the inherent advantage of probing the fundamental absorption features in the mid-IR using quantum cascade lasers can fully be exploited.

Mid-Infrared spectrometer featuring μ-second time resolution based on dual-comb quantum cascade laser frequency combs

We present a dual-comb spectrometer based on QCL frequency combs. It features a large optical bandwidth and high-resolution. One key benefit of this instrument is the ability to measure broadband μs time-resolved mid-IR spectra.

Self-detection of MIR QCL frequency combs (Withdrawal Notice)

In the recent years, we demonstrated Quantum Cascade Laser (QCL) based dual comb spectroscopy in the mid-IR by combining two comb beams on a fast detector. It was shown as well that, by using a RF bias-tee between the QCL current driver and the QCL, the RF spectrum containing the comb repetition frequency can be recorded. In this work, we demonstrate a new integrated approach for dual comb spectroscopy based on a self-detection scheme. We inject the optical beam of a QCL comb directly into another QCL which produces as well his comb. By properly choosing slightly different frequency repetition rates for both combs, we can obtain the multi-heterodyne beat spectrum through the RF port of a bias-tee placed between the current driver and the injected QCL. In this manner, this QCL acts both as a source and a detector, allowing to simplify considerably the dual comb spectroscopy setup by removing the fast detector. An optical isolator as well as a variable attenuator were placed after the injecting QCL to prevent the formation of an external cavity and gain stability. The QCL combs have a frequency centered around 7.6 μm. The difference of frequency repetition rate is of about 5.3 MHz and approximately 150 lines are visible on the multi-heterodyne beat signal. This corresponds to a coverage of 50 cm-1 in the optical spectra with a resolution of 0.33 cm-1. Those preliminary results are extremely encouraging and spectroscopy measurements will be performed using this self-detection dual comb setup.