Common mode frequency instability in internally phase-locked terahertz quantum cascade lasers

The study of quantum cascade lasers (QCL) in the terahertz frequency region by the use of THz time-domain spectroscopy has gained a lot of interest within the last few years [1]. Good physical insight was obtained for THz QCLs employing a single plasmon waveguide (surface plasmon waveguide) [2,3]. In comparison to that the double plasmon or metal-metal waveguide QCL uses a metal/semiconductor/metal structure as a waveguide. THz QCLs with this type of waveguide are showing the highest operating temperatures so far [4]. The metal-metal THz QCLs confine the whole mode in the active region of subwavelength size. In order to investigate metal-metal THz QCLs by THz pulses an efficient method of THz pulse launching needs to be employed. Several methods for coupling of free space THz pulses into the subwavelength waveguide, such as horn antenna, gratings or lenses can be used. A further method is a waveguide emitter [5,6] which benefits from the near infrared excitation beam focus which is much smaller than the corresponding THz focus.

Quantum cascade (QC) lasers opens new prospects for powerful sources operating at THz frequencies. Up to now the best THz QC lasers are based on intersubband emission in GaAs/AlGaAs quantum well (QW) heterostructures. The maximum operating temperature is 200 K, which is too low for wide-spread applications. This is due to the rather low LO-phonon energy (36 meV) of GaAs-based materials. Indeed, thermal activation allows non-radiative path through electron-phonon interaction which destroys the population inversion. Wide band gap materials such as ZnO have been predicted to provide much higher operating temperatures because of the high value of their LO-phonon energy. However, despite some observations of intersubband absorption in c-plane ZnO/ZnMgO quantum wells, little is known on the fundamental parameters such as the conduction band offset in such heterostructures. In addition the internal field inherent to c-plane grown heterostuctures is an handicap for the design of QC lasers and detectors. In this talk, we will review a systematic investigation of ZnO/ZnMgO QW heterostructures with various Mg content and QW thicknesses grown by plasma molecular beam epitaxy on low-defect m-plane ZnO substrates. We will show that most samples exhibit TM-polarized intersubband absorption at room temperature linked either to bound-to-quasi bound inter-miniband absorption or to bound-to bound intersubband absorption depending on the Mg content of the barrier material. This systematic study allows for the first time to estimate the conduction band offset of ZnO/ZnMgO heterostructures, opening prospects for the design of QC devices operating at THz frequencies. This was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement #665107.

Terahertz quantum cascade (QC) lasers are well suited for the exploration of active metamaterial concepts in the terahertz frequency range. Terahertz composite right/left handed (CRLH) transmission line metamaterials can be integrated with quantum cascade laser gainmaterial in order to compensate for losses, and enable laser waveguides with new functionality. In particular, we consider the use of metamaterial transmission lines as travelling wave antennas. After presenting the characteristics of a 2.5 THz quantum-cascade laser, calculated radiation characteristics and beam patterns for a leaky-wave antenna based upon a balanced 1D CRLH transmission line waveguide are shown.

The technological solutions and optical configurations employed to tune the emission frequency of quantum cascade lasers (QCLs) operating in the Terahertz range are reviewed. The gain spectrum of THz QCLs can be engineered by quantum design to be broadband over a range even larger than 1 THz, or, alternatively, to be electrically tunable, for instance exploiting the Stark shift of the lasing transition. Fabry-Perot lasers can then display very ample multimode spectra, but wide, though discontinuous and unpredictable, tunability through mode hopping can also be achieved. For reliable and repeatable single-mode tuning, QCL emitters based on external cavities can be realized using either a movable mirror to control the cavity length or a rotating grating for wavelength dependent feedback. First-order and second-order distributed feedback (DFB) THz QCLs also provide stable single-mode emission, whose frequency can be tuned by temperature and current, though usually on a more limited range of only a few GHz. It has been recently shown, however, that second-order THz DFB lasers can be coupled to an external microcavity, exploiting the anti-crossing of the respective eigenfrequencies to achieve wider tuning ranges by mechanically changing the size of the external microcavity. As an alternative, if fabricated in a narrow (~λ/3) ridge configuration, double-metal THz DFB lasers represent an ideal device where to manipulate the lateral size of the lasing mode via electro-mechanical components. In this way, a new and robust method for wide and continuous tuning of the laser emission has been recently demonstrated, displaying a record large bandwidth of over 330 GHz.

Their inherently broadband gain and high value of the third order nonlinearity makes quantum cascade lasers (QCLs) ideal candidates for frequency comb generation in the terahertz (THz) and mid-infrared (midIR) spectral ranges. In recent years, it has been demonstrated that free running QCLs can produce broad comb spectra, even without any special endeavour to induce phase-locking of the lasing modes [1]. Unfortunately, such “coherent” regimes of multimode operation seem to span only a very narrow portion of the full dynamic range of the devices, which hinders the practical usability of this technology [1, 2].

The generation of frequency combs in the mid-infrared and terahertz regimes from compact and potentially cheap sources could have a strong impact on spectroscopy, as many molecules have their rotovibrational bands in this spectral range. Thus, quantum cascade lasers (QCLs) are the perfect candidates for comb generation in these portions of the electromagnetic spectrum. Here we present a theoretical model based on a full numerical solution of Maxwell-Bloch equations suitable for the simulation of such devices. We show that our approach captures the intricate interplay between four wave mixing, spatial hole burning, coherent tunneling and chromatic dispersion which are present in free running QCLs. We investigate the premises for the generation of QCL based terahertz combs. The simulated comb spectrum is in good agreement with experiment, and also the observed temporal pulse switching between high and low frequency components is reproduced. Furthermore, non-comb operation resulting in a complex multimode dynamics is investigated.

We demonstrate phase locking of a 4.7 THz distributed feedback quantum cascade laser (QCL) to a stable laboratory synthesizer. The QCL operates at 4747.9 GHz with 1 mW output power, mounted on the first stage of a pulse-tube cryocooler with mechanical dampers. Despite residual mechanical vibrations, the QCL radiation is successfully coupled out through a series of cold and warm mirrors and focused onto a room-temperature superlattice diode (SLD). The SLD is pumped by a 182.6 GHz signal from a diode frequency multiplier chain and generates the 26th harmonic at 4748.2 GHz for harmonic mixing as a phase reference. This produces a 327 MHz intermediate frequency (IF) that is amplified, filtered, and fed to a balanced diode mixer phase detector. The phase-locked loop employs a loop filter to generate correction signals for QCL current adjustment, reducing the unlocked linewidth from approximately 10 MHz to 1 Hz (limited by resolution bandwidth of spectrum analyzer). In addition of providing absolute frequency reference and ability of introducing fine frequency steps, the system demonstrates remarkable robustness, maintaining phase lock for several hours and even under low signal-to-noise conditions such as 1 dB of signal over noise at 3 MHz resolution bandwidth.This robustness comes from tailored PLL design, incorporating a diode ring mixer as phase detector and a linear phase narrow band filter, and proves its suitability for practical heterodyne array receivers targeting the [OI] line at 4.7 THz.

Coupling of broadband terahertz pulses into metal-metal terahertz quantum cascade lasers is presented. Mode matched terahertz transients are generated on the quantum cascade laser facet of subwavelength dimension. This method provides a full overlap of optical mode and active laser medium. A longitudinal optical-phonon depletion based active region design is investigated in a coupled cavity configuration. Modulation experiments reveal spectral gain and (broadband) losses. The observed gain shows high dynamic behavior when switching from loss to gain around threshold and is clamped at total laser losses.

Gold rectangular waveguides suitable for single mode operation at frequencies near 3 THz have been micromachined using silicon microfabrication methods. The design, fabrication, and loss characteristics of these waveguides will be presented. On-chip integration of these waveguide structures with THz quantum cascade laser (QCL) sources has been achieved and the characteristics of QCL beams guided by waveguide will be discussed.

We present an investigation into the vertical transport through 13 different superlattice structures, where the well and barrier widths, doping concentration, dopant position, and contact layers were varied. Although superlattices have been extensively studied since 1970, there is a lack of publications on transport through superlattices similarly low doped as THz quantum cascade lasers (QCLs), for which the doping is in the $3--5\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$ range. The superlattices presented are doped in the same range as THz QCLs, with contact layers and fabrication comparable to high-temperature THz QCLs. The temperature-dependent current-voltage characteristics were measured starting from 5 K and an anomalous temperature effect was observed at the first plateau. The measured current through the superlattice first decreases before increasing again with increasing temperature, resulting in the lowest current occurring at 75--110 K. This behavior is also observed in some THz QCLs. The effect disappears for thinner barriers, higher quantum well doping, or modified contact layers, indicating a strong dependency on band bending, due to the large difference in the doping of the contact layers and the superlattice, which is confirmed with multiscattering B\"uttiker simulations.

We investigated and demonstrated bio-medical imaging using a THz quantum cascade laser. With the THz quantum cascade laser (QCL) at 3.8 THz, we obtained large dynamic range and high spatial resolution in the transmission imaging technique. The various tissues images, such as lung, liver, and brain sections from the laboratory mouse were obtained and studied. The most important factor for this imaging scheme is to obtain high contrast with different absorption characteristics in tissues. We explored distinct images from the fat, muscles and tendon from the freshly cut tissues and investigated absorption coefficient and compared with FTIR measurement. We also demonstrated the image of distinct region of tumors progressed and normal tissues using this technique. The comparison of frequency dependent medical imaging with utilizing different wavelength of QCLs has been addressed.

Two-dimensional terahertz strong-field spectroscopy reveals wave-mixing processes up to eighth order in a free-running quantum cascade laser, unravelling its sub-cycle gain dynamics and nonlinearities in a regime of negative absorption.

Recently, a novel type of molecular THz laser with ∼mW-level output power was demonstrated [1,2]. This laser is optically pumped by a mid-IR Quantum Cascade Laser (QCL), therefore resulting in a much more compact, frequency agile and deployable source compared to standard, CO 2 -laser pumped, far-infrared gas lasers. This achievement has revived the interest for molecular THz lasers, in particular for their exploitation as low-phase noise THz local oscillators (LOs) operating at room temperature. Compared to CO 2 -laser pumping, a QCL pump offers an unrivalled frequency agility that allows accessing an unprecedented number of molecular transitions. On the other hand, its lower frequency stability can potentially impact the frequency noise of the THz laser source [3–5]. Here we report the first measurement of the frequency noise power spectral density (PSD) of a 1.073THz ammonia laser pumped by a 10.3μm QCL. This is achieved by using a commercial harmonic mixer to generate a beating between the ammonia laser frequency and the 1080th harmonic of the 1GHz repetition rate of a custom-made, 1560nm frequency comb (FC). We then exploit a voltage-controlled oscillator to track the beat-note signal and realize the frequency to amplitude conversion [6]. The measured frequency noise PSD is shown in Fig.1. In the range ∼30–100kHz we find a frequency noise 2 -pumped, 2.5THz methanol laser, reaching 0.1Hz2/Hz (−110dBc) at 40kHz from the carrier, limited by the frequency noise of the FC harmonic. Finally, we will show that it is possible to actively phase-lock the QCL-pumped ammonia laser to the FC repetition rate harmonic by controlling the QCL drive current, demonstrating a sub-Hz linewidth. These results pave the way to the development of QCL-pumped molecular lasers as compact, low-phase noise THz sources operating a room temperature.

This talk starts with an overview of present challenges for TERA-MIR devices and the state of the art of the most promising device for this range: the quantum cascade laser (QCL). The need for advanced quantum statistical mechanics, many particle and nonequilibrium Physics to describe these devices will be explained and results of our leading edge simulator will be presented. In the last part of the talk progress on intersubband lasing without inversion will be discussed, since those designs can mitigate some of the most crucial problems faced by THz QCLs. In particular the influence of increasing dephasing on the k-space filtering mechanism will be discussed.

The THz quantum cascade laser (THz-QCL) based two-beam self mixing (SM) interference is achieved by re-injecting optical feedbacks from two independent targets into the laser cavity. The two-beam SM effect manifests itself in the variation of laser terminal voltage. Information of the relative position change of the two targets can be obtained by analyzing the amplitude and phase of the SM signal. Using this method, micron-scale thickness variation of a GaAs sample, about one percent of the laser wavelength, is accurately measured. Our study provides an available way to develop new terahertz nondestructive testing technology.