Quantum Cascade Laser Frequency Combs Research Articles

Surface-emitting THz quantum cascade laser frequency comb with tunable external mirror dispersion compensation

We present a surface emitting THz quantum cascade laser frequency comb with an adjustable chromatic dispersion compensation via a mechanically tunable GTI cavity. Surface emission and high optical feedback into the laser cavity are achieved by a planarized ridge waveguide design with low reflectivity facets and two broadband patch array antennas for coupling to an external mirror (back side) and for power extraction (front side). We demonstrate direct and reproducible manipulation of the frequency comb state, specifically the comb stability and beatnote frequency tuning, by controlling the position of an external movable mirror.

Self‐Induced Phase Locking of Terahertz Frequency Combs in a Phase‐Sensitive Hyperspectral Near‐Field Nanoscope (Adv. Sci. 28/2022)

Hyperspectral Nanoimaging A hyperspectral terahertz near field optical microscope comprising a miniaturized quantum cascade laser frequency comb, acting simultaneously as a source and as a detector is presented. Quantum nanomaterials and related collective mode excitations can be probed at the nanoscale. More details can be found in article number 2200410 by Miriam Serena Vitiello and co-workers.

Terahertz quantum cascade laser frequency combs with optical feedback.

Optical feedback exists in most laser configurations and strongly affects laser performances depending on the feedback strength, length, and phase. In this paper, we investigate the frequency comb behaviour of a semiconductor quantum cascade laser emitting around 4.2 THz with external optical feedback. A periodic evolution of the laser inter-mode beatnote from single-line to multiple-line structures is experimentally observed with a minor change of optical feedback length (phase) on the wavelength scale. The comb stability of the laser with feedback is also measured and compared with the same laser without feedback. Furthermore, our simulations reveal that the dynamical oscillations invoked by optical feedback are responsible for the measured multiple-line beatnotes. It is found that the characteristic feedback period is determined by the half wavelength of the laser, while the comb operation is maintained at most feedback length positions. Therefore, terahertz quantum cascade laser combs are robust against the minor position vibration of the feedback mirror in practice, owing to the much smaller feedback phase change than that of common near-infrared laser diodes.

All in One‐Chip, Electrolyte‐Gated Graphene Amplitude Modulator, Saturable Absorber Mirror and Metrological Frequency‐Tuner in the 2–5 THz Range

AbstractLayered 2D materials display unique optical and electrical properties that can enable manipulation, propagation, and detection of electromagnetic waves over a broad spectral range, with a high level of control, offering the potential to activate different functionalities, by optical or electrical means, in a single chip. Here, a compact optoelectronic device behaving as an amplitude modulator, saturable absorber mirror (SA mirror), and frequency‐tuner is conceived at terahertz (THz) frequencies. It comprises a gate‐tunable single layer graphene (SLG), embedded in a quarter‐wave cavity, operating in the 1–5 THz range. The use of electrolyte ionic liquid gate ensures 40% optical amplitude modulation depth. Z‐scan self‐mixing interferometry reveals 60% reflectivity modulation, with ≈4.5 W cm−2 saturation intensity. By integrating the modulator/SA mirror with a heterogeneous THz quantum cascade laser frequency comb, in an external cavity configuration, fine‐tuning of the intermode beatnote frequency is also demonstrated. This opens intriguing perspectives for short pulse generation, phase‐locking, frequency tuning/chirping, phase modulation, and metrological referencing, inter alia.

Analog FM free-space optical communication based on a mid-infrared quantum cascade laser frequency comb.

Quantum cascade laser frequency combs are nowadays well-appreciated sources for infrared spectroscopy. Here their applicability for free-space optical communication is demonstrated. The spontaneously-generated intermodal beat note of the frequency comb is used as carrier for transferring the analog signal via frequency modulation. Exploiting the atmospheric transparency window at 4 µm, an optical communication with a signal-to-noise ratio up to 65 dB is realized, with a modulation bandwidth of 300 kHz. The system tolerates a maximum optical attenuation exceeding 35 dB. The possibility of parallel transmission of an independent digital signal via amplitude modulation at 5 Mbit/s is also demonstrated.

Frequency axis for swept dual-comb spectroscopy with quantum cascade lasers.

In dual-comb spectroscopy, there is a one-to-one map between the frequencies of the measured beat notes and the frequencies of the optical comb lines. Its determination usually involves the use of one or more reference lasers with known frequencies. Quantum cascade laser frequency combs, however, are often operated in a free-running mode, and without a reference, the determination of the RF-to-optical frequency map is not trivial. Here, we propose a method by which the comb shift is measured with an unbalanced Mach-Zehnder interferometer, and the spectral point spacing is determined through the intermode beat measured on the laser electrodes. The frequency axis is accurate within ∼ 0.001 cm-1.

Field-enhancing tapered planarized waveguides for THz quantum cascade laser frequency combs

We present a new planarized waveguide geometry for THz quantum cascade laser frequency combs with improved waveguide losses, RF and thermal dissipation properties. Ridge devices display broadband free-running comb states, and the THz emission can be further broadened by RF injection. Tapered waveguide devices feature a strong field-enhancement effect, which results in an improved comb performance. This includes free-running comb states with strong single beatnotes up to nearly -30 dBm at 90 K, almost three orders of magnitude stronger than for ridge devices. Improved comb operation is maintained also for high operating temperatures, up to 115 K.

Near-Full Current Dynamic Range Thz Quantum Cascade Laser Frequency Comb

Near-Full Current Dynamic Range Thz Quantum Cascade Laser Frequency Comb

Octave-spanning low-loss mid-IR waveguides based on semiconductor-loaded plasmonics

Plasmonic waveguides are crucial building blocks for integrated on-chip mid-infrared (mid-IR) sensors, which have recently attracted great interest as a sensing platform to target enhanced molecular sensing. However, while hosting a wide range of applications from spectroscopy to telecommunication, the mid-IR lacks suitable broadband solutions that provide monolithic integration with III-V materials. This work reports a novel concept based on hybrid semiconductor-metal surface plasmon polariton waveguides, which result in experimentally demonstrated low loss and broadband devices. Composed of a thin germanium slab on top of a gold layer, the waveguiding properties can be directly controlled by changing the geometrical parameters. The measured losses of our devices are as low as 6.73 dB/mm at 9.12 µm and remain <15 dB/mm in the mid-IR range of 5.6–11.2 µm. The octave-spanning capability of the waveguides makes them ideal candidates for combination with broadband mid-IR quantum cascade laser frequency combs and integrated spectroscopic sensors.

Controlling Quantum Cascade Laser Optical Frequency Combs through Microwave Injection (Laser Photonics Rev. 15(12)/2021)

Quantum Cascade lasers In article number 2100242, Barbara Schneider and co-workers present strong microwave injection in quantum cascade laser frequency combs as a means for controlling the lasing state. Varying modulation frequency and power enables us to broaden and shift the spectral output as well as modify the phase profile. Moreover, the fast gain dynamics enable rapid switching between separate spectral regions with just a single device.

Controlling Quantum Cascade Laser Optical Frequency Combs through Microwave Injection

AbstractIn this work, control over the precise state emitted by quantum cascade laser frequency combs through strong radio‐frequency current modulation close to their repetition frequency is demonstrated. In particular, broadening of the spectrum from about 20 cm−1 to an average bandwidth of 60 cm−1 can be achieved throughout most of the current dynamical range while preserving the coherence, as measured by shifted wave interference Fourier transform spectroscopy (SWIFTS). The required modulation frequency to achieve this broadening is red‐shifted compared to the free‐running beatnote frequency at increasing modulation powers starting from 25 dBm, whereas the range where it occurs narrows. Outside of this maximum‐bandwidth range, the spectral bandwidth of the laser output is gradually reduced and the new center frequency is red‐ or blue‐shifted, directly dependent on the detuning of the modulation frequency. By switching between two modulation frequencies detuned symmetrically with respect to the free‐running beatnote, two multiplexed spectral regions with negligible overlap from the same device at rates of at least 20 kHz can be generated. In the time‐domain, with both SWIFTS and interferometric autocorrelation (IAC) measurements a transition from quasi‐continuous output to long‐pulsed ( ps) output is shown by ramping up the injection power to 35 dBm.

Spectrally resolved linewidth enhancement factor of a semiconductor frequency comb

The linewidth enhancement factor (LEF) has recently moved into the spotlight of research on frequency comb generation in semiconductor lasers. Here we present a novel modulation experiment that enables direct measurement of the spectrally resolved LEF in a laser frequency comb. By utilizing a phase-sensitive technique, we are able to extract the LEF for each individual comb mode in any laser type. We first investigate and verify this universally applicable technique using Maxwell–Bloch simulations. Following, we present the experimental demonstration on a quantum cascade laser frequency comb, confirming the predicted key role of the LEF in frequency comb dynamics.

Engineering the spectral bandwidth of quantum cascade laser frequency combs.

Quantum cascade lasers (QCLs) facilitate compact optical frequency comb sources that operate in the mid-infrared and terahertz spectral regions, where many molecules have their fundamental absorption lines. Enhancing the optical bandwidth of these chip-sized lasers is of paramount importance to address their application in broadband high-precision spectroscopy. In this work, we provide a numerical and experimental investigation of the comb spectral width and show how it can be optimized to obtain its maximum value defined by the laser gain bandwidth. The interplay of nonoptimal values of the resonant Kerr nonlinearity and cavity dispersion can lead to significant narrowing of the comb spectrum and reveals the best approach for dispersion compensation. The implementation of high mirror losses is shown to be favorable and results in proliferation of the comb sidemodes. Ultimately, injection locking of QCLs by modulating the laser bias around the round trip frequency provides a stable external knob to control the frequency-modulated comb state and recover the maximum spectral width of the unlocked laser state.

Coherently-averaged dual comb spectrometer at 7.7 µm with master and follower quantum cascade lasers.

We demonstrate coherent averaging of the multi-heterodyne beat signal between two quantum cascade laser frequency combs in a master-follower configuration. The two combs are mutually locked by acting on the drive current to control their relative offset frequency and by radio-frequency extraction and injection locking of their intermode beat signal to stabilize their mode spacing difference. By implementing an analog common-noise subtraction scheme, a reduction of the linewidth of all heterodyne beat notes by five orders of magnitude is achieved compared to the free-running lasers. We compare stabilization and post-processing corrections in terms of amplitude noise. While they give similar performances in terms of signal-to-noise ratio, real-time processing of the stabilized signal is less demanding in terms of computational power. Lastly, a proof-of-principle spectroscopic measurement was performed, showing the possibility to reduce the amount of data to be processed by three orders of magnitude, compared to the free-running system.

Quantum Cascade Lasers: Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning (Laser Photonics Rev. 15(6)/2021)

In article number 2000417, Luigi Consolino and co-workers demonstrate a novel degree of freedom for controlling the emission of quantum cascade lasers (QCLs). It is based on small optical frequency tuning (SOFT) by means of a white LED. The slightly perturbative regime accessible in these conditions allows tweaking the semiconductor complex refractive index. The resulting additional tuning knob is capable of phase stabilizing the modes spacing of a QCL comb.

Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning

AbstractFull phase control of terahertz (THz)‐emitting quantum cascade laser (QCL) combs has recently been demonstrated, opening new perspectives for even the most demanding applications. In this framework, simplifying the set‐ups for control of these devices will help to accelerate their spreading in many fields. This study reports a new way to control the emission frequencies of a THz QCL comb by small optical frequency tuning (SOFT), using a very simple experimental setup, exploiting the incoherent emission of an ordinary white light‐emitting diode. The slightly perturbative regime accessible in these conditions allows tweaking the complex refractive index of the semiconductor without destabilizing the broadband laser gain. The SOFT actuator is characterized and compared to another actuator, the QCL driving current. The suitability of this additional degree of freedom for frequency and phase stabilization of a THz QCL comb is shown and perspectives are discussed.

Microsecond-Resolved Infrared Spectroscopy on Nonrepetitive Protein Reactions by Applying Caged Compounds and Quantum Cascade Laser Frequency Combs.

Infrared spectroscopy is ideally suited for the investigation of protein reactions at the atomic level. Many systems were investigated successfully by applying Fourier transform infrared (FTIR) spectroscopy. While rapid-scan FTIR spectroscopy is limited by time resolution (about 10 ms with 16 cm-1 resolution), step-scan FTIR spectroscopy reaches a time resolution of about 10 ns but is limited to cyclic reactions that can be repeated hundreds of times under identical conditions. Consequently, FTIR with high time resolution was only possible with photoactivable proteins that undergo a photocycle. The huge number of nonrepetitive reactions, e.g., induced by caged compounds, were limited to the millisecond time domain. The advent of dual-comb quantum cascade laser now allows for a rapid reaction monitoring in the microsecond time domain. Here, we investigate the potential to apply such an instrument to the huge class of G-proteins. We compare caged-compound-induced reactions monitored by FTIR and dual-comb spectroscopy by applying the new technique to the α subunit of the inhibiting Gi protein and to the larger protein-protein complex of Gαi with its cognate regulator of G-protein signaling (RGS). We observe good data quality with a 4 μs time resolution with a wavelength resolution comparable to FTIR. This is more than three orders of magnitude faster than any FTIR measurement on G-proteins in the literature. This study paves the way for infrared spectroscopic studies in the so far unresolvable microsecond time regime for nonrepetitive biological systems including all GTPases and ATPases.

Mid-infrared quantum cascade laser frequency combs with a microstrip-like line waveguide geometry

In this work, a design for a mid-infrared quantum cascade laser frequency comb source that enhances the high frequency response and the comb characteristics of the device is presented. A state-of-the-art active region, grown on a heavily n-doped InP:Si substrate, was processed into a buried heterostructure with a microstrip-like line waveguide. As a result, the repetition rate frequency frep, around 11.09 GHz, can be locked to an injected narrow-linewidth radio frequency (RF) signal, over a range of more than 200 kHz with an injected power of −10 dBm, which outperforms normal buried heterostructure schemes by an order of magnitude. Moreover, under RF injection at powers higher than 20 dBm, the lasing spectrum is flattened and significantly broadened, from 24 cm−1 to 65 cm−1 in bandwidth, while at the same time, the coherence of the comb is maintained and verified.

QCL Dual‐Comb Spectroscopy Matures for Applications

AbstractDual‐comb spectroscopy is a powerful direct absorption spectroscopy technique and has attracted considerable attention, with high precision spectroscopy applications being the most prominent since the invention of the frequency comb was awarded the 2005 Nobel Prize in physics. The application range is continuously broadening and here we review recent advances achieved with quantum cascade laser frequency comb spectrometers in process monitoring and reaction kinetics applications.

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).