Dominic Bachmann

Short pulse generation and mode control of broadband terahertz quantum cascade lasers

We report on a waveguide engineering technique that enables the generation of a bandwidth up to 1 THz and record ultra-short pulse length of 2.5 ps in injection seeded terahertz quantum cascade lasers. The reported technique is able to control and fully suppress higher order lateral modes in broadband terahertz quantum cascade lasers by introducing side-absorbers to metal-metal waveguides. The side-absorbers consist of a top metalization set-back with respect to the laser ridge and an additional lossy metal layer. In continuous wave operation the side-absorbers lead to octave spanning laser emission, ranging from 1.63 to 3.37 THz, exhibiting a 725 GHz wide at top within a 10 dB intensity range as well as frequency comb operation with a bandwidth of 442 GHz. Numerical and experimental studies have been performed to optimize the impact of the side-absorbers on the emission properties and to determine the required increase of waveguide losses. Furthermore, these studies have led to a better understanding of the pulse formation dynamics of injection-seeded quantum cascade lasers.

Spectral gain profile of a multi-stack terahertz quantum cascade laser

The spectral gain of a multi-stack terahertz quantum cascade laser, composed of three active regions with emission frequencies centered at 2.3, 2.7, and 3.0 THz, is studied as a function of driving current and temperature using terahertz time-domain spectroscopy. The optical gain associated with the particular quantum cascade stacks clamps at different driving currents and saturates to different values. We attribute these observations to varying pumping efficiencies of the respective upper laser states and to frequency dependent optical losses. The multi-stack active region exhibits a spectral gain full width at half-maximum of 1.1 THz. Bandwidth and spectral position of the measured gain match with the broadband laser emission. As the laser action ceases with increasing operating temperature, the gain at the dominant lasing frequency of 2.65 THz degrades sharply.

Broadband terahertz amplification in a heterogeneous quantum cascade laser

We demonstrate a broadband terahertz amplifier based on ultrafast gain switching in a quantum cascade laser. A heterogeneous active region is processed into a coupled cavity metal-metal waveguide device and provides broadband terahertz gain that allows achieving an amplification bandwidth of more than 500 GHz. The temporal and spectral evolution of a terahertz seed pulse, which is generated in an integrated emitter section, is presented and an amplification factor of 21 dB is reached. Furthermore, the quantum cascade amplifier emission spectrum of the emerging sub-nanosecond terahertz pulse train is measured by time-domain spectroscopy and reveals discrete modes between 2.14 and 2.68 THz.

Dispersion in a broadband terahertz quantum cascade laser

We present dispersion data of a broadband terahertz quantum cascade laser with a heterogeneous active region. The experimental method to extract the group velocity dispersion of the entire laser cavity, including the contributions of the active region, the semiconductor material, and the waveguide relies on a time-domain spectroscopy system. The obtained group velocity dispersion curves exhibit oscillations with amplitudes up to 1 × 105 fs2/mm between 2.0 and 3.0 THz and strongly depend on the driving conditions of the laser. This indicates that the group velocity dispersion is mainly determined by the intersubband gain in the active region. The obtained dispersion data are compared to a dispersion model based on multiple Drude-Lorentz gain media yielding a significant correlation.

High-Power Growth-Robust InGaAs/InAlAs Terahertz Quantum Cascade Lasers

We report on high-power terahertz quantum cascade lasers based on low effective electron mass InGaAs/InAlAs semiconductor heterostructures with excellent reproducibility. Growth-related asymmetries in the form of interface roughness and dopant migration play a crucial role in this material system. These bias polarity dependent phenomena are studied using a nominally symmetric active region resulting in a preferential electron transport in the growth direction. A structure based on a three-well optical phonon depletion scheme was optimized for this bias direction. Depending on the sheet doping density, the performance of this structure shows a trade-off between high maximum operating temperature and high output power. While the highest operating temperature of 155 K is observed for a moderate sheet doping density of 2 × 1010 cm–2, the highest peak output power of 151 mW is found for 7.3 × 1010 cm–2. Furthermore, by abutting a hyperhemispherical GaAs lens to a device with the highest doping level a record output power of 587 mW is achieved for double-metal waveguide structures.

Heterogeneous terahertz quantum cascade lasers exceeding 1.9 THz spectral bandwidth and featuring dual comb operation

Abstract We report on a heterogeneous active region design for terahertz quantum cascade laser based frequency combs. Dynamic range, spectral bandwidth and output power have been significantly improved with respect to previous designs. When individually operating the lasers, narrow and stable intermode beatnote indicate frequency comb operation up to a spectral bandwidth of 1.1 THz, while in a dispersion-dominated regime a bandwidth up to 1.94 THz at a center frequency of 3 THz can be reached. A self-detected dual-comb setup has been used to verify the frequency comb nature of the lasers.We report on a heterogeneous active region design for terahertz quantum cascade laser based frequency combs. Dynamic range, spectral bandwidth as well as output power have been significantly improved with respect to previous designs. When operating individually the lasers act as a frequency comb up to a spectral bandwidth of 1.1 THz, while in a dispersed regime a bandwidth up to 1.94 THz at a center frequency of 3 THz can be reached. A self-detected dual-comb setup has been used to verify the frequency comb nature of the lasers.

Coupled cavity terahertz quantum cascade lasers with integrated emission monitoring

We demonstrate the on-chip generation and detection of terahertz radiation in coupled cavity systems using a single semiconductor heterostructure. Multiple sections of a terahertz quantum cascade laser structure in a double-metal waveguide are optically coupled and operate either as a laser or an integrated emission monitor. A detailed analysis of the photon-assisted carrier transport in the active region below threshold reveals the detection mechanism for photons emitted by the very same structure above threshold. Configurations with a single laser cavity and two coupled laser cavities are studied. It is shown that the integrated detector can be used for spatial sensing of the light intensity within a coupled cavity.

All-Electrical Thermal Monitoring of Terahertz Quantum Cascade Lasers

A key limitation, especially for the continuous-wave operation of terahertz quantum cascade lasers, is the large amount of heat dissipated in the active region. We demonstrate an all-electrical technique for monitoring the lattice temperature and characterizing the thermal properties of the active region, using the waveguide of the device as a temperature sensor. We report a measured temperature difference between the heat sink and top waveguide layer of up to 27 K during the continuous-wave operation of GaAs/Al0.15Ga0.85As-based devices, lasing at 2.4 THz. A thermal model of the devices is used to determine the thermal conductivity of the active region perpendicular to the semiconductor interfaces to be 7.3 W/(m·K).

Terahertz quantum cascade lasers frequency combs: Wide bandwidth operation and dual-comb on a chip

Terahertz (THz) quantum-cascade lasers (QCLs) constitute a very promising candidate for compact, wide bandwidth, integrated frequency combs [1, 4-7]. QCLs based on heterogeneous cores display the widest spectral coverage reaching more than one octave [2]. The possibility to engineer the gain profile turns out to be fundamental also with respect to dispersion compensation in order to extended the comb spectral bandwidth [2, 7]. In the effort of extending the comb operation to a full-octave to implement the laser self-referencing [3], we present here a new here a new THz QCL active region that allows the generation of a frequency comb with a spectral bandwidth exceeding of 1 THz centered at 3.1 THz. It fully exploits the capability of QCLs to integrate different active region designs within one laser cavity. The used building block is the three-active region design reported in Ref. [2], where a design at 3.4 THz has been added to increase the bandwidth towards higher frequencies. The four designs have central frequencies of 2.3, 2.6, 2.9, and 3.4 THz, the number of periods per design has also been rearranged in order to provide a flat gain resulting in a similar threshold for all the active regions and more dynamic range. Additionally, the doping level has been increased. Laser performance results in a largely extended dynamic range with respect to the original three-stack structure. Peak powers above 8 mW are recorded at 30 K and the lasing spectrum spans over 1.94 THz from 1.88 THz to 3.82 THz covering more than a full octave in frequency (Fig.1(a, b)). Dry-etched lasers with side-absorbers for lateral mode suppression similar as in [5] were fabricated for continuous wave (CW) operation and comb operation is probed through the beatnote analysis. Fig. 1(c, d) shows the beatnote as a function of the injected current with a maximum comb span of 1.1 THz which is the broadest demonstrated so far. Further proof of comb regime comes from the simultaneous measurements of two laser ridges on the same chip that show multiheterodyne spectra working in a dual-comb configuration [6].

Low effective electron mass InGaAs/InAlAs for high power terahertz quantum cascade lasers

Quantum cascade lasers (QCLs) are powerful sources of coherent radiation covering the frequency range from mid-infrared to terahertz. In the terahertz frequency range the active region is normally realized using a GaAs/AlxGa1−xAs semiconductor heterostructure. This material system enables a variable conduction band offset by changing the Al-content in the barrier layers without introducing a significant lattice mismatch between the barrier and well material. In comparison to the standard GaAs-based material system, active regions based on material systems with a lower effective electron mass are highly beneficial for the design of terahertz QCLs as the optical gain increases for a lower effective electron mass [1]. Promising material systems are based on InGaAs or InAs with an effective electron mass of 0.043 and 0.023, respectively, compared to that of GaAs (0.067) [2, 3].