Study of thick active region in a terahertz quantum-cascade laser

Developing a high-temperature terahertz (THz) quantum cascade laser (QCL) has been one of the major challenges in the THz QCL field over recent decades. The maximum lasing temperature of THz QCLs has gradually been increased, arguably by shortening the length of repeating periods of the quantum structure in the device’s active region from 7 wells/14 layers to 2 wells/4 layers per period. The current highest operating temperature of 250 K was achieved in a two-well direct-phonon design. In this paper, we propose a potential and promising novel quantum design scheme named the quasi one-well (Q1W) design, in which each quantum cascade period consists of only three semiconductor layers. This design is the narrowest of all existing THz QCL structures to date. We explore a series of the Q1W designs using the non-equilibrium green function (NEGF) and rate-equation (RE) models. Both models show that the Q1W designs exhibit the potential to achieve sufficient optical gain with low-temperature sensitivity. Our simulation results suggest that this novel Q1W scheme may potentially lead to relatively less temperature-sensitive THz QCLs. The thickness of the Q1W scheme is less than 20 nm per period, which is the narrowest of the reported THz QCL schemes.

The terahertz (THz) frequency quantum cascade laser (QCL) is a semiconductor heterostructure laser that has attracted much research interest over the past decade. We report on the high performance of THz QCLs based on a three-well resonant-phonon (RP) depopulation active region (AR) and operating in the frequency range 2.7 THz to 4.0 THz. Devices, processed into surface-plasmon waveguides, lased up to 116 K in pulsed mode with threshold current densities as low as 840 Acm−2. The effects of the design frequency and laser cavity length on performance are discussed. We also report on the operation of QCLs with reduced AR thicknesses, and show, for the first time, that the AR thickness of RP QCLs processed in a surface plasmon waveguide can be reduced to as little as 5 µm. Finally, we demonstrate the use of an electrically tuneable THz QCL, based on a heterogeneous AR, for spectroscopic imaging of the high-explosive pentaerythritol tetranitrate.

We present superconducting waveguides for terahertz (THz) quantum cascade lasers (QCLs). Double-metal waveguides provide high confinement of the optical mode and low waveguide losses which are dominated by absorption of the radiation in the metal layers. Implementing novel waveguide materials like superconductors is one way to reduce these losses. In order to prove the compatibility with the THz QCL active region and waveguide we have replaced the commonly used gold or copper layers by superconducting niobium (Nb). We have simulated the temperature distribution inside the THz QCL in order to evaluate the operation conditions at which the critical temperature of the Nb layers is not exceeded. Experimental results of THz QCLs with Nb waveguides are presented which show lasing emission despite the fact that the energy of the THz radiation of the investigated active region f = 2.5 THz = 10.3 meV is higher than the superconducting energy gap of Nb 2Δ = 2.8 meV. Calculations show that improvements in terms of lower waveguide losses can be achieved using a superconductor with higher critical temperature and thus wider superconducting gap e.g., NbTiN or MgB2.

We demonstrate continuous-wave (cw) operation of terahertz (THz) quantum cascade lasers emitting at 3.2 THz based on bound-to-continuum active region and semi-insulating surface-plasmon waveguide design. Optical power of 62 mW with a threshold current density of 285 A/cm2 is obtained at 10 K from a 130-μm-wide and 1.5-mm-long laser in cw operation. Maximum cw operation temperature is up to 60 K. In pulsed mode, peak optical power more than 100 mW at 10 K and 2.1 mW at 85 K are observed from a 230-μm-wide and 2-mm-long device.

PDT with genetically encoded photosensitizer miniSOG on a tumor spheroid model: A comparative study of continuous-wave and pulsed irradiation

In Cermet coatings such as WC reinforced NiCr metal matrix, distribution of WC particles with effective bonding at particle-matrix interface plays an important role in retention of high hardness and toughness. Formation of secondary carbide phases are found to be detrimental in reducing hardness and there by rendering low wear and erosion resistance of the coating. Although, Laser cladding in continuous-wave (CW) energy mode can be utilized to control heat input to reduce melting of WC particles in NiCr matrix as compared to other conventional methods, the problem of melting of these particles still persists. A method of laser-cladding under pulsed mode was found to provide a better alternative to effectively control heat input leading to controlled melting of WC in NiCr matrix with reduction in formation of deleterious secondary carbides. In the present work, laser cladding of NiCr with 25% WC addition on SS310 steel substrate has been carried out using a 6kW diode laser and an off-axis powder feeding nozzle under argon atmosphere. Coatings were produced in both pulsed and CW modes at similar average laser powers. Coatings were characterized for microstructure, hardness and also analysed for erosive wear resistance. Clads produced under pulsed-mode of processing showed relatively higher amount of unmelted WC particles in γ-Ni+Ni3B eutectic matrix as compared to CW mode of processing. Relatively higher amounts of secondary carbides and other meta-stable phases were observed in CW mode as compared to Pulsed counterpart. A 30% improvement in clad hardness and 35% improvement in erosive wear resistance have been obtained in pulsed-clad coating than CW clad one.In Cermet coatings such as WC reinforced NiCr metal matrix, distribution of WC particles with effective bonding at particle-matrix interface plays an important role in retention of high hardness and toughness. Formation of secondary carbide phases are found to be detrimental in reducing hardness and there by rendering low wear and erosion resistance of the coating. Although, Laser cladding in continuous-wave (CW) energy mode can be utilized to control heat input to reduce melting of WC particles in NiCr matrix as compared to other conventional methods, the problem of melting of these particles still persists. A method of laser-cladding under pulsed mode was found to provide a better alternative to effectively control heat input leading to controlled melting of WC in NiCr matrix with reduction in formation of deleterious secondary carbides. In the present work, laser cladding of NiCr with 25% WC addition on SS310 steel substrate has been carried out using a 6kW diode laser and an off-axis powder feeding no...

Tuning the emission frequency of a 2 THz quantum cascade laser by altering the total thickness of the structure

Fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair using quantum cascade lasers (QCLs) as a gain medium. However, no demonstrations of harmonic modelocking have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at harmonics of the cavity’s fundamental round trip frequency. Here, using time resolved THz techniques, we show for the first time harmonic injection, and active and passive mode-locking where THz QCLs are modulated at harmonics of the round-trip frequency. Furthermore, using the unique ultrafast nature of our approach, we show that passive or self-starting harmonic modelocking originates from the QCL self-generating a harmonic microwave modulation. The latter auto-modulates the gain and loss in the system, spontaneously forcing the QCL to operate up to its 15th harmonic and opening up prospects of passive THz short pulse generation.

The distribution of charge carriers inside the active region of a terahertz (THz) quantum cascade laser (QCL) has been measured with scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). Individual quantum well-barrier modules with a 35.7-nm single module thickness in the active region of the device have been resolved for the first time using high-resolution SSRM and SCM techniques at room temperature. SSRM and SCM measurements on the quantum well-barrier structure were calibrated utilizing known GaAs dopant staircase samples. Doping concentrations derived from SSRM and SCM measurements were found to be in quantitative agreement with the designed average doping values of the n-type active region in the terahertz quantum cascade laser. The secondary ion mass spectroscopy provides a partial picture of internal device parameters, and we have demonstrated with our results the efficacy of uniting calibrated SSRM and SCM to delineate quantitatively the transverse cross-sectional structure of complex two-dimensional terahertz quantum cascade laser devices.

In recent years, Terahertz (THz) quantum cascade lasers (QCLs) have undergone a fast development, showing high power, ultra-broadband gain and quantum-limited linewidth. For many applications, THz QCLs need to operate in continuous-wave (CW), with a tight control of the emission spectrum and highly collimated beam profiles. These requirements are usually addressed by exploiting distributed feedback (DFB), photonic crystals or micro-cavity architectures, which can allow tailoring either the laser beam divergence or the emission frequency, and eventually both of them simultaneously, as in the case of third-order DFBs. Here we report on an original design in which a wire DFB THz QCL, engineered with a lateral sinusoidal corrugation providing feedback and frequency control, and an array of surface hole provides light outcoupling. This new photonic structure has led to the achievement of low-divergence beams (10°), single-mode emission, high slope efficiencies (250 mW/A), and stable CW operation.

We present the stabilization of terahertz (THz) quantum cascade lasers (QCLs) to a harmonic of the repetition rate of femtosecond fiber lasers. The technique that we have developed is 1) inherently broadband, i.e., it allows the stabilization of virtually any THz QCL regardless its emission frequency, and 2) can be implemented using different detection techniques such as electrooptic or photoconductive detection. Using electrooptic detection, we demonstrate phase locking of a 2.5-THz QCL, producing an RF beatnote with 70 dB of dynamic range in a bandwidth of 100 Hz. In addition, we show how the broadband character of the phase-locking technique can be used to coherently sample the emission of a multimode THz QCL that is actively mode-locked by current modulation at the roundtrip frequency.

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].

In this study, the interaction between diode laser radiation and chicken soft tissue was studied in vitro by a high-speed digital video camera. We used a diode laser with a wavelength of (980 ± 10) nm and average power of 10 W. The diode laser was operated in continuous wave (CW) and pulsed modes. In CW mode, the average laser radiation power was 10 W; in pulsed mode, the average laser radiation power was 10 W and the peak power was 20 W. Diode laser radiation was delivered to soft tissue (chicken meat) using a quartz optical fiber with either a clear distal end (clear tip) or a distal end containing an optothermal converter (hot tip). Application of the diode laser in pulsed mode resulted in crater depths and areas of collateral damage in soft tissue about 1.6 times greater than those observed in CW mode at treatment with the clear tip. Significant differences in the crater depth and collateral damage width of chicken meat were not found after hot-tip treatment with the diode laser in CW and pulsed modes. Soft tissue treated with the hot tip showed crater depths about 3.4 times greater than those observed after treatment with the clear tip. Hot tip treatment further resulted in collateral damage widths about 2.7 times lower than those obtained after treatment with the clear tip.

Terahertz (THz) quantum cascade lasers (QCLs) are semiconductor-based electrically pumped laser sources that are based on electron transitions between the subbands in the conduction band of a multiple-quantum-well heterostructure. Being such, the wavelength of THz QCL can be tuned by varying the thickness of the quantum wells, covering the frequency range of ∼1–5 THz. THz QCLs are also high-power light sources in the THz region, with highest output power in the 100 mW and watt levels for the continuous mode and pulsed mode operation, respectively. However, there are also limitations that hinder their widespread applications, which include low operating temperature, large beam divergence, the intrinsic carrier dynamics, etc. Nevertheless, continuous efforts have been devoted to this area and pushing the frontier further and further. In this paper, we will review both the aforementioned limitations and some of the key recent developments in THz QCLs in the past few years.

Considerable interest exists for sensing and imaging technologies in the terahertz (THz) spectral range, in particular for the interrogation of materials of an organic or biological nature. Development in THz quantum cascade lasers is seeing higher operating temperatures and peak output powers in pulsed mode, accentuating their place as the preferred source of coherent THz frequency radiation. Technological development of interferometric sensing schemes continues to take advantage of practical improvements in THz quantum cascade lasers. In this Summary, we give a brief overview of some recent developments in this regard.