Plasmonic lasers generate coherent long-range or localized surface-plasmon-polaritons (SPPs), where the SPP mode exists at the interface of the metal (or a metallic nanoparticle) and a dielectric. Metallic-cavities sup- porting SPP modes are also utilized for terahertz quantum-cascade lasers (QCLs). Due to subwavelength apertures, plasmonic lasers have highly divergent radiation patterns. Recently, we theoretically and experimentally demonstrated a new technique for implementing distributed-feedback (DFB), which is termed as an antenna- feedback scheme, to establish a hybrid SPP mode in the surrounding medium of a plasmonic laser’s cavity with a large wavefront. This technique allows such lasers to radiate in narrow beams without requirement of any specific design considerations for phase-matching. Experimental demonstration is done for terahertz QCLs that show beam-divergence as small as 4-degrees. The antenna-feedback scheme has a characteristic feature in that refractive-index of the laser’s surrounding medium affects its radiative frequency in the same vein as refractive- index of the cavity. Hence, any perturbations in the refractive-index of the surrounding medium could lead to large modulation in the laser’s emission frequency. Along this line, we report ~57 GHz reversible, continuous, and mode-hop-free tuning of such QCLs operating at 78 K based on post-process deposition/etching of a dielectric on an already mounted QCL chip. This is the largest tuning range achieved for terahertz QCLs when operating much above the temperature of liquid-Helium. We review the aforementioned experimental results and discuss methods to increase optical power output from terahertz QCLs with antenna-feedback. Peak power output of ~13 mW is realized for a 3.3 THz QCL operating in a Stirling cooler at 54 K. A new dual-slit photonic structure based on antenna-feedback scheme is proposed to further improve output power as well as provide enhanced tunability.
The development of terahertz quantum cascade lasers (QCLs) has progressed significantly in the past ten years. Widely different types of QCLs have been demonstrated covering a frequency range from 1:2 THz to 5 THz (when operating without the existence of an external magnetic field). Improvement of operating temperatures of terahertz QCLs is one of the primary goals to make such devices viable for important terahertz applications. Some of the best techniques to obtain high operating temperatures have relied on electron-phonon scattering assisted depopulation. This paper reviews terahertz QCLs operating in a frequency range of 1:4 THz to 4:7 THz with such design schemes. Operation above a temperature of 160 K has been obtained across a broad range of frequencies from 1:8 THz - 4:3 THz. While the temperature degradation mechanisms are still not completely understood, it is speculated that collisional broadening of subbands may result in degradation of resonant tunneling transport at higher temperatures, which is critical to establishing population inversion in the QCL structure. The recently developed scattering-assisted injection techniques may mitigate subband broadening effects at higher temperatures, which is supported by experimental results. Further advances in the active region design as well as choice of different materials for growth and design of superlattices may result in even higher operating temperatures for terahertz QCLs.
The equations for the threshold-current density Jth, differential quantum efficiency ηd and maximum wallplug efficiency
ηwp,max for quantum-cascade lasers (QCLs) have been modified for electron leakage and backfilling. We used a thermalexcitation
model of "hot" injected electrons from the upper laser state to upper active-region energy states to calculate
leakage currents. Then the calculated characteristic temperature T0 for Jth was found to agree well with experiment for
both conventional and deep-well QCLs. The characteristic temperature T1 for ηd was deduced to be due to both electron
leakage and an increase in the waveguide-loss coefficient. For conventional mid-infrared QCLs ηwp,max is found to be
strongly temperature dependent which explains experimental data. By using a new concept: tapered active-region (TA),
deep-well QCLs have been optimized for virtual suppression of the electron-leakage currents. In turn, at room
temperature, for continuous-wave (CW)-operating, 4.5-5.0 μm-emitting TA QCLs we estimate the threshold current to
decrease by ~ 25 %, the active-region temperature rise at the ηwp,max point to decrease by ~ 30 %, and the single-ended,
ηwp,max value to become at least 22 %. Preliminary results from TA QCLs include T1 values as high as 454 K, over the
20-60 oC heatsink-temperature range.
We demonstrate a terahertz quantum-cascade laser (QCL) operating significantly above the temperature of
hv/kB, which had so-far been been an empirical limitation for the maximum operating temperature of these
devices. With a design that employs a new scattering-assisted injection scheme, a 1.8 THz QCL operating up
to a temperature of 1.9hv/kB (163 K) is realized with more than 3 mW of peak optical power output at 150 K.
We also demonstrate continuous tunability over a frequency range of 137 GHz of a single-mode QCL operating
at 3.8 THz in metal-metal waveguides. A unique concept of altering the lateral mode profile of the "wire laser"
waveguide geometry was implemented to achieve tuning despite the strong mode confinement of metal-metal
waveguides at terahertz frequencies.
The equations for threshold-current density Jth and external differential quantum efficiency d of quantum cascade lasers (QCLs) are modified to include electron leakage and the electron-backfilling term corrected to take into account hot electrons in the injector. We show that by introducing both deep quantum wells and tall barriers in the active regions of 4.8-µm-emitting QCLs, and by tapering the conduction-band edge of both injector and extractor regions, one can significantly reduce electron leakage. The characteristic temperatures for Jth and d, denoted by T0 and T1, respectively, are found to reach values as high as 278 and 285 K over the 20 to 90°C temperature range, which means that Jth and d display 2.3 slower variation than conventional 4.5- to 5.0-µm-emitting, high-performance QCLs over the same temperature range. A model for the thermal excitation of hot injected electrons from the upper laser level to the upper active-region energy states, wherefrom some relax to the lower active-region states and some are scattered to the upper miniband, is used to estimate the leakage current. Estimated T0 values are in good agreement with experiment for both conventional QCLs and deep-well QCLs. The T1 values are justified by increases in both electron leakage and waveguide loss with temperature
High-resolution heterodyne spectrometers operating at above 2 THz are crucial for detecting, e.g., the HD line at 2.7
THz and oxygen OI line at 4.7 THz in astronomy. The potential receiver technology is a combination of a hot electron
bolometer (HEB) mixer and a THz quantum cascade laser (QCL) local oscillator (LO).Here we report the first highresolution
heterodyne spectroscopy measurement of a gas cell using such a HEB-QCL receiver. The receiver employs a
2.9 THz free-running QCL as local oscillator and a NbN HEB as a mixer. By using methanol (CH3OH) gas as a signal
source, we successfully recorded the methanol emission line at 2.92195 THz. Spectral lines at IF frequency at different
pressures were measured using a FFTS and well fitted with a Lorentzian profile. Our gas cell measurement is a crucial
demonstration of the QCL as LO for practical heterodyne instruments. Together with our other experimental
demonstrations, such as using a QCL at 70 K to operate a HEB mixer and the phase locking of a QCL such a receiver is
in principle ready for a next step, which is to build a real instrument for any balloon-, air-, and space-borne observatory.
In this work we show that by using both deep quantum wells and tall barriers in the active regions of quantum cascade
(QC)-laser structures and by tapering the conduction-band edge of both injector an extractor regions one can
significantly reduce the leakage of the injected carriers. Threshold-current, Jth and differential-quantum efficiency, ηd
characteristic temperatures, T0 and T1, values as high as 278 K and 285 K are obtained to 90 °C heatsink temperature,
which means that Jth and ηd vary ~ 2.5 slower over the 20-90 °C temperature range than in conventional QC devices.
Modified equations for Jth and ηd are derived. In particular, the equation for ηd includes, for the first time, its dependence
on heatsink temperature. A model for the thermal excitation of injected carriers from the upper lasing level to upper
active-region energy states from where they relax to lower active-region energy states or get scattered to the upper Γ
miniband is employed to estimate carrier leakage. Good agreement with experiment is obtained for both conventional
QC lasers and deep-well (DW)-QC lasers.
We summarize recent development of terahertz quantum-cascade lasers (QCLs) based on a resonant-phonon
active region design and metal-metal waveguides for mode confinement. Maximum pulsed operating temperature
of 169 K is demonstrated for a 2.7 THz design. Lasers processed with the semi-insulating surface-plasmon (SISP)
waveguides and the metal-metal (MM) waveguides are experimentally compared. Whereas the SISP waveguides
have higher out-coupling efficiencies, the MM waveguides demonstrate improved temperature performance owing
to their lower-loss and near unity mode confinement; however, this comes at the cost of poor radiation patterns
and low output power. The beam quality and the out-coupling efficiency of the MM waveguides is shown to be
significantly improved by abutting a silicon hyperhemispherical lens to the cleaved facets of ridge lasers. Whereas
peak pulsed power of 26 mW at 5 K was detected from a 4.1 THz laser without the lens (device Tmax = 165 K),
the detected power increased to 145 mW with the lens with only a 5 K degradation in the maximum operating
temperature (device Tmax = 160 K).
We summarize recent results in the development of terahertz quantum cascade lasers (QCLs) based on resonant-phonon
active region designs. First, we describe attempts to improve high-temperature operation of terahertz QCLs by the use
of double-phonon depopulation in order to prevent thermal backfilling of the lower radiative state. While the best of the
three tested devices displayed a threshold current density of Jth=170 A/cm2 at 5 K and lased up to 138 K in pulsed
mode, no temperature advantage was observed compared to single-phonon designs. Also, we describe high power
operation of two different THz QCLs that emit up to 248 mW (pulsed) and 135 mW (continuous-wave) at 4.3-4.5 THz,
and 75 mW (pulsed) at 4.8-5.0 THz.
Quantum cascade lasers that operate in the underdeveloped terahertz spectral range (1-10 THz) promise to contribute to applications in sensing, spectroscopy, and imaging. We describe our development of terahertz quantum cascade lasers based on the resonant-phonon depopulation concept and that use low-loss metal-metal waveguides for optical confinement. Two- and three-dimensional finite-element simulations of terahertz metal-metal waveguides are used to demonstrate their high modal confinement even for very
narrow ridges. Also, simulations predict high facet reflectivities due to the modal impedance mismatch with free space
at the sub-wavelength waveguide aperture of these metal-metal waveguides. Finally, we report the demonstration
of a 2.8 THz laser that operates up to 97 K in continuous-wave mode fabricated using a Cu-Cu thermocompression bonding technique.
The recent extension of quantum cascade lasers (QCLs) from the mid-infrared to the terahertz frequency range (1-10 THz) promises to help address the relative lack of compact, coherent radiation sources in this spectral regime. We report our recent development of terahertz QCLs based on a resonant phonon depopulation scheme coupled with high-confinement, low-loss, metal-metal waveguides for mode confinement. A 3.2 THz laser (λ≈ 93.4 μm) is presented that operates in continuous wave mode up to a temperature of 93 K and up to 133 K in pulsed mode. Also presented is a 2.1 THz laser (λ ≈ 141 μm) that lases up to 40 K in continuous wave mode and 72 K in pulsed mode.