Kazuue Fujita

Theory of the Intrinsic Linewidth of Quantum-Cascade Lasers: Hidden Reason for the Narrow Linewidth and Line-Broadening by Thermal Photons

We have developed a theory of the intrinsic linewidths of laser output of single-mode quantum-cascade (QC) lasers in mid-infrared and terahertz (THz) ranges. In the theoretical treatment, the concept of an effective coupling efficiency of spontaneous emission, given by a fractional rate of spontaneous emission coupled into a lasing mode to total nonlasing relaxation, is introduced to clarify a hidden reason for the narrowness of the linewidths. A narrow linewidth (12-kHz) reported with a frequency-stabilized 8.5- distributed-feedback QC laser is successfully interpreted in terms of an extremely small effective coupling efficiency of spontaneous emission, caused by ultrafast nonradiative scatterings. The present theory predicts the presence of a minimum ldquolinewidth floorrdquo in a high-injection-current region and the independence of linewidth on detuning between gain-peak and emission wavelengths. The theoretical treatment is expanded to derive the further modified Schawlow-Townes formula including the line-broadening by black body radiation in a THz QC laser. The linewidth of a THz QC laser is predicted to be considerably broadened by black body radiation.

Indirect pump scheme for quantum cascade lasers: dynamics of electron-transport and very high T0-values.

An alternative pump scheme, named indirect pump one is proposed to clarify its own feasibility. The high device performances of 8 microm quantum cascade lasers with cavity lengths of 4 mm and 1.5 mm are demonstrated: low threshold current densities of 2.7 and 3.3 kA/cm2 and maximum output powers of 362 and 50 mW at room temperature, and high T0-values of 243 and 303 K around room temperature. The higher T0-value, 303 K is the highest record ever reported with quantum cascade lasers. The high stability for temperature changes is interpreted in terms of the indirect pumping model.

High-performance, homogeneous broad-gain quantum cascade lasers based on dual-upper-state design

A broad-gain quantum cascade laser design with dual-upper-state is proposed to clarify its own feasibility. Devices employing the proposed active region design exhibit homogeneously wide (>330 cm−1) electroluminescence spectra of which shapes are insensitive to voltage changes. A buried heterostructure laser, emitting at λ∼8.4 μm, demonstrates a high continuous-wave output power of 152 mW together with a high constant slope efficiency of 518 W/A at room temperature. In addition, the device performance is observed to be very insensitive to temperature change; T0-values of ∼306 K and constant slope efficiency over the wide temperature range, 280–400 K.

Room temperature, continuous-wave operation of quantum cascade lasers with single phonon resonance-continuum depopulation structures grown by metal organic vapor-phase epitaxy

We propose a new quantum-cascade laser structure with single phonon resonance-continuum depopulation and demonstrate room temperature, continuous-wave operation of the proposed 7.9μm quantum-cascade laser. The laser grown by metal organic vapor-phase epitaxy emits a cw output power of 36mW at 30°C, exhibiting a threshold current density of 2.23kA∕cm2. The cw operation is reported for higher temperatures up to slightly above 60°C. The proposed structure may lead to a successful commercial mass production of the lasers.

Extremely high T0-values (∼450 K) of long-wavelength (∼15 μm), low-threshold-current-density quantum-cascade lasers based on the indirect pump scheme

The high device performance of a long-wavelength (∼15 μm), InGaAs/InAlAs, Fabry–Perot quantum-cascade laser based on the indirect pump scheme is reported. As a result of felicitous designing of the active region and waveguide structure, a low threshold-current-density of ∼3.5 kA/cm2, a high maximum output power of ∼216 mW, and a high slope efficiency of ∼346 mW/A, all at room temperature are obtained. The observed extremely high characteristic temperature of threshold current, T0∼450 K over wide temperature range, 320–380 K is ascribed to a strong suppression of electron populations in injectors, which are specifically visualized in the indirect pump scheme.

Terahertz generation in mid-infrared quantum cascade lasers with a dual-upper-state active region

We report the performance of room temperature terahertz sources based on intracavity difference-frequency generation in mid-infrared quantum cascade lasers with a dual-upper-state (DAU) active region. DAU active region design is theoretically expected to produce larger optical nonlinearity for terahertz difference-frequency generation, compared to the active region designs of the bound-to-continuum type used previously. Fabricated buried heterostructure devices with a two-section buried distributed feedback grating and the waveguide designed for Cherenkov difference-frequency phase-matching scheme operate in two single-mode mid-infrared wavelengths at 10.7 μm and 9.7 μm and produce terahertz output at 2.9 THz with mid-infrared to terahertz conversion efficiency of 0.8 mW/W2 at room temperature.

Broad-gain (Δλ/λ 0 ~0.4), temperature-insensitive (T 0 ~510K) quantum cascade lasers

Broad-gain operation of λ~8.7 μm quantum cascade lasers based on dual-upper-state to multiple-lower-state transition design is reported. The devices exhibit surprisingly wide (~500 cm(-1)) electroluminescence spectra which are very insensitive to voltage and temperature changes above room temperature. With recourse to the temperature-insensitivity of electroluminescence spectra, the lasers demonstrate an extremely-weak temperature-dependence of laser performances: T0-value of 510 K, associated with a room temperature threshold current density of 2.6 kA/cm2. In addition, despite such wide gain spectra, room temperature, continuous wave operation of the laser with buried hetero structure is achieved.

High-performance quantum cascade lasers with wide electroluminescence (∼600 cm−1), operating in continuous-wave above 100 °C

The authors report high temperature continuous-wave (cw) operations of broad-gain quantum cascade lasers based on the anticrossed dual-upper-state to multiple-lower-state design. The devices exhibit extremely wide electroluminescence (>600 cm−1) and subthreshold amplified spontaneous emission (∼570 cm−1) spectra at room temperature. Despite showing such broad electroluminescence spectra, the high-reflection coated, buried heterostructure lasers operating at 6.8 μm demonstrate a low threshold current density of ∼1.5 kA/cm2 and a high power of >500 mW with a high slope efficiency of ∼1.6 W/A in cw mode at 300 K. The maximum cw operating temperature of above 100 °C is achieved.

Indirectly pumped 3.7 THz InGaAs/InAlAs quantum-cascade lasers grown by metal-organic vapor-phase epitaxy

Device-performances of 3.7 THz indirect-pumping quantum-cascade lasers are demonstrated in an InGaAs/InAlAs material system grown by metal-organic vapor-phase epitaxy. The lasers show a low threshold-current-density of ~420 A/cm2 and a peak output power of ~8 mW at 7 K, no sign of parasitic currents with recourse to well-designed coupled-well injectors in the indirect pump scheme, and a maximum operating temperature of Tmax ~100 K. The observed roll-over of output intensities in current ranges below maximum currents and limitation of Tmax are discussed with a model for electron-gas heating in injectors. Possible ways toward elevation of Tmax are suggested.

Extremely temperature-insensitive continuous-wave quantum cascade lasers

Conspicuous temperature performances of λ ∼ 8.7 μm quantum cascade lasers with anticrossed dual-upper laser states are reported. The lasers characterized by strong super-linear current-light output curves exhibit an extremely high characteristic temperature for the threshold current density above 330 K (T0 ∼ 750 K). The slope efficiency grows with increasing temperature (a negative T1-value). In addition, for the pulsed operation of a short 1 mm length laser, the T0-value reaches a value of 1085 K above 340 K. These distinctive characteristics are attributable to optical absorption quenching in the injector as well as to suppression of carrier leakage in the active region.