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: T 0-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.
© 2011 OSA
Quantum cascade lasers  (QCLs) are ideal light sources for spectroscopic applications in mid-infrared spectrum region. External cavity systems  including QCLs are the most promising configuration for broadband wavelength tuning. Recently, QCLs with multiple stacks of active regions [3–5] have been developed to expand the tuning wavelength range of external cavity QCLs. However, in QCLs of a number of stacks with different resonant wavelengths, a serious mode competition  inhibiting stable single mode operation may take place unless gain curves of different cascade stages overlap sufficiently. In this view, an inherently broad and homogeneous gain profile of the laser medium with translational symmetry is preferable for single mode operation in the external cavity application of QCLs. At this moment, the most promising active-region-design for broad-gain is bound-to-continuum  (BTC) which leads to high laser performance as well as broad electroluminescence (EL). However, in BTC transition, the EL spectrum exhibits asymmetric shape and the EL linewidth decreases steeply with increasing voltage. On the other hand, as an alternative approach, transitions from continuum states may also produce possibly broad-gain. However, the wide energy width of an upper miniband consisting of many upper states raise problems, such as difficulty of selective injection for each state, leakage current to higher spurious states and inhomogeneous broadening of spectra. In fact, λ~8 μm QCLs with transitions from continuum states that are formed by complicate mixing of many states originated from active parts and injector , have exhibited a relatively high threshold current density with a low T 0-value and strong voltage-dependence of EL linewidth. Although much wider EL linewidth of λ~4.8 μm QCLs has been reported recently , the EL linewidth from QCLs without multiple stacks of active regions is limited to Δλ/λ0~0.2. On the other hand, we have recently proposed a broad-gain quantum cascade laser design,  “dual-upper-state to single-lower-state transition (DAU/SS)”, consisting of the same type of coupled (anti-crossed) two upper laser states in all stacks, in which shapes of wavefunctions and energy separations can be well-optimized for desired electron populations as well as nearly equal transition strength from both the upper-states. In addition, the two upper states are energetically separated from higher parasitic states for selective injection into the higher upper state. These carefully-designed structural features of the DAU/SS laser, unlike “Continuum-to-bound” and “Continuum-to-Continuum” active region designs [8,9], give rise to very distinctive device-performances: a weak voltage-dependence of EL linewidth as well as a broad symmetric EL curve, a very high T 0-value and super-linear behavior in I-L. In this paper, we report a broad-gain QCL design based on the dual-upper-state to multiple-lower-state transition (DAU/MS) design, and demonstrate an extremely broad EL spectrum (Δλ/λ0~0.4) with its weak dependence on voltage and temperature-insensitive laser performances (T 0~510K).
2. Design of active region and device structure
The conduction-band diagram of injector-active-injector parts is shown in Fig. 1 . The coupled two upper states  are characterized by the wavefunctions labeled numbers 4, and 3. In the DAU design, after turn-on, electrons are injected into the higher upper state, state 4, via resonant tunneling from ground state, state 1'. In fact, we have, apparently, experienced that the energy alignment between state 1' and state 4 (not state 3) at the turn-on voltage results in successfully broad gain spectra. In this situation, electrons are quickly distributed in the two-upper-laser states by LO-phonon scatterings and/or by electron-electron scatterings. Such fast relaxation processes are expected, importantly, to eliminate dynamical spectral hole-burnings above threshold in a real laser operation. The electron populations in the both upper states are basically equal, due to the small energy gap between the two upper states, which is same as that in DAU/SS design (E 43~20 meV). On the other hand, the lower laser states consist of multiple states (miniband: 2mb), in which wavefunctions extend over the whole active-injector parts. Thus, in this design, the transitions take place from states 4 and 3 to the miniband 2mb and such many transition channels lead to a broad-gain spectrum. The energy separation between the upper-laser-states and parasitic state 5 can be designed to be large (E 54~60 meV), which is larger than the value of the continuum upper state design (~ 40 meV) ; this preserves inherent characters of the DAU design such as high T 0-value and super linear I-L character, actually observed in our previous report .
All the layer structures were grown on an n type InP substrate by metal organic vapor phase epitaxy technique. The 40-period active regions with translational symmetry were used as the emitting region and sandwiched between two 0.25 μm thick n-In0.53Ga0.47As layers (Si, 5x1016 cm−3). The upper cladding layer consists of a 3.5 μm thick n-InP (Si, 1x1017 cm−3) followed by a 0.3 μm thick n +- InP (Si, ~1018 cm−3) cap layer. After the growth, the wafer was processed into ridge structure. Finally, the evaporation of the top Ti/Au contacts was followed by electroplating of a thick 5 μm Au layer on top of the ridge.
3. Electroluminescence measurement
We measured EL spectra of a mesa device without stripe structure, of which results at various voltages at room temperature are shown in Fig. 2(a) . The linewidth γ of the spectra taken with pulsed injection currents of 500 ns width at a repetition rate of 100 kHz was observed to be a highly wide, γ ~62 meV (~500 cm−1), which is wider than the value of not only conventional QCLs but also a 2-stack-BTC-QCL operating at similar wavelength . The ratio of EL linewidth and center wavelength is also very large, Δλ/λ0~0.4. The wide EL curves, which are attributed to the transitions from the two-upper-state to multiple-lower-state, obviously show top wide- and flat-shapes over the whole voltage range. The EL linewidth of the present device together with of bound-to-bound , DAU/SS  and BTC  devices, for a comparison, are shown in Fig. 2(b). The linewidth of the present device is revealed to be overwhelmingly wider than the value of conventional devices over the whole voltage range and its insensitivity to voltage changes since the anti-crossed wave functions of the both upper laser states are basically located in the same space.
In order to investigate temperature-dependence of the gain, in particular to clarify whether gain of the present QCL is spectrally homogeneous or inhomogeneous, we measured temperature-dependences of the EL spectra. The EL spectra at various temperatures at fixed current (500 mA) are shown in Fig. 3(a) . Although at low temperature, 78~200 K, the EL curves split into two peaks because of narrow linewidth for each peaks, above room temperature the EL curves are kept to be top flat-shape and almost spectrally homogeneous. Temperature-dependences of ratios of reciprocal linewidth, 1/γ, and peak intensities, which are normalized by the values at 78 K, are also shown in Fig. 3(b). In general, when the ratios are constant to temperature change, the EL curves are viewed to be spectrally homogeneous. In the case of the device with bound-to-bound vertical transition, the ratios range from 1.0 to 0.6 between 78 K and 300 K. Above 250 K the ratios for the bound-to-bound device are nearly constant to the temperature change within the temperature range; this suggests the EL spectra of the bound-to-bound device behave homogeneous above room temperature. Below room temperature the stronger temperature-dependences of the ratios for the bound-to-bound device is attributed to inhomogeneous components because of the fluctuation in subband energies for each cascade stage. Actually, the different temperature-dependence of EL linewidth for bound-to-bound QCLs with the different number of cascade stages has been observed in Ref. 13. On the other hand, the ratio for the present device much steeply decreases from 1.0 to 0.4 between 78 K and 300 K, because of inhomogeneous spectral behavior at low temperature. For the higher temperature (>250 K), it shows weaker temperature-dependence and is regarded to be basically homogeneous broadening. Under such homogeneous situation, the temperature-dependence of the reciprocal linewidth represents that of gain coefficient as gcpeak∝1/γ. Figure 3(c) shows the reciprocal linewidth as a function of temperature. The weak temperature-dependence of the reciprocal linewidth of the DAU devices has been observed to be weak, T c =1130 K for the DAU/SS device and T c =2745 K for the DAU/MS device, as shown in Fig. 3(c), where the characteristic temperature T c is defined by the relation: 1/T c = (1/γ)(dγ /dT). These high T c-values of DAU devices originate from its wide EL linewidth since increase of linewidth with temperature change is basically in the same range for the every QCL. The T c-value of the DAU/MS QCL plays a key role in the laser performance in terms of temperature stability.
4. Laser performance
For current-light output characterization, the peak output power was measured with a calibrated thermopile detector. The pulsed current-light output (I-L) characteristics at different temperatures as well as the voltage-current (V-I) characteristics for an 14 μm-wide, 4 mm-long, HR-coated device, are shown in Fig. 4(a) . A threshold current density at 300 K was observed to be ~2.6 kA/cm2 and an optical output power of 930 mW with a slope efficiency of ~1.0 W/A was obtained at 300 K in the pulsed operation: 100 kHz, 100 ns. At high temperatures (>380K), the I-L curves show super-linear behavior around maximum current, which may be ascribed to the dynamics of electron transport . Consequently, as in the DAU/SS case, the slope efficiency at threshold is very insensitive to the temperature changes above 300 K (corresponding T 1~10000 K). In the spectral measurements (Fig. 4(b)), despite the wide EL linewidth the relatively narrow spectrum has been observed, ~60 cm−1, at the current of I 0/I th~1.3 in contrast to 180 cm−1 of the 2-stack-BTC-QCL.4
The threshold current densities for various devices as a function of temperature are shown in Fig. 5 . In contrast to the linear temperature-dependence for the bound-to-bound device , the temperature-dependences for both the present device and the DAU/SS device show different behavior at low and high temperatures. Below room temperature the characteristic temperature T 0 deduced by the exponential function, J th = J 0 exp(T/T 0) is identified to be T 0~200 K. On the other hand, above room temperature for the DAU/MS device the T 0-value is an astonishingly high value, T 0~510 K, which is roughly twice as high as the value of conventional QCLs. As a consequence, although the threshold current density of the DAU/MS device at low temperature is higher than the value of conventional devices because of the broad EL linewidth, at high temperature, eventually the threshold current densities of the DAU devices are close to conventional QCL values. As regards high T 0-values, we have recently demonstrated another high T 0 operation (T 0~450 K) of the λ~15 μm QCL based on indirect pump scheme, despite temperature dependent peak gain comparable to direct pumping QCLs, the indirectly pumped QCL in which high T 0-value mostly originates from the optical absorption quenching in the injector due to electron dynamics [14,15]. On the other hand, the high T 0-value of the present QCL is mainly led by the weak temperature-dependence of the peak gain as shown in Fig. 3, though it may be brought about by electron dynamics ; the origin of the high T 0-value for the present device is very similar to quantum dot laser case , in which the gain cross section is also weakly temperature-dependent, as a result of the quantization of electronic density-of-states due to three dimensional confinement. In other words, the origins of the high T 0-values in DAU/MS and indirect pump cases are essentially different.
For cw operation above room temperature, buried hetero (BH) structure lasers were processed with electroplated gold as the top contact metallization. The HR-coated, BH structured laser epi-side down mounted on an AlN sub-mount. As a result of the high T 0-value above room temperature, the present laser, despite the broad EL linewidth, has operated successfully in cw operation at room temperature. Figure 6(a) shows cw current-light curves of the device at different heat-sink temperatures. A threshold current density at 20°C was observed to be 3.7 kA/cm2 and a cw output power of 95 mW together with a high constant slope efficiency of ~500 mW/A was obtained at 20°C in cw operation. The device has operated in cw mode at higher temperatures up to 60°C. The temperature-dependence of the cw threshold current density has been observed to be weak, T 0~250 K, as shown in the inset of Fig. 6(a), which is not only nearly twice as high as the value of the 2-stack-BTC-QCL in cw operation , but also still higher than the values of conventional direct pumping QCLs in pulsed operation, despite showing higher thermal resistance (15.2 K/W). The subthreshold spectra of the DAU/MS devices at different current are shown in Fig. 6(b). A spectral narrowing with increasing current and a wide spectral width of over 380 cm−1 which is a reference value for the tunable wavelength range of external cavity QCL, have been observed. In addition, the spectral measurements reveal no sign of multiple wavelength emissions unlike QCLs with multiple stacks of active regions [3,5]. These results suggest that the DAU design is highly suitable for broadband tuning applications.
The high performance, broad-gain QCLs based on dual-upper-state to multiple-lower-state transition design have been reported. As a result of felicitous designing of the active region, the devices exhibit an extremely wide EL spectrum (~500cm−1, Δλ/λ0~0.4) as well as its voltage stability. Furthermore, the high performance of the ridge laser with HR-coating has been demonstrated: a low threshold current density of 2.6 kA/cm2 and a high maximum output power of 930 mW both at room temperature, and a maximum lasing temperature of above 400 K, and a very high characteristic temperature of 510 K over the all temperature range of above room temperature. The EL linewidth (Δλ/λ0~0.4) and the T 0-value (510 K) are far superior to the values of QCLs with conventional broad-gain designs. The observed high characteristic temperature has been interpreted in terms of a weak temperature-dependence of gain coefficient due to its spectrally homogeneous behavior and broad EL linewidth. Furthermore, as a result of the high T 0-value above room temperature, the HR-coated, BH structured laser, despite the broad EL linewidth, has operated successfully in cw operation up to slightly above 60°C.
The dual-upper-state design may open up a new opportunity for broad-gain applications since the design promises the high laser performance with very high T 0-value as well as the highly broad-gain with weak voltage-dependence which offers the great advantages for the broadband tuning of an external cavity QCL.
The authors express their thanks to T. Ochiai, A. Itoh and Y. Kaneko for carrying out the laser processing. They also wish to acknowledge N. Akikusa, HAMAMATSU PHOTONICS K.K. for his discussion on applications of the present broad-gain quantum cascade lasers.
References and links
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