We present ultra-broadband room temperature monolithic terahertz quantum cascade laser (QCL) sources based on intra-cavity difference frequency generation, emitting continuously more than one octave in frequency between 1.6 and 3.8 THz, with a peak output power of ~200 μW. Broadband terahertz emission is realized by nonlinear mixing between single-mode and multi-mode spectra due to distributed feedback grating and Fabry-Perot cavity, respectively, in a mid-infrared QCL with dual-upper-state active region design. Besides, at low temperature of 150 K, the device produces a peak power of ~1.0 mW with a broadband THz emission centered at 2.5 THz, ranging from 1.5 to 3.7 THz.
© 2016 Optical Society of America
The terahertz (THz) spectral range (1–10 THz) is preferable for many applications, such as imaging, communications, and chemical/biological sensing due to non-ionizing properties . The realization of a room-temperature broadband THz source is an important target to be accomplished. A THz quantum cascade laser (QCL) is a good candidate because of its high output power and design flexibility in active region structures [2–4]. In fact, ultra-broadband spectra from THz-QCLs have been achieved by integrating in the same laser core different active region designs engineered for different frequencies . In addition, THz-QCLs are suitable for direct comb operation without requiring an additional locking mechanism [6,7]. However, THz-QCLs still require cryogenic cooling to operate, despite recent progress for higher temperature operation .
In 2007, monolithic THz sources based on intracavity difference-frequency generation (DFG) in mid-infrared (mid-IR) QCLs known as THz DFG-QCLs, were demonstrated . Upon application of a bias current, they generate two mid-IR frequencies that are converted via DFG into THz frequency in the same laser cavity. Such an approach does not require maintaining a population inversion across THz transitions in a QCL, which is the only technology that results in electrically pumped monolithic semiconductor sources operable at room temperature [9,10]. THz DFG-QCLs can be designed to have a broad gain bandwidth and a giant nonlinear susceptibility over a broad THz frequency range. In fact, THz DFG-QCLs have demonstrated broadband operations: spectral tuning of THz DFG-QCLs from 1.2 to 5.9 THz using an external cavity setup , and broadband THz emission from a Fabry-Perot (FP) THz DFG-QCL  in which, however, THz peak power is limited to ~5 μW.
As a result of recent development, the performance of THz DFG-QCLs has improved considerably. By adoption of the Cherenkov phase matching scheme [13,14], researchers at Northwestern University have recently achieved a few mW-level peak THz output power in pulsed mode and continuous-wave (CW) operation at room temperature [15,16]. Furthermore, the dual-upper-state (DAU) active region can be designed to enhance strong optical nonlinearity χ(2) [17,18]. THz DFG-QCL based on a DAU active region has demonstrated a high mid-IR-to-THz conversion efficiency of 0.8 mW/W2 . Since the DAU active region possesses broadband gain width, compared to other active region design with a single upper state [19–21], a heterogeneous approach is unnecessary to operate dual-wavelength emission. This could result in a reduction of threshold current density; in fact, very recently, low-threshold (<2 kA/cm2) CW DFG-QCL with a homogeneous active region, which is conceptually similar to DAU active region design, has been achieved .
Here we report high performance broadband THz DFG-QCL with 90-period DAU active region, in which broad emission has been obtained from the nonlinear mixing between single mode emission and multi-mode emission due to FP cavity. The devices demonstrate a THz peak output power of ~200 μW and operate in the THz range with an emission spanning more than one octave, from 1.6 to 3.8 THz. The THz emission presents no spectral holes across the entire 2.2 THz-wide emission bandwidth. Another device exhibits narrower THz spectral width, ~1.2 THz, around 3 THz at room temperature, and demonstrates THz peak power of ~300 μW with mid-IR-to-THz conversion efficiency of 1.2 mW/W2, which is a record high number for THz DFG-QCL.
2. Design of active/injector structures
The conduction band diagram as well as nonlinear processes for the DAU active region is shown in Fig. 1(a). The waveguide core in present devices consists of identical DAU active regions  with 90-period cascade stages, designed for emission around 11 μm to obtain a high nonlinear susceptibility. In general, dipole matrix elements for transitions, which are important for high nonlinearity, can be designed to be large in the longer wavelength active region. The homogeneous DAU active region structure is virtually identical to that of our previous THz DFG-QCLs based on DAU active region, except for increasing the number of active region periods from 50 to 90. In general, QCLs with a large number of cascade stages are expected to lead to high optical confinement factors and high peak power . The refractive index profile for mid-IR frequency as well as mid-IR waveguide mode in the present device is shown in Fig. 1(b). The confinement factor increases from 0.57 to 0.78 with increasing the stage number from 50 to 90. The QCL active region with a larger number of cascade stages promises higher slope efficiency that is proportional to the number of cascade stages, due to carrier recycling in the active region. In order to achieve broadband THz emission, the THz DFG is created between single-mode and multi-mode spectra due to distributed feedback (DFB) grating and mirror feedback, respectively. For single-mode THz DFG-QCL with the dual-period or two section gratings [10,14], the highly optimized coupling coefficient of the DFB grating is required to operate pure dual single-mode mid-IR pumps. Although two single modes with appropriate spectral positions are ideal for THz DFG as mid-IR pumps , the product of mid-IR pump powers could also be maximized for the product of mid-IR pumps with single-mode and multi-mode operation. On the other hand, broadband THz emission has been obtained for an FP THz DFG-QCL . However, nonlinear mixing in the FP device takes place between small components in the mid-IR FP spectrum. The product of mid-IR pumps corresponding to THz emission will be small, so that high THz output power cannot be anticipated.
All the layer structures were grown on a semi-insulating InP substrate by the metal organic vapor phase epitaxy technique. The laser waveguide was designed to provide dielectric confinement for mid-IR and THz DFG emission at Cherenkov angle of approximately 20° into the semi-insulating InP substrate [13,14]. The growth starts with a 200 nm-thick InGaAs current spreading layer (Si, 1.0 × 1018 cm−3), and a 5.0 μm-thick n-InP (Si, 1.5 × 1016 cm−3) is used as a lower cladding layer. The lattice matched InGaAs/InAlAs active regions were sandwiched between the 250 nm-thick lower n-In0.53Ga0.47As layer (Si, 1.5 × 1016 cm−3) and the 450 nm-thick lower n-In0.53Ga0.47As layer (Si, 1.5 × 1016 cm−3). The uniform single-period buried grating was defined for single wavelength emission and was etched 200 nm deep into the upper n-In0.53Ga0.47As guide layers. The spectral position of the emission line due to the grating has been determined by performing the wavelength tuning using an external cavity setup, after measuring the FP emission spectra of FP lasers fabricated from the same wafer. We note that a wide wavelength tuning for similar DAU QCLs has already been demonstrated . The grating period was designed to provide feedback at the mid-IR wavelength of λDFB~10.7 μm, far away from the lasing center of FP emission at λFP~12 μm, by ~100 cm−1, in order to obtain THz emission by DFG. In this case, the FP emission is not suppressed by the fabricated grating effectively and so that the output power of the FP lasing is expected to be high enough. On the other hand, since the gain at the DFB lasing wavelength is significantly smaller than the gain peak (center of the FP emission), the high grating coupling coefficient is definitely required for lasing in the DFB cavity. The grating coupling coefficient κ is calculated to be ~7 cm−1. Then, the wafer was etched (standard dry and wet etchings) to form 14 μm-wide stripes and buried with a semi-insulating InP (Fe doped) layer. The upper cladding layer consists of a 5.0 μm-thick n-InP (Si, 1.5 × 1016 cm−3) followed by a 15 nm-thick n+- In0.53Ga0.47As (Si, ~1018 cm−3) cap layer. Then, the evaporation of the top Ti/Au contact was followed by electroplating of a thick 5 μm Au layer on top of the QC laser structure. After the wafer processing, the front facet is polished to 20° to facilitate Cherenkov THz radiation. Measurements of the present devices were taken using 200 ns current pulses at 50 kHz repetition rate for gathering both mid-IR and THz data. Emission spectra of the device were measured with a Fourier transform infrared (FTIR) spectrometer. The values of the maximum THz output powers of our devices were obtained by bringing a 6 mm-diameter high-resistivity Si hyper-hemispherical lens in near-contact with DFG-QCL chips and recording THz output power with a Golay cell (Tydex Instruments). The THz power data were not corrected for any correction efficiency. The mid-IR power was measured with a 10-mm-diameter calibrated thermopile detector positioned approximately 1 mm away from the laser facet and we assumed approximately 100% collection efficiency for this measurement configuration. Optical filters were used to distinguish between the power outputs at different frequencies.
3. Device performances
The pulsed current-light output (I-L) characteristics for mid-IR and THz signals for a 14 μm-wide, 3 mm-long device at room temperature are shown in Fig. 2. The DFG-QCL exhibits a mid-IR peak power of 2.14 W (combined power of the mid-IR FP and DFB pumps) and a THz peak power of approximately 200 μW at 1% duty cycle, despite producing a broadband spectrum in Fig. 3(b). The improvement in the THz power output is realized through the increase of mid-IR pump power, due to increasing the number of active region stages , as well as the change of the mid-IR pump approach. The slope efficiency of the total mid-IR output is actually enhanced, compared to our previous DAU DFG-QCLs , from 0.9 W/A to 2.2 W/A. Figure 2 (inset) shows the maximum THz average power as a function of duty cycle at 240 K and 295 K. The maximum THz average power of 14 μW at 295 K is obtained at a duty cycle of ~10%. For comparison, we also measured the room-temperature performance of a 3.0 mm-long by 14 μm-wide FP source taken from same wafer in which a maximum THz peak power of 9 μW was measured, as shown in Fig. 2, similar to the device reported in . In addition, we show in Fig. 3(b) (bottom) THz DFG spectra simulated using Mid-IR spectra for both types of DFG-QCLs, by assuming the same total mid-IR pump intensity for DFB/FP and FP devices. After the FP lasing at around λFP = 12 μm, the DFB emission is observed at λDFB = 10.7 μm, as shown in Fig. 3(a). Threshold current densities for FP and DFB emissions were observed to be 2.0 kA/cm2 and 2.9 kA/cm2 for λFP and λDFB respectively. The FP spectra broaden as the current increased. The behavior is observed for most of the samples measured, and seems to be attributable to broad gain bandwidth of DAU active region. Figure 3(b) displays the THz emission spectra of the DFB device at different biases in linear scale; the broadband emission extends from 1.6 THz to 3.8 THz, covering more than one octave at room temperature, which is a consequence of the nonlinear mixing between distributed-feedback single-mode lasing and multi-mode emission due to FP cavity. THz spectra are slightly different to the DFG spectrum expected from mid-IR spectra, shown in Fig. 3(a), in which signal components around lower THz frequency (~1 THz) are expected. The difference between measured THz spectra and that expected from mid-IR spectra may be due to several factors, such as the high free carrier absorption in the QCL waveguide, the frequency-dependencies of the DFG efficiency (proportional to the square of THz frequency) and optical nonlinearity .
Figure 4(a) depicts the temperature dependence of THz I-L characteristics for the device described in Fig. 2, in the temperature range of 150–295 K and in the same pulsed mode as described above. It shows a THz output power of approximately 1 mW at 150 K and ~0.4 mW at 240 K. The power at 150 K is comparable with that of typical THz-QCLs at cryogenic temperatures, though several state-of-the-art THz-QCLs demonstrate higher output power around 150 K. Additionally, we have confirmed that average power of more than 100 μW is possible at cryogenic temperature when the duty cycle increases. The mW-level high THz output power at low temperature is attributed to the increased output pump power, as shown in Fig. 4(b); though the conversion efficiency is slightly increased due to corresponding changes in the value of nonlinearity, such as transition linewidth broadening, mid-IR pump frequencies. Figure 5 shows mid-IR (a) and THz DFG (b) spectra of the present device collected at different temperatures, in which temperature-dependent behavior is observed and the THz emission bandwidth remains over the whole temperature range.
We also tested another DFG-QCL, showing narrower FP spectra in order to obtain higher THz power, since the spectra of the broadband DFG-QCL presented in Figs. 2 and 3 include low-frequency components for which the mid-IR-to-THz conversion efficiency will substantially be low. In fact, for the broadband DFG-QCL, the low-frequency components do not reflect mid-IR FP spectra. The measured device was chosen from the same wafer used for the broadband DFG-QCL. The difference is only the target wavelength of DFB grating. We confirm that mid-IR FP spectra vary from device to device, and thus we choose a device exhibiting narrow FP spectral width. Room-temperature performance results are presented in Fig. 6(a) for a 3.0 mm-long, 14 μm-wide buried hetero structured DFG-QCL. A peak power of up to 1.61 W and threshold current density of 2.1 kA/cm2 are obtained. Mid-IR emission is observed at two group of wavelengths centered round 860 cm−1 (λFP~11.6 μm) and at 960 cm−1 (λDFB~10.4 μm), as shown in Fig. 6(a). A maximum THz peak power of ~300 μW is achieved from the device. The obtained THz peak power is significantly higher than the value (76 μW) of the state-of-the-art CW DFG-QCL with the similar current density range and mid-IR pump power, 1.74 W . Figure 6(b) (inset) displays the THz spectrum taken at near roll over points, and multi-mode emission is centered at ~3 THz and spans ~1.2 THz bandwidth. The THz maximum wall-plug efficiency (WPE) is obtained to be 0.6 × 10−5, which is very close to the record for THz DFG-QCLs , despite the relatively low active region doping and broadband spectrum. Figure 6(b) shows THz output power versus the product of the two mid-IR pump powers. For the device with narrow FP spectra, a linear increase of THz power is observed with record high conversion efficiency of 1.2 mW/W2, which is six times higher than that of the broadband DFG-QCL in Fig. 2. This device also shows 1.5 times higher mid-IR-to-THz conversion efficiency compared to the previous proof-of-principle DFG-QCL with DAU active region . The higher conversion efficiency is attributed to the frequency-dependent nonlinearity and square of THz frequency , as the main components of the THz spectrum in Fig. 6 inset span 2.8–3.5 THz, which corresponds to the spectral region giving higher conversion efficiency for THz DFG-QCL [11,14]. The THz spectrum of the device in Fig. 6 corresponds to the frequency difference between the peaks in the mid-IR spectrum.
In addition to output power conversion efficiency, generally used in THz DFG-QCL, photon-flux conversion efficiency would be more essential to comprehend THz DFG process, since the energy difference between mid-IR and THz frequencies is significantly large (from more than 100 meV to ~10 meV). In fact, nonlinear process can be regarded as frequency down conversion. According to the Manly-Rowe relations , the nonlinear process in DFG-QCL described in Fig. 1(a), can be expressed as:14]. It should be noted that the observed photon flux for THz power is nearly equal to that of the power of 20 mW in the near IR region, λ = 1.3 μm. The high THz photon flux for THz DFG-QCL is mainly attributed to the large photon flux generated from the mid-IR QCL. Unlike other semiconductor lasers, the large photon flux of QCL is caused by the very high rate for stimulated emission in QCL owing to the high non-radiative rate, which is observed as a huge jump of the photon population at the threshold for lasing . Obviously, this is the fundamental advantage of the QCL-based THz DFG scheme, and from the conversion efficiency point of view, further improvement in terms of device performance is possible through the nonlinear active region, heat dissipation, and waveguide design optimizations.
In conclusion, we have reported the high-performance, ultra-broadband operation of room-temperature THz sources based on homogeneous DAU-QCLs with single distributed feedback grating. The THz DFG device demonstrates broadband spectra continuously more than one octave in frequency between 1.6 and 3.8 THz, with a peak output power of ~200 μW at room temperature. Furthermore, a device showing narrower THz spectral width, ~1.2 THz, produces a peak power of ~300 μW and displays a high mid-IR-to-THz conversion efficiency of ~1.2 mW/W2 as well as a high flux conversion efficiency of 1.5% at room temperature. The presented broadband THz DFG-QCLs may have great potential for broadband applications such as THz sensing and imaging technologies. Moreover, by achieving CW operation, THz frequency comb generation would be possible using the proposed concept here, as room-temperature frequency comb generation in mid-IR QCL has been reported .
The authors express their thanks to Atsushi Sugiyama, Takahide Ochiai, and Yuji Kaneko for assistance with carrying out the device fabrication. They also wish to acknowledge Prof. Mikhail A. Belkin, Dr. Seungyong Jung, and Yifan Jiang, University of Texas at Austin for many fruitful discussions and suggestions.
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