Widely tunable terahertz source based on intra-cavity frequency mixing in quantum cascade laser arrays Aiting Jiang, Seungyong Jung, Yifan Jiang, Karun Vijayraghavan, Jae Hyun Kim, and Mikhail A. Belkin Citation: Applied Physics Letters 106, 261107 (2015); doi: 10.1063/1.4923374 View online: http://dx.doi.org/10.1063/1.4923374 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in InAs/AlSb widely tunable external cavity quantum cascade laser around 3.2 μm Appl. Phys. Lett. 102, 011124 (2013); 10.1063/1.4774088 Room temperature single-mode terahertz sources based on intracavity difference-frequency generation in quantum cascade lasers Appl. Phys. Lett. 99, 131106 (2011); 10.1063/1.3645016 Terahertz sources based on intracavity frequency mixing in mid-infrared quantum cascade lasers with passive nonlinear sections Appl. Phys. Lett. 98, 151114 (2011); 10.1063/1.3579260 Surface-emitting terahertz quantum cascade laser source based on intracavity difference-frequency generation Appl. Phys. Lett. 93, 161110 (2008); 10.1063/1.3009198 Single-frequency coherent terahertz-wave generation using two Cr:forsterite lasers pumped using one Nd:YAG laser Rev. Sci. Instrum. 79, 036101 (2008); 10.1063/1.2884926
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APPLIED PHYSICS LETTERS 106, 261107 (2015)
Widely tunable terahertz source based on intra-cavity frequency mixing in quantum cascade laser arrays Aiting Jiang,1 Seungyong Jung,1 Yifan Jiang,1 Karun Vijayraghavan,1,2 Jae Hyun Kim,1 and Mikhail A. Belkin1,a)
1 Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78712, USA 2 ATX Photonics, 10100 Burnet Rd., Austin, Texas 78758, USA
(Received 6 May 2015; accepted 19 June 2015; published online 29 June 2015) We demonstrate a compact monolithic terahertz source continuously tunable from 1.9 THz to 3.9 THz with the maximum peak power output of 106 lW at 3.46 THz at room temperature. The source consists of an array of 10 electrically tunable quantum cascade lasers with intra-cavity terahertz difference-frequency generation. To increase fabrication yield and achieve high THz peak power output in our devices, a dual-section current pumping scheme is implemented using two electrically isolated grating sections to independently control gain for the two mid-IR pumps. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4923374] V
Compact, widely tunable and mass-producible room temperature terahertz (THz) sources are highly desired for imaging and spectroscopy applications in 0.3–6 THz spectral range. Electrically pumped monolithic semiconductor sources, similar to diode lasers in ultraviolet, visible, and infrared or quantum cascade lasers (QCLs) in mid-infrared, would be the most desired solution owing to their compactness and low-cost mass-production potential. However, THz power generated by semiconductor electronic devices rolls off with frequency and is well below the milliwatt level above 1 THz; moreover, these sources are not widely tunable.1–4 THz QCLs are still constrained to cryogenic operation temperatures and have demonstrated a maximum tuning range of 380 GHz.5–7 THz sources based on intra-cavity difference-frequency generation (DFG) in mid-infrared (mid-IR) QCLs, first demonstrated in 2007 and known as THz DFG-QCLs,8,9 are currently the only mass-producible monolithic electrically pumped semiconductor sources of 1–6 THz radiation that can operate at room temperature. Upon application of bias current, these devices generate two mid-IR pump frequencies x1 and x2 that are converted into THz radiation at xTHz ¼ x1 x2 by DFG inside of the laser cavity. The active region in THz DFG-QCLs is quantum-engineered to provide broadband mid-IR gain and giant optical nonlinearity for THz DFG. With the introduction of laser waveguides designed for Cherenkov DFG phase matching,10,11 these devices are now capable of room temperature operation with nearly 2 mW of peak power output12 and show single-mode tuning over nearly the entire 1–6 THz range using an external cavity setup.13 Monolithic electronically tunable semiconductor terahertz sources are most desirable for low-cost mass-production. Monolithic THz DFG-QCL single-mode tuners have recently been demonstrated by our group using two electrically isolated grating sections to independently control and electrically tune the frequencies of the two mid-IR pumps (x1 and x2) which result in tunable THz DFG output xTHz ¼ x1 x2.14 A tuning of approximately 600 GHz in a)
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the 3.4–4 THz range was achieved. THz DFG-QCLs with even larger tuning range of 2.6–4.2 THz were demonstrated recently by Razeghi group using a three-section device configuration with one section controlling the wavelength of one mid-IR pump via a distributed Bragg reflector grating and the other two sections providing tuning of the second mid-IR pump based on Vernier tuning mechanism in a sampled grating.15 We note that single-mode operation of tunable QCLs based on sampled gratings is very challenging and the side mode suppression ratio in these devices is typically worse than 10–20 dB.16,17 In this report, we follow the QCL array approach, first demonstrated for monolithic mid-IR tuners,18 to produce monolithic THz source with continuous spectral tuning from 1.9 to 3.9 THz. We integrate 10 monolithic THz DFG-QCL tuners described in Ref. 14, each designed for a slightly different center THz emission frequency, in a single laser array. Figures 1(a) and 1(b) show the schematic and scanning electron microscope image of a THz DFG-QCL array. The array consists of ten 22-lm-wide ridge-waveguide THz DFG-QCLs, each designed to emit at a specific THz frequency spanning 1.9–3.9 THz range. The structure of an individual device in the array is shown in Fig. 1(c). The devices contain two first order distributed feedback (DFB) grating sections fabricated to select two mid-IR pump frequencies. The surface grating scheme11,19–21 is used for DFB gratings. Grating periods are in the range from 1.4 lm to 1.6 lm; grating duty cycle is set to 50%. The gratings are defined in the top cladding layer and are approximately 145 nm deep. Such etching depth was computed to provide nearly the same coupling constant of 11 cm1 for both mid-IR pumps, based on the analysis in Ref. 21. The grating in the front section of the device is designed for short wavelength (x1) mid-IR pump selection, and the grating at the back section is designed for long wavelength (x2) mid-IR pump selection. Each grating section is 1.2-mm-long, and the two sections are separated by a 300 lm gap for electrical isolation.14 The device wafer is grown by a commercial foundry on a semi-insulating InP substrate. The layer sequence is similar
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FIG. 1. (a) Schematic of a 10-ridge THz DFG-QCL array (only two ridges are shown). (b) Scanning electron microscope image of the fabricated THz DFGQCL array. Inset: image of an individual device. (c) Schematic of an individual device structure with the two DFB sections and the direction of THz output shown.
to that of the devices reported in Ref. 11. Lateral currentextraction scheme is used, similarly to devices in Refs. 10, 11, and 14. THz radiation is extracted at a Cherenkov angle into the semi-insulating InP substrate.10 The front substrate facet is polished at 30 degree angle, similar to devices in Refs. 10, 11, and 14 to facilitate Cherenkov THz DFG radiation outcoupling in forward direction. Devices are tested at room-temperature with 70 ns current pulses at 40 kHz repetition frequency. Mid-IR and THz power are measured using thermopile and calibrated bolometer, respectively, from the front facet using a nitrogen-purged setup with two 2-in.-diameter off-axis parabolic-mirrors: one with focal length of 50 mm to collimate the light from the device and the second one with 100 mm focal length to refocus the light onto the detector. Recorded mid-IR power is corrected for an estimated 70% collection efficiency of our setup; THz power is not corrected for any collection efficiency. Fourier-transform infrared spectrometer (FTIR) is used for spectral measurements. Dual-wavelength mid-IR lasing is required to produce THz DFG output. Due to gain competition, dual color DFB lasing requires very similar threshold gain for both of frequencies, which is difficult to achieve in practice.13 To fabricate a 10-element DFB THz DFG-QCL array fabrication with any reasonable yield, we need to be able to fabricate individual dual-color devices with nearly 100% yield. To that end, we implement independent control of the net gain for the two mid-IR pumps in our devices using the configuration shown in Fig. 2. The two grating sections shown in Fig. 2 are connected to 50-X coaxial power lines in series
FIG. 2. Device biasing scheme. Shown are the front DFB section for the short wavelength mode selection (right), the back DFB section for the long wavelength mode selection (left), and the electrical biasing scheme with variable attenuators. Power and spectrum measurements are all taken from the right (front) facet.
with 50 X resistors that provide impedance matching (differential resistance of QCLs at operating bias is only a few Ohms). The power lines are connected to the power supply through 50-X-impedance-matched variable attenuators.22 The power supply is made of a DEI HV1000 pulse generator (IXYS Colorado Corp.) connected to a 1000 V DC voltage source (Kepco Corp.). Current is measured using coil current sensors. In this biasing configuration, we can control the portion of current from the power supply that goes to the front and back device section by adjusting attenuation values on the two attenuators. Independent control of the current density through the two grating sections is crucially important to enable dualcolor lasing with high yield. Figures 3(a)–3(d) plot the power of mid-IR pumps, collected from the front facet of the device, and THz output for device #6 in the array designed for emission at 3 THz. This device performance is typical for the other elements in our array. Panels 3(a) and 3(b) show the power of the short wavelength (x1) and long wavelength (x2) mid-IR pumps, respectively, as a function of current density through the front and back sections of the device. Mid-IR inference filters (Spectrogon, Inc.) were used to separate the pump powers. Figure 3(a) indicates that the device predominantly lases at the short wavelength x1, which implies that the threshold gain at x1 is lower than that at x2. White dashed line in all the panels in Fig. 3 represents the uniform current pumping condition (same current density for both sections), and only one mid-IR pump is lasing in this case. To enable dual-color mid-IR emission, we need to apply different current bias to the front and the back section as shown in Fig. 3(b). To provide further detail of the device mid-IR performance, insets in Figs. 3(a) and 3(b) show the emission spectra of the laser for the case of uniform pumping density at 16 kA/cm2 and non-uniform pumping density with 12 kA/cm2 pump current through the x1 DFB section and 16 kA/cm2 pump current through the x2 DFB section, respectively. Figures 3(c) and 3(d) show the product of the two mid-IR output powers and the power of THz output of the device #6 as a function of the current density through the two DFB sections. As expected, THz radiation is generated only when both mid-IR pumps are lasing. Maximum THz power is measured to be 46 lW for this device. To understand the results in Fig. 3, we note that the midIR modes x1 and x2 only interact strongly with front and back gratings, respectively. Neglecting feedback from cleaved mirrors x1-mode is expected to be localized predominantly in the front DFB section, while the mode at x2 is expected to be localized predominantly in the back DFB section. Thus, stronger electrical pumping of the front (back)
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FIG. 3. (a) Peak power output of the short-wavelength mid-IR pump, (b) peak power output of the longwavelength mid-IR pump, (c) the product of the peak powers of the two midIR pumps, and (d) peak THz power output as a function of current density through the front and back sections of the device #6 in the array. White dashed line in (a)–(d) corresponds to the uniform current pumping situation. Insets in panels (a) and (b) display mid-IR emission spectra of the device for the injection current densities through the front and back section indicated by yellow circles in the panels.
section is expected to predominantly increase the round-trip gain of the x1 (x2) pump. The feedback from the cleaved facets, which are uncoated in case of our devices, may distort the mode distribution.23,24 Detailed theoretical analysis of our device performance in this case is beyond the scope of this paper. We note, however, that even in the presence of facet reflections, the x1- and x2-mode distribution along the laser waveguide is expected to remain dissimilar and the two modes are expected to benefit differently from the front- and back-section pumping. Similar search of optimal combination of pump current densities through the two DFB sections for maximum THz output is performed for other devices in the array. As a result, dual-color lasing and THz output are achieved for all devices in the array. We emphasize that fewer than 50% of our devices showed dual-color mid-IR emission under uniform pump density and thus dual-section pumping was crucial for high yield THz DFG-QCL array fabrication. Figure 4(a) plots the mid-IR lasing spectra of the entire 10-ridge array. The mid-IR pump frequencies are selected so as to produce THz emission lines spaced by approximately 210 GHz. Spectral tuning of mid-IR frequencies x1 and x2 and hence THz frequency xTHz ¼ x1 x2 are achieved by applying DC bias current to either the front or the back DFB section of the device through a bias tee as described in Ref. 14. Figure 4(b) shows the THz emission of all devices in the array with zero DC bias (solid lines) and with DC bias applied to front section only (dashed lines) so as to produce THz emission precisely in-between the emission lines of the array at zero DC bias. Maximum THz output for each line is also shown. Maximum THz peak power from the array was recorded to be 106 lW for a device #8 at zero DC tuning bias with a mid-IR-to-THz conversion efficiency of 0.4 mW/W2, similar to that observed in external-cavity THz DFG-QCL systems with QCL chips based on a similar active
FIG. 4. (a) Normalized mid-IR emission spectra of all 10 devices in the array recorded at optimal currents densities through the two DFB sections in each device. (b) THz emission spectra (bottom panel) and the maximum THz peak power (top panel) for all devices in the array without applying DC bias for tuning (solid lines and stars) and with DC tuning bias applied to the front grating section so as to produce THz emission in-between the emission lines of the array at zero DC bias (dashed lines and triangles).
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FIG. 5. Mode-hope free THz tuning using device #8 in the array as an example. (a) Emission spectra of device #8 in the array for different values of DC tuning current though the front section. Also shown is the THz emission spectrum of the adjacent device #7 at zero DC tuning current. (b) THz tuning behavior of device #8 around the region circled in panel (a). Mode hoping is observed around 3.43 THz. (c) Mode-hope-free tuning of device #8 within the circled region shown in (b) by applying DC bias to both the front and the back DFB sections of the device #8. From left to right, the spectra shown in (c) correspond to the front/back DC tuning current of 110 mA/0 mA, 100 mA/0 mA, 100 mA/50 mA, 140 mA/100 mA, and 90 mA/0 mA.
region design.13 Figures 5 shows continuous tuning capability of our array source. Figure 5(a) shows emission spectra of the device #8 for different DC tuning bias currents applied to the front DFB section. Tuning range is wide enough to cover the gap between the emission frequencies of the two adjacent devices in the array. Figure 5(b) shows further details of the DC current tuning around the circled region in Fig. 5(a). As discussed in Ref. 14, due to cleaved facet reflectivity, mid-IR pumps experience mode-hoping which leads to a gap in THz frequency tuning. As further described in Refs. 14 and 25, continuous THz tuning can still be obtained by applying a second DC bias to the back DFB section. The experimental demonstration is presented in Fig. 5(c). To summarize, we demonstrated continuously tunable monolithic room-temperature THz DFG-QCL source made of an array of 10 DFG-QCL devices. Independent current pumping scheme for two grating sections was implemented to enable dual-color mid-IR pump emission in all devices in the array with nearly 100% yield. The array achieves the maximum THz peak power output of 106 lW at 3.46 THz and provides continuous spectral coverage between 1.9 and 3.9 THz. Further improvement of tuning range is expected by increasing the number of lasers in the array and/or by further increasing the frequency spacing between the emitters in the array. We expect that, similar to tunable mid-IR sources based on QCL arrays,26–28 tunable THz sources based on this technology will provide a compact low-cost THz source option for spectroscopic applications. The University of Texas at Austin group acknowledges support from the National Science Foundation Grant Nos. ECCS-1150449 (CAREER) and ECCS-1408511. Sample fabrication was carried out in the Microelectronics Research Center at the University of Texas at Austin, which is a member of the National Nanotechnology Infrastructure Network. 1
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