Home Search Collections Journals About Contact us My IOPscience External cavity terahertz quantum cascade laser sources based on intra-cavity frequency mixing with 1.2 5.9 THz tuning range This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Opt. 16 094002 (http://iopscience.iop.org/2040-8986/16/9/094002) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 129.116.140.242 This content was downloaded on 09/09/2014 at 22:57 Please note that terms and conditions apply.
Journal of Optics J. Opt. 16 (2014) 094002 (9pp) doi:10.1088/2040-8978/16/9/094002 INVITED ARTICLE External cavity terahertz quantum cascade laser sources based on intra-cavity frequency mixing with 1.2 5.9THz tuning range Yifan Jiang 1, Karun Vijayraghavan 1, Seungyong Jung 1, Frederic Demmerle 2, Gerhard Boehm 2, Markus C Amann 2 and Mikhail A Belkin 1,3 1 Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78758, USA 2 Walter Schottky Institut, Technische Universität München, Am Coulombwal 3, D-85748, Garching, Germany E-mail: mbelkin@ece.utexas.edu Received 28 April 2014, revised 24 June 2014 Accepted for publication 7 July 2014 Published 3 September 2014 Abstract We discuss the design and operation of widely-tunable terahertz sources based on Cherenkov intra-cavity difference-frequency generation in mid-infrared quantum cascade lasers. Laser chips are integrated into a Littrow-type external cavity system. Devices demonstrate continuous terahertz emission tuning at room temperature with a record tuning range from 1.2 THz to 5.9 THz and peak power output varying between 5 and 90 μw, depending on the operating frequency. Beam steering of terahertz Cherenkov emission with frequency is suppressed and mid-infrared-to-terahertz conversion efficiency is improved by bonding devices onto highresistivity silicon substrates that have virtually no refractive index dispersion and vanishinglysmall optical loss in 1 6 THz range. Keywords: external cavity, DFG THz, QCL, broadly tunable (Some figures may appear in colour only in the online journal) 1. Introduction Widely-tunable terahertz (THz) sources are highly desired for spectroscopy, sensing, and spectroscopic imaging applications [1]. The lack of compact high-performance broadlytunable room-temperature semiconductor THz emitters is one of the main factors limiting the growth of THz applications. While THz quantum cascade lasers (QCLs) [2] are an attractive option, they still require cryogenic cooling to 3 Author to whom any correspondence should be addressed. operate [3] and have a limited tuning range [4]. Recent developments of THz sources based on intra-cavity difference-frequency generation (DFG) in dual wavelength midinfrared (mid-ir) QCLs offer a promising alternative [5 11]. Not only can these THz DFG-QCL sources work at roomtemperature, but they also have the intrinsic capability for operation in the entire 1 6 THz region of the electromagnetic spectrum [8 10]. Single-frequency distributed feedback (DFB) THz DFG-QCL emitters [9 11] and monolithic THz DFG-QCL tuners reported recently [12] have limited tuning range. External cavity (EC) THz DFG-QCL systems, in 2040-8978/14/094002+09$33.00 1 2014 IOP Publishing Ltd Printed in the UK
Figure 1. (a) Schematics of the external cavity (EC) system setup used in the experiments. (b) Simulated reflectivity of the antireflection (AR) coating (dashed blue line and left axis) of the laser facet and a room-temperature lasing spectrum of an uncoated 22 μm wide 2.1 mm long Fabry Perot ridge waveguide device pumped at a current density of 7 ka cm 2 (solid red line and right axis) (c) light output current characteristic of the Fabry Perot ridge-waveguide device in (b) before (thick red line) and after (thin blue line) back facet AR coating. Also shown is the light output current characteristic (circles) of the same AR-coated device placed into the EC setup with external grating tuned to operate at 1080 cm 1, close to the laser gain peak. The current voltage characteristic (dashed line) of the device is also shown. (d) Electroluminescence spectrum (dashed line) of the laser material at current density of 7.5 ka cm 2 and emission spectra of the device described in (b), (c) at different EC grating position (peaks of different colors). Also shown is the peak mid-ir power output of the device for different EC grating positions (red squares). The laser was operated in pulsed mode at room temperature with pump current density of 8.0 ka cm 2. contrast, can achieve extremely wide tuning range, spanning multiple octaves in THz as shown recently [10]. The EC THz DFG-QCL source can be operated by fixing one mid-ir pump frequency (ω 1 ) with a monolithic DFB grating etched in the laser cavity while tuning the other mid- IR pump frequency (ω 2 ) with an external diffraction grating as shown schematically in figure 1(a) [10]. Terahertz radiation at frequency ωthz = ω2 ω1 is emitted into the substrate based on a Cherenkov phase matching scheme [8, 10]. In [10], we reported an EC THz DFG-QCL system for the first time and demonstrated tuning from 1.7 THz to 5.25 THz with the peak power output varying between 5 and 40 μw at room temperature, depending on the operating frequency. This proof-of-principle EC system, however, did not have any mid-ir anti-reflection (AR) coating on the back facet of QCL chips which limited tuning range. Another drawback of the system was significant THz beam steering during tuning due to changes in Cherenkov phase matching angle θ C, see figure 1(a) associated with refractive index dispersion of the InP substrate at terahertz frequencies. Here we describe an optimized widely tunable room temperature EC THz DFG-QCL system with record continuous tuning from 1.2 THz to 5.9 THz and dramatically reduced beam steering. Improvement in the tuning range is achieved principally by depositing a dielectric AR coating on the back facet of the laser chip facing the external grating and a carefully selecting the spectral position of the mid-ir pump fixed by the DFB grating relative to the peak of the laser gain. THz beam steering is suppressed by bonding devices to a high-resistivity silicon substrate that has virtually no refractive index dispersion in 1 6 THz range. For comparison, we also present the EC performance of an AR-coated DFG-QCL chip on an InP substrate that displays similar tuning range, but shows significant THz beam steering. The power output of the device bonded onto a high-resistivity silicon substrate varies between 5 and 45 μw while the power output of the device on an InP substrate varies between 10 and 90 μw, depending on the THz emission frequency. In section 2 we describe the construction of the EC setup, details of the AR coating design, and discuss EC tuning of the 2
mid-ir pumps. In section 3 we present and discuss tunable THz emission of a DFG-QCL source on a semi-insulating InP substrate. In section 4 we discuss the fabrication and performance of an EC THz DFG-QCL system employing a device bonded to a high-resistivity Si substrate that shows virtually no beam steering. 2. Laser chip and external cavity system design EC configurations are widely used to produce tunable single-mode emission from semiconductor diode lasers and QCLs. Both Littrow-type and Littman Metcalf EC schemes have been demonstrated with mid-ir QCLs [13 17]. To enable THz DFG emission tuning in our device, we need to configure an EC system to continuously tune either one mid-ir pump frequency, while having the second mid-ir pump frequency fixed, or both mid-ir pump frequencies. In our implementation, shown in figure 1(a), we fix one mid-ir pump frequency by a DFB grating etched into the QCL waveguide and tune the second mid-ir pump frequency with an external grating in a Littrow-type EC configuration. Due to spatial hole burning, gain competition between the two mid-ir pumps is reduced and simultaneous two-color emission can be achieved experimentally [18]. Optical components used in the EC system include an aspheric AR coated collimating lens with focal length of 1.87 mm and numerical aperture of 0.85 (LightPath Technologies), a 150 grooves mm 1 gold diffraction grating (Optometrics Corporation model ML-303), and a computer-controlled Newport rotation stage. Both mid-ir and THz emission were collected from the front facets of QCL chips. The active region and waveguide structure of the QCLs used in this work is the same as that of the device reported by us in [10]. The waveguide core of the devices contains two stacks of bound-to-continuum heterogeneous active regions designed to have giant intersubband nonlinearity χ (2) for THz DFG. Devices were grown by the molecular beam epitaxy on a 350 μm thick semi-insulating (SI) InP substrate. Wafers were fabricated into deep-etched 22 μm -wide Fabry Perot and DFB ridge waveguide devices using dry etching process, followed by a 400 nm thick silicon nitride film deposition for sidewall insulation and the formation of metal contacts with a layer of Ti/Au (10 nm/ 600 nm). A side contact current extraction scheme was used due to the non-conductive nature of the SI substrate. Firstorder 140 nm-deep surface Bragg gratings with a 50% duty cycle and uniform grating period were fabricated into the top most waveguide layer using electron beam lithography, followed by reactive ion etching and overlaying it with the top-contact metal [10]. All devices shown in this paper were mounted epi-side up on copper blocks. The grating coupling coefficient was calculated to be κ 35 + 7i cm 1 using COMSOL Multiphysics simulations based on the approach described in [19]. A two-layer mid-ir AR coating made of a 650 nm thick layer of YF 3 followed by 360 nm thick layer of ZnSe was deposited by electron beam evaporation on the laser back facet [20]. Calculated reflectivity of the AR coated facet as a function of wavelength is shown in figure 1(b). Figure 1(c) shows the light output current (L I) and current voltage (I V) characteristics of a 2.1 mm long Fabry Perot device before and after AR coating was applied to the back facet. Given the calculated waveguide loss of α wg =10cm 1 and mirror loss of 5.9 cm 1 for the device with uncoated facets and assuming the laser gain scales linearly with current density, we calculate the AR-coated facet in our device has approximately 1.5% facet reflectivity around the gain peak at approximately 1080 cm 1. Figure 1(c) also provides the L I for the same AR-coated device integrated into the EC setup with external grating tuned to lase at gain peak 1080 cm 1. The threshold current density in this case is nearly the same as that for the Fabry Perot device with uncoated facets, indicating that the optical feedback of the EC system results in an effective reflectivity of approximately 30%. All measurements in this report were done at room temperature with 100 ns wide current pulses at a 50 khz repetition frequency. The mid-ir spectra were collected using the rapid-scan mode of a Bruker Fourier transform infrared spectrometer (FTIR) equipped with a deuterated L-alanine doped triglycine sulphate detector (DTGS) with a spectral resolution of 0.2 cm 1. THz spectra were obtained with the same FTIR using a time-resolved step scan and He-cooled silicon bolometer as an external detector. Power measurements were done using a two parabolic mirrors set-up: one with a two inch focal length to collect light from the devices and the other with a six inch focal length to refocus it onto a thermopile for mid-ir and a Golay cell for THz measurements. Mid-IR powers were corrected for 70% collection efficiency of this setup. THz powers were measured in a N 2 - purged environment and were not corrected for collection efficiency. The mid-ir gain spectrum of our DFG-QCL chips was initially determined by electroluminescence measurements and then by EC mid-ir tuning of Fabry Perot devices with ARcoated backfacets. Figure 1(d) shows the electroluminescence spectrum from a structure fabricated into a wet etched cylindrical mesa, as well as the mid-ir tuning performance of a 2.1 mm long, back facet AR coated Fabry Perot laser in the external cavity system. Mid-IR tuning from 927 cm 1 to 1197 cm 1 has been achieved, indicates a gain bandwidth of 8.1 THz, corresponding to 25% of the center frequency. 3. Broadly tunable external cavity DFG-THz system In QCLs the gain recovery process is faster than carrier diffusion and spatial hole burning of the laser gain is significant, favoring multimode operation for both longitudinal and transverse modes [21]. The requirement of strong modal overlap for efficient THz DFG [5] dictates that the two mid- IR pumps need to operate in TM 00 waveguide modes which have nearly identical intensity profiles for both ω 1 and ω 2 3
Figure 2. (a) Light output-current characteristics of the mid-ir and THz emission (see key) of the THz EC system with a Cherenkov DFG- QCL on semi-insulating InP substrate as described in the text. Data is shown for operation at 3.8 THz. Also shown is the current voltage characteristic of the device. (b) Mid-IR emission spectra and power output of the two mid-ir pumps for the EC THz DFG-QCL system described in (a) at different EC diffraction grating positions taken at a current density of 8.0 ka cm 2. Also shown is the mid-ir power for ω 1 (blue circles) and ω 2 (red triangles) pumps as a function of ω 2 pump wavenumber. The dashed line shows the transmission spectrum of the mid-ir long-pass filter used for power measurements. (c) THz emission spectra of the EC THz DFG-QCL system in taken at a current density of 8.0 ka cm 2. Also plotted are the THz peak power (blue circles and right axis) and mid-ir-to-thz conversion efficiency (red triangles and left axis) as a function of THz frequency. (d) Vertical far-field THz emission profile of the EC THz DFG-QCL system described in (a) (c) at 3.08 THz (grey stars), 3.44 THz (red diamonds), 3.76 THz (green circles), 4.06 THz (blue squares), and 4.38 THz (orange triangles). Vertical angle θ is defined relative to the direction normal to the laser facet as shown in the top-right inset. Top-left inset shows calculated dependence of the Cherenkov emission angle in a SI InP substrate as a function of THz frequency (solid line) and the Cherenkov emission angles in the substrate deduced from experimental measurements of the far field at different THz frequencies (circles). frequencies [5, 6, 8]. The tunable THz DFG-QCL system is created by placing a DFB DFG-QCL device in the EC system, so that two mid-ir pumps one at frequency ω 1, determined by the DFB grating, and the other at frequency ω 2, determined by the position of the external diffraction grating can achieve lasing at similar threshold current densities. The transverse TM 00 pump mode is enforced by using the DFB grating designed for TM 00 mode at ω 1 and by natural feedback discrimination between TM 00 and higher-order modes in the EC setup for ω 2 pump. For the devices tested in this work, the best results are obtained when the spectral position of the DFB mode ω 1 is fixed at 963 cm 1, approximately 100 150 cm 1 away from the laser gain peak at approximately 1050 1100 cm 1 as indicated by the electroluminescence and external cavity measurements in figure 1(d). Experimental results for an EC system with a 2.3 mm long, back facet AR-coated ridge waveguide THz DFG-QCL that contains a 1.6 mm long DFB grating section is shown in figure 2. The DFB grating was designed to fix ω 1 pump at 963 cm 1. The substrate front facet of the device was polished at a 30 angle to avoid total internal reflection of the Cherenkov THz emission and allow for THz extraction in the forward direction [8]. Figure 2(a) displays the L I characteristics for the ω 1 and ω 2 pumps for the case when the EC grating is positioned to select ω 2 pump frequency at approximately 1090 cm 1 (corresponding to a difference-frequency of approximately 127 cm 1 or 3.8 THz) close to device gain peak. Also shown is the L I characteristic of the THz emission for the same EC grating configuration. Figure 2(b) shows the emission spectra and power output of the two mid-ir pumps at different EC grating positions at a 4
Figure 3. (a) Refractive index (dashed red line and right axis) and absorption coefficient (solid black line and left axis) of SI InP as a function of THz frequency. The refractive index data was taken from [26]; the absorption coefficient was measured experimentally using SI InP device substrates. (b) Refractive index (dashed red line and right axis) and absorption coefficient (solid black line and left axis) of high-resistivity silicon as a function of THz frequency. The data is taken from [26]. 8.0 ka cm 2 current density through the device. A long pass interference filter was used to spectrally separate the two mid- IR pumps for the measurements. The mid-ir power was corrected for the filter transmission shown in figure 2(b). The spectral data shows that, as expected, the DFB pump frequency ω 1 stays fixed at 963 cm 1 while the EC pump frequency ω 2 is tuned continuously from 1009 cm 1 to 1182 cm 1, resulting in the available difference-frequency tuning from 1.4 THz to 5.9 THz. Due to gain competition, the mid- IR emission switches from dual-frequency (ω 1 and ω 2 )to single-frequency (ω 1 ) output as the EC grating mode is tuned towards the DFB frequency ω 1 as shown in figure 2(b). Figure 2(b) also shows that the ω 1 pump power increases somewhat as the ω 2 pump power drops. Gain competition between the two mid-ir pumps, not fully suppressed by spatial hole burning [21], is likely to be the mechanism behind this effect. Detailed modelling of the gain competition between the pumps is, however, beyond the scope of this work. On the other side of the ω 2 tuning range, the power of ω 2 is decreased as it tunes away from the gain peak until the ω 2 lasing stops. The THz emission spectra and peak power for different EC grating positions taken at a current density of 8.0 ka cm 2 through the device are displayed in figure 2(c). Also shown is mid-ir-to-thz nonlinear conversion efficiency, defined as the ratio of the measured THz peak power to the product of the two mid-ir pump powers, at a current density of 8.0 ka cm 2. A THz peak power of 90 μw and a mid-ir-to-thz conversion efficiency of nearly 250 μww 2 were observed at 3.8 THz. The conversion efficiency in figure 2(c) peaks in the 3.7 4.5 THz range of frequencies and falls off at both high- and low-frequency ends of the tuning curves. On the high-frequency end, efficiency of THz generation is limited by the onset of high optical losses in InGaAs/AlInAs/InP materials due to tails of LO-phonon absorption bands (Reststrahlen band). On the low-frequency end, THz generation efficiency is principally limited by high free carrier absorption in the QCL waveguide and the ( ωthz) 2 dependence of the DFG efficiency [5, 22]. Additional factors include spectral dependence of intersubband optical nonlinearity and residual absorption in SI InP substrate through which THz radiation is extracted, see figure 3(a). Detailed theoretical analysis of spectral dependence of mid-ir-to-thz conversion efficiency in THz DFG- QCLs will be presented elsewhere. The far field THz emission profile of a typical device on a SI InP substrate is shown in figure 2(d). The data was obtained at selected THz emission frequencies by placing the bolometer 15 cm away from the laser facet and monitoring the received THz power while sweeping the bolometer in the x z plane, see figure 1(a) for the coordinate system. Significant beam steering in the vertical direction is observed. To explain the origin of beam steering, we need to consider the angle of Cherenkov DFG emission into the substrate which is given as [8, 10] sub θ C 1 = cos ( β k ) nl THz. (1) Here kthz = nthzωthz/ c is the propagation constant of the THz wave in the substrate, with n sub THz being the refractive index of the substrate, and β nl is the propagation constant of the nonlinear polarization wave at the terahertz frequency in the laser waveguide. The right-going nonlinear polarization wave (cf figure 1(a)) can be written as (2) ω ω β β εχ * j( 2 1) t ( P (, z t) E E e 2 1) x, (2) ωthz 0 (2) zzz 2z 1z ( ) (2) where χ zzz is the intersubband nonlinear susceptibility for THz DFG in the laser active region, E 1z and E 2z are the z- components of the electric fields of TM-polarized mid-ir pumps, β 1 and β 2 are the propagation constants of the mid-ir pumps, and we assume the z-axis to be normal to the 5
Figure 4. (a) Schematic of the device bonded to a high-resistivity silicon substrate. (b) Scanning electron microscope images of the laser bar bonded to a high-resistivity silicon substrate (left panel) and the InP/SU-8/silicon interface. SU-8 bonding layer was measured to be 520 nmthick. Figure 5. (a) Power transmission coefficient of TM polarized Cherenkov THz wave from SI InP substrate to silicon substrate as a function of SU-8 bonding layer thickness. The data is shown for the case of 3.8 THz emission. (b) Expected THz power improvement factor for a device bonded to 1 mm thick silicon substrate compared to an identical device on 350 μm thick SI InP substrate. Here we assumed a 2.3 mm long device bonded using 500 nm thick SU-8 layer and plot the improvement factor as a function of remaining InP substrate thickness of the silicon-bonded device. heterostructures layers and x-axis to be directed along the waveguide as shown in figure 1(a). From equation (2) we see that β nl = β 2 β 1 and its value can be written as [8, 23] ω β = β β n g nl 2 1 c THz, (3) where ωthz = ω2 ω1 and ng( ω) = neff( ω) + ω d neff/dω is the group effective refractive index for mid-ir pumps. The expression for the Cherenkov emission angle in equation (1) then becomes 1 θ = ( sub n n ) C g THz cos /. (4) For mid-ir QCLs with a broad gain spectrum, similar to the devices reported here, the value of n g is nearly constant over the gain bandwidth [24, 25], and for our devices n g 3.37. The value of n sub THz for InP substrate, on the other hand, change significantly from n sub THz = 3.5 at 1 THz to 3.8 at 6 THz [26] as shown in figure 3(a). According to equation (4) such a change leads to over a 10 change in θ C in the substrate and translates into nearly a 40 change in THz far field emission direction in 1 6 THz range for a device with a 30 polished substrate. The inset in figure 2(d) compares Cherenkov emission calculated with equation (4) using InP THz refractive index values from [26] with the values of θ C deduced from experimentally-measured far-field emission profiles. The experimental data is in excellent agreement with theory. 4. EC THz DFG-QCL system with suppressed beam steering To suppress THz beam steering in our system, the InP substrate of devices was replaced with a high-resistivity silicon substrate. Due to the lack of strong optical phonon absorption, silicon has significantly smaller refractive index dispersion in the 1 6 THz range compared to InP [26], see figure 3(b). A 6
Figure 6. (a) Light output-current characteristics of the mid-ir and THz emission (see key) of the THz EC system with a Cherenkov DFG- QCL on a high-resistivity silicon substrate as described in the text. Data is shown for operation at 3.8 THz. Also shown is the current voltage characteristic of the device. (b) Mid-IR emission spectra and power output of the two mid-ir pumps for the EC THz DFG-QCL system described in (a) at different EC diffraction grating positions taken at a current density of 7.5 ka cm 2. Also shown is the mid-ir power for ω 1 (blue circles) and ω 2 (red triangles) pumps as a function of ω 2 pump wavenumber. (c) THz emission spectra of the EC THz DFG-QCL system in taken at a current density of 7.5 ka cm 2. Also plotted are the THz peak power (blue circles and right axis) and mid-ir-to-thz conversion efficiency (red triangles and left axis) as a function of THz frequency. (d) Vertical far-field THz emission profile of the EC THz DFG-QCL system described in (a) (c) at 3.08 THz (black stars), 3.44 THz (red diamonds), 3.76 THz (green circles), 4.06 THz (blue squares), and 4.38 THz (orange triangles). Vertical angle θ is defined relative to the direction normal to the laser facet as shown in the top-right inset. Top-left inset shows calculated dependence of the Cherenkov emission angle in a silicon substrate as a function of THz frequency (solid line) and the Cherenkov emission angles in the substrate deduced from experimental measurements of the far field at different THz frequencies (circles). 350 μm thick SI InP substrate in some devices was thinned down to 120 μm thickness and the devices were bonded, substrate-down, to a 1 mm thick high-resistivity silicon substrate using a 500 nm thick adhesion layer of SU-8 photoresist. The assembly was then cured at 65 C for 30 min, then at 95 C for 30 min and finally at 140 C for 10 min under the pressure of 4 MPa. The back facet of the silicon substrate was aligned with the laser back facet and the front side was polished to allow for THz Cherenkov wave emission in forward direction. The schematic of our design structure is shown in figure 4(a) and the scanning electron microscope images of the devices and the InP/SU-8/silicon bond interface are shown in figure 4(b). The images in figure 4(b) indicates a uniform interface and good bond quality. The angle of THz emission in the silicon substrate can be calculated as n = n cos θ = n cos θ, (5) g InP THz C InP Si THz where n InP THz and θ InP Si C (n THz and θ Si C ) are the refractive index of and the Cherenkov THz emission angle in InP (silicon), respectively. This small refractive index dispersion ensures a constant Cherenkov emission angle of approximately 10 into the silicon substrate over the entire 1 6 THz range. A 14 silicon substrate polishing angle will then produce THz outcoupling in forward direction. Due to the lack of suitable polishing wedges, silicon substrates were polished at a 10 facet angle, resulting in a THz emission approximately 10 from forward direction. C Si 7
In addition to displaying very low refractive index dispersion in 1 6 THz range, high-resistivity silicon is also highly-transparent at THz frequencies [26] with THz absorption coefficient being more than an order of magnitude smaller than SI InP, as shown in figure 3. As a result one may expect more efficient THz radiation extraction for devices bonded to silicon substrates. Full analysis needs to take into account the reflection loss of THz radiation at InP/SU-8/ silicon interface as well as partial blocking of Cherenkov THz beam by the unpolished section of InP substrate, cf figure 4(a). Figure 5(a) shows the calculated transmission of TM-polarized Cherenkov wave at 3.8 THz though InP/SU-8/ silicon interface as a function of SU-8 layer thickness. For this calculation we used the refractive indices of InP and silicon as shown in figure 3 and assumed that the refractive index of SU-8 to be 1.7 (similar to the table values in near-infrared) and no loss at 3.8 THz. Thinner SU-8 bonding layers lead to higher THz transmission through the interface. However, devices bonded on silicon substrates with 200 300 nm-thick layer of SU-8 showed numerous bonding imperfections, so we use a 500 nm thick SU-8 bonding layer for devices reported here. The thickness of the remaining SI InP substrate is another critical parameter that determines the THz power of devices bonded to silicon. SI InP substrates have substantial THz loss and the unpolished InP facet partly blocks THz Cherenkov wave from outcoupling to free space, see figure 3(a). Figure 5(b) shows the expected THz power improvement factor for devices bonded to 1 mm thick silicon substrates compared to identical devices on 350 μm thick InP substrates. Here we considered a 2.3 mm long device bonded using 500 nm thick SU-8 bonding layer and plot the improvement factor as a function of the remaining InP substrate thickness of the device bonded to silicon. These calculations assume that the mid-ir pump powers stays unchanged as devices are bonded to silicon. We note that thick silicon substrates could in principle allow efficient THz extraction from devices much longer than 2.3 mm. To exclude the effect of variation of mid-ir pump powers in different devices, we can consider the change in mid-ir-to-thz conversion efficiency between devices on SI InP substrates and devices bonded to high-resistivity silicon. For the case of our devices (bonded with 500 nm thick SU-8 layer and having 120 μm thick SI InP substrate remaining) the mid-ir-to-thz conversion efficiency is expected to increase by approximately a factor of 1.3 at 3.8 THz. The performance of a 2.3 mm long, back facet AR coated ridge waveguide device with a 1.5 mm long surface grating section near the front facet bonded to a 2.8 mm long, 1 mm thick silicon substrate is detailed in figure 6. Similar to the devices on SI InP substrates described in the previous section, the DFB grating for this device was designed to fix ω 1 at 963 cm 1. The I V and L I curves for both mid-ir and THz emission of this device are shown in figure 6(a) for the case when the EC grating is positioned to select ω 2 pump frequency at approximately 1090 cm 1 (corresponding to a difference-frequency of approximately 127 cm 1 or 3.8 THz) close to the mid-ir gain peak. The device provided smaller mid-ir and THz power output, compared to the device on SI InP substrate reported in figure 2, possibly due to excessive heating. The heating problem may, in principle, be solved by mounting devices epi-side down on copper blocks [11]. External cavity mode ω 2 could be tuned from 1004 cm 1 to 1185 cm 1 as shown in figure 6(b), which also displays mid- IR pump powers at different grating position at a pump current of 7.5 ka cm 2. THz performance of the device at this pump current is shown in figure 6(c). The THz emission could be tuned from 1.2 THz to 5.9 THz. We note that, while we could tune ω 2 pump so as to produce difference frequency as high as 6.7 THz (222 cm 1 ), we observed no detectable THz emission from this device at frequencies above approximately 5.9 THz. This is likely due to strong THz radiation absorption at the active region and the substrate by the tails of optical phonon absorption bands in InP, InGaAs and AlInAs materials. Figure 6(c) also shows terahertz power output and mid-ir-to-thz conversion efficiency at different THz frequencies. A maximum mid-ir-to-thz conversion efficiency of 0.35 mw W 2 was achieved at 3.8 THz, which represent a factor of 1.4 improvement over the reference device on SI InP substrate, see figure 2(c). The experimental improvement factor is close to that predicted in figure 5(b). Far field profiles in the vertical direction plane (cf figure 2(d)) for this device at selected THz frequencies are shown in figure 6(d). Virtually no beam steering is observed in close agreement with theoretical prediction shown in the inset to figure 6(d). Overall, theoretical analysis predicts only a 1.2 change in the far field profile peak for the devices bonded to high-resistivity silicon as the emission is tuned from 1 THz to 6 THz. 5. Conclusion We discussed the design and performance of a widely tunable EC THz DFG-QCL system with suppressed THz beam steering. Tunable THz sources based on THz Cherenkov DFG extraction through a SI InP substrate shows significant beam steering. We demonstrated that the beam steering can be suppressed and the mid-ir-to-thz conversion efficiency can be improved by bonding devices to high-resistivity silicon substrates that have virtually no refractive index dispersion and vanishingly-small THz loss in the entire 1 6 THz range. By applying a mid-ir antireflection coating and bonding devices to high-resistivity silicon substrates, we produced an EC THz DFG-QCL source with a record 4.7 THz (1.2 5.9 THz) tuning bandwidth and virtually no beam steering. Acknowledgements The university of Texas group acknowledge support from the DARPA Young Faculty Award grant number N66001-12-1-4241 and the National Science Foundation grant number ECCS-0935217 (CAREER). Walter Schottky Institute group acknowledges financial support from the excellence cluster 8
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