A typical single mode device performance is shown in Fig. 1(a). We have also demonstrated that THz DFG-QCL chips can be continuously tuned over nearly the entire 1-6 THz range using an external cavity setup [3-5] or in monolithic configurations [6,7], see Fig. 2(b). We have further recently confirmed that, in continuous-wave operation, these devices provide a narrow (sub-MHz) emission linewidth which makes them suitable to be used as local oscillators for heterodyne spectroscopy [8]. The THz DFG-QCL platform holds a high promise for developing efficient broadband THz frequency comb sources for spectroscopic and metrological applications.

Our ongoing work is aimed at improving the THz radiation outcouping efficiency (estimated to be below ~5% in the current devices) [9,10], enhancing the optical nonlinearity in the QCL active regions to increase THz output power, improving device thermal packaging, generating THz frequency combs, developing miniature broadly-tunable THz systems, and demonstrating compact semiconductor-based THz instrumentation such as spectroscopy and imaging systems, dual-comb spectrometers, and microscopy systems.

REFERENCES

1. K. Fujita, S. Jung, Y. Jiang, J.H. Kim, A Nakanishi, A. Ito, M. Hitaka, T. Edamura, and M.A. Belkin, “Recent progress in terahertz difference-frequency quantum cascade laser sources,” Nanophoton. 7, 1795-1817 (2018).
2. M.A. Belkin and F. Capasso, “New frontiers in quantum cascade lasers: high performance room temperature terahertz sources,” Phys. Scr. 90, 118002 (2015).
3. K. Vijayraghavan, Y. Jiang, M. Jang, A. Jiang, K. Choutagunta, A. Vizbaras, F. Demmerle, G. Boehm, M. C. Amann, and M. A. Belkin, “Broadly tunable terahertz generation in mid-infrared quantum cascade lasers,” Nature Commun. 4, 2021 (2013).
4. Y. Jiang, K. Vijayraghavan, S. Jung, F. Demmerle, G. Boehm, M.-C. Amann, and M.A. Belkin “External cavity terahertz quantum cascade laser sources based on intracavity frequency mixing with 1.2-5.9 THz tuning range,” J. Opt. 16, 094002 (2014).
5. Y. Jiang, K. Vijayraghavan, S. Jung, A. Jiang, J.H. Kim, F. Demmerle, G. Boehm, M.C. Amann, and M.A. Belkin, “Spectroscopic study of terahertz generation in mid-infrared quantum cascade lasers,” Sci. Rep. 6, 21169 (2016).
6. S. Jung, A. Jiang, Y. Jiang, K. Vijayraghavan, X. Wang, M. Troccoli, and M.A. Belkin, “Broadly tunable monolithic room-temperature terahertz quantum cascade laser sources,” Nature Commun. 5, 4267 (2014).
7. A. Jiang, S. Jung, Y. Jiang, K. Vijayraghavan, J. Kim, and M.A. Belkin, “Widely tunable terahertz source based on intra-cavity frequency mixing in quantum cascade laser arrays,” Appl. Phys. Lett. 106, 261107 (2015).
8. L. Consolino, S. Jung, A. Campa, M. De Regis, S. Pal, K. Fujita, A. Ito, M. Hitaka, S. Bartalini, P. De Natale, M.A. Belkin, and M.S. Vitiello, “Spectral purity and tunability of terahertz quantum cascade laser sources based on intra-cavity difference frequency generation,” Sci. Adv. 3, e160331 (2017).
9. J.H. Kim, S. Jung, Y. Jiang, K. Fujita, M. Hitaka, A. Ito, T. Edamura, and M.A. Belkin “Double-metal waveguide terahertz difference-frequency generation quantum cascade lasers with surface grating outcouplers,” Appl. Phys. Lett. 113, 161102 (2018).
10. S. Jung, J. Kim, Y. Jiang, K. Vijayraghavan, and M. A. Belkin, "Terahertz difference-frequency quantum cascade laser sources on silicon," Optica 47, 38-43 (2017).

Figure 2(a) shows the electron microscope image of a mid-IR QCL epi-transferred to Silicon-on-Sapphire (SOS) wafers using 250-nm-thick SU-8 epoxy as an adhesive. Efficient coupling of mid-IR light from the laser cavity to silicon waveguides was achieved using tapered QCL waveguides as shown in Fig. 3(a) after Ref. [1]. Similar transfer-printing methods can be used to integrate other mid-IR light sources, such as diode lasers and interband cascade lasers (ICLs) with silicon waveguides.

Hybrid mid-IR systems based on III-V laser materials transferred to silicon photonic waveguides are expected to perform well with mid-IR diode lasers and ICLs that have relatively low (few tens of milliwatts) optical powers and thermal power dissipation. However, long-wavelength room-temperature operation of these devices is limited to the 5-6 microns spectral range. In addition, the use of the Si-based platforms limits the spectral range of mid-IR PICs to below 7 μm due to optical absorption in Si.

QCLs are currently the only room-temperature electrically pumped semiconductor light sources that can operate in the entire mid-IR range (from 3 to 15 microns) with optical powers over 1 W. Given the very high thermal dissipation in QCL active regions, achieving long-term reliability and continuous-wave operation of heterogeneously integrated devices on silicon platforms is challenging. Therefore, we have recently started working on homogeneous integration of mid-IR QCLs with low-loss III-V passive waveguides epitaxially grown on InP substrates, see Fig. 2(b). The homogeneous integration approach uses materials, growth, and processing steps nearly identical to those used for conventional high-performance mid-IR QCLs, which offers superior reliability and performance of photonic integrated circuits. Devices integrated with III-V waveguides show more than an order of magnitude higher output power from passive waveguides, compared to devices integrated with the SOS waveguides as shown in Fig. 3.

Ongoing and planned work include efforts to reduce the waveguide losses and the development of continuous-wave mid-IR PICs for high-speed light modulation, beam steering, on-chip sensing, and nonlinear generation of new frequencies.

REFERENCES

1. S. Jung, J. Kirch, J.H. Kim, L.J. Mawst, D. Botez, and M.A. Belkin, “Quantum cascade lasers transfer-printed on silicon-on-sapphire,” Appl. Phys. Lett. 111, 211102 (2017).
2. S. Jung, D. Palaferri, K. Zhang, F. Xie, Y. Okuno, C. Pinzone, K. Lascola, and M.A. Belkin, “Homogeneous photonic integration of mid-infrared quantum cascade lasers with low-loss passive waveguides on InP platform,” Optica 6, 1023-1030 (2019).

The basic schematic of the metasurface is given in Fig. 4 and its operation is explained in the figure caption. We achieved over 0.075 % of mid-IR second-harmonic power conversion efficiency using only 15 kW/cm² of pump intensity [1,2]. The second-order nonlinear susceptibility of our latest metasurfaces at low pump intensity is estimated to be over 10⁶ pm/V, 4 orders of magnitude higher than that of traditional nonlinear materials such as LiNbO3. From the fundamental standpoint, our metasurfaces provide the foundation for the “flat nonlinear optics paradigm” in which we can simultaneously produce high nonlinear conversion efficiency and achieve full control of the wavefront of the nonlinear output, as demonstrated by us in Ref. [3].

We are now investigating numerous directions in which the effective nonlinearity and conversion efficiency of the metasurfaces may be further improved, particularly, by optimizing plasmonic nanoresonator designs to achieve higher field enhancement and higher nonlinear overlap integral [4,5] and by increasing the intersubband nonlinearity and saturation intensity of the semiconductor heterostructure using more sophisticated MQW designs. Theoretical simulations indicate that up to 10% of power conversion efficiency is possible in fully-optimized structures using input intensities ~10-100 kW/cm². Additional ongoing and planned future research on this topic include

 

• Design and experimental testing of metasurfaces for THz difference-frequency generation [6];
• Metasurfaces for efficient up-conversion of mid-IR and THz signals for detection and imaging;
• Nonlinear metasurfaces for mid-IR and THz Raman lasing and other four-wave mixing processes such as phase conjugation and all-optical control;
• Investigating quantum effects in individual highly-nonlinear nanoscale optical resonators [7].

REFERENCES

1. J. Lee, M. Tymchenko, C. Argyropoulos, P. Y. Chen, F. Lu, F. Demmerle, G. Boehm, M. C. Amann, A. Alu, and M.A. Belkin, “Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions,” Nature 511, 65 (2014).
2. J. Lee, N. Nookala, J. S. Gomez-Diaz, M. Tymchenko, F. Demmerle, G. Boehm, M.-C. Amann, A. Alù, and M.A. Belkin “Ultrathin second-harmonic metasurfaces with record-high nonlinear optical response,” Adv. Opt. Mat. 4, 664-670 (2016).
3. N. Nookala, J. Lee, J.S. Gomez-Diaz, M. Tymchenko, F. Demmerle, G. Boehm, K. Lai, G. Shvets, M.-C. Amann, A. Alù, and M.A. Belkin, “Ultrathin gradient nonlinear metasurface with a giant nonlinear response,” Optica 3, 283-288 (2016).
4. N. Nookala, J. Xu, O. Wolf, S. March, R. Sarma, S. Bank, J. Klem, I. Brener, and M. A. Belkin, “Mid-infrared second-harmonic generation in ultra-thin plasmonic metasurfaces without a full-metal backplane,” Appl. Phys. B. 124, 132 (2018).
5. R. Sarma, D. de Ceglia, N. Nookala, M. Vincenti, S. Campione, O. Wolf, M. Scalora, M. Sinclair, M. Belkin and I. Brener, "Broadband and efficient second-harmonic generation from a hybrid dielectric metasurface/semiconductor quantum-well structure," ACS Photon. 6, 1458 (2019).
6. M. Tokman, Z. Long, S. Al Mutairi, Y. Wang, V. Vdovin, M. Belkin, and A. Belyanin, “Purcell enhancement of the parametric down-conversion in two-dimensional nonlinear materials,” APL Photonics 4, 034403 (2019).
7. M. Tymchenko, J. S. Gomez-Diaz, J. Lee, M.A. Belkin, and A. Alù, “Highly-efficient THz generation using nonlinear plasmonic metasurfaces,” J. Opt. 19, 104001 (2017).

Due to current unavailability of CW VCSELs operating in the wavelength range of the fundamental molecular roto-vibrational absorption lines, devices operating at shorter near-infrared (near-IR) wavelengths (often around 1550 nm) are used to target the absorption lines corresponding to “overtones” (higher-order harmonics) of fundamental molecular vibrations. However, these lines are typically about 2 orders of magnitude weaker than the fundamental absorption lines. Figure 5(a) shows the absorption spectrum of methane displaying both fundamental absorption lines around 3300 nm and overtone lines at 1550 nm and 2300 nm. Additionally, the overtone line spectrum of molecular compounds is more complex and spectrally dense compared to the spectrum of the fundamental lines. As a result, it is often difficult to select an overtone absorption line of a target molecule that is free from interference produced by overtone lines of other gases.

Our research efforts focus on developing CW room-temperature mid-IR VCSELs operating in the 3-5 µm wavelength range using InAlGaAsSb heterostructures grown on GaSb substrates. The schematic of a VCSEL structure is shown in Fig. 5(b). The Chair has produced devices that define the current state of the art in the mid-IR VCSEL technology, including CW VCSELs at 3 µm wavelength [1] and at 4 µm wavelength [2] with near-room-temperature operation, see Fig. 6.

Current research efforts are focused on improving the active region design of the devices using both type-I and type-II active region schemes, improving the mirror design and the current injection aperture design in these devices, optimizing thermal packaging and more. The development of the first CW mid-IR VCSELs is also expected to lead to various opportunities in application of these devices for gas sensing and spectroscopy.

REFERENCES

1. A. Andrejew, S. Sprengel, and M.-C. Amann, "GaSb-based vertical-cavity surface-emitting lasers with an emission wavelength at 3 μm", Opt. Lett. 41, 2799 (2016).
2. G.K. Veerabathran, S. Sprengel, A. Andrejew, and M.-C. Amann, "Room-temperature vertical-cavity surface-emitting lasers at 4 μm with GaSb-based type-II quantum wells", Appl. Phys. Lett. 110, 071104 (2017).