Walter Schottky Institute
Center for Nanotechnology and Nanomaterials

Head of Group: Prof. Dr. Mikhail Belkin


Our research projects are focused on developing compact room-temperature optoelectronic and integrated photonics devices and systems operating in the mid-infrared (mid-IR, λ ≈ 2.5-30 µm) and terahertz (THz, λ ≈ 30-300 µm) spectral range. We also exploit opportunities offered by the new mid-IR and THz photonics technologies for applications. Selected current research projects in the group are listed below.

Widely-tunable room-temperature THz quantum cascade laser sources

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(Left) A laser bar with several THz DFG-QCLs on a copper heatsink. (Right) Schematic of a Cherenkov THz DFG-QCL. The active region (light grey) is designed to provide both mid-IR gain and giant nonlinearity for THz DFG. THz radiation is emitting into the substrate in the so-called Cherenkov phase-matching scheme.

THz spectral range is teeming with mainstream concepts. However, it is still in need of a convenient and compact semiconductor source and detector technology. In particular, THz radiation sources are bulky, complex in operation, and expensive to manufacture. Real-world applications require room-temperature broadly-tunable or frequency-comb THz sources that are similar in operation simplicity and mass producibility to diode lasers and mid-IR quantum cascade lasers (QCLs).

We have recently achieved significant progress in the development of such sources [1,2]. Our devices are based on efficient frequency mixing inside of dual-wavelength mid-IR QCLs. Their active regions are quantum-engineered to provide a giant nonlinearity for difference-frequency generation (DFG) with population inversion [1,2] and their waveguides are designed for Cherenkov phase-matching of DFG that enables THz extraction through the substrate [3]. As a result, these devices (referred to as THz DFG-QCLs) can now provide up to nearly 2 mW of peak THz power output and over 10 µW of continuous-wave THz power at room temperature [1,2]. A schematic of a Cherenkov THz DFG-QLC is shown in the figure on the right. Show more


Integrated mid-infrared photonics

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Mid-IR QCLs transfer-printed to silicon-on-sapphire.

Near-infrared (wavelengths approximately in the range of 1-2.5 microns) photonic integrated circuits (PICs) based on the silicon-on-insulator (SOI) or III-V platforms have undergone a tremendous expansion in recent years, driven initially by applications in fiber-optics communications and optical interconnects and later expanding to beam combining and steering, chemical and biological sensing, and frequency comb generation. Near-infrared PIC systems are now commercialized for several different applications by companies such as Infinera, Sisco Systems, SICOYA, and many others.

In contrast to the near-infrared spectral range, mid-IR laser-based systems have so far been designed around free-space optics. Integration of a semiconductor laser with a suitable mid-IR photonics platform will enable the development of mid-IR PICs for a wide range of applications from spectroscopy and sensing to beam steering aund ab nd new frequency generation. We have recently started investigating approaches for developing mid-IR PICs using both silicon and III-V platforms. Show more


Quantum-engineered nonlinear metamaterials

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Nonlinear metasurface for second-harmonic generation.

The mid-IR and THz regions are particularly suitable for creating engineered materials based on the concepts of quantum-engineering of electron states, plasmonics, and metamaterials. We have recently demonstrated the potential of this approach by creating large-area ultrathin metasurfaces with record-high nonlinear optical response. The metasurfaces operate by coupling modes in electromagnetically-engineered plasmonic nanoresonators with quantum-engineered intersubband nonlinearities in a thin semiconductor heterostructure.

Subwavelength thickness of our metasurfaces precludes phase matching constrains associated with traditional nonlinear optical crystals and thus allows for broadband frequency conversion. Furthermore, since very low optical intensity is required to produce strong nonlinear effects, our metasurfaces may be pumped using compact continuous-wave semiconductor lasers such as QCLs or diode lasers. Continuously-pumped metasurfaces may be used, for example, to achieve self-referencing and frequency shift of low-power microresonator-based optical frequency comb sources to anywhere in mid-IR and THz, to generate large amounts of THz radiation using high-power mid-IR pump lasers (e.g., CO2 lasers or high-power QCLs), and to up-convert mid-IR and THz optical signals for focal-plane-array imaging. Show more


Mid-infrared vertical cavity surface emitting lasers (VCSELs)

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A portion of the mid-IR molecular fingerprint region with absorption lines of selected gases.

Gas sensing based on direct laser diode absorption spectroscopy is one of the most sensitive and selective detection methods and it is widely used in industry. Because of their geometry, vertical cavity surface emitting lasers (VCSELs) are known to have significant advantages over edge-emitting diode lasers, including extreme compactness, about two orders of magnitude lower power consumption (due to smaller active region volume), round (rather than elliptical) beam shape, and intrinsic single-mode operation without the need for complex distributed feedback gratings used in single-mode edge-emitting lasers (due to much smaller laser cavity size).

Mid-IR spectral range is often called the ‘molecular fingerprint region’ because any chemical compound can be uniquely described by its molecular absorption fingerprint. The development of continuous-wave (CW) mid-IR VCSELs is highly desired for the creation of compact chemical sensors with high detectivity and specificity. Currently, however, such VCSELs do not exist. We are working on the development of CW room-temperature mid-IR VCSELs in the wavelength range 3-5 µm. Show more


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