Freely engineering the operation frequency of frequency comb sources is crucial for various applications, e.g., high-precision spectroscopy, ranging, communications, and so on. Here, by employing ...band structure simulations, group velocity dispersion (GVD) analysis, and experimental verifications, we demonstrate that the operation frequency of terahertz (THz) quantum cascade laser frequency combs can be engineered from 4.2 to 4.0 THz. First of all, from the viewpoint of the band structure engineering, we shift the frequency corresponding to the optical transitions in the active region from 4.2 to 4.0 THz by slightly altering the thicknesses of quantum wells. Meanwhile, a GVD analysis is applied to evaluate the potential comb performance. Finally, experimental characterizations, e.g., emission spectra, inter-mode beatnote, dual-comb operation, are performed to validate the exceptional comb operation at 4.0 THz. The advancement in simulations and experimental results present a comprehensive method to customize the desired THz radiative frequency for comb generation, which facilitates the practical development of broadband, high-precision THz comb sources.
High performance terahertz imaging devices have drawn wide attention due to their significant application in healthcare, security of food and medicine, and nondestructive inspection, as well as ...national security applications. Here we demonstrate a broadband terahertz photon-type up-conversion imaging device, operating around the liquid helium temperature, based on the gallium arsenide homojunction interfacial workfunction internal photoemission (HIWIP)-detector-LED up-converter and silicon CCD. Such an imaging device achieves broadband response in 4.2-20 THz and can absorb the normal incident light. The peak responsivity is 0.5 AW
. The light emitting diode leads to a 72.5% external quantum efficiency improvement compared with the one widely used in conventional up-conversion devices. A peak up-conversion efficiency of 1.14 × 10
is realized and the optimal noise equivalent power is 29.1 pWHz
. The up-conversion imaging for a 1000 K blackbody pin-hole is demonstrated. This work provides a different imaging scheme in the terahertz band.
Optical frequency combs, consisting of well‐controlled equidistant frequency lines, have been widely used in precision spectroscopy and metrology. Terahertz combs have been realized in quantum ...cascade lasers (QCLs) by employing either an active mode‐locking or phase seeding technique, or a dispersion compensator mirror. However, it remains a challenge to achieve the passive comb formation in terahertz semiconductor lasers due to the insufficient nonlinearities of conventional saturable absorbers. Here, a passive terahertz frequency comb is demonstrated by coupling a multilayer graphene sample into a QCL compound cavity. The terahertz modes are self‐stabilized with intermode beat note linewidths down to a record of 700 Hz and the comb operation of graphene‐coupled QCLs is validated by on‐chip dual‐comb measurements. Furthermore, the optical pulse emitted from the graphene‐coupled QCL is directly measured employing a terahertz pump–probe technique. The enhanced passive frequency comb operation is attributed to the saturable absorption behavior of the graphene‐integrated saturable absorber mirror, as well as the dispersion compensation introduced by the graphene sample. The results provide a conceptually different graphene‐based approach for passive comb formation in terahertz QCLs, opening up intriguing opportunities for fast and high‐precision terahertz spectroscopy and nonlinear photonics.
Enhanced passive frequency comb operation and pulse generation in terahertz semiconductor lasers, i.e., quantum cascade lasers, are successfully demonstrated by coupling a multilayer graphene saturable absorber into a 6 mm long terahertz laser. Dual‐comb and pump–probe techniques are implemented to verify the frequency comb and pulsed operations (16 ps optical pulses) of graphene‐coupled terahertz lasers.
Heterogeneous integration of compound semiconductors on a Si platform leads to advanced device applications in the field of Si photonics and high frequency electronics. However, the unavoidable ...bubbles formed at the bonding interface are detrimental for achieving a high yield of dissimilar semiconductor integration by the direct wafer bonding technology. In this work, lateral outgassing surface trenches (LOTs) are introduced to efficiently inhibit the bubbles. It is found that the chemical reactions in InP–Si bonding are similar to those in Si–Si bonding, and the generated gas can escape via the LOTs. The outgassing efficiency is dominated by LOTs’ spacing, and moreover, the relationship between bubble formation and the LOT’s structure is well described by a thermodynamic model. With the method explored in this work, a 2-in. bubble-free crystalline InP thin film integrated on the Si substrate with LOTs is obtained by the ion-slicing and wafer bonding technology. The quantum well active region grown on this Si-based InP film shows a superior photoemission efficiency, and it is found to be 65% as compared to its bulk counterpart.
In this paper, we presented single mode terahertz quantum cascade lasers (THz QCLs) with sampled lateral grating emitting approximately 3.4 THz. Due to strong mode selection, the implementation of ...sampled lateral grating on THz QCL ridges can result in stable single longitudinal mode emission with a side-mode suppression ratio larger than 20 dB. The measured peak power of the grating laser is improved by about 11.8% compared to the power of devices with uniform distributed feedback gratings. Furthermore, the far-field pattern of the presented device is uninfluenced by grating structures.
Compared with other typical terahertz (THz) detectors, the quantum-well photodetector (QWP) has the advantages of high detection sensitivity, fast response, mature fabrication, small size, and easy ...integration. Therefore, it is suitable for high-speed detection and imaging applications at the THz band. Researchers, both domestic and overseas, have systematically studied material design and device performance of the THz QWP. The design of the device is such that the peak frequency error is within 8%, the maximum peak responsibility is 5.5 A/W, the fastest response speed is 6.2 GHz, the best noise equivalent power is ∼10
−13
W/Hz
0.5
, and the spectrum range is 2.5–6.5 THz. In this article, firstly the basic principles and theoretical calculations of the THz QWP are described, and then the research progress of the THz QWP in our research group at imaging and communication is reviewed, which looks forward to its future development.
We demonstrate the successful implementation of a terahertz (THz) quantum-well photodetector (QWP) for effective signal collection in a scattering-type scanning near-field optical microscope (s-SNOM) ...system. The light source is an electrically pumped THz quantum cascade laser (QCL) at 4.2 THz, which spectrally matches with the peak photoresponse of THz QWP. The sensitive THz QWP has a low noise equivalent power (NEP) of about 1.1 pW/Hz0.5 and a spectral response range from 2 to 7 THz. The fast-responding capability of the THz QWP is vital for detecting the rapidly tip-modulated THz light which can effectively suppress the background noise. The THz images of the nanostructure demonstrate a spatial resolution of about 95 nm, corresponding to ∼λ/752 at 4.2 THz. We experimentally investigate and theoretically interpret the formation of the fringes which appear at the edge position of a gold stripe in the THz near-field image.
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•THz scattering-type scanning near-field optical microscope with a high-power THz QCL•Highly sensitive THz quantum-well photodetector for effective signal collection•Nanoscale spatial resolution reveals local optical properties in THz range•Experimentally investigate and theoretically interpret the formation of the edge fringes
Radiation physics; Nanotechnology; Quantum physics
Broadband dual‐comb spectroscopy has attracted increasing interests due to its unique advantages in high spectral resolution, fast detection, and so on. Although the dual‐comb technique is relatively ...mature in the infrared wavelengths, it is, currently, not commercially capable of practical applications in the terahertz regime due to the lack of high‐performance broadband terahertz dual‐comb sources. In the terahertz frequency range, the electrically pumped quantum cascade laser (QCL) is a suitable candidate for the dual‐comb operation. Although the resonant microwave injection locking is widely used to broaden the emission spectra of terahertz QCLs, it is challenging to be employed to obtain broadband dual‐comb sources that can fully exploit the laser gain bandwidth due to the large phase noise induced by the resonant injection and nonideal microwave circuits. Herein, an off‐resonant microwave injection to significantly broaden the dual‐comb bandwidth of a terahertz QCL dual‐comb source is used. The measured optical dual‐comb bandwidth is broadened from 147 GHz in free running to >450 GHz under the off‐resonant injection. By performing a numerical analysis based on a rate equation model, the broadband dual‐comb operation under the off‐resonant microwave injection can result from a wider lasing bandwidth and a higher degree of phase matching.
It is much demanding to develop broadband dual‐comb sources covering absorption lines of various molecules for spectroscopic and imaging applications. Herein, an off‐resonant microwave injection is employed to significantly broaden the optical bandwidth of a quantum cascade laser dual‐comb source emitting around 4.2 THz. The technique provides a powerful tool to achieve broadband terahertz dual‐comb sources for high‐precision applications.
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