Fig 2 - uploaded by Jeffrey Hesler
Content may be subject to copyright.
A typical power spectrum of the beat signal of the THz QCL that is phase locked to a microwave reference recorded by the spectrum analyzer with a low resolution bandwidth (RBW) of 100 KHz. For comparison, a spectrum of the free-running QCL is also shown. The inset shows a relative frequency shift of the free-running QCL emission line versus the biasing current at 5 K. The starting frequency is 2.735 THz.
Source publication
We demonstrate the phase locking of a 2.7 THz metal-metal waveguide quantum cascade laser (QCL) to an external microwave signal. The reference is the 15th harmonic, generated by a semiconductor superlattice nonlinear device, of a signal at 182 GHz, which itself is generated by a multiplier chain (x12) from a microwave synthesizer at approximately 1...
Similar publications
This work focuses on surface plasmon quantum cascade lasers (QCL) as a local oscillators (LO) for heterodyne receivers operating above 2 THz. The far-field beam pattern of a 214 μm wide and 1500 μm long 2.84 THz surface plasmon QCL is measured and found to be less divergent compared to metal-metal waveguide QCLs, which is preferable for coupling th...
We report a set of measurements to demonstrate a new type of surface emitting distributed feedback (DFB) quantum cascade laser (QCL) operated at 3.5 THz as a local oscillator by pumping a superconducting hot-electron bolometer (HEB) mixer. Based on the Bragg gratings incorporated into the waveguide, the second order DFB surface emitting THz QCL sho...
We report the demonstration of a heterodyne receiver for detection of OH lines at 3.5 THz. The receiver uses a superconducting NbN hot electron bolometer integrated with a tight winding spiral antenna as mixer and a THz distributed feedback quantum cascade laser operating at 3.42 THz as local oscillator. We measured a double sideband receiver noise...
We demonstrate for the first time the integration of a superconducting hot electron bolometer (HEB) mixer and a quantum cascade laser (QCL) on the same 4-K stage of a single cryostat, which is of particular interest for terahertz (THz) HEB/QCL integrated heterodyne receivers for practical applications. Two key issues are addressed. Firstly, a low p...
We report a heterodyne receiver using a superconducting NbN hot electron bolometer (HEB) integrated with a tight winding spiral antenna as mixer and a distributed feedback (DFB) terahertz quantum cascade laser (QCL) operating at 3.42 THz as local oscillator. The aim is to demonstrate the readiness of both devices for the detection of OH lines at 3....
Citations
... For example, this was achieved by mixing the QCL signal with a tone of a subharmonic Schottky diode mixer [5][6][7] or superlattice mixer [8]. Locking to the output of a 1.5 THz and 2.7 THz Schottky diode frequency multiplier chain (FMC) has also been demonstrated utilizing cryogenic superconducting mixers [9,10]. Optical frequency combs have also been used to lock THz QCLs via the generation of an RF beat note in a photoconductive mixer [11], electrooptic sampling in a ZeTe crystal [12], or a superconducting mixer [13]. ...
Optical injection locking of a metasurface quantum-cascade (QC) vertical-external-cavity surface-emitting laser (VECSEL) is demonstrated at 2.5 THz using a Schottky diode frequency multiplier chain as the injection source. The spectral properties of the source are transferred to the laser output with a locked linewidth of ∼1 Hz, as measured by a separate subharmonic diode mixer, and a locking bandwidth of ∼300 MHz is achieved. The large locking range is enabled by the microwatt power levels available from modern diode multipliers. The interplay between the injected signal and feedback from external reflections is studied and demonstrated to increase or decrease the locking bandwidth relative to the classic locking range depending on the phase of the feedback.
... Frequency-and phase-locking of ridge wave waveguide THz QCLs has been demonstrated using a variety of techniques including: referencing the QCL to a more stable gas laser or frequency multiplier chain (FMC) source using a superconducting hot electron bolometer [12,13] or diode mixer [14,15], locking to an optical or THz frequency comb [16][17][18], locking to a subharmonic diode mixer with an RF synthesized pump [19,20], direct injection locking with signal from a THz photomixer [21], and locking to a gas cell [22][23][24]. ...
... While it is difficult to directly compare these free running results to those from other studies due to varying setups and measurement techniques, typical free-running linewidths for ridge waveguide THz QCLs have been reported to be 1-10 MHz [12,15,42,43]. As it was established in Section II that the impact of QC-material refractive index on the lasing frequency for the QC-VECSEL should be similar that of a ridge waveguide, the relatively large frequency instability of the QC-VECSEL is suggestive that it is indeed more subject to mechanical instabilities in the external optics than most ridge waveguides. ...
p>High resolution frequency study and phase-locking have been performed on terahertz quantum-cascade vertical-external-cavity surface-emitting-lasers at 2.5 THz and 3.4 THz. A subharmonic diode mixer is used to downconvert the THz signal to a gigahertz intermediate frequency (IF). Feedback from reflections at the mixer are observed to have a strong influence on the free-running QC-VECSEL frequency stability as a result of efficient coupling to free-space compared to more typical ridge waveguide lasers. Instabilities in feedback result in free-running linewidths of tens of MHz, and at times >100 MHz. The QC-VECSEL IF signal is phase locked to a 100 MHz reference using the bias on the device as a means of error correction. Between 90-95% of the QC-VECSEL signal is locked within 2 Hz of the multiplied RF reference, and amplitude fluctuations on the order of 1-10% are observed, depending on the bias point of the QC-VECSEL. The bandwidth of the locking loop is ~1 MHz. Many noise peaks in the IF signal corresponding to mechanical resonances in the 10 Hz-10 kHz range are observed. These peaks are generally -30 to -60 dB below the main tone, and are below the phase noise level of the multiplied RF reference which ultimately limits the phase noise of the locked QC-VECSEL.</p
... Frequency-and phase-locking of ridge wave waveguide THz QCLs has been demonstrated using a variety of techniques including: referencing the QCL to a more stable gas laser or frequency multiplier chain (FMC) source using a superconducting hot electron bolometer [12,13] or diode mixer [14,15], locking to an optical or THz frequency comb [16][17][18], locking to a subharmonic diode mixer with an RF synthesized pump [19,20], direct injection locking with signal from a THz photomixer [21], and locking to a gas cell [22][23][24]. ...
... While it is difficult to directly compare these free running results to those from other studies due to varying setups and measurement techniques, typical free-running linewidths for ridge waveguide THz QCLs have been reported to be 1-10 MHz [12,15,42,43]. As it was established in Section II that the impact of QC-material refractive index on the lasing frequency for the QC-VECSEL should be similar that of a ridge waveguide, the relatively large frequency instability of the QC-VECSEL is suggestive that it is indeed more subject to mechanical instabilities in the external optics than most ridge waveguides. ...
p>High resolution frequency study and phase-locking have been performed on terahertz quantum-cascade vertical-external-cavity surface-emitting-lasers at 2.5 THz and 3.4 THz. A subharmonic diode mixer is used to downconvert the THz signal to a gigahertz intermediate frequency (IF). Feedback from reflections at the mixer are observed to have a strong influence on the free-running QC-VECSEL frequency stability as a result of efficient coupling to free-space compared to more typical ridge waveguide lasers. Instabilities in feedback result in free-running linewidths of tens of MHz, and at times >100 MHz. The QC-VECSEL IF signal is phase locked to a 100 MHz reference using the bias on the device as a means of error correction. Between 90-95% of the QC-VECSEL signal is locked within 2 Hz of the multiplied RF reference, and amplitude fluctuations on the order of 1-10% are observed, depending on the bias point of the QC-VECSEL. The bandwidth of the locking loop is ~1 MHz. Many noise peaks in the IF signal corresponding to mechanical resonances in the 10 Hz-10 kHz range are observed. These peaks are generally -30 to -60 dB below the main tone, and are below the phase noise level of the multiplied RF reference which ultimately limits the phase noise of the locked QC-VECSEL.</p
... This value is considerably smaller than the free-running linewidths of waveguide-based THz QC-lasers, which are typically on the order of a few MHz when measured over times of more than a few seconds and are primarily limited by temperature and current fluctuations. [14][15][16][17] This is expected, since in a QC-VECSEL, most of the cavity mode resides within vacuum and not the QC-active material and, hence, is less sensitive to refractive index fluctuations of the semiconductor. This is consistent with the fact that the tuning coefficient is measured to be 1.7 kHz/mA, which is 3-4 orders of magnitude smaller than that reported for waveguide-based QClasers or even short-cavity VECSELs. ...
... starting point for further stabilization using method such as frequency locking to gas absorption lines 30 or high-finesse optical cavities, 31 and phase-locking to optical frequency combs 32 or harmonics of a microwave frequency standard. 15 However, this comes at the expense of a limited electrical frequency tuning range; future work will be required to combine this approach with frequency agility. ...
We report continuous wave (cw) operation of a terahertz quantum-cascade vertical-external-cavity surface-emitting laser with an external cavity length of approximately 30 mm, benefited by an intra-cryostat focusing cavity. Compared to previous plano–plano cavities, an off-axis paraboloid mirror is introduced into the external cavity as a focusing element to reduce the diffraction loss and to enable cw lasing using small-area metasurfaces and long cavity lengths. The device shows lasing operation in the cw mode up to 111 K, and cw output power up to 11.5 mW at 77 K (0.5% wall-plug efficiency). A circular, directive beam pattern is collected, and free-running linewidths on the order of tens of kHz are measured over tens of seconds.
... Among the most important master oscillators (MO) employed in combination with THz QCLs we can include molecular far-Infrared lasers, [51] frequency standards from microwave sources, [56,57] frequency standards from optical or near-infrared sources, as frequency combs. Microwave references provided a very narrow and absolute-frequency reference, suitable for mixing with THz QCLs onto sensitive cryogenic detectors, like hotelectron bolometers even with a few pW of MO radiation power. ...
Quantum cascade lasers (QCLs) represent the most fascinating achievement of quantum engineering, showing how artificial materials can be generated through quantum design, with tailor‐made properties. Their inherent quantum nature deeply affects their core physical parameters. QCLs indeed display intrinsic linewidths approaching the quantum limit, and show spontaneous phase‐locking of their emitted modes via intracavity four‐wave‐mixing, meaning that they can naturally operate as miniaturized metrological frequency rulers, also in frontier frequency domains, as the far‐infrared, yet unexplored in quantum science. Here, the authors discuss the fundamental quantum properties of QCLs operating at terahertz frequencies and their key technological performances, highlighting future perspectives of this frontier research field in disruptive areas of quantum technologies such as quantum sensing, quantum metrology, quantum imaging, and photonic‐based quantum computation.
... Frequency converters that can operate at ambient temperatures enable operation for a long lifetime and eliminate the necessity of bulky cryostats. In 2009, Khosropanah et al. demonstrated phase locking of a 2.7-THz QCL using a superlattice mixer [10]. Later in 2013, Hayton et al. [11] reported both, frequency and phase, locking of a 3.4-THz QCL to a ×15-harmonic signal generated by a superlattice harmonic mixer operating at room temperature. ...
Efficient and compact frequency converters are essential for frequency stabilization of terahertz sources. In this paper, we present a 3.5-THz, x6-harmonic, integrated Schottky diode mixer operating at room temperature. The designed frequency converter is based on a single-ended, planar Schottky diode with a sub-micron anode contact area defined on a suspended 2-um ultra-thin GaAs substrate. The dc-grounded anode pad was combined with the radio frequency E-plane probe, which resulted in an electrically compact circuit. At 200 MHz intermediate frequency, a mixer conversion loss of about 59 dB is measured resulting in a 40 dB signal-to-noise ratio in a phase-locking application. Using a quasi-static diode model combined with electromagnetic simulations, good agreement with the measured results was obtained. Harmonic frequency converters without the need of cryogenic cooling will help in the realization of highly sensitive space and air-borne heterodyne receivers.<br
... The Schottky diode has also been applied as a frequency mixer for producing the RF beat signal from ν THz up to 2.9 THz [22,23]. On the other hand, a semiconductor-superlattice electron device (SLED) with negative differential conductivity at room temperature, which was predicted by Esaki and Tsu in the early 1970s [24], has been demonstrated for THzwave generation [25][26][27] and THz-frequency downconversion [28][29][30] while competing with the Schottky diode. The SLED is a compact, easy-to-use, and room-temperature operational device. ...
We have developed a broadband and high-precision terahertz (THz) frequency counter based on a semiconductor-superlattice harmonic mixer (SLHM). Comparison of two THz frequencies determined using two independent counters and direct measurement of frequency-stabilized THz-quantum cascade lasers by a single counter showed a measurement uncertainty of less than 1 × 10 ⁻¹⁶ over a four-octave range from 120 GHz to 2.8 THz. Further extension of this measurable range was indicated by the research regarding the higher-harmonics generation of a local oscillator for the SLHM. This compact and easy-to-handle THz counter operating at room temperature is available for high-resolution spectroscopy of ultracold molecules proposed for detecting temporal changes in physics constants as well as many THz applications requiring a wide measurement range without a bulky cryogenic apparatus.
... The intrinsic linewidth of the terahertz QCL is of the order of a few hundreds of Hz [26,27] but thermal, electric, and mechanical instabilities typically result in free-running linewidths of a few MHz [28]. This has led to the development of several techniques to stabilize the frequency of QCLs, including locking to a gas absorption line [29], a multiplied microwave source [30], femtosecond laser frequency combs [31,32], and a femtosecond frequency comb stabilized to a primary frequency reference [33]. All of these methods only make use of an electrical feedback to stabilize the frequency of the QCL to a stable reference source. ...
... Furthermore, the combination of both locking schemes allows us to maintain lower phase error at low frequencies via the electronic PLL, while the addition of terahertz injection provides a wider locking range [34]. Hence, the combined implementation of the terahertz injection phase lock loop (IPLL) provides improved tracking of environmental fluctuations and low phase error variance compared to systems that use either IL [35] or only the electronic PLL configuration [29][30][31][32]. ...
The accuracy of high-resolution spectroscopy depends critically on the stability, frequency control, and traceability available from laser sources. In this work, we report exact tunable frequency synthesis and phase control of a terahertz laser. The terahertz laser is locked by a terahertz injection phase lock loop for the first time, with the terahertz signal generated by heterodyning selected lines from an all-fiber infrared frequency comb generator in an ultrafast photodetector. The comb line frequency separation is exactly determined by a Global Positioning System-locked microwave frequency synthesizer, providing traceability of the terahertz laser frequency to primary standards. The locking technique reduced the heterodyne linewidth of the terahertz laser to a measurement instrument-limited linewidth of ${\lt}1\;{\rm Hz}$ < 1 H z , robust against short- and long-term environmental fluctuations. The terahertz laser frequency can be tuned in increments determined only by the microwave synthesizer resolution, and the phase of the laser, relative to the reference, is independently and precisely controlled within a range $\pm{0.3}\pi$ ± 0.3 π . These findings are expected to enable applications in phase-resolved high-precision terahertz gas spectroscopy and radiometry.
... Furthermore, SSLs host rich dynamics in the presence of a driving field, which include the formation of Stark ladders [19], the manifestation of Bragg reflections and Bloch oscillations [20]. From the viewpoint of applications, SSLs have attracted great interest because they allow the development of devices which operate at microwave [21] and far-infrared frequencies [5,21] suitable for high precision spectroscopic studies and detection of submillimeter waves. In addition, a considerable number of studies have tackled the task of engineering parametric amplifiers [22,23] and frequency multipliers [24,25,26] based on superlattice periodic structures. ...
... Furthermore, SSLs host rich dynamics in the presence of a driving field, which include the formation of Stark ladders [19], the manifestation of Bragg reflections and Bloch oscillations [20]. From the viewpoint of applications, SSLs have attracted great interest because they allow the development of devices which operate at microwave [21] and far-infrared frequencies [5,21] suitable for high precision spectroscopic studies and detection of submillimeter waves. In addition, a considerable number of studies have tackled the task of engineering parametric amplifiers [22,23] and frequency multipliers [24,25,26] based on superlattice periodic structures. ...
Semiconductor superlattices are strongly nonlinear media offering several technological challenges associated with the generation of high-frequency Gigahertz radiation and very effective frequency multiplication up to several Terahertzs. However, charge accumulation, traps and interface defects lead to pronounced asymmetries in the nonlinear current flow, from which high harmonic generation stems. This problem requires a full non-perturbative solution of asymmetric current flow under irradiation, which we deliver in this paper within the Boltzmann-Bloch approach. We investigate the nonlinear output on both frequency and time domains and demonstrate a significant enhancement of even harmonics by tuning the interface quality. Moreover, we find that increasing arbitrarily the input power is not a solution for high nonlinear output, in contrast with materials described by conventional susceptibilities. There is a complex combination of asymmetry and power values leading to maximum high harmonic generation.
... Furthermore, SSLs host rich dynamics in the presence of a driving field, which include the formation of Stark ladders [21], the man- * mauro.pereira@ku.ac.ae ifestation of Bragg reflections and Bloch oscillations [22]. From the viewpoint of applications, SSLs have attracted great interest because they allow the development of devices which operate at microwave [23] and far-infrared frequencies [5,23] suitable for high precision spectroscopic studies and detection of submillimeter waves. In addition, a considerable number of studies have tackled the task of engineering parametric amplifiers [24,25] and frequency multipliers [26][27][28] based on superlattice periodic structures. ...
... Furthermore, SSLs host rich dynamics in the presence of a driving field, which include the formation of Stark ladders [21], the man- * mauro.pereira@ku.ac.ae ifestation of Bragg reflections and Bloch oscillations [22]. From the viewpoint of applications, SSLs have attracted great interest because they allow the development of devices which operate at microwave [23] and far-infrared frequencies [5,23] suitable for high precision spectroscopic studies and detection of submillimeter waves. In addition, a considerable number of studies have tackled the task of engineering parametric amplifiers [24,25] and frequency multipliers [26][27][28] based on superlattice periodic structures. ...
In this paper we solve the Boltzmann-Bloch equation within a path integral approach, delivering general, non-perturbative solutions of high harmonic generation in semiconductor superlattices with asymmetric current flow. The system is treated non-perturbatively in the illuminating field by employing local boundary conditions which allow the inclusion of asymmetric relaxation rates. The spectroscopic properties of the high harmonic generation are demonstrated by calculations of the nonlinear response in both frequency and time domain. We show that asymmetric currents affect the spontaneous emission and can result in a significant enhancement of even harmonics by tuning the interface quality.
