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We report a novel laser cavity design in third-order distributed feedback (DFB) terahertz quantum-cascade lasers based on a perfectly phase-matching technique. This approach substantially increases the usable length of the third-order DFB laser and leads to narrow beam patterns. Single frequency emissions from 151 apertures (5.6 mm long device) are coherently added up to form a narrow beam with (FWHM≈6×11°) divergence. A similar device with 40 apertures shows more than 5 mW of optical power with slope efficiency ∼140 mW/A at 10 K pulsed operation.

We report on a heterodyne receiver designed to observe the astrophysically important neutral atomic oxygen [OI] line at 4.7448 THz. The local oscillator is a third-order distributed feedback Quantum Cascade Laser operating in continuous wave mode at 4.741 THz. A quasi-optical, superconducting NbN hot electron bolometer is used as the mixer. We recorded a double sideband receiver noise temperature (T DSB rec ) of 815 K, which is ∼7 times the quantum noise limit ( hν 2k B ) and an Allan variance time of 15 s at an effective noise fluctuation bandwidth of 18 MHz. Heterodyne performance was confirmed by measuring a methanol line spectrum. a)

Phase locking of an array of lasers is a highly effective method in beam shaping because it increases the output power and reduces the lasing threshold. Here, we show a conceptually novel phase-locking mechanism based on 'antenna mutual coupling' in which laser elements interact through far-field radiations with definite phase relations. This allows a longrange global coupling among the array elements to achieve a robust phase locking in two-dimensional laser arrays. The scheme is ideal for lasers with a deep subwavelength confined cavity, such as nanolasers, whose divergent beam patterns could be used to achieve a strong coupling among the elements in the array. We demonstrated experimentally such a scheme based on subwavelength short-cavity surface-emitting lasers at terahertz frequencies. More than 37 laser elements that span over ∼8 λ o were phase locked to each other, and delivered up to 6.5 mW (in a pulsed operation) single-mode radiation at ∼3 THz, with a maximum 450 mW A -1 slope efficiency and a near-diffraction-limited beam divergence. P hase locking of an array of lasers is a highly effective way to combine coherently the output radiations from individual lasers to achieve beam shaping and a higher output power. Moreover, the interaction between array elements could lead to a significant reduction in the lasing threshold. Recently, a new genre of laser cavity with deep subwavelength confinement in two or three dimensions, such as nanolasers or spasers 1-3 and photonic wire lasers 4 , has found various potential applications in fields such as optical-information processing, short-distance communication between integrated circuits and optical sensing 5 . However, substantial developments in reducing the excess lasing threshold and improving the coupling of light into a well-defined free-space mode are still required before these devices can be truly useful. If the phase-locking technique could be properly applied here, it will be a key method to overcome the aforementioned shortcomings.A robust phase locking requires strong couplings among the individual lasers so that otherwise-independent oscillators are forced to oscillate in sync. Currently, there are four demonstrated coupling schemes to phase-lock integrated diode laser arrayslaser ridges are coupled through exponentially decaying fields outside the high refractive-index dielectric core (evanescent-wave coupled 6 ) or through the Talbot feedback from external reflectors (diffraction-wave coupled 7,8 ), or by connecting two ridges to one single-mode waveguide (Y-coupled 9,10 ) or through lateral propagating waves (Leaky-wave coupled 11-13 ). Here we present a novel coupling scheme for phase-locking 2D laser arrays through 'global antenna mutual coupling'. This scheme is distinctly different from the other two phase-locking mechanisms for non-contacting laser elements. In contrast to the evanescent-wave coupled scheme, in which the coupling is through near-field electromagnetic fields, as in the antenna mutual coupled scheme, the coupling is establis...

A terahertz pulse emitter monolithically integrated with a quantum cascade laser ͑QCL͒ is demonstrated. The emitter facet is excited by near-infrared pulses from a mode-locked Ti:sapphire laser, and the resulting current transients generate terahertz pulses that are coupled into an electrically isolated QCL in proximity. These pulses are used to measure the gain of the laser transition at ϳ2.2 THz, which clamps above threshold at ϳ18 cm −1 and has a full width at half-maximum linewidth of ϳ0.67 THz. The measurement also shows the existence of absorption features at different biases that correspond to misalignment of the band structure and to absorption within the two injector states. The simplicity of this scheme allows it to be implemented alongside standard QCL ridge processing and to be used as a versatile tool for characterizing QCL gain media.

We report the demonstration of phase-locked arrays of surface-emitting distributed-feedback (DFB) terahertz quantum-cascade lasers with single-mode operations. Carefully designed “phase sector” locks several surface-emitting DFB laser ridges in-phase, creating tighter beam-patterns along the phased-array direction with full width at half maximum (FWHM)≈10°. In addition, the phase sector can be individually biased to provide a mechanism of frequency tuning through gain-induced optical index change, without significantly affecting the output power levels. A tuning range of 1.5 GHz around 3.9 THz was achieved. This fine tunability could be utilized to frequency- or phase-lock the DFB array to an external reference.

A frequency tunable terahertz heterodyne spectrometer, based on a third-order distributed feedback quantum cascade laser as a local oscillator, has been demonstrated by measuring molecular spectral lines of methanol ͑CH 3 OH͒ gas at 3.5 THz. By varying the bias voltage of the laser, we achieved a tuning range of ϳ1 GHz of the lasing frequency, within which the molecular spectral lines were recorded. The measured spectra show excellent agreement with modeled ones. By fitting we derived the lasing frequency for each bias voltage accurately. The ultimate performance of the receiver including the resolution of noise temperature and frequency is also addressed. © 2011 American Institute of Physics. ͓doi:10.1063/1.3599518͔Driven by the demands of astronomical observations and atmospheric remote sensing in the terahertz ͑THz͒ frequency range, we have recently developed a high resolution heterodyne spectrometer using a quantum cascade laser ͑QCL͒ at 2.9 THz as a local oscillator ͑LO͒ and a NbN hot electron bolometer ͑HEB͒ as a mixer.1 However, such a spectrometeris not yet adequate for operation in a telescope because of a number of drawbacks noted during the previous experiment. First, the QCL used previously was based on a metal-metal waveguide Fabry-Perot cavity design, which has no mode control of the lasing frequency and has virtually zero tuning range by the bias voltage because of a resulting strong reduction in the output power. The tuning capability is, in general, highly desirable for application in spectroscopy as it is crucial for targeting more molecular lines, and also a means to identify unknown spectral lines when a heterodyne receiver is operated in the double sideband ͑DSB͒ mode. 1 Second, a 4 He flow cryostat was used to operate the QCL. For any balloon-borne and space mission, the use of a liquid-He based cryostat can impose a serious obstacle due to the relatively high DC power dissipation of the laser. Therefore, a dry, liquid cryogen-free cooler such as a pulse tube cryocooler or a Stirling cooler 2 is preferred. One of the challenges in using a dry cooler is the mechanical stability. As demonstrated in Ref. 3, the vibration in the cooler can introduce deviations in the operating point of the detector, leading to instability of the receiver.In this letter we report on a high-resolution heterodyne molecular spectroscopic experiment applying a 3.5 THz QCL as a LO. In comparison with the previous work, 1 there are three key differences; ͑a͒ a single mode, third-order distributed feedback ͑DFB͒, tunable QCL ͑Refs. 4 and 5͒ is used as a LO; ͑b͒ a pulse tube cryocooler is applied to operate the QCL; and ͑c͒ a theoretical model for methanol molecular lines has been verified at 3.5 THz, which was not possible until now because of the lack of a heterodyne technique at such a frequency. By using the third-order periodic structure with strong refractive index contrast gratings, not only is the single mode emission achieved in the DFB laser, but also the radiation power out-coupling from the laser to the free...

We report on the phase locking of a 3.4 THz third-order distributed feedback quantum cascade laser (QCL) using a room temperature GaAs/AlAs superlattice diode as both a frequency multiplier and an internal harmonic mixer. A signal-to-noise level of 60 dB is observed in the intermediate frequency signal between the 18th harmonic of a 190.7 GHz reference source and the 3433 GHz QCL. A phase-lock loop with 7 MHz bandwidth results in QCL emission that is 96% locked to the reference source. We characterize the QCL temperature and electrical tuning mechanisms and show that frequency dependence of these mechanisms can prevent phase-locking under certain QCL bias conditions. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4817319]Since the development of the first THz quantum cascade lasers (QCLs), 1 there has been considerable progress made in their development. They are compact and offer lasing at any frequency between roughly 1 and 5 THz with high output power in the range of tens of milliwatts, making them highly suitable for many applications from local oscillator (LO) sources for high resolution heterodyne spectroscopy to gas sensing and terahertz imaging.One of the key applications driving the development of the THz QCL is heterodyne spectroscopy in the superterahertz which is loosely defined as 2 to 6 THz. For frequencies beyond 2 THz, there are few solid state sources available. The commonly used LO below 2 THz is the multiplier based source which, to date, has demonstrated output power of a microwatt at up to 2.7 THz. For an LO source, single mode emission is crucial. A 3rd order distributed feedback (DFB) laser, as to be explained, can offer not only a robust single mode operation, but also a relatively narrow single-lobe beam. The latter is of practical importance for efficient coupling of the radiation to a mixer or mixer array.Frequency locking of a THz QCL was first demonstrated by Betz et al. 2 in 2005. Since then, it has been well established that to apply a QCL as an LO in a real receiver system, either frequency stabilization or phase locking is required. For this reason, many frequency or phase locking experiments have been reported in the literature. Those demonstrations can be mainly divided into a few cases: (a) phase locking of Fabry-Perot (FP) based QCLs with the use of a cooled superconducting detector as the mixing element 3-5 or by the use of a frequency comb generated from a mode-lock femtosecond laser. 6,7 The latter is operated at room temperature but requires relatively bulky and high power consumption electronics; (b) frequency locking of an FP or 3rd order DFB laser using a gas absorption line as the reference; 8 (c) frequency locking of an FP laser using a Schottky-diode harmonic mixer, 9 which was operated at room temperature, but requires high THz input power from the QCL in the order of several mW and has so far been demonstrated only below 3 THz.In this paper we report on a phase locking demonstration of a 3.4 THz 3rd order DFB laser QCL using a roomtemperature component,...