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The purpose of this article is to explore to what extent international students may be affected by a lack of study/work-life balance or study-work-life balance amongst those who study (full-time) and work part-time. International students tend to face more pressures due to usually lacking a support network, at least for some time once arriving in the host country. Such pressures may be financial or social and may be due to a lack of awareness of host country norms and regulations or due to language barriers. An online survey informed by work-life balance theory was completed by 42 international students. The findings from the analysis of responses were that while most respondents were satisfied with their study-work-life balance, some although managing overall, faced pressures. A key finding was that the students managed due to the student visa condition restricting employment to a maximum of twenty hours per week. This seems to have helped respondents to focus more time on their studies, however, may add to financial pressures or put international students at a disadvantage over their domestic peers in terms of gaining work experience. Further, findings have uncovered that although working while studying provides additional pressures, it creates benefits through the ability to build a support network.

This research is exploratory, designed to investigate emerging themes in order to examine perceptions of precarious employment and social procurement in the construction industry for Māori and Pasi...

Quantum cascade (QC) lasers operating at terahertz frequencies were demonstrated two years ago, and, since then, their development has proceeded at a very rapid pace. The gain medium of the first devices was based on chirped superlattices, and a resonator relying on the surface plasmon concept was employed to achieve a large optical confinement with concomitant low propagation losses. Laser action was obtained at 4.4 THz, in pulsed mode and at temperatures up to 50 K. Improved fabrication allowed continuous-wave (cw) operation and increased the operating temperature to 75 K. Using a similar active region, lasing at 3.5 THz was achieved. More recently, various groups have introduced several new design concepts such as bound-to-continuum transitions and extraction of carriers via resonant phonon scattering, leading to pulsed operation up to 140 K, output powers of up to 50 mW, and cw operation up to 93 K. The lowest emission frequency is now 2.1 THz, tackling the technologically important region of 1.5-2.5 THz. Stable single-mode emission under all operating conditions has also recently become a reality thanks to the adoption of distributed feedback resonators. This rapid and substantial progress underlines the growing potential of QC lasers in THz photonics.

Quantum cascade lasers (QCL) are unipolar injection lasers based on intersubband transitions in a modular semiconductor heterostructure. The first THz QCL, operating at 67 mu m (4.3 THz), was demonstrated in 2002; the wavelength range now extends beyond 250 mu m (1.2 THz) and is entering the sub-terahertz frequency range for devices operated in external magnetic field. Although a number of different quantum designs have been demonstrated, increasing the operating temperature remains a major challenge: the maximum temperature is still similar to 195 K, and recently approached 225 K in high magnetic fields. Nevertheless, compact continuous wave systems operating within Sterling coolers already ensure ample portability and turn-key operation and QCLs represent then the THz solid-state radiation source that actually shows the best performance in terms of optical output power, which can reach more than 100 mW average, and linewidth, typically in the tens of kHz for single mode devices. THz QCLs have then a realistic chance to deeply impact technological applications such as process monitoring, security controls, and bio-medical diagnostics. They are ideally suited though for plasma polarimetry and interferometry, thanks to their high polarization selectivity, excellent stability and ruggedness, and ease of high-speed modulation. Their compact size and monolithic cavity arrangement allows placement in the very proximity of the plasma to be monitored, easing requirements of stability against vibrations etc. Furthermore, the long coherence lengths should be easily compatible with interferometric arms of even very different lengths, a geometry ideal for coupling to a plasma reactor. The possibility of direct current modulation at MHz if not GHz frequencies ensures then an excellent temporal resolution of the meaurements, and a large low-frequency noise rejection. New analysis schemes also become feasible, for instance employing two-color lasers, operating at the same time at two appropriately chosen wavelengths, or exploiting the laser emission tunability for implementing frequency modulation techniques. This paper will discuss the current state-of-the-art in THz QCL technology and applications, focusing on those aspects of greatest relevance for plasma diagnostics. Quantum cascade lasers (QCL) are unipolar injection lasers based on intersubband transitions in a modular semiconductor heterostructure. The first THz QCL, operating at 67 μm (4.3 THz), was demonstrated in 2002; the wavelength range now extends beyond 250 μm (1.2 THz) and is entering the sub-terahertz frequency range for devices operated in external magnetic field. Although a number of different quantum designs have been demonstrated, increasing the operating temperature remains a major challenge: the maximum temperature is still ∼ 195 K, and recently approached 225 K in high magnetic fields. Nevertheless, compact continuous wave systems operating within Sterling coolers already ensure ample portability and turn-key operation and QCLs represent then the THz solid-state radiation source that actually shows the best performance in terms of optical output power, which can reach more than 100 mW average, and linewidth, typically in the tens of kHz for single mode devices. THz QCLs have then a realistic chance to deeply impact technological applications such as process monitoring, security controls, and bio-medical diagnostics. They are ideally suited though for plasma polarimetry and interferometry, thanks to their high polarization selectivity, excellent stability and ruggedness, and ease of high-speed modulation. Their compact size and monolithic cavity arrangement allows placement in the very proximity of the plasma to be monitored, easing requirements of stability against vibrations etc. Furthermore, the long coherence lengths should be easily compatible with interferometric arms of even very different lengths, a geometry ideal for coupling to a plasma reactor. The possibility of direct current modulation at MHz if not GHz frequencies ensures then an excellent temporal resolution of the meaurements, and a large low-frequency noise rejection. New analysis schemes also become feasible, for instance employing two-color lasers, operating at the same time at two appropriately chosen wavelengths, or exploiting the laser emission tunability for implementing frequency modulation techniques. This paper will discuss the current state-of-the-art in THz QCL technology and applications, focusing on those aspects of greatest relevance for plasma diagnostics. © 2012 IOP Publishing Ltd and SISSA.

We report on the experimental observation of non-equilibrium longitudinal (LO) and interface transverse-like (IF-TO) optical phonons populations associated with electron transport in resonant-phonon THz quantum-cascade lasers. The measured LO-phonon occupation numbers are in excellent agreement with the result of Monte Carlo simulations at a phonon in-plane wave number q = 4.2 x 10(+4) cm(-1), and they remain about 40% larger than the IF-TO ones. (C) 2012 American Institute of Physics. [doi:10.1063/1.3687913]

As part of a study for a second generation heterodyne spectrometer on SOFIA we are developing a 2.5-THz frontend which is implemented in a pulse-tube cooler . It consists of a quantum-cascade laser as local oscillator and a phonon-cooled NbN hot electron bolometric mixer. Frequency stabilization to below 300 kHz full width at half maximum is achieved by locking to a molecular absorption line. We will present the design and performance of the frontend and discuss the prospects for an application on board of SOFIA.

We present a planar laser diode based on a simple fabrication scheme compatible with virtually any geometry accessible by standard semiconductor lithography technique. We show that our lasers exhibit similar to 1 GHz -3 dB-modulation-bandwidth already in this prototypical implementation. Directions for a significant speed increase are discussed. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3672438]

The frequency of a terahertz quantum-cascade laser is stabilized to the absorption line of methanol gas at a frequency of 2.55 THz. The method is based on frequency modulation of the laser emissionm across the absorption line. The resulting derivativelike signal is used as an error signal for a control loop that keeps the laser frequency at maximum absorption. The unstabilized laser that is operated in a pulse tube cooler has frequency fluctuations of 15 MHz, which are reduced to 300 kHz with the control loop in action. The line shape of the locked signal is Gaussian.

Although lasers have found numerous applications, their design is often still based on the concept of a gain medium within a mirror cavity. Exceptions to this are distributed feedback lasers1, in which feedback develops along a periodic structure, or random lasers, which do not require any form of cavity2. Random lasers have very rich emission spectra, but are difficult to control. Distributed feedback devices, conversely, have the same limited design possibilities of regular lasers. We show, by making use of a quasi-crystalline structure in an electrically pumped device, that several advantages of a random laser can be combined with the predictability of a distributed feedback resonator. We have constructed a terahertz quantum cascade laser based on a Fibonacci distributed feedback sequence, and show that engineering of the self-similar spectrum of the grating allows features beyond those possible with traditional periodic resonators, such as directional output independent of the emission frequency and multicolour operation. Researchers have constructed a terahertz quantum cascade laser using quasi-periodic distributed feedback gratings based on the Fibonacci sequence. Features that go beyond traditional distributed feedback lasers are demonstrated, such as directional output independent of the emission frequency and multicolour operation.

We show that a distributed-feedback terahertz quantum cascade laser can be tuned with a coupled microcavity by anti-crossing of the respective eigenfrequencies. In this proof-of-concept experiment, a tuning range of 20 GHz is obtained, in good agreement with a simple finite element model, which shows that the tuning is determined by the coupling strength between the resonators. The concept could be applied to any laser cavity, but becomes progressively more attractive the lower the emission frequency.

A superconducting bolometer with an on-chip lithographic terahertz antenna has been characterized in passive and active mode. Direct detection measurements made with 300K/77K blackbody sources yeld an optical efficiency of 10%. Time response of the bolometer has been obtained at 2.5 and 4.4 THz by illuminating the detector with quantum cascade lasers (QCL) showing a time constant of 1.2 mus. Here we report on the fabrication and the optical characterization of an antenna-coupled hot-spot microbolometer operating in vacuum at liquid helium temperature.

In this paper we investigate, both theoretically and experimentally, nonequilibrium electron and phonon effects in quantum-cascade devices. In particular, we have developed a Monte Carlo-based global kinetic approach describing the complete interacting electronic subsystem (i.e., the full set of active-region and injector subbands) coupled to out-of-equilibrium longitudinal polar-optical (LO) phonons, which in turn will decay anharmonically into thermalized acoustic modes. Simulated results obtained for a prototypical terahertz emitting device show a very good agreement with measured data, evidencing how the nonequilibrium LO phonon population affects the electro-optical device performances. The latter may be qualitatively reproduced in terms of a global effective temperature of the heated phononic system. (C) 2010 American Institute of Physics. [doi:10.1063/1.3464977]

A superconducting bolometer with an on-chip lithographic terahertz antenna has been illuminated by two quantum cascade lasers operating at 2.5 and 4.4 THz to be used for wide dynamic range Terahertz imaging applications.

The periodic scattering of the surface plasmon modes employed in the waveguide of terahertz quantum cascade lasers is shown to be an efficient method to control the properties of the laser emission. The scatterers are realized as thin slits in the metal and top contact layer carrying the surface plasmon. This technique provides larger coupling strengths than previously reported and can be used in various device implementations. Here the method is applied to realize a distributed feedback resonator without back-facet reflection, to achieve vertical emission of the radiation with second-order gratings, and to increase the facet reflectivity by fabricating passive distributed Bragg reflectors. (c) 2006 Optical Society of America.

We analyze surface-emitting distributed feedback resonators for Terahertz quantum cascade lasers fabricated from double-metal waveguides. We explain the influence on resonances and surface-emission properties of the finite length and width of the gratings in connection with absorbing boundary conditions, and show that, contrary to the infinite case, the modes on either side of the photonic band-gap have finite surface losses. The lateral design of the resonator is shown to be important to avoid transverse modes of higher order and anti-guiding effects. Experimental findings are indeed in excellent agreement with the simulations. Both modeling and fabrication can easily be applied to arbitrary gratings, of which we discuss here a first interesting example.

In microdisk lasers(1-3) a ring resonator is formed by successive total internal reflections inside a circularly shaped waveguide. The photon lifetime of the resulting whispering gallery optical modes is limited mainly by the waveguide absorption. Light is usually coupled out by tunnelling owing to the disk curvature or through imperfections at the border, but the output power is hard to exploit in a potential application because the emission is mainly in the disk plane and isotropic. Here we realize vertically emitting whispering gallery lasers by implementing appropriate diffraction gratings along the disk circumference. We use terahertz quantum cascade structures(4,5) and demonstrate a 50-fold increase in the optical power compared to devices without gratings, while at the same time engineering the lasing spectrum according to the grating rotational symmetry. This concept will allow the fabrication of compact arrays of single-mode terahertz sources with regular beam profiles and high output power.

In this paper, the spectral behavior of two terahertz (THz) quantum cascade lasers (QCLs) operating both pulsed and cw is characterized using a heterodyne technique. Both lasers emitting around 2.5 THz are combined onto a whisker contact Schottky diode mixer mounted in a corner cube reflector. The resulting difference frequency beatnote is recorded in both the time and frequency domain. From the frequency domain data, we measure the effective laser linewidth and the tuning rates as a function of both temperature and injection current and show that the current tuning behavior cannot be explained by temperature tuning mechanisms alone. From the time domain data, we characterize the intrapulse frequency tuning behavior, which limits the effective linewidth to approximately 5 MHz. (C) 2009 Optical Society of America

A liquid cryogen-free terahertz heterodyne receiver in a pulse tube cooler has been realized. The receiver operates at 2.5 THz. It is based on a quantum cascade laser (QCL) as local oscillator and a hot electron bolometric mixer. A detailed study of the QCL beam quality yielded a beam propagation factor of 1.1-1.2. The double sideband noise temperature of the system is 2000 K and when corrected for optical losses in the signal path it is similar to 800 K. (C) 2008 American Institute of Physics.

We report on progress towards a solid state 2.5-THz heterodyne receiver. The front-end of the receiver is integrated in a pulse tube cooler. It consists of a quantum cascade laser (QCL) as local oscillator and a phonon-cooled NbN hot electron bolometric mixer. The QCL is mounted on the first stage of the PTC and operates at a temperature of about 45 K while the HEB is mounted on the second stage of the PTC (temperature ~5 K). Design and performance of the receiver will be reported.

Various applications in astronomy and security need highly sensitive heterodyne receiver with a quantum cascade laser or a gas laser as local oscillator and a quasioptically coupled hot-electron bolometer mixer. Here we report on the development and characterization of the receiver components for frequencies up to 2.5 THz.

Quantum cascade lasers operating at about 2.5 terahertz have been implemented in a spectrometer for high resolution molecular spectroscopy. Linewidth, frequency tunability and frequency stability of the lasers were investigated by mixing their radiation with radiation from a 2.5 terahertz gas laser. Two first laser characteristics were found to be sufficient for Doppler-limited spectroscopy. For a demonstration of the spectrometer, absorption at rotational transitions of methanol was measured. The cascade lasers served as radiation source. A gas laser was used to determine the absolute frequency simultaneously with the detection of absorption signal by mixing the gas laser radiation with a part of that from cascade lasers. Amplitude as well as frequency modulation of the output power of the cascade lasers was used. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519 terahertz were determined.

Quasi-periodic surface plasmon gratings based on the Fibonacci sequence are studied as distributed feedback resonators for terahertz quantum cascade lasers. The simulations indicate a quality factor comparable with standard DFB resonators. The fabricated devices show grating-dependent emission, but suppression of higher transverse waveguide modes will be necessary to unambiguously identify the fundamental resonance frequencies of the grating. (C) 2007 Elsevier B.V. All rights reserved.

Spectral characteristics of quantum cascade lasers operating at about 2.5 terahertz have been investigated by means of high-resolution mixing spectroscopy. Linewidth, frequency tunability and frequency stability of the lasers were measured by mixing their radiation with radiation from a 2.5 terahertz gas laser (heterodyne mixing). The results of the homodyne mixing of different modes of the laser indicate on laser mode switching and large variations of the effective refractive index in the active medium caused by the laser drive current.

The design and performance of a local oscillator (LO) for an airborne terahertz heterodyne receiver is reported. The LO is based on a THz gas laser, which is transversely excited by radiation from a CO 2 pump laser. Frequency stabilization is achieved by cascaded locking loop. where the frequency of the CO 2 laser is stabilized to a Fabry-Perot interferometer, which in turn is stabilized to a HeNe laser. The LO is compact and mechanically rugged in order to comply with the harsh environment on board of an aircraft. It also fulfills all physical requirements with respect to output power, beam profile and frequency stability. A THzquantum cascade laser (QCL) is a potential alternative to a gas laser LO. We report on the performance of a 2.5-THz QCL with respect to beam profile, frequency tuning behaviour and width of its emission line. The QCL has been used as LO to pump a hot electron bolometric mixer at 2.5 THz.

The linewidth enhancement factor (LEF) of terahertz quantum cascade lasers is measured using an optical feedback self-mixing technique. As expected, a much lower LEF is found than is common for interband lasers, but instead of the predicted value of zero, the LEF depends on the laser conditions and can be as high as 0.5. The measured value tends to increase with increasing current. Cross absorption within the laser active region is suggested as a possible cause for the nonzero LEF observed. (C) 2008 American Institute of Physics.

We have studied the current- and temperature-driven frequency tunability of quantum cascade lasers operating at 2.5 THz. The design of the active medium follows the so-called bound-to-continuum approach. One laser has a Fabry-Perot type resonator. It operates on several modes simultaneously. The other has a distributed feedback structure on top of the active medium which leads to single mode emission. High-resolution spectra of the laser emission by heterodyne mixing with the radiation from a THz gas laser were obtained. In the case of the Fabry-Perot laser, homodyne mixing of its modes was performed too. The emission frequency increases with current for both lasers and all modes. The analysis of the spectra shows up to 2% variation of the difference frequency due to changes of the laser current. The origins of these shifts are discussed.

We have used an optical self-mixing technique to measure the linewidth enhancement factor of a terahertz quantum cascade laser. We find that contrary to previous predictions of a zero linewidth enhancement factor, it has a value of ~0.5.

We report simulations and experimental results of THz QCLs with quasiperiodic resonators based on a Fibonacci sequence. Measurements of the linewidth enhancement factor in a THz QCL are also discussed.

In this paper, the frequency characteristics of a tera-hertz quantum-cascade laser (QCL) operating around 2.5-THz in the pulsed regime have been studied by means of a heterodyne detection by using an optically pumped molecular laser as the local oscillator. The absolute frequency of the QCL can be determined with an accuracy of 1 MHz due to the pulse-to-pulse jitter of the QCL. The frequency tunability as a function of the injection current of the QCL is estimated to be around +2 GHz/A.

For THz quantum cascade lasers to prove useful for applications, certain requirements for their spectral performance will have to be met. Here, we focus on the provision of single mode operation. Distributed feedback devices lasing on a single longitudinal mode are reported using both first and second order gratings. We also report the operation of terahertz master oscillator power amplifier structures with the potential to increase the output power which is available in a single mode.

We have implemented quantum cascade lasers (QCLs) operating at about 2.5 THz in a spectrometer for high resolution molecular spectroscopy. One QCL has a Fabry-Perot resonator while the other is a distributed feedback laser. Linewidth and frequency tunability of both QCLs were investigated by mixing the radiation from the QCL with that from a 2.5 THz gas laser. Both were found sufficient for Doppler limited spectroscopy. Rotational transitions of methanol were detected in absorption using a QCL as radiation source. Amplitude as well as frequency modulation of the output power of the QCL were used. The absolute frequency was determined simultaneously with the absorption signal by mixing a small part of the radiation from the QCL with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519 THz were measured in order to demonstrate the performance of the spectrometer.

An anti-reflection coated 4.7 terahertz quantum cascade laser coupled to an external cavity formed by a single moving mirror frequency tunes up to 4 wavenumbers with mode hops and around 0.4 wavenumbers without mode hops.

Tunable quantum cascade lasers operating in the terahertz frequency range are demonstrated. By using an external cavity based on the reflection from a movable mirror, both broad and fine tuning of the frequency are achieved by varying the cavity length. Coarse tuning up to 3 cm(-1) is obtained near the center frequency of 4.8 THz (similar to 160 cm(-1)), and continuous mode-hop-free tuning is observed over similar to 0.4 cm(-1), nearly corresponding to the cavity free spectral range. (c) 2007 American Institute of Physics.

We have performed high resolution spectroscopy of rotational lines of 12CH3 16OH in a THz spectrometer based on a 2.5 THz distributed feedback quantum cascade laser (QCL). The absolute frequency was determined simultaneously with the absorption measurement by mixing part of the radiation from the QCL with the radiation from a THz gas laser in a Schottky diode.

The quantum cascade laser is a powerful, narrow linewidth, and continuous wave source of terahertz radiation. The authors have implemented a distributed feedback device in a spectrometer for high-resolution gas phase spectroscopy. Amplitude as well as frequency modulation schemes have been realized. The absolute frequency was determined by mixing the radiation from the quantum cascade laser with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519 THz were measured in order to demonstrate the performance of the spectrometer. (c) 2006 American Institute of Physics.

Quantum cascade lasers (QCLs) operating at 2.5 THz have been used for gas phase spectroscopy and as local oscillator in a heterodyne receiver. One QCL has a Fabry-Perot resonator while the other has a distributed feedback resonator. The linewidth and frequency tunability of both QCLs have been investigated by either mixing two modes of the QCL or by mixing the emission from the QCL with the emission from a 2.5 THz gas laser. The frequency tunability as well as the linewidth is sufficient for Doppler limited spectroscopy of methanol gas. The QCLs have been used successfully as local oscillators in a heterodyne receiver. Noise temperature measurements with a hot electron bolometer and a QCL yielded the same result as with a gas laser as local oscillator.

Quantum cascade (QC) lasers operating at terahertz frequencies were demonstrated two and a half years ago, and, since then, their development has proceeded at a very rapid pace. Most of the research has focused on new concepts for the quantum design of the gain medium as well as on improving device structure by optimizing fabrication and waveguide technology. These efforts by various groups have led to maximum operating temperatures of about 140 K in pulsed mode, output powers of up to 50 mW, and lasing in continuous wave up to 93 K. Such advances are making THz QC lasers more and more appealing for applications in various fields like chemical sensing, astronomy, spectroscopy, and imaging. For their successful implementation, specific requirements have to be addressed, particularly concerning the spectral properties of the emission. Here we report some latest developments in this direction. We demonstrate the perfect control of laser design for the realization of devices with precisely defined emission frequency in the whole range from 2.3 THz to 4.8 THz. Additionally, single-mode THz lasers with distributed feedback resonators have been achieved and a new technique involving surface plasmon gratings has been developed to improve performances. The latter offers also the possibility of constructing distributed Bragg gratings as a replacement for high-reflection coatings. Finally, solutions allowing broad tuneability are examined, with preliminary results illustrating the viability of external cavity set-ups.

A periodic array of thin slits opened on a metallic surface can act as a one-dimensional photonic crystal for the propagation of surface-plasmon waves. We have used such structure for the implementation of distributed feedback resonators in quantum cascade lasers emitting near 2.5 THz. Single-mode emission, stable at all injection currents and operating temperatures, was achieved both in pulsed and continuous wave. The devices exhibited output powers of several milliwatts with low threshold current densities of similar to 100 A/cm(2). (C) 2005 American Institute of Physics.

Terahertz quantum cascade lasers have been investigated with respect to their performance as a local oscillator in a heterodyne receiver. The beam profile has been measured and transformed in to a close to Gaussian profile resulting in a good matching between the field patterns of the quantum cascade laser and the antenna of a superconducting hot electron bolometric mixer. Noise temperature measurements with the hot electron bolometer and a 2.5 THz quantum cascade laser yielded the same result as with a gas laser as local oscillator. (C) 2005 Optical Society of America.

Distributed feedback terahertz quantum-cascade lasers emitting at 4.34 and 4.43 THz are presented. Mode selection is based on a complex-coupling scheme implemented into the top-contact layer by a combination of wet chemical etching and ohmic-contact deposition. Single-mode emission stable at all injection currents and operating temperatures is shown, with a side-mode suppression ratio exceeding 20 dB. Peak output powers of up to 1.8 mW are obtained at low temperatures. (C) 2004 American Institute of Physics.

We present a surface emitting distributed feedback terahertz laser resonator based on a dual-slit configuration. With the spacing of the slits as a parameter, the surface losses of the lasers can be chosen over a large range, while keeping slits narrow for optimum current injection. Quantum cascade devices based on a double metal waveguide showed slope efficiencies up to 40 mW/A and peak powers of more than 20 mW. The efficient radiation out-coupling does not depend on boundary conditions, and therefore would allow the application to large-area devices, making this an ideal resonator for high-power surface emission.

A superconducting bolometer with an on-chip lithographic terahertz antenna has been illuminated by two quantum cascade lasers operating at 2.5 and 4.4 THz. The detector displays a 1.2 mu s time constant, a noise equivalent power of 20 fW/Hz(1/2) and a 60 dB dynamic range. We fabricated a monolithic prototype detector array of five elements. This scalable detector is a suitable candidate for terahertz spectroscopic imaging systems, as it can measure both full illuminator power and strongly attenuated or diffuse reflected signals in subsequent frames.

We report on the experimental observation of non-equilibrium longitudinal and transverse optical phonons populations associated with electron transport in resonant-phonon THz quantum-cascade lasers and compare the results with the outcome of Monte Carlo simulations.

Terahertz quantum cascade lasers (THz QCLs) were first demonstrated in 2002 [1] and have since undergone rapid development [2]. One of the most active directions of research has been the development of devices emitting in a single longitudinal mode through the use of distributed feedback (DFB) resonators. Because the optical mode in these devices arises through a surface plasmon, it is possible to implement them with periodically arranged slits in the top metallization. This has been demonstrated for both single- and double-metal waveguides [3–5]. More recently, such gratings have been applied to microdisk lasers [6], where they lead to a circular far-field in the vertical direction, whose angular spread depends on the ratio of wavelength and device diameter. To this end, we have modeled and fabricated THz QCLs in a circular double-metal DFB resonator (see Fig 1a) of much larger diameter, for which we compute the far-field shown in the inset. Devices are designed using a finite element solver. A second order grating with a double slit configuration was found to offer the best compromise between electrical and optical properties of the top electrode. Fig. 1b) shows an L-I characteristics of a fabricated device. Even without collection optics employed, the devices show peak power up to 10 mW, with a slope efficiency of ∼25mW/A, demonstrating the good radiative efficiency of this resonator and directionality of the emitted power. The inset of Fig. 1b) shows the spectrum of the device. We observe singlemode emission with a side-mode suppression of more than 20 dB. Measured far-fields of the devices will also be discussed. We believe the current work will further simplify the integration of THz QCLs into possible applications.

A quantum cascade laser operating at 2.5 THz has been investigated with respect to performance parameters which are relevant for a local oscillator in a heterodyne receiver. The beam profile has been measured and transformed in to a close to Gaussian profile resulting in a good matching between the field patterns of the quantum cascade laser and the antenna of a superconducting hot electron bolometric mixer. Noise temperature measurements with the hot electron bolometer and a 2.5 THz quantum cascade laser yielded the same result as with a gas laser as local oscillator.