Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions

Adam, A.J.L.; Kašalynas, Irmantas; Hovenier, J. N.; Klaassen, T.O.; Gao, J. R.; Orlova, E. E.; Williams, Benjamin S.; Kumar, Sushil; Hu, Qing; Reno, John L.

doi:10.1063/1.2194889

Cited by 113 publications

(57 citation statements)

References 12 publications

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“…It requires low LO power (< 300 nW). This turns out to be crucial for our phase-locking experiment because the QCL has a divergent far-field beam with strong interference patterns [18], resulting in a limited amount of usable power coupled into the mixer. Gunn diode and YIG oscillators).…”

Section: Methodsmentioning

confidence: 99%

Phase-locking of a 2.7-THz quantum cascade laser to a mode-locked erbium-doped fibre laser

et al. 2010

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Abstract-We demonstrate phase-locking of a 2.7-THz metalmetal 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 (x2x3x2) from a microwave synthesizer at 15 GHz. Both laser and reference radiations are coupled into a hot electron bolometer mixer, resulting in a beat signal, which is fed into a phase-lock loop. Spectral analysis of the beat signal (see fig. 1) confirms that the QCL is phase locked. This result opens the possibility to extend heterodyne interferometers into the far-infrared range.

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Section: Methodsmentioning

confidence: 99%

Phase-locking of a 2.7-THz quantum cascade laser to a mode-locked erbium-doped fibre laser

et al. 2010

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“…However, for typically used THz QCL waveguides the output aperture is on the order of 10 µm, while the emission wavelength is about 100 µm. This, in fact, leads to a very divergent output beam, which is additionally distorted by interference effects [10,11]. Since external optical elements such as lenses or antennas are difficult to handle on a large scale [12,13], several monolithic concepts have been pursued to address this issue.…”

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confidence: 99%

Random lasers for broadband directional emission

Schönhuber

Brandstetter

Hisch

et al. 2016

Optica

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Broadband coherent light sources are becoming increasingly important for sensing and spectroscopic applications, especially in the mid-infrared and terahertz (THz) spectral regions, where the unique absorption characteristics of a whole host of molecules are located. The desire to miniaturize such light emitters has recently lead to spectacular advances with compact on-chip lasers that cover both of these spectral regions. The long wavelength and the small size of the sources result in a strongly diverging laser beam that is difficult to focus on the target that one aims to perform spectroscopy with. Here, we introduce an unconventional solution to this vexing problem relying on a random laser to produce coherent broadband THz radiation as well as an almost diffraction limited far-field emission profile. Our random lasers do not require any fine-tuning and thus constitute a promising example of practical device applications for random lasing.Various spectroscopic techniques rely on the specific absorption features of numerous molecules within the midinfrared and terahertz (THz) spectral regions, which allow their unambiguous identification. Although broadband coherent sources of radiation are already available within these frequency regions [1][2][3][4][5], a compact size and electrically pumped operation are additionally desired for actual applications. These criteria are fulfilled by quantum cascade lasers (QCLs), semiconductor sources which are able to provide broadband emission at mid-infrared [6,7] and THz [8,9] frequencies. However, for typically used THz QCL waveguides the output aperture is on the order of 10 µm, while the emission wavelength is about 100 µm. This, in fact, leads to a very divergent output beam, which is additionally distorted by interference effects [10,11]. Since external optical elements such as lenses or antennas are difficult to handle on a large scale [12,13], several monolithic concepts have been pursued to address this issue. To improve the far-field for facet emission, e.g., a 3rd order distributed feedback (DFB) grating can be used [14]. Another approach is to coherently emit from a large area on the laser surface. In contrast to interband semiconductor lasers, QCLs can only generate in-plane radiation due to intersubband selection rules, preventing the realization of VCSELtype resonators. Thus several concepts have been developed to couple out the in-plane cavity mode in vertical direction, including 2nd order DFB gratings [15,16] , and photonic crystal (PhC) cavities [17]. Recently, also non-periodic resonator structures such as graded photonic heterostructures [18], or quasicrystal cavities [19,20] have been developed. However, all on-chip techniques providing surface emission demonstrated so far are designed to support a single laser mode only, while for many spectroscopic applications a broadband coherent light source is necessary. To achieve the objective of a broadband and at the same time very collimated laser light emission in the THz regime, we propose here a radical ...

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“…The spatial structure of wire laser radiation differs drastically from that of conventional lasers with large apertures. It was found to be highly divergent, with strong intensity modulations observed both in far [4] and near field [5], with the pattern of modulations more dense for longer lasers. It was shown that the far-field pattern of wire lasers is formed by the interference of radiation from the longitudinal distribution of sources along the laser waveguide [6] and is similar to that of antennas of traveling wave.…”

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confidence: 99%

Image beam from a wire laser

et al. 2015

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We demonstrate the formation of a narrow beam from a long (L λ) laser with subwavelength transverse dimensions (wire laser) as an image of the subwavelength laser waveguide formed by a spherical lens. The beam is linearly diverging with the angle determined by the ratio of the wavelength to the lens radius, while the minimum beam spot size is the same as that of the image of a point source. We realize such a beam experimentally using a terahertz quantum cascade wire laser. , and multi-quantum-well structures [3] have a wire geometry with transverse dimensions smaller than the emission wavelength and a length much larger than the wavelength. The spatial structure of wire laser radiation differs drastically from that of conventional lasers with large apertures. It was found to be highly divergent, with strong intensity modulations observed both in far [4] and near field [5], with the pattern of modulations more dense for longer lasers. It was shown that the far-field pattern of wire lasers is formed by the interference of radiation from the longitudinal distribution of sources along the laser waveguide [6] and is similar to that of antennas of traveling wave. Thus it depends strongly on the longitudinal phase velocity of the waveguide mode. Concentration of radiation into a narrow, axially symmetric beam along the laser axis with the divergence determined by the ratio of the wavelength to the laser length has been predicted by the antenna model for the modes of wire lasers with longitudinal phase velocity close to that of light in air, where all the sources along the laser emit in phase in the direction of the longitudinal axis. However, realization of such modes is hindered due to their low confinement and high radiation losses. Directive emission from wire lasers has been achieved using gratings with appropriate periodicity, acting as a discrete array of phased sources along the laser waveguide [7][8][9][10]. However, this approach not only implies a complicated waveguide design, but also leads to an increase of radiation losses; thus it is not always possible. Transformation of radiation with external optical elements is free from these complications. Beam shaping is a wellestablished technique for lasers with transverse dimensions much bigger than the wavelength [11]. Lenses have been used to collimate the radiation from wire lasers [12] and to provide their optical images [5,13], yet little is known about peculiarities of external transformation of wire laser radiation. In this Rapid Communication we analyze the structure of the beam formed as an image of a wire laser placed on the axis of a spherical lens behind the focal plane (Fig. 1). According to geometrical theory of optical images, the image of the laser is stretched along the lens axis. The length of the image increases with reduction of the distance between the lens and the laser, and the image becomes semi-infinite when the end of the laser touches the focal plane. The distribution of the radiation field in the vicinity of the image of a wire laser ...

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Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions

Cited by 113 publications

References 12 publications

Phase-locking of a 2.7-THz quantum cascade laser to a mode-locked erbium-doped fibre laser

Phase-locking of a 2.7-THz quantum cascade laser to a mode-locked erbium-doped fibre laser

Random lasers for broadband directional emission

Image beam from a wire laser

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