Single-Mode Surface-Emitting Terahertz Quantum-Cascade Lasers Operating up to ~ 150 K
ABSTRACT We report robust single-mode operation of surface-emitting distributed-feedback terahertz quantum-cascade lasers in metal-metal waveguides. Grating devices span a range of 0.35THz around 2.9THz, with 149K maximum pulsed operating temperature, and >6mW continuous-wave power at 5K.
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ABSTRACT: 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.Optics Express 07/2006; 14(12):5335-45. · 3.55 Impact Factor
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ABSTRACT: Semiconductor devices have become indispensable for generating electromagnetic radiation in everyday applications. Visible and infrared diode lasers are at the core of information technology, and at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications. But the terahertz region (1-10 THz; 1 THz = 10(12) Hz) between these ranges has remained largely underdeveloped, despite the identification of various possible applications--for example, chemical detection, astronomy and medical imaging. Progress in this area has been hampered by the lack of compact, low-consumption, solid-state terahertz sources. Here we report a monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure. The prototype demonstrated emits a single mode at 4.4 THz, and already shows high output powers of more than 2 mW with low threshold current densities of about a few hundred A cm(-2) up to 50 K. These results are very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems.Nature 06/2002; 417(6885):156-9. · 38.60 Impact Factor
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ABSTRACT: We report the demonstration of a terahertz quantum-cascade laser that operates up to 164 K in pulsed mode and 117 K in continuous-wave mode at approximately 3.0 THz. The active region was based on a resonant-phonon depopulation scheme and a metal-metal waveguide was used for modal confinement. Copper to copper thermocompression wafer bonding was used to fabricate the waveguide, which displayed improved thermal properties compared to a previous indium-gold bonding method.Optics Express 06/2005; 13(9):3331-9. · 3.55 Impact Factor
Single-Mode Surface-Emitting Terahertz Quantum-Cascade
Lasers Operating up to ∼ 150 K
Sushil Kumar, Benjamin S. Williams, Qi Qin, Alan W. M. Lee, Qing Hu
Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology,
Cambridge, MA 02139
PH: 617-253-2431, FAX: 617-258-7864, email: email@example.com
John L. Reno
Sandia National Laboratories, Center of Integrated Nanotechnologies, MS 1303, Albuquerque, NM 87185-1303
Abstract: We report robust single-mode operation of surface-emitting distributed-feedback tera-
hertz quantum-cascade lasers in metal-metal waveguides. Grating devices span a range of 0.35THz
around 2.9THz, with 149K maximum pulsed operating temperature, and >6mW continuous-wave
power at 5K.
c ? 2006 Optical Society of America
OCIS codes: (140.3070) Far-infrared lasers, (140.3490) Distributed-feedback lasers, (250.7270) Vertical emitting lasers
Terahertz quantum-cascade lasers (QCLs) were initially developed in the “semi-insulating surface-plasmon” (SISP)
waveguides , which are able to provide greater output power and tighter beam patterns. However, they are limited
in their temperature performance due to a higher waveguide loss, which arises due to a large fraction of the mode
that propagates in the substrate that can be lossy in the terahertz. The best terahertz QCLs in terms of high tempera-
ture operation have been demonstrated in the “metal-metal” (MM) waveguides [2, 3], which provide strong mode
confinement and low waveguide losses. However, due to the sub-wavelength localization in the vertical dimension,
the end-facet reflectivities are high (∼ 0.7–0.9) resulting in small output power, and the emitted beam is highly diver-
gent . Additionally, higher order modes in the width dimension are easily excited even for very narrow (< 50 µm)
In this paper, we present implementation of second-order distributed feedback (DFB) grating by having apertures
in the top-metal of the MM waveguides, to couple the laser beam out from the surface. The objective of the scheme
is to preserve the low waveguide-loss advantage of the metal-metal waveguides, but increase their output power
levels and improve the beam patterns. Due to sub-wavelength mode confinement in the vertical direction, there is a
large mode-mismatch at each grating step, which makes the DFB strongly-coupled. Consequently, the conventional
coupled-mode theory can not be applied for DFB design. Moreover, the mode behavior depends sensitively on the
location of the facets due to their high reflectivities. We have obtained robust single-mode operation and single-lobed
beam patterns in such structures, by implementing a combination of techniques including precise control of phase of
reflection at the facets, and use of metal on the sidewalls to eliminate higher-order lateral modes. Whereas surface-
emitting terahertz QCLs operating up to ∼ 45 K have been demonstrated in SISP waveguides , we have obtained
operation up to 149 K by the use of metal-metal DFB structures in combination with a resonant-phonon terahertz
QCL active-region .
Fig. 1(a) shows the schematic of the grating design. A finite sized rectangular waveguide is enclosed by metal on all
sides. A Λ/2 defect (where Λ is the grating period) is implemented in the center of the grating to achieve a single
lobed beam pattern for the desired grating mode. The end-length δ plays a critical role for correct grating operation
and only a limited range of values allow the desired grating mode to be excited. Detailed grating analysis was done
using finite-element simulations and a value of δ ∼ 3Λ/4 + 2 µm was finally chosen. To obtain outward sloped side-
walls for covering them with metal, a special wet-etching technique was developed with the etch mask of the mesas
aligned at 45◦angle to the cleave direction (i.e. along ?100?) on the (100) GaAs substrate. With this technique, out-
ward sloped sidewalls are obtained in all directions by wet-etching. Fig. 1(c) shows the continuous-wave (cw) spec-
tra from three different devices, each with a different grating period. Single-mode operation is obtained at all bias
conditions. For comparison, multi-mode cw spectrum from a Fabry-Perot ridge laser from the same die is shown in
Fig. 1(d). Maximum temperatures of pulsed operation for various devices are also indicated in the figure. Pulsed and
cw L-Is from the Λ = 30 µm device are shown in Fig. 2(a). This device lased up to 149 K in pulsed mode and up to
78 K in cw mode, emitting more than 6 mW of cw power at 5 K, which is about a factor of 2 greater than that from
edge emitting Fabry-Perot lasers. A thermal tuning of ∼ 20 GHz was obtained from 5 K to 147 K. The representative
beam pattern measured from the same device with a Ge:Ga photoconductor at a distance of ∼ 13 inches from the
device is also plotted. All the three grating devices showed a similar beam pattern.
2.4 2.5 2.6 2.7 2.8 2.93 3.1 3.2 3.3 3.4
2.4 2.5 2.6 2.7 2.8 2.933.1 3.2 3.3 3.4
125 120 115 110 105 100 95 90
Fig. 1. (a),(b) Grating schematic, and an SEM picture from near the facet. (c) Cw spectra measured from three different grating devices at different
bias. (d) Cw spectra from a Fabry-Perot ridge device from the same die.
0100 200 300 400 500 600 700 800 900
Current Density (A/cm2)
Optical Power (a.u.)
Optical Power (mW)
Fig. 2. (a) Pulsed and cw L-I curves measured from the Λ = 30 µm device. Pulsed spectra measured at 5 K and 147 K is shown in the inset. (b)
Beam pattern of the same device measured at a distance of ∼ 13 inches from the device. The FWHM in the longitudinal (z) direction is ∼ 5.1◦.
This work is supported by AFOSR, NASA, and NSF. Sandia is a multiprogram laboratory operated by Sandia
Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under Contract DE-AC04-94AL85000.
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semiconductor-heterostructure laser,” Nature 417, 156 (2002).
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