J. Palomo

Room-temperature operation of λ≈7.5μm surface-plasmon quantum cascade lasers

We report the pulsed, room-temperature operation of λ≈7.5μm quantum cascade lasers (QCLs) in which the optical mode is a surface-plasmon polariton excitation. Previously reported devices based on this concept operate at cryogenic temperatures only. The use of a silver-based electrical contact with reduced optical losses at the QCL emission wavelength allows a reduction of the laser threshold current by a factor of 2 relative to samples with a gold-based contact layer. As a consequence, the devices exhibit room-temperature operation with threshold current densities ∼6.3kA∕cm2. These devices could be used as all-electrical surface-plasmon generators at midinfrared wavelengths.

Optical Mode Control of Surface-Plasmon Quantum Cascade Lasers

Surface-plasmon waveguides based on metallic strips can provide a two-dimensional optical confinement. This concept has been successfully applied to midinfrared quantum cascade lasers, processed as ridge waveguides, to demonstrate that the lateral extension of the optical mode can be influenced solely by the width of the device top contact. In this configuration, the waveguide mode has a reduced interaction with the top metal and the ridge sidewalls. This results in lower propagation losses and higher performances. For devices operating at a wavelength of lambdaap7.5 mum, the room-temperature threshold current density was reduced from 6.3 to 4.4 kA/cm2 with respect to larger devices with full top metallization

Echo-Less Photoconductive Antenna Sources for High-Resolution Terahertz Time-Domain Spectroscopy

Interdigitated photoconductive antennas are powerful and easy-to-use sources of terahertz radiation for time-resolved spectroscopy. However, the emission of unwanted echoes, resulting from reflections of the emitted pulse in the antenna substrate, inherently limits the spectroscopic frequency resolution. A novel interdigitated photoconductive antenna that suppresses unwanted echoes from the substrate, without power losses, is proposed and demonstrated. This is realized through a buried metal geometry where a metal plane is placed at a sub-wavelength thickness below the surface antenna structure and GaAs active layer. In a reflection geometry this effectively eliminates echoes, permitting high resolution spectroscopy to be performed. As a proof-of-principle, the 1 01 -2 12 and the 2 12 -3 03 rotational lines of water vapor have been spectrally resolved with the new buried metal antenna, which are unresolvable with a standard antenna. In addition, as no THz field is lost to the substrate and reflections, the THz peak electric field amplitude is enhanced by a factor of three compared to a standard design in the equivalent reflection geometry.

Diffraction-limited ultrabroadband terahertz spectroscopy

Diffraction is the ultimate limit at which details of objects can be resolved in conventional optical spectroscopy and imaging systems. In the THz spectral range, spectroscopy systems increasingly rely on ultra-broadband radiation (extending over more 5 octaves) making a great challenge to reach resolution limited by diffraction. Here, we propose an original easy-to-implement wavefront manipulation concept to achieve ultrabroadband THz spectroscopy system with diffraction-limited resolution. Applying this concept to a large-area photoconductive emitter, we demonstrate diffraction-limited ultra-broadband spectroscopy system up to 14.5 THz with a dynamic range of 103. The strong focusing of ultrabroadband THz radiation provided by our approach is essential for investigating single micrometer-scale objects such as graphene flakes or living cells, and besides for achieving intense ultra-broadband THz electric fields.

Ultra-low threshold THz microcavity lasers with sub-wavelength mode volumes

We demonstrate terahertz microcavity lasers at lambda=112 mum with ultra-low current thresholds of 4 mA and with mode volumes of less than one-cubic-wavelength. Confinement in the longitudinal direction is obtained using almost-circular micro-disk resonators. The guiding properties of surface-plasmons are exploited to guide the mode with the metal contact. The devices laser up to 70 K in pulsed mode, and up to 60 K in continuous-wave.

THz surface plasmon polariton modes coupled to complementary metasurfaces tuned by inter meta-atom distance

Tailoring the electro-magnetic response of materials beyond naturally occurring properties is possible with the concept of meta-materials [1]. Subwavelength elements which are usually closely spaced can influence the electro-magnetic response and form a fundamental building block of modern optics. The influence of the spacing of the meta-atoms has been investigated for direct meta-materials [2] but only little for complementary metamaterials [3], which are of interest e.g. in ultra-strong coupling experiments at THz frequencies [4]. The effective medium condition is changing due to the presence of a metal sheet in between the meta-atoms which has a very high refractive index in the THz.

Monolithic echo-less photoconductive switches as a high-resolution detector for terahertz time-domain spectroscopy

Interdigitated photoconductive (iPC) switches are powerful and convenient devices for time-resolved spectroscopy, with the ability to operate both as sources and detectors of terahertz (THz) frequency pulses. However, reflection of the emitted or detected radiation within the device substrate itself can lead to echoes that inherently limit the spectroscopic resolution achievable for their use in time-domain spectroscopy (TDS) systems. In this work, we demonstrate a design of low-temperature-grown-GaAs (LT-GaAs) iPC switches for THz pulse detection that suppresses such unwanted echoes. This is realized through the growth of a buried multilayer LT-GaAs structure that retains its ultrafast properties, which, after wafer bonding to a metal-coated host substrate, results in an iPC switch with a metal plane buried at a subwavelength depth below the LT-GaAs surface. Using this device as a detector, and coupling it to an echo-less iPC source, enables echo-free THz-TDS and high-resolution spectroscopy, with a resolution limited only by the temporal length of the measurement governed by the mechanical delay line used. As a proof-of-principle, the 212-221 and the 101-212 rotational lines of water vapor have been spectrally resolved, demonstrating a spectral resolution below 10 GHz.

Echo-less Photoconductive Antenna sources for High-resolution Terahertz Time-domain Spectroscopy

A novel terahertz photoconductive antenna that totally suppresses unwanted echoes is demonstrated. This is achieved with a buried metal plane placed below the surface antenna structure and active layer, realizing a sub-wavelength cavity.

THz microcavity lasers with sub-wavelength mode volumes and thresholds in the milli-Ampere range

THz quantum cascade (QC) lasers with an active-region thickness of 5.86 mum was demonstrated. The current threshold was then reduced by reducing the device surface. The laser resonator is based on a micro-cylindrical geometry. In general micro-cylindrical resonators rely on total-internal reflection on the cavity sidewalls in order to confine the light. The small (r=25 mum) and medium (r=37.5 mum) microdisk lasers show a single mode emission. All the lasers operate in continuous-wave mode up to 60 K, and in pulsed mode up to 70 K.

Optical mode control of surface‐plasmon quantum cascade lasers

Surface‐plasmon waveguides based on metallic strips can provide a two dimensional optical confinement. This concept has been successfully applied to quantum cascade lasers, processed as ridge waveguides, to demonstrate that the lateral extension of the optical mode can be influenced solely by the width of the device top contact. For devices operating at a wavelength of λ ≈7.5 μm, the room‐temperature threshold current density was reduced from 6.3 kA/cm2 to 4.4 kA/cm2 with respect to larger devices with full top metallization.