Christoph Walther

High-power surface emission from terahertz distributed feedback lasers with a dual-slit unit cell

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.

Strong light-matter coupling at terahertz frequencies at room temperature in electronic LC resonators

Strong light-matter coupling at room temperature in the terahertz (THz) range is demonstrated. The studied system consists of electronic intersubband transitions in AlxGa1−xAs parabolic quantum wells coupled to an electronic LC microcavity resonator allowing strong subwavelength confinement of the cavity mode. The measured Rabi frequency is 0.48 THz, corresponding to 14% of the center frequency, independent of the temperature of the system in the range 10–300 K.

Step well quantum cascade laser emitting at 3 THz

A resonant-phonon THz quantum cascade laser based on Al<inf>0.03</inf>Ga<inf>0.097</inf>As step well is demonstrated. Maximum pulsed operating temperature is 123 K, with threshold current densities as low as 110 A/cm² at 10 K and 175 A/cm² at 100 K.

Finite size effects in surface emitting Terahertz quantum cascade lasers.

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.

Purcell effect in the inductor-capacitor laser

The recently demonstrated inductor-capacitor (LC) laser has a strongly subwavelength mode volume. In this Letter we investigate the Purcell effect in the LC laser resonator. An average Purcell factor of 17 is computed for the LC laser. The laser rate equations are formulated and solved for the LC laser. Evidence of a large Purcell factor in agreement with the theoretical one is obtained through comparison of the threshold and the emission characteristics of the LC laser with the rate equations.

Scattering processes in terahertz InGaAs/InAlAs quantum cascade lasers

The Quantum cascade laser (QCL) is a compact source of coherent radiation in the terahertz spectral region (FIR). A possible solution for improvement of the maximum operating temperature is use of the InGaAs/InAlAs/InP (referred as InP based) material system instead of GaAs/AlGaAs. This material has a lower electron effective mass that leads to a larger matrix dipole element and larger optical gain for a similar structure.

Hot electron effects and nanoscale heat transfer in Terahertz quantum cascade lasers

We show that the internal quantum efficiency and the wall-plug efficiency of quantum cascade lasers (QCL) are correlated with the hot-electron cooling associated with photon emission. The experimental procedure for the assessment of these key device parameters is based on micro-probe photoluminescence (PL) that allows high resolution measurements of the electronic and local lattice temperatures in operating QCLs. By using a terahertz QCL as a prototype we demonstrate that the electronic distributions are Fermi-Dirac functions characterized by temperatures significantly larger than the lattice one. The lattice temperature is in turn well above the one of the heat sink bath. Combining the above observation with time-resolved PL experiments we assessed the characteristic time constants controlling the heating and cooling processes of terahertz QCLs that are limited by the presence of a high density of interfaces that causes phonon interference effects. The correlation between the above constants, the thermal diffusivities and the diffusion lengths have been extracted from the comparison with the outcome of a transient heat diffusion model.

THz Intersubband Polaritons in LC Resonator Structures

Strong light matter coupling in the THz spectral range is demonstrated. A material excitation is coupled to a novel electronic feedback microcavity, which allows strong subwavelength confinement of the cavity mode and tunability via change of the electrical size of the components. Intersubband transitions in conventional square quantum wells as well as parabolically graded quantum wells are used as material excitations. With the square wells, a splitting of 0.94 THz was found at cryogenic temperatures. The parabolic quantum well samples show a splitting of 0.96 THz up to room temperature.

Recent progress in THz quantum cascade lasers

Recent progress in the design and realization of quantum cascade lasers emitting at THz frequencies will be discussed. High temperature and high power operation are reported and broadband emitters are discussed. THz lasers oscillating in micro-resonators with extremely small modal volumes will be also presented.

Distributed feedback ring resonators for vertically emitting terahertz quantum cascade lasers

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.