Ji-Hua Xu

Tunable terahertz quantum cascade lasers with an external cavity

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 3cm−1 is obtained near the center frequency of 4.8THz (∼160cm−1), and continuous mode-hop-free tuning is observed over ∼ 0.4cm−1, nearly corresponding to the cavity free spectral range.

Linewidth enhancement factor of terahertz quantum cascade lasers

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.

Surface plasmon photonic structures in terahertz quantum cascade lasers

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.

High efficiency coupling of Terahertz micro-ring quantum cascade lasers to the low-loss optical modes of hollow metallic waveguides

We demonstrate that azimuthally polarized surface emitting Terahertz quantum cascade lasers fabricated in a micro-ring resonator geometry can be coupled to cylindrical hollow aluminum waveguides reaching efficiencies as high ≈98%, when a collimating lens is used. By placing the waveguide in close contact with the QCL in a simple back-to-back geometry, the laser mode can be perfectly matched with the low loss TE(01) waveguide mode showing attenuation losses as low as ≈2.3-2.7 dB/m at 3.2 THz.

Characterization and modeling of quantum cascade lasers based on a photon-assisted tunneling transition

A detailed characterization and modeling of long-wavelength (/spl lambda//spl sim/10 /spl mu/m) quantum cascade (QC) lasers based on a photon-assisted tunneling transition are presented. In particular, the influence of the finite lifetime of the lower state of the laser transition on the current-voltage and threshold current versus temperature characteristics have been studied both theoretically and experimentally. It is shown that, for our structure, the value of the lower state lifetime can be extracted from the voltage-current curve; the value we found was 2.6 ps. In addition, this model allows to understand the abrupt degradation of the performance of the device for T>150 K. Low temperature (T=10 K) threshold current densities of 1.1 kA/cm/sup 2/ and a tuning range of 85 cm/sup -1/ in pulsed mode are reported. In continuous-wave mode, the emission linewidth of a free-running laser was determined to be 3.9 MHz.

Spectral behavior of a terahertz quantum-cascade laser

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.

Wide dynamic range terahertz detector pixel for active spectroscopic imaging with quantum cascade lasers

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 μs time constant, a noise equivalent power of 20 fW/Hz1/2 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.

Guiding a terahertz quantum cascade laser into a flexible silver-coated waveguide

We report on a promising experimental approach to efficiently couple quantum cascade laser (QCL) sources fabricated in either surface emitting micro-ring resonator or standard edge emitting geometry with terahertz (THz) hollow waveguides. We show that the THz beam of a QCL can be guided into flexible silver-coated polycarbonate waveguides having length in the range 4–12 cm with coupling efficiencies >80%. The dominant optical modes propagating through the waveguides can be selected by varying the polarization of the incoming QCL beam, the coupling geometries, and/or bending the hollow waveguide. Either the lowest loss TE11 or the TE01 mode can be selected and guided through the waveguide with propagation losses in the range 2.1–4.4 dB/m and bending losses lower than 1.2 dB.

Advances in THz quantum cascade lasers: fulfilling the application potential

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.

Terahertz confocal microscopy with a quantum cascade laser source

We report the realization of a confocal THz microscope based on a ~ 3 THz quantum cascade laser source. A resolution as low as 67 μm is achieved, with a large contrast enhancement allowed by the confocal geometry. The capability of imaging overlayed objects on close-by planes is also demonstrated.