Gain recovery dynamics and photon-driven transport in quantum cascade lasers.

Quantum cascade lasers are semiconductor devices based on the interplay of perpendicular transport through the heterostructure and the intracavity lasing field. We employ femtosecond time-resolved pump-probe measurements to investigate the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery is determined by the time-dependent transport of electrons through both the active regions and the superlattice regions connecting them. As the laser approaches and exceeds threshold, the component of the gain recovery due to the nonzero lifetime of the upper lasing state in the active region shows a dramatic reduction due to the onset of quantum stimulated emission; the drift of the electrons is thus driven by the cavity photon density. The gain recovery is qualitatively different from that in conventional lasers due to the superlattice transport in the cascade.

Terahertz quantum cascade lasers based on two-dimensional photonic crystal resonators.

We demonstrate high spectral control from surface emitting THz Quantum Cascade Lasers based on a two-dimensional photonic crystal cavity. The perforated top metallic contact acts as an in plane resonator in a tight double-metal plasmonic waveguide providing a strong optical feedback without needing three-dimensional cavity features. The optical far-field patterns do not exhibit the expected symmetry and the shape of the cavity mode. The difference is attributed to a metal surface plasmon mediated light outcoupling mechanism also responsible for the relatively low extraction efficiency.

Room-temperature continuous-wave operation of an external-cavity quantum cascade laser.

Room-temperature, continuous-wave operation of an external-cavity quantum cascade laser (EC-QCL) is reported. Single-mode tuning range of 120 cm(-1) was achieved, from 7.96 to 8.84 microm. The gain chips utilized are based on the bound to continuum design and were fabricated as buried heterostructure lasers. Gap-free tuning (mode hops only on the external-cavity modes) is demonstrated for an antireflection-coated laser, just by grating rotation. The EC-QCL was implemented in a Littrow setup and an average power of 1.5 mW was obtained at 20 degrees C, while a peak power of 20 mW was obtained for a modified Littrow setup with the back extraction of light.

Design and fabrication of photonic crystal quantum cascade lasers for optofluidics.

We present novel designs and demonstrate a fabrication platform for electrically driven lasers based on high quality-factor photonic crystal cavities realized in mid-infrared quantum cascade laser material. The structures are based on deep-etched ridges with their sides perforated with photonic crystal lattice, using focused ion beam milling. In this way, a photonic gap is opened for the emitted TM polarized light. Detailed modeling and optimization of the optical properties of the lasers are presented, and their application in optofluidics is investigated. Porous photonic crystal quantum cascade lasers have potential for on-chip, intracavity chemical and biological sensing in fluids using mid infrared spectroscopy. These lasers can also be frequency tuned over a large spectral range by introducing transparent liquid in the photonic crystal holes.

Fabrication and characterization of molecular beam epitaxy grown thin-film GaAs waveguides for mid-infrared evanescent field chemical sensing.

Thin-film GaAs waveguides were designed and fabricated by molecular beam epitaxy for use in mid-infrared (MIR) evanescent field liquid sensing. Waveguides were designed to facilitate the propagation of a single mode at a wavelength of 10.3 microm emitted from a distributed feedback quantum cascade laser, which overlaps with molecular selective absorption features of acetic anhydride. The characterization of the waveguides shows transmission across a broad MIR band. Evanescent field absorption measurements indicate a significant sensitivity enhancement in contrast to multimode planar silver halide waveguides.

Characterization of a near-room-temperature, continuous-wave quantum cascade laser for long-term, unattended monitoring of nitric oxide in the atmosphere.

We report on power, spectral linewidth, and mode purity for a cw 5.3 microm quantum cascade laser operated on a thermo-electric cooler. A totally noncryogenic nitric oxide monitor was constructed by integrating this laser with an astigmatic multipass cell and a thermo-electrically cooled infrared detector. The resulting instrument is capable of continuous unattended monitoring of ambient, atmospheric nitric oxide for several weeks with no operator intervention. The detection method was rapid sweep, direct absorption spectroscopy. A detection sensitivity of 0.03 parts in 10(9) is achieved with 30 s averaging time with a path length of 210 m, corresponding to an absorbance path length product of 1.5 x 10(-10) cm(-1).

Mid-infrared single-photon counting.

We report a procedure to detect mid-infrared single photons at 4.65 microm by means of a two-stage scheme based on sum-frequency generation, by using a periodically poled lithium niobate nonlinear crystal and a silicon avalanche photodiode. An experimental investigation shows that, in addition to a high timing resolution, this technique yields a detection sensitivity of 1.24 pW with 63 mW of net pump power.

Turn-key compact high temperature terahertz quantum cascade lasers: imaging and room temperature detection.

Terahertz quantum cascade lasers have been investigated as a turn-key terahertz source for widespread applications. Two lasers were mounted in a small liquid nitrogen-cooled dewar and combined with a sophisticated pulse driver. We present a detailed analysis in respect to current-voltage characteristics, emission wavelengths, polarization, pulse lengths and repetition rates. We have measured the laser power with a germanium photoconductor and compared the results to a Golay detector evaluating potential artifacts. We have studied mode profiles in the far-field which mirror the internal mode structure. Potential applications have been illustrated by imaging optical elements and a simple test object. Video rate room temperature imaging has been demonstrated in concept.

Continuous-wave operation of a broadly tunable thermoelectrically cooled external cavity quantum-cascade laser.

Continuous-wave operation of an external cavity quantum-cascade laser on a thermoelectric cooler is reported. The active region of the gain element was based on a bound-to-continuum design emitting near 5.15 microm. The external cavity setup was arranged in a Littrow configuration. The front facet of the gain chip was antireflection coated. The laser could be tuned over more than 170 cm(-1) from 4.94 to 5.4 microm and was single mode over more than 140 cm(-1). The output power was in excess of 10 mW over approximately 100 cm(-1) and in excess of 5 mW over approximately 130 cm(-1) at -30 degrees C.

Imaging at 3.4 Thz with a quantum-cascade laser.

We have assembled a single-frequency imaging system at 3.4 THz with a quantum-cascade laser. Images of electronic and biological applications are demonstrated. We operate the laser with a peak output power of 2.5 mW at a 7% duty cycle and a 22 K operating temperature. The minimum spot size is 340 microm. The signal is detected with a single-element deuterated triglycine sulfate detector, and images are captured by scanning of the sample.

Design, fabrication and optical characterization of quantum cascade lasers at terahertz frequencies using photonic crystal reflectors.

We designed, fabricated and characterised electrically injected quantum cascade lasers with photonic crystal reflectors emitting at terahertz frequencies (3.75 THz). These in-plane emitting structures display typical threshold current densities of 420 A/cm2 and output powers of up to 2 mW under pulsed excitation. The emission characteristics are shown to be robust, as with increasing current the emission remains singlemode with no drift in wavelength, this results from the narrow reflectivity band of the photonic crystal reflectors.

Terahertz quantum cascade lasers.

Recent developments in terahertz quantum cascade lasers are reviewed. Structures operating from a wavelength of lambda = 66 microm down to lambda = 87 microm are demonstrated. These devices used either a three-quantum-well chirped-superlattice active region or an active region based on a bound-to-continuum transition. The comparison between structures grown in a waveguide based on a single interface plasmon and a buried contact and (non-lasing) structures using a double plasmon waveguide demonstrates the importance of waveguide design on the operation of such devices. Continuous-wave operation up to a maximum temperature of 55 K with up to 15 mW output power at 10 K was demonstrated.

Imaging with a Terahertz quantum cascade laser.

We demonstrate bio-medical imaging using a Terahertz quantum cascade laser. This new optoelectronic source of coherent Terahertz radiation allows building a compact imaging system with a large dynamic range and high spatial resolution. We obtain images of a rat brain section at 3.4 THz. Distinct regions of brain tissue rich in fat, proteins, and fluid-filled cavities are resolved showing the high contrast of Terahertz radiation for biological tissue. These results suggest that continuous-wave Terahertz imaging with a carefully chosen wavelength can provide valuable data on samples of biological origin; these data appear complementary to those obtained from white-light images.