Peter Q. Liu

Importance of interface roughness induced intersubband scattering in mid-infrared quantum cascade lasers

The electron transit time of many different quantum cascade lasers has been measured and compared to the calculated upper laser level lifetimes with and without taking into account interface roughness induced intersubband scattering. A significantly better correlation is found between the experimental results and the calculation when including the contribution from the interface roughness (corr. coeff.: 0.79 vs. 0.43 with and without the consideration of interface roughness, respectively). This suggests that in addition to longitudinal optical phonons, interface roughness is also crucial in determining the intersubband lifetimes in mid-infrared quantum cascade laser and should routinely be included in design.

Quantum cascade laser open-path system for remote sensing of trace gases in Beijing, China

Exploiting several key characteristics of quantum cascade (QC) lasers, including wide tunability and room- temperature operation, the Quantum Cascade Laser Open- Path System (QCLOPS) was designed for the detection of a range of trace gases and for field deployment in urban environments. Tunability over a wavelength range from 9.3 to 9.8 μm potentially provides the capability for monitoring ozone, ammonia, and carbon dioxide, a suite of trace gases important for air quality and regional climate applications in urban environments. The 2008 Olympic Games in Beijing, China drew attention to air quality problems in urban environ- ments. Prior to and during the Olympic games, regional air quality modifications through factory shutdowns, car restric- tions, and construction halts in Beijing and its surrounding areas created a unique test bed for new sensor technolo- gies such as the QCLOPS sensor. We report the design of this novel, open-path air quality sensor and the results of both laboratory tests and field trials during the 2008 Olympic Games in Beijing, China. C 2010 Society of Photo-Optical Instrumen-

Single-mode quantum cascade lasers based on a folded Fabry-Perot cavity

We demonstrate single-mode quantum cascade lasers employing a folded Fabry-Perot cavity consisting of two straight sections connected by a semicircular section in a “hairpin” shape. These folded cavity lasers emitting at ∼4.5 μm are fabricated with identical processes as those for plain Fabry-Perot ridge lasers, and show a strong suppression of the comb of Fabry-Perot cavity modes, leading to tunable single-mode emission with up to 27 dB side mode suppression ratio and a single-mode operating current range of up to 60% above the threshold current when operated in pulsed mode; single-mode emission is achieved from 80 to ∼240 K.

Femtosecond Carrier Dynamics and Nonlinear Effects in Quantum Cascade Lasers

Quantum cascade lasers (QCLs) are complicated unipolar semiconductor devices based on intersubband transitions and resonant tunneling. In this study, femtosecond mid-infrared (Mid-IR) pulses are employed to investigate the nature of carrier transport through the active and injector regions of a room temperature, pulse biased ultrastrong coupling design QCL. Despite the low average power (<;1 mW) of femtosecond Mid-IR pulses, the efficient coupling of these pulses into the QCL waveguide made the study of nonlinear effects in QCLs possible. Biased just below threshold, we observed ultrafast gain recovery within the first 200 fs mainly contributed by electrons resonant tunneling through a much thinner injector barrier than that of conventional designed QCLs, which overcomes the interface-roughness-induced detuning of resonant tunneling. Oscillation or overshooting within the first picosecond is caused by electron relaxation from continuum region excited by strong pump beam, as well as coherent electron tunneling transport from injector to active region. The former feature is supported by the observation of second harmonic generation (SHG) with emission of λ≈2.2 μm pulses and measured positive photoconductivity. The transport of electrons through the injector region contributes to a slower gain recovery. A much longer recovery (hundreds of picoseconds) can be explained as electrons are depleted from upper stages down to lower stages in real space.

Electrically controlled terahertz magneto-optical phenomena in continuous and patterned graphene

The magnetic circular dichroism and the Faraday rotation are the fundamental phenomena of great practical importance arising from the breaking of the time reversal symmetry by a magnetic field. In most materials, the strength and the sign of these effects can be only controlled by the field value and its orientation. Furthermore, the terahertz range is lacking materials having the ability to affect the polarization state of the light in a non-reciprocal manner. Here we demonstrate, using broadband terahertz magneto-electro-optical spectroscopy, that in graphene both the magnetic circular dichroism and the Faraday rotation can be modulated in intensity, tuned in frequency and, importantly, inverted using only electrostatic doping at a fixed magnetic field. In addition, we observe strong magneto-plasmonic resonances in a patterned array of graphene antidots, which potentially allows exploiting these magneto-optical phenomena in a broad THz range.

Single-mode quantum cascade lasers employing asymmetric Mach-Zehnder interferometer type cavities

We employ properly designed asymmetric Mach-Zehnder interferometer structures as effective wavelength filters and monolithically integrate them in conventional Fabry-Perot cavities to facilitate single-mode operation of the lasers. With such asymmetric Mach-Zehnder interferometer type laser cavities, continuously tunable single-mode operation of quantum cascade (QC) lasers is achieved in pulsed mode from 80 K up to room temperature and in continuous-wave mode with side-mode suppression ratio up to ∼35 dB. These devices are fabricated with the same process as simple ridge lasers, therefore providing a promising solution to achieving more cost-effective single-mode QC lasers.

Short Injector Quantum Cascade Lasers

We report our study on the effects of shortened quantum cascade (QC) laser injector regions. While conventional short-wavelength QC lasers typically have around seven or more injector region quantum wells, we investigate QC structures with three and two injector wells. Improvements in threshold currents, output powers, and wall-plug efficiencies are expected for fundamental reasons. At heat sink temperatures near 80 K, we observe threshold current densities less than 0.5 kA/cm2, nearly 4 W peak output power, and wall-plug efficiencies in excess of 20%. At room temperature, we see threshold current densities around 2.3 kA/cm2, output powers in excess of 1 W, and wall-plug efficiencies around 7.6%. We also observe new effects in midinfrared QC lasers, such as a pronounced negative differential resistance, pulse instabilities, and multiple and varied turn-off mechanisms. These effects result from the greatly abbreviated injector regions with highly discrete states.

Single-mode quantum cascade lasers employing a candy-cane shaped monolithic coupled cavity

We demonstrate single-mode quantum cascade lasers emitting at ∼4.5 μm by employing a monolithic “candy-cane” shaped coupled-cavity consisting of a straight section connecting at one end to a spiral section. The fabrication process is identical to those for simple Fabry-Perot-type ridge lasers. Continuously tunable single-mode emission across ∼8 cm−1 with side mode suppression ratio up to ∼25 dB and a single-mode operating current range of more than 70% above the threshold current is achieved when the lasers are operated in pulsed-mode from 80 K to 155 K.

High power spiral cavity quantum cascade superluminescent emitter.

Quantum cascade devices have been shaped into compact, yet long spiral cavities to increase mid-infrared superluminescence power. A peak power of ~57 mW at 250 K is obtained with a coherence length of ~107 μm.

Second harmonic generation in quantum cascade lasers pumped by femtosecond mid-infrared pulses

Second harmonic generation (SHG) pulses at λ ∼ 2.25 µm have been obtained from a 4.5 µm quantum cascade laser (QCL) when it is resonantly pumped by transverse magnetic polarized 120 fs, λ = 4.5 µm pulses through the QCL’s front facet at room temperature. The measured SHG spectrum narrows when the bias across the QCL increases due to the electron population re-distribution and subband realignment. The expected quadratic dependence of the SHG with pump power is observed but saturates at higher pump powers. The linear to nonlinear power conversion efficiency is calculated (∼2 µW/W2) and compared with theoretical calculation. This experiment provides an alternative way of investigating the carrier dynamics in QCLs.