TERAHERTZ GENERATION IN QUANTUM CASCADE LASERS

The terahertz spectral range (1-10 THz) has historically been devoid of a compact source of coherent radiation. There are many applications for coherent THz sources, including biology and medical imaging, security screening, heterodyne receivers, spectroscopy, and trace gas detection. Recently, qcls were developed as a novel source for terahertz radiation and have rapidly covered the frequency range 2-5 THz; there is strong interest to push the operating temperature to a range supported by thermoelectric coolers (240K).

Surface emission for single-mode operation and low divergence

We investigated the implementation of surface emission via a second order grating in terahertz quantum cascade lasers with double-metal waveguides. In these structures, radiation emits from the entire length of the device, yielding enhanced power levels and emission directionality. Absorbing edge structures are designed to enforce anti-reflecting boundary conditions, which ensure distributed feedback in the cavity. Fabricated devices demonstrate measured output powers up to an order of magnitude greater than that from conventional edge-emitting double metal waveguide structures. Surface emitting lasers show single mode behavior which is a very important feature for real-world application, with a beam divergence of approximately six degrees.

Jonathan Fan, Qi Jie Wang and Mikhail Belkin

Optical mode and far-field manipulations

The far-field profile in current state-of-the-art edge-emitting structures is highly non-directional both vertically and laterally, and slope efficiencies are poor. Therefore, we investigated the suppression of higher order lateral modes in edge emitting devices. Terahertz quantum cascade lasers with wide-ridge metal-metal waveguides are prone to lasing in high-order lateral modes, which reduce the maximum light output power from these devices. We have demonstrated, theoretically and experimentally, that the output power can be improved several-fold by introducing ‘side absorbers’ into the waveguide structure, which enforce lasing in the TM00 mode with minor temperature performance deterioration. This concept may be adapted to surface emitting devices. Lasers without side absorbers operate up to 168K, a record as of October 2008 for devices processed using indium/gold wafer bonding.

Jonathan Fan, Qi Jie Wang and Mikhail Belkin

High temperature operation

We also reported terahertz quantum cascade lasers with double metal waveguides comprising copper, which is less lossy compared to gold. They utilize a three-quantum-well active region design with resonant-phonon depopulation, and they are designed around an emission frequency of 3 THz. These devices have been demonstrated to operate in pulsed mode at maximum temperature of 178 K, which is a world record as of October 2008.

Qi Jie Wang, Jonathan Fan and Mikhail Belkin

THz generation through difference frequency generation in QCLs

Quantum-well semiconductor structures provide a unique opportunity to manipulate the nonlinear optical response of a medium by tailoring the energies and strengths of electronic resonances. These structures with giant nonlinear second order susceptibilities can be incorporated into the quantum cascade laser active region. This concept of intracavity nonlinear light generation can be used to fabricate compact semiconductor THz sources working at high temperatures. The trick to getting a usable amount of terahertz light out at room temperature is to take advantage of what the quantum-cascade laser does best: emit mid-infrared light. Thus, the lasers are designed to emit two different wavelengths of mid-infrared light. Due to the properties of the special active region, the incoming infrared photons are partly converted into terahertz radiation. Based on this concept we recently demonstrated the first semiconductor source of coherent THz radiation working at RT and we are working currently on the optimization of the band structure and waveguide designs to improve the performance. In particular, we work on the implementation of second-order diffraction gratings in the waveguide to improve the out-coupling efficiency of these devices.

Christian Pflügl, Markus Geiser and Qi Jie Wang in collaboration with Hamamatsu Photonics, M. Belkin (UT Austin), A. Belyanin (Texas A&M), J. Faist (ETH Zurich)

MULTIMODE OPERATION, COHERENT INSTABILITIES AND MODELOCKING IN QUANTUM CASCADE LASERS

Coherent coupling of multiple transverse modes of QCLs

Quantum cascade lasers are a unique laboratory for studying nonlinear laser dynamics because of their high intracavity intensity, strong intersubband optical nonlinearity, and an unusual combination of relaxation timescales. In this project we investigate the nonlinear coupling between the transverse modes of quantum cascade lasers. It was observed in the experiments that the laser spectra are consisting of more than one set of longitudinal modes, indicating excitation of several transverse modes. It was observed that over a large dynamic range of injection currents the far-field and near-field mode profiles are not symmetric. These are evidences for stable phase coherence of multiple transverse modes. We explain the observed phase coherence by a four-wave mixing interaction originating from the strong optical nonlinearity of the gain transition, which cancels waveguide dispersion of the cavity modes. The phase locking conditions predicted by theory are supported by experimental observations.

Nanfang Yu and Laurent Diehl

Near-field imaging of quantum cascade laser transverse modes

Direct monitoring of the mode pattern on the facet of a quantum cascade laser would be a powerful technique to investigate multi-transverse mode operation of the device. We constructed a mid-infrared apertureless near-field scanning optical microscope to characterize the modes on the laser facet. A very stable mode pattern corresponding to the fundamental TM00 mode was observed as function of increasing driving current for a narrow active region quantum cascade laser. Higher order modes were observed for devices with a larger active region width-to-wavelength ratio operated in pulsed mode close to threshold. A theoretical model is proposed to explain why specific transverse modes are preferred for a certain waveguide structure. The model is in good agreement with the experimental results.

Nanfang Yu and Laurent Diehl

HIGH POWER QUANTUM CASCADE LASERS

High performance QCLs grown by MOVPE

The group of Prof. Capasso, in collaboration with Agilent Technologies showed that QCLs with state-of-the-art performance can be grown using a deposition method known as Metalorganic Vapor Phase Epitaxy (MOVPE). MOVPE is one of the most common and versatile method for mass-producing technology for semiconductor lasers, circuits and other photonics components for communications. This technique has significant advantages over other growth techniques, since MOVPE offers excellent stability, higher growth rates and considerably shorter machine downtime, ideal characteristics for industrial production. The work of the Harvard-Agilent team has received a lot of attention in particular from the private sector and inspired a number of private companies and semiconductor foundries to start an effort in QCLs.

Laurent Diehl, Christian Pflügl and Qi Jie Wang in collaboration with Agilent Technologies, Pranalytica Inc., Adtech Optics and Hamamatsu Photonics

QCL SPECTROMETER - DISTRIBUTED FEEDBACK QUANTUM CASCADE LASER ARRAYS

Quantum Cascade Laser Array Spectrometer

Our device is based on an array of distributed feedback QCLs, fabricated monolithically on the same chip. We fabricated a device with 32 lasers on a single chip. Each individual laser in the array is designed to emit at a different wavelength. The emission wavelength of each individual laser can be temperature tuned in a small range so that it can cover the entire region between the emission wavelengths of its neighbors. Together, all the lasers in the array can cover a broad range of wavelengths; in the case of our fabricated devices, the 32 lasers cover a wavelength range from 8.73 to 9.43 microns. Thus a single device can be used to target all the molecular absorption lines in the range 8.73 to 9.43 microns for sensing. We demonstrated the spectroscopic applications of our array by performing infrared absorption spectroscopy on various fluids - isopropanol, acetone, methanol.

Ben Lee, Christian Pflügl and Laurent Diehl

QUANTUM CASCADE LASER PLASMONICS

Mid-IR plasmonic laser antenna

The purpose of this research work is to design and fabricate suitable antenna structures to concentrate incident mid-IR illumination to a sub- wavelength region with significant field enhancement, which may find application in infrared near field microscopy and chemical analysis. The antenna is fabricated on the facet of a QCL. A scanning near field setup based on an atomic force microscope is constructed to characterize the field distribution around the antenna structure.

Nanfang Yu, Mikhail Belkin, and Laurent Diehl in collaboration with Ken Crozier (Harvard)

Small divergence semiconductor lasers by plasmonic collimation

Surface plasmons offer the exciting possibility of improving the functionality of optical devices through the subwavelength manipulation of light. We show that surface plasmons can be used to shape the beams of edge-emitting semiconductor lasers and greatly reduce their large intrinsic beam divergence. Using quantum cascade lasers as a model system, we show that by defining subwavelength apertures and metallic gratings on their facet, a small beam divergence angle can be achieved in directions both perpendicular and parallel to the laser waveguide layers. Divergence angles as small as a few degrees are obtained, representing a reduction in beam spread by more than one order of magnitude compared with the original lasers used. Despite having a patterned facet, our collimated lasers do not suffer significant reductions in output power. Plasmonic collimation provides a means of efficiently coupling the output of a variety of lasers into optical fibres and waveguides, or to collimate them for applications such as free-space communications, ranging, and remote sensing.

Nanfang Yu, Romain Blanchard and Christian Pflügl

SURFACE ACOUSTIC WAVE QUANTUM CASCADE LASER

GHz Surface Acoustic Wave generation on ZnO thin film deposited on III-V semiconductor

Surface acoustic wave (SAW) devices have been widely used in wireless communications and signal processing applications. Recently, there have been growing interests to use SAWs in optoelectronics for optical and/or electrical modulations. Efficient generation of SAWs at frequencies above 1 GHz on III-V semiconductors would not only broaden the area of compact system-on-a-chip solutions for wireless communication, high-frequency filtering and sensing applications, but would also provide interesting opportunities for monolithically-integrated SAW-modulated optoelectronic devices. This also allows us to directly modulate semiconductor devices like quantum cascade lasers (QCLs) to achieve a single-mode broadly-tunable laser output. This, however, has so far been difficult due to the weak piezoelectricity and low acoustic phase velocities of III-V semiconductor materials.

Qi Jie Wang and Christian Pflügl in collaboration with Prof. Masamichi Yamanishi (Hamamatsu Photonics, Japan)