Mid-IR Physics and Devices : Quantum Cascade Lasers
InGaAs/AlAs(Sb) QCLs on InP
The 3-4 μm wavelength range is of key technological importance for a wide range of applications. Many important hydrocarbon species, for example ethane, methane, acetone, formaldehyde and butane and other molecules such as hydrogen halides have their strongest absorption features in this region. This leads to many potential applications in areas such as clinical diagnostics (breath analysis), process monitoring, control of outdoor and indoor pollution, and remote detection of oil and gas deposits.
At longer mid IR wavelengths, quantum cascade lasers (QCLs) display spectacular levels of performance, and they have already been adopted for a range of sensing applications. However, the short wavelength 3.0 μm < λ<3.8 μm range has proved to be very difficult to access with any high performance semiconductor laser technology. For QCLs, difficulties arise because the "conventional" InGaAs/InAlAs materials system does not provide sufficiently deep quantum wells to support the required high energy intersubband transitions. This has led to significant recent interest in a new QCL material such as InGaAs/AlAsSb which has conduction band offset (ΔEC) of around 1.6 eV. The InGaAs/AlAsSb system offers both a very high ΔEC and lattice-matched compatibility with InP.Recent highlights of research on short wavelength InGaAs/AlAsSb/InP based QCLs have included first InGaAs/AlAsSb strain compensated QCL growth (Appl. Phys. Lett, 90 151105 (2007)), the shortest wavelength room temperature InP-based QCL operation at around 3.0 μm (Appl. Phys. Lett, 94 031106 (2009)), novel high performance structures incorporating AlAs barriers (for example, Appl. Phys. Lett, 95 111113 (2009)), high power (several Watts) pulsed operation of λ ~ 3.3-3.8 μm InGaAs/AlAsSb QCLs up to 400 K (Fig.1 and Fig.2) (Appl. Phys. Lett, 97 031108 (2010)) and single mode room temperature λ ~ 3.35 μm QCLs with third order buried distributed feedback grating (Appl. Phys. Lett, 97 111113 (2010)). All of these advances were achieved at Sheffield.
Fig.1. Light-current characteristics for a 10 μm wide 4 mm long strain compensated λ ~ 3.3 μm InGaAs/AlAs(Sb)/InP QCL with its back facet covered with high reflectivity coating.
Fig.2. Laser spectra at 80 and 300K.
This work has attracted much attention across the whole QCL community and has been the subject of several invited presentations at international conferences in recent years. The details of the invited talk given at the International conference on Semiconductor Mid-IR Materials and Optics held in Poland in 2010 can be found here.The deep quantum well InGaAs/AlAsSb QCLs offer especially strong opportunities in their capability for intracavity sum/difference frequency generation provided by the very high non-linear optical susceptibilities of intersubband transitions. This has emerged as a major topic of our current activity. One of the examples includes the extension of QCL technology to telecoms wavelengths by intracavity second harmonic generation in λ ~ 3 μm lasers. The particular use of InGaAs/AlAsSb QCLs is highly advantageous since, unlike InAs/AlSb QCL devices (which have very deep quantum wells as well), they can be produced with relatively wide band gap waveguide cladding material (ie InP) which is optically transparent at near IR wavelengths.
Probing the electronic and optical properties of quantum cascade lasers under operating conditions
Recently, a new intersubband transmission spectroscopy technique, which provides details of the electronic carrier distribution inside operating QCLs over the entire range of mid-infrared transition energies has been developed in our group (Appl. Phys. Lett, 88 131105 (2006)). In these experiments, the broadband thermal "Globar" emission from the FTIR spectrometer was used as an incident light. This emission was precisely focused onto one facet of QCL under investigation and only the light transmitted through the waveguide was collected from the other side of the laser ridge and detected with cooled MCT detector. This novel technique allows spectroscopic study of light transmission through the waveguide of QCLs at various temperatures (4-300 K) and in a very broad spectral range (~1.5-12 μm), limited only by the detector response and by the interband absorption materials used in QCL core region. Waveguide transmittance spectra have been studied for both TE and TM polarisation, for a range of InGaAs/InAlAs/InP and InGaAs/AlAsSb/InP QCLs with different active region designs emitting from 4.5 to 10.3μm. The transmission measurements clearly show the depopulation of the lower laser levels as bias is increased, the onset and growth of optical amplification at the energy corresponding to the laser transitions as current is increased towards threshold, and the thermal filling of the second laser level and decrease of material gain at high temperatures. This technique also allows direct determination of key parameters such as the exact temperature of the laser core region under operating conditions, as well as the modal gain and waveguide loss coefficients.
By using this technique we have recently observed dispersive gain or Bloch gain in a QCL (simultaneous coexistence of gain and losses on a single intersubband transition) (Appl. Phys. Lett, 92 081110 (2008)). In general, such an effect can happen only under certain conditions: in particular, the electron concentrations on the upper and lower laser levels have to be similar. In our experiments it is possible to control these concentrations of the electrons on these levels almost independently: the increase of temperature results in more electrons on the lower laser level, while the increasing bias leads to higher electron population on the upper laser level. Fig.3 demonstrates the evolution of the transmission spectra for various temperatures and bias conditions. The energy positions of the amplification peaks (corresponded to the gain on the laser transition) are indicated by the red circles and the energy positions of the absorption deeps (corresponded to the losses on the laser transitions) are indicated by the blue squares. Individual spectra are vertically shifted for clarity. The dispersive gain effect is more pronounce at higher temperatures (cases (c) - (e) in Fig.3). To mark the precise energy positions of the intersubband transition for the corresponding temperatures the lasing spectra (in thick red) are shown on top of each series of the transmission spectra. (The additional absorption peaks at ~305 meV correspond to another electron transition in the active region of the QCL.)
Fig.3. Transmission spectra taken for TM polarization normalized to those taken for TE polarization at various bias conditions and temperatures.
MOVPE Grown QCLs
Quantum Cascade Lasers have become important, high-quality sources of laser light in the mid-infrared region of the spectrum. Fabrication of these devices is demanding in terms of crystal growth, requiring precise control of large numbers of thin (nm-sized) layers of semiconducting material forming the active regions. Until recently, growth of QCLs has been confined to the high-vacuum technique of molecular beam epitaxy (MBE). It was our group, which in 2003 demonstrated the first lattice matched InGaAs/InAlAs and GaAs/AlGaAs QCLs grown by metal-organic vapour phase epitaxy (MOVPE) (Applied Physics Letters 82 (24), p4221 (2003)), a technique which has several potential advantages over MBE. These include reactors that can be readily maintained without the need for elaborate baking cycles to recover from atmospheric contamination, and the ability to controllably alter growth rates over a wide range within a growth run. A talk on the very first development of MOVPE grown QCLs was presented at CLEO 2004.
Since first demonstration of MOVPE grown QCLs, several lattice-matched and strain compensated InGaAs/InAlAs/InP QCLs operating between λ ~ 4 and 10.5μm have been produced in Sheffield. Power levels of several Watts and typical pulsed threshold current densities (Jth) of 0.6 kA/cm2 and ~2 kA/cm2 have been measured for cryogenic and room temperatures, respectively. These values are comparable with similar state-of-the-art MBE grown samples highlighting the competitiveness of MOVPE growth.
Light-current (L-I) characteristics (Fig. 4) and room temperature emission spectra (Fig. 5) are shown below for an as-cleaved strain compensated (the layer strain of 1.5 %) In0.7Ga0.3As/In0.34Al0.66As/InP QCL grown by MOVPE. The laser has Jth = 2.5kA/cm2 (at 300K) and operates up to at least 400 K. The maximum wall plug efficiency of 9 % has been observed at room temperature in pulsed regime.
Fig.4. L-I characteristics (for the emission from both laser facets) for a 10μm wide 3 mm long In0.7Ga0.3As/In0.34Al0.66As/InP QCL.
Fig.5. Laser (just above the threshold) and sub-threshold electroluminescence spectra for a QCL at 300 K.
The aim of this study is to construct an Optical Coherence Tomography (OCT) system which operates at λ ~ 6-12 μm, demonstrating real-time imaging with a spatial resolution better than 100 μm on engineered tissues grown in bioreactors.
Broadband laser emission is achieved by extending the idea of a conventional QC laser. Since the emission wavelength of QCL can be tailored by means of band structure engineering, a net broadband gain can be achieved over a target wavelength range by incorporating a series of active regions with different emission wavelengths. For example, our broadband QCL designed to emit between 6 and 8 μm contains 11 different active regions, each repeated either 3 or 4 times to give 36 active stages in total. The material system used is InGaAs/AlInAs latticed matched to InP and is grown by MBE.
Several broadband QCLs designed to operate between 6-8 μm have been fabricated and characterised. Peak power levels in the order of several hundred milliWatts and typical pulsed Jth in the order of 1.4 kA/cm2 at cryogenic temperatures have been measured.
The laser spectrum for a broadband QCL device operating at 4.8A is shown in Fig. 6. The emission spectrum shows continuous broadband lasing over the range of 6.0 - 8.0 μm. The laser performance is: Jth ~ 1.35 kA/cm2 (10 K) and ~5.1 kA/cm2 (300 K) with a characteristic temperature T0 ~ 198 K. Fig. 7 exhibits the coherence length measurements using a Michelson Interferometer. The estimated coherence length at a current density of 6.5 kA/cm2 is ~25 μm. A requirement of OCT is a short coherence length and work is underway to make the emission more continuous over the designed wavelength range and to extend the operating wavelength into the 8-10 μm range to achieve even shorter coherence lengths.
Fig.6. Emission spectra of broadband QCL. Continuous lasing is observed across the 6 to 8 μm range.
Fig.7. Coherence length measurement in air, laser driven at 15 kHz with 100 ns long current pulses.