Unipolar Semiconductor Lasers Research Articles

Modeling techniques for quantum cascade lasers

Quantum cascade lasers are unipolar semiconductor lasers covering a wide range of the infrared and terahertz spectrum. Lasing action is achieved by using optical intersubband transitions between quantized states in specifically designed multiple-quantum-well heterostructures. A systematic improvement of quantum cascade lasers with respect to operating temperature, efficiency, and spectral range requires detailed modeling of the underlying physical processes in these structures. Moreover, the quantum cascade laser constitutes a versatile model device for the development and improvement of simulation techniques in nano- and optoelectronics. This review provides a comprehensive survey and discussion of the modeling techniques used for the simulation of quantum cascade lasers. The main focus is on the modeling of carrier transport in the nanostructured gain medium, while the simulation of the optical cavity is covered at a more basic level. Specifically, the transfer matrix and finite difference methods for solving the one-dimensional Schrödinger equation and Schrödinger-Poisson system are discussed, providing the quantized states in the multiple-quantum-well active region. The modeling of the optical cavity is covered with a focus on basic waveguide resonator structures. Furthermore, various carrier transport simulation methods are discussed, ranging from basic empirical approaches to advanced self-consistent techniques. The methods include empirical rate equation and related Maxwell-Bloch equation approaches, self-consistent rate equation and ensemble Monte Carlo methods, as well as quantum transport approaches, in particular the density matrix and non-equilibrium Green's function formalism. The derived scattering rates and self-energies are generally valid for n-type devices based on one-dimensional quantum confinement, such as quantum well structures.

On Line Sensing of Ultrafast Laser Microdrilling Processes by Optical Feedback Interferometry

Abstract The instantaneous measurement of both ablation front displacement and removal rate during ultrafast laser microdrilling is demonstrated by on line sensing technique based on optical feedback interferometry in both unipolar and bipolar semiconductor laser. The dependence of laser ablation dynamics on pulse duration, energy density and working pressure has been investigated, thus allowing a significant advancement of the basic understanding of the ultrafast laser-material interactions. Moreover, the detection system results high-sensitive, compact, and easily integrable in most industrial workstations, enabling the development of real-time control to improve ablation efficiency and quality of laser micro- machining processes.

Peculiarities of electron spectrum rearrangement for the double-well heterostructure GaAs/AlGaAs with a variable dimensionality of electronic states in an external electric field

Peculiarities of the electron spectrum rearrangement for the double-well heterostructure GaAs/AlGaAs with a variable dimensionality of electronic states in an external electric field are investigated theoretically and experimentally. The structure is an important part of the active element of a quantum-well unipolar semiconductor laser proposed by the authors earlier. The possibility of controlling the dimensionality of the lower laser subband in such an active element by an external electric field is demonstrated.

High reflectivity metallic mirror coatings for mid-infrared (λ ≈ 9 μm) unipolar semiconductor lasers

We present a detailed study on the improvement of the performance of λ ∼ 9 μm quantum cascade lasers by using a metallic high reflectivity facet coating. Higher operating temperatures, lower operating currents and higher peak powers are all seen as a consequence for this relatively straightforward technology. Furthermore, a dramatic approximately fourfold increase of average power is observed when operated on a Peltier cooler at −30 °C.

Lateral current spreading in unipolar semiconductor lasers

Lateral current spreading in shallow ridge processed unipolar semiconductor lasers is described using a two-dimensional flow model. In these devices, contrary to bipolar diode lasers, the density of carriers can be considered constant also in the active region. Therefore electron diffusion is a negligible effect and the spatial distribution of the current can be obtained by solving a two-dimensional differential equation for the electric potential. Our calculations prove that the major contribution to the current spreading takes place right before electrons enter the active region and is caused by the discontinuity of the conductivity at the cladding–active region interface.

Intrinsic population inversion in biased multiband superlattices

The electric field induced carrier redistribution in multiband superlattices is studied within an equation of motion analysis of the density matrix. Semiclassical intrasubband and quantum-mechanical tunneling contributions are identified. It is shown that there is no global population inversion for superlattices with only two minibands. However, we predict that an intrinsic global population inversion may occur at particular electric field strengths in superlattices with at least three minibands. Conditions under which global population inversion is expected to emerge are discussed. It is proposed to exploit the global population inversion that may appear due to resonant tunneling into a wide third miniband to fabricate unipolar midinfrared semiconductor lasers without any injector regions.

Unipolar semiconductor lasers on asymmetric quantumwells

We propose an original design of the active element ofa quantum unipolar semiconductor laser for optical pumping.The peculiarities of the proposed design are stronglyasymmetric barriers surrounding a double-well active element.The suppression of intersubband transitions to the lowerworking subband can be readily achieved if the transformationpoint of electronic state dimensionality for the lower subbandoccurs at small momentum. By this means the populationinversion conditions in this system can be easily realized. Theresults of photoluminescence studies of the individual elementsof the proposed structure are presented.

Analysis of absorption and amplification in a unipolar semiconductor structure with quantum dots

The allowed intraband optical transitions in a quantum dot are considered in the context of their application in quantum-well IR detectors and unipolar semiconductor lasers. The atom-like quantum dot is described by using the harmonic oscillator model. An expression is obtained for the absorption cross section which is independent of the wavelength and the quantum-dot size. Optical absorption and amplification are estimated.

Low-loss Al-free waveguides for unipolar semiconductor lasers

A promising waveguide design for midinfrared (λ=5–20 μm) unipolar semiconductor lasers is proposed and demonstrated in (Al)GaAs quantum cascade structures. In the latter, the active region is embedded between two GaAs layers, with an appropriate doping profile which allows optical confinement, with low waveguide losses and optimal heat dissipation. Low internal cavity losses of 20 cm−1 have been measured using different techniques for lasers with emission wavelength at ∼9 μm. At 77 K, these devices have peak output power in excess of 550 mW and threshold current of 4.7 kA/cm2.

Influence of DX centers on the performance of unipolar semiconductor lasers based on GaAs-Al/sub x/Ga/sub 1-x/As

Optical and electrical characteristics of quantum cascade lasers, made in the GaAs-Al/sub x/Ga/sub 1-x/As material system, show persistent changes at low temperatures, when devices are illuminated with an external white light. The magnitude of the effect is a function of the exposure time and finally saturates. The induced variation on the threshold current density of the lasers is more than 30%. This effect is related to the presence of deep donors (DX centers) in the high aluminum content AlGaAs cladding layers, which under illumination release electrons, thus increasing waveguide losses. By analyzing the light induced variations of the optical characteristics we were able to deduce the waveguide losses of our devices.

Terahertz bandwidth prediction for amplitude modulation response of unipolar intersubband semiconductor lasers

An analysis is performed of the direct-current (dc) amplitude modulation response of unipolar intersubband semiconductor lasers. It is shown that unipolar lasers have the potential for achieving terahertz dc modulation bandwidths.

Gain calculations for unipolar semiconductor lasers

The optical gain spectra of a single triple quantum well (TQW) mid-infrared (MIR) laser element suitable for incorporation in a quantum cascade configuration are investigated. The kinetics of the carrier transport in the TQW structure are provided using a four-level rate equation model. The low gain per element at MIR frequencies illustrates the need for several such elements. Preliminary gain calculations for a near-infrared (NIR) structure are also performed.

High-performance superlattice quantum cascade lasers

Superlattice quantum cascade (QC) lasers based on optical transitions between conduction minibands are unipolar semiconductor lasers with high-current-carrying capability and attendant high optical power due to miniband transport in the active and in the injector regions. Other advantages include the intrinsic population inversion associated with the large interminiband-to-intraminiband relaxation time ratio and the high oscillator strength of the laser transition at the superlattice Brillouin zone boundary. This oscillator strength is significantly larger than that of intersubband transitions in double-quantum-well active regions of conventional cascade lasers, particularly at long infrared wavelengths (/spl ges/10 /spl mu/m). Following a brief review of results on conventional QC lasers, design considerations for superlattice cascade lasers are discussed, along with recent advances in long-wavelength (11 /spl mu/m) structures with doped active regions. Dopants broaden the gain spectrum and increase the laser threshold. Two laser designs that avoid doping of the active regions without causing electric-field-induced localization of the superlattice states are then presented, along with experimental results. In the first one, modulation doping creates a space-charge electric field that compensates the voltage drop across the undoped superlattice active regions. In the second scheme, the latter are designed with quantum wells of varying thickness (chirped superlattice), so that under application of the external field, the localized quantum well states overlap, forming minibands. Both schemes lend to considerably lower threshold current densities than devices with doped active regions, as well as to much higher peak optical power and to room-temperature operation. A record peak power of 500 mW at /spl lambda/=7.6 /spl mu/m at room temperature is obtained with the chirped design. The latter also leads to the longest operating wavelength of any other QC laser (17 /spl mu/m). The last section of the paper describes superlattice cascade lasers operating simultaneously at two or more widely different wavelengths.

Self-consistent analysis of the dc modulation response of unipolar semiconductor lasers

Abstract A self-consistent rate equation analysis of the dc modulation response is performed through a calculation of the relevant carrier and tunnelling lifetimes for a prototype triple-quantum-well structure suitable for incorporation in intersubband lasers. Opportunities for accessing predicted terahertz modulation response capabilities in such devices are examined.

Self-consistent analysis of the dc modulation response of unipolar semiconductor lasers

A self-consistent rate equation analysis of the dc modulation response is performed through a calculation of the relevant carrier and tunnelling lifetimes for a prototype triple-quantum-well structure suitable for incorporation in intersubband lasers. Opportunities for accessing predicted terahertz modulation response capabilities in such devices are examined.

The quantum cascade laser. A device based on two-dimensional electronic subbands

This paper reviews the state-of-the-art of quantum cascade lasers. These new light sources are unipolar semiconductor lasers based on electron resonant tunnelling and optical transitions between quantized conduction band states. Quantum engineering of the electronic energy levels and tailoring of the wavefunctions are used to obtain the correct matrix elements and scattering rates which enable population inversion and the optimization of the overall laser performance. The laser wavelength can be selected over a broad range of the infrared spectrum using the same heterostructure material by tailoring the active layer thickness. Using MBE-grown AlInAs-GaInAs heterostructures, room temperature operation at and has been achieved and lasers in the 4- range were demonstrated at a temperature of 200 K.

Long-wavelength (≈15.5 μm) unipolar semiconductor laser in GaAs quantum wells

A unipolar semiconductor laser emitting in the mid-infrared spectral region is demonstrated. The laser scheme relies on a simple three-level system in GaAs/AlGaAs asymmetric coupled quantum wells. Population inversion between excited states is achieved by optical pumping of electrons from the ground state with a CO2 laser. Long-wavelength (≈15.5 μm) laser emission is demonstrated. The laser is operated in the pulsed regime up to a temperature of 110 K and with an output peak power ≈0.4 W at 77 K. Unipolar quantum well semiconductor lasers based on this principle are capable of covering the long wavelength mid-infrared spectral region above 12 μm.

Unipolar semiconductor laser for the terahertz frequency range

In this paper a new design is proposed for a unipolar semiconductor laser with longitudinal transport of electrons and a stepped variation in the thickness of quantum wells, in which population inversion is obtained, and as a result the electrons of the first subband in the narrow part of the well, with injection into the wide part, preferentially populate its second subband.

Quantum cascade laser: A new optical source in the mid-infrared

A unipolar intersubband semiconductor laser operating above liquid nitrogen temperature at 4.26 μm wavelength is reported. The measured optical power in pulsed operation for a 1.2 mm long device is 22 mW at 10 K and 9 mW at 90 K. Strong line-narrowing above threshold and well resolved longitudinal modes are observed. This new light source, called quantum cascade laser, is based on sequential photon emission in a staircase/coupled quantum well AlInAs/GaInAs structure grown by molecular beam epitaxy. As the transitions arise between quantum confined states, its wavelength can be tailored in an extremely wide wavelength region while keeping the same heterostructure material.

Quantum cascade laser: An intersub-band semiconductorlaser operating above liquid nitrogen temperature

A unipolar intersub-band semiconductor laser operating above liquid nitrogen temperature at 4.3 µm wavelength is reported. The measured optical power in pulsed operation is 30 mW at 102 K and the slope efficiency 0.1 W/A. Strong line-narrowing above threshold and well resolved longitudinal modes are observed up to 125 K. This new light source, called the quantum cascade laser, is based on sequential photon emission in a staircase/coupled quantum well AlInAs/GaInAs structure grown by MBE and its wavelength can be tailored in an extremely wide region.