Quantum Cascade Lasers: Structure, Growth, Theory, and Experiments, Study notes of Electrical and Electronics Engineering

Download Quantum Cascade Lasers: Structure, Growth, Theory, and Experiments and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! Quantum Cascade Lasers ECE183 Term Paper Winter 2006 David Shrekenhamer Introduction The continued development of controlling optical systems has been at the forefront of physics for over a hundred years. Since the discovery of black body radiation during Einstein’s era, to the theoretical predictions of a semiconductor laser by Nicolay Basov and Ali Javen who’s Nobel prize winning paper on MASER system lead to the experimental work shortly thereafter of Robert Hall in 1962, in which he produced the first laser diode. Since that time semiconductor lasers have been experimentally and theoretically well defined, and the products of this have been more famously produced as hand held lasers as well as LED’s used in millions of devices. Along with these devices other ideas were explored, eventually leading to the topic of this paper, Quantum Cascade (QC) Laser Systems. Quantum Cascade Lasers Introduced Q.C. Lasers were first predicted in 1974 by Rudolf Kazarinov and were not first produced experimentally untill 1994 by Jerome Faist and other Bell Lab researchers. In principle they act as a tunable photon emitter in the ranges of 3.5 to 24 microns and are intrinsically high power in nature. Typical structure consists of 25 to 75 stages of injector to active regions (Fig. 1), that emit the photons necessary for lasing. The way that these regions are designed are through multiple layers of varying semiconducting material. These layers are designed as quantum barriers that influence the wave function in real space by producing uniform discrete levels of transition for each stage included. The idea here is the creation of a domino effect. That for every electron injected through a current source there are N photons emitted, where N is the number of stages.cite: 1 6 0 n m 5 2 0 m E v En er gy D is ta n c e Figure 1 Quantum Cascade Laser Layout Growth Process The process of growth is mainly through Molecular Beam Epitaxy (MBE). In this process many layers (~500) of varying doped semiconductor are grown as very thin flat smooth layers. This is crucial for the functionality of Q.C. Laser so that the quantum stages are exact, and so that all electron passing through the stages undergo the same potential change. The MBE operates in a vacuum chamber where the pressure is such that there are no oxidation layers or other defects that can occur at standard room temperatures and pressure. Containers filled with different alloy materials are dispersed onto a growth area through nanosecond shutters that allow for monolayer growth. These layers are on the scale of several nanometers (Fig. 2), and Energy (meV) G ai n (c m -1 ) G ai n (c m -1 ) G ai n (c m -1 ) Figure 5 Low Concentration Gain Model B. High Concentration Model In the second model there is an overall change in the system behavior that deals with bulk carrier concentrations well above 1017 cm−3. As a result of these high concentrations the ee scattering process dominates over the optical phonon emission, resulting in huge loss in the gain. Boltzmann statistics can be applied to simplify down the model introduced for the low concentration model. The resulting gain equation contains similar terms to that of the low concentration model, but the scattering rate, , behaves differently as well the change in the subband concentrations that are accounted for in the Boltzman approximation. The obvious difference can be seen in Figure 6 where the overall gain peak is significantly lower, by an order of magnitude for optimal temperature. Temperatures exceeding this are clearly unable to produce gain sufficient for any lasing capabilities.cite: 2 kBTc − T  Jn g  4e 2|z12 |2n2 3ac  kBTc  exp −kBTc  |−12 |2| |2 1 − f1f2 d f1 f2  n1m2n2m1 exp  kBTc 1 − m2m1  Energy (meV) G ai n (c m -1 ) G ai n (c m -1 ) Figure 6 High Concentration Gain Experimental Works The following section consists of a broad spectrum of results. Part A consists of experimental verification that Q.C. lasers act in single mode and have wavelength dependence on the temperature that they are operating at. Part B shows example of spectroscopy that is used with a Q.C. laser and how it’s tunability and high gain allow for analysis of complex systems. Part C explains the study of implementing a photonic crystal structure with a Q.C. laser as a mechanism to further increase the efficiency of the device. A. The Tunability of Quantum Cascade Laser Systems The ability for Q.C. Lasers to be able to be single mode and have tunability are two features that have been very difficult to achieve before. In a regular semiconductor laser a liquid crystal typically would be used to tune the laser pulse into a different frequency, but again this does not actually allow for real time tunability. Q.C. Lasers have proven that they can retain single mode characteristic (See Fig. 7), and tune wavelength by changing the temperature of the laser (See Fig. 8). The temperature has the effect of shifting the subbands closer together with increasing temperature, as well as change the gain profile as discussed in the theoretical models.cite: 1 Figure 7 Single Mode Figure 8 Optical Wavelength Temperatre Dependence B. Open Path Ozone Measurements : Q.C. Laser Spectroscopy Recent work has shown that Q.C. Lasers can be used in spectroscopic work for complex systems including ozone layers. In this research a Q.C. Laser has been designed to operate at on a pulsed current that allows for a temperature change and simultaneous wavelength change in the output laser mode. The ability to have the wavelength to be linearized with time (see Fig. 9) is remarkable. This relationship allows for a direct relationship between time and wavelength. This Q.C. Laser system has several other key advantages that include a compact system and non cryogenic setup. All of this allows for real time measurements of the different ozone layers and can detect various elements (CH4, C2H6, NO, NO2, CO, NH3 and O3) existing in as low a concentration as parts per billion (ppb). The measurements conducted on ozone layers 440m to 5800m away, and particle densities of 10 to 70 ppb. Procedure setup involves calibration in a closed environment in which a controlled local threshold is measured experimentally and recorded (See Fig. 10), this also allows for calculation of time to wavelength value. In field measurements made of ozone layers at a distance of 5800m with 18 second pulse length (See Fig. 11), show significant absorption at higher concentrations of ozone layer, which is similar to effects seen to water vapor in basic spectroscopy. cite: 3 Figure 14 SEM Side View Wavenumber (cm-1) O pt ic al P ow er (a .u .) O pt ic al P ow er (a .u .) Figure 15 Tuning Ability Conclusion Quantum Cascade Lasers in their brief history in the world of science are clearly showing their versatility in the different areas of research that are optical in nature. The theory behind the experimental work is still being developed in the hope to be able to develop more complicated and efficient designs. As a result of these capabilities one should not be surprised to see that in the future that history repeats itself and that Q.C. Lasers become the basic Semiconductor Lasers of today’s world. 1. (ref1) J. Faist et. al. Appl. Phys. Lett. 68, 3680 (1996) 2. (ref2) V. Gorfinkel et. al. IEEE Journal of Quantum Electronics 32, 1995 (1996) 3. (ref3) M.Taslakov et. al. Appl. Phys. B 82, 501 (2006) 4. (ref4) R. Colombelli et. al. Science, 302, 1374(2003)