8.4 8.5 8.6 8.7 8.8 8.9 9.0 0.0
0.5 1.0 1.5 2.0
Wavelength ( µm)
Mode Gain (norm.)
T1=200 K, T
2=400 K T1=100 K, T
2=200 K T1=300 K, T2=300 K T1=300 K, T
2=200 K
Figure 4.9: Emission modes for two segment vertical transition structure QCL for different temperatures at applied bias ofV1=50kV/cm andV2 =60kV/cm.
atures. While temperature tuning is almost linear for single segment device, it is non-linear for multi-segment variable temperature devices.
• If the applied bias and temperature both are varied in multi-segment QCL the multi- color output mode positions also change. The tuning is dependent on both the applied bias and the temperature variation and has no specific pattern.
Chapter 5 Conclusion
Quantum cascade lasers are seen as ideal sources of coherent radiation in the mid-infrared region. The past decade has seen an outstanding improvement in QCL performance. Now a days, a large interest is shown in the usage of this devices for applications like imaging, high resolution spectroscopy, or gas and chemical analysis. For many of these applications apart from high power and good beam quality, a wide tuning range with a multi-color output is required. In this thesis, we have presented a theoretical analysis on multi-color broadly tunable multi-segment quantum cascade lasers. Using a multi-segment cavity instead of a single segment, we could get multi-color broadband output and the output could be tuned by varying the bias and the temperature of the individual segments.
In our proposed theoretical model for multi-segment QCLs, first the gain spectrum in each segment under each bias condition was calculated individually using the gain equation.
Then the peak emission mode for each segment was obtained by considering the modes that sustain in the entire cavity not just within the segment length as the reflecting mirrors are placed at the two ends along the entire cavity length. The net gain experienced by each sustainable mode in the cavity is dependent upon the gain it receives in each segment and the modal overlap factor. We noted that if the peak emission mode in each segment received sufficient gain throughout the laser cavity it is found as a useful output.
From the analysis of a two-segment QCL, it is found that the gain received by each of the segment modes depends on the spatial distribution of the gain spectrums created by each of the segments. A highly overlapping gain spectrum ensures a stronger output but with wavelengths near each other, while widely separated wavelengths end up providing weaker outputs. A third, otherwise suppressed mode is also found at the output from the interaction of the two gain regions. The third mode can be controlled by the applied voltages on the two segments. This provide means of tuning using input bias voltage for this sort of de- vice. Also varying the length of the segments also affects the over-all gain received by each output mode. For largely different segment lengths the larger segment gives the stronger output and the the third output mode closely follows the larger segment, and sometimes if the segment lengths are significantly different we might get monochromatic output as only one mode receives sufficient gain from the device. So for multi-color output segments of comparable lengths are preferred.
The number of segments that would prove to be useful is limited by the quantum mechani- cal design of the gain medium and the segment length to segment separation length ratio to be maintained to provide uniform electric field in each segment. Multiple segments which do not emit at significantly different wavelengths is not of practical use in spectroscopic applications as wavelengths spaced by less than0.2 µm can not be used for detection of multiple molecules. Each active region is usually designed to emit at a certain wavelength region (0.3–1.0 µm)and works only within a certain operating voltage range. This also limits the number of different effective input bias that we can apply to the device to get various significantly different wavelengths from a single device. From our analysis of the three different structures we found that segmenting the device beyond four to five segments is not practical.
We have studied three QCL structures using the theoretical model developed in this work.
The lattice matched structures emit in the 5µmand 8µm range. The diagonal structure provides a wider tuning range compared to the vertical structure but provides smaller gain than its vertical counter part. The strain-compensated structure emits in the all important 4µmregion. The particular structure we worked on can be successfully used to detect trace gases such asCO2, CO, andN2O. Choice of gain medium material and well-barrier width for a multi-segment QCL will depend on the application of the device. The theory holds for all gain mediums and the simulation tool is applicable to all InGaAs/InAlAs based material systems and can be extended to include other material systems by enriching the materials library.
Wavelength tuning of multi-segment QCL is achieved by temperature variation in each seg- ment. Temperature tuning is the most common tuning method for QCL devices. While in single segment devices the wavelength increases almost linearly with increase in tempera- ture of the device under constant bias, the change of wavelength in a multi-segment QCL is non-linear and does not follow a strict patter as the tuning mechanism is slightly differ- ent. Changing the temperature effects the refractive index of the material most significantly.
The change of refractive index changes the mode spacing. Each segment under different temperature creates a different set of sustainable modes, and the modes that finally are able to sustain in the cavity are the subsets of all mode sets. The final output mode is determined by the gain received by each mode in the cavity.
Varying only the segment temperatures under a constant bias, we get a monochromatic out- put light controllable by the segment temperatures. All the structures displayed a significant tuning range. Varying the temperature and the segment bias voltage creates room for mul- tiple mode emission and tuning of the multiple modes. Depending on the temperature and the bias applied the output mode separation, gain received, and also the individual wave- length changes.
The results presented in this thesis show that a multi-segment QCL can emit multi-color light, which is tunable by the bias voltages and the temperatures applied to the segments.
The vertical transition structure shows to support a maximum of three to four useful seg- ments, with a tuning range of about0.6µm with constant bias and variable temperature, and0.3µmtuning range of each individual modes with variable bias and temperature. The analysis has been extended to the diagonal and lattice matched structures as well, which yield similar results. Based on the obtained results, we can conclude that a multi-segment QCL can be used to obtain multi-color widely tunable output emission.
Appendix A
QCL Structures
A.1 Lattice Matched Vertical Transition Structure
The lattice matched two phonon resonance vertical transition structure is made up of al- ternate layers ofIn0.53Ga0.47AsQWs andIn0.52Al0.48Asbarriers lattice matched to its InP substrate. It has a four quantum well active region and has a barrier height of 0.52 eV. The structure is engineered to emit around 8.5µm[27]. Table A.1 gives its layer sequence in detail.
A.2 Lattice Matched Diagonal Transition Structure
The lattice matched diagonal transition structure is made up of alternate layers ofIn0.53Ga0.47As QWs andIn0.52Al0.48Asbarriers lattice matched to its InP substrate. It has a single quan- tum well active region and designed to emit around5.5µm[28]. Table A.2 gives its layer sequence in detail.
Table A.1: Layer sequence of lattice matched two phonon resonance vertical transition structure starting from active region quantum well.
Material Thickness (Å) Doping (cm−3) Active Region
InGaAs 19 i
InAlAs 07 i
InGaAs 58 i
InAlAs 09 i
InGaAs 57 i
InAlAs 09 i
InGaAs 50 i
Injector Region
InAlAs 22 i
InGaAs 34 i
InAlAs 14 i
InGaAs 33 i
InAlAs 13 i
InGaAs 32 i
InAlAs 15 4×1017
InGaAs 31 4×1017
InAlAs 19 4×1017
InGaAs 30 4×1017
InAlAs 23 i
InGaAs 29 i
InAlAs 25 i
InGaAs 29 i
InAlAs 40 i
Table A.2: Layer sequence of lattice matched diagonal transition structure starting from active region quantum well.
Material Thickness (Å) Doping (cm−3) Active Region
InGaAs 19 i
InAlAs 27 i
InGaAs 44 i
Injector Region
InAlAs 33 i
InGaAs 35 i
InAlAs 23 i
InGaAs 26 4×1017
InAlAs 22 4×1017
InGaAs 20 4×1017
InAlAs 20 4×1017
InGaAs 20 i
InAlAs 25 i
InGaAs 18 i
InAlAs 27 i
InGaAs 19 i
InAlAs 35 i
A.3 Strain Compensated Structure
The strain compensated structure is made up of alternate layers of In0.61Ga0.39As QWs (0.51% compressive strain) and In0.45Al0.55As(0.48% tensile strain) barriers. Table A.3 gives its layer sequence. Table A.3 gives its layer sequence in detail.
Table A.3: Layer sequence of strain compensated structure starting from active region bar- rier.
Material Thickness (Å) Doping (cm−3) Material Thickness (Å) Doping (cm−3)
Active Region InGaAs 11 i
InAlAs 46 i InAs 02 i
InGaAs 10 i InGaAs 02 i
InAlAs 14 i InAs 02 i
InGaAs 20 i InGaAs 10 i
InAs 02 i InAlAs 07 i
InGaAs 21 i AlAs 02 i
InAlAs 07 i InAlAs 02 i
AlAs 02 i AlAs 02 i
InAlAs 07 i InAlAs 08 i
InGaAs 19 i InGaAs 12 i
InAs 02 i InAs 02 i
InGaAs 18 i InGaAs 12 i
InAlAs 08 i InAlAs 10 i
AlAs 02 i AlAs 02 i
InAlAs 07 i InAlAs 10 i
InGaAs 13 i InGaAs 24 1.4×1017
InAs 02 i InAlAs 23 1.0×1017
InGaAs 02 i InGaAs 22 1.4×1017
InAs 02 i InAlAs 25 1.0×1017
InGaAs 13 i InGaAs 21 i
Injector Region InAlAs 30 i
InAlAs 9 i InGaAs 20 i
AlAs 2 i InAlAs 33 i
InAlAs 2 i InGaAs 19 i
AlAs 2 i InAlAs 37 i
InAlAs 9 i InGaAs 18 i
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