OSA's Digital Library

Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 3, Iss. 11 — Nov. 1, 2013
  • pp: 1872–1884
« Show journal navigation

Recent advances in mid infrared (3-5µm) Quantum Cascade Lasers

Manijeh Razeghi, Neelanjan Bandyopadhyay, Yanbo Bai, Quanyong Lu, and Steven Slivken  »View Author Affiliations


Optical Materials Express, Vol. 3, Issue 11, pp. 1872-1884 (2013)
http://dx.doi.org/10.1364/OME.3.001872


View Full Text Article

Acrobat PDF (1455 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

Quantum cascade laser (QCL) is an important source of electromagnetic radiation in mid infrared region. Recent research in mid-IR QCLs has resulted in record high wallplug efficiency (WPE), high continuous wave (CW) output power, single mode operation and wide tunability. CW output power of 5.1 W with 21% WPE has been achieved at room temperature (RT). A record high WPE of 53% at 40K has been demonstrated. Operation wavelength of QCL in CW at RT has been extended to as short as 3µm. Very high peak power of 190 W has been obtained from a broad area QCL of ridge width 400µm. 2.4W RT, CW power output has been achieved from a distributed feedback (DFB) QCL. Wide tuning based on dual section sample grating DFB QCLs has resulted in individual tuning of 50cm−1 and 24 dB side mode suppression ratio with continuous wave power greater than 100mW.

© 2013 OSA

Introduction

The quantum cascade laser [1

1. M. Razeghi, “High-Performance InP-based Mid-IR Quantum Cascade Lasers,” IEEE J. Sel. Top. Quantum Electron. 15(3), 941–951 (2009). [CrossRef]

] is a semiconductor laser where electromagnetic radiation is achieved by intersubband transitions between energy levels inside superlattice quantum wells. In comparison to conventional interband lasers in near infrared, the energy of intersubband transitions can be tuned over a very wide wavelength range from 3 to 15µm by tuning the compositions and thickness of wells and barriers. The intersubband lasers are free from Auger recombination which limited high temperature operation of conventional lasers in mid-infrared region. Lastly, multiple stages can be cascaded in series to achieve a large output power for various applications. Due to all these attractive properties, QCL is a compact semiconductor source of choice in the wavelength range of 3-5µm, corresponding to the first atmospheric window, inside mid-infrared spectrum. Specifically, a lot of research has taken place around 4.5-5µm which is technologically important for infrared countermeasure and spectroscopic application, and it has resulted in record room temperature (RT) continuous wave (CW) output power [2

2. Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. 98(18), 181102 (2011). [CrossRef]

] and wall plug efficiency (WPE) [3

3. Y. Bai, S. Slivken, S. Kuboya, S. R. Darvish, and M. Razeghi, “Quantum cascade lasers that emit more light than heat,” Nat. Photonics 4(2), 99–102 (2010). [CrossRef]

]. The wavelength 3-4µm, also known as the spectroscopic finger print region of many important hydrocarbons, also attracted the attention of the QCL community. The wavelength of CW operation at RT has been extended to 3µm [4

4. N. Bandyopadhyay, Y. Bai, S. Tsao, S. Nida, S. Slivken, and M. Razeghi, “Room temperature continuous wave operation of λ ~ 3-3.2 μm quantum cascade lasers,” Appl. Phys. Lett. 101(24), 241110 (2012). [CrossRef]

], which is extremely attractive for spectroscopic detection of large number of molecules.

Highest Power CW wave, RT operation of QCL

The QCL wafer was processed into double channel geometry of width 19µm and 8 µm for pulsed and CW operation, respectively. A laser with a cavity length of 3mm was tested under pulsed mode operation with a pulse width of 500ns and a duty cycle of 5%. It yielded a maximum pulsed WPE of 27%. Buried ridge processing was performed on 8µm wide ridge for CW operation and cavity length of 5mm was cleaved. A maximum output power of 5.1W and WPE of 21% is obtained in CW mode at RT. The power current voltage curve and WPE curve for a QCL under CW operation at RT are shown in Fig. 1 (b).

Highest WPE QCL

Figure 2 (a) shows the conduction band edge and wave function diagram. Figure 2 (b) shows the WPE vs current characteristic of a λ~5µm QCL having ridge width of 6µm and cavity length of 2 mm. Inset of (a) shows the graph of maximum WPE vs heat sink temperature, which clearly demonstrates rapid backfilling of lower laser level with temperature.
Fig. 2 (a) Conduction band edge and wave function diagram for single well injector QCL. (b) WPE vs current characteristic of a device having ridge width of 6µm and cavity length of 2 mm at 40K. Inset of (b) shows maximum WPE vs heat sink temperature. Maximum WPE of 53% occurs around 40K.

Watt level CW operation of 3.76 um QCL at RT

One of the ways to implement 3-4um QCL is to apply the knowledge of QCLs obtained from the higher wavelength side of 4-5 µm. Similar to its longer wavelength counterpart, 3 well active region or single phonon resonance was used to design the active region since in comparison to double phonon resonance or bound to continuum designs, the upper laser level is lower down in terms of absolute energy in quantum well system. This creates better confinement of electrons and increases the activation energy of electron escape through optical phonon emission and thermal emission to the continuum. The strain of the superlattice was increased and Ga0.24In0.76As/ Al0.76In0.24As superlattice was grown, to accommodate a large optical transition at shorter wavelength [5

5. N. Bandyopadhyay, Y. Bai, B. Gokden, A. Myzaferi, S. Tsao, S. Slivken, and M. Razeghi, “Watt level performance of quantum cascade lasers in room temperature continuous wave operation at λ ~ 3.76 μm,” Appl. Phys. Lett. 97(13), 131117 (2010). [CrossRef]

].

The QCL structure was grown in a single growth run on n- doped (Si, ~2E17cm−3) InP substrate, in a gas source molecular beam epitaxy reactor. The layer sequence and average doping levels are as follows: 1µm buffer layer acting as the lower cladding (Si, ~2E16cm−3), 30 period laser core (Si, ~2.8E16cm−3), 3µm upper cladding (Si, ~2E16cm−3), and 1µm cap layer (Si, ~1E19cm−3). The experimental and simulated (X’Pert Epitaxy) x-ray diffraction curves for the laser core are shown in Fig. 3.
Fig. 3 Experimental and simulated x-ray diffraction curve of the laser core. The satellite peaks have minimum FWHM of 21.2 arc seconds.
Excellent agreement has been found between the two curves, confirming the material compositions. The increase in strain in the superlattice was accompanied by sharp interface as was determined by low background and sharp higher order superlattice peaks in x-ray, with the smallest full width at half maximum (FWHM) of the satellite peak being 21.2 arc sec.

The wafer was processed into buried ridge geometry with ridge width of 8.3µm and a device with cavity length of 4mm was cleaved. The maximum RT, CW output power was 1.1 W, with a threshold current density of 1.67KA/cm2 and slope efficiency near threshold of 2.16 W/A. A maximum RT, WPE of 6% and 10% was obtained in CW and pulsed operation, respectively. Figure 4 represents the P-I-V curve and corresponding WPE for of a HR coated 8.3µm wide and 4mm QCL, at RT.
Fig. 4 Pulsed (dashed) and CW (solid line) P-I-V curve for a HR coated 8.3µm wide and 4mm long, buried ridge heterostructure QCL at RT. Corresponding WPE is also plotted. Inset shows spectrum in CW operation at 0.6A.

Greater than 400 mW of RT, CW output power at 3.4-3.55 µm QCL

Many important hydrocarbons have their fundamental modes of vibration in the 3-3.6 um electromagnetic spectrum range. As a result, an efficient source of radiation within this wavelength range is essential in spectroscopic detection of trace hydrocarbons. The 3-3.6um QCL has to accommodate a large energy transition within the quantum well. At present there are three competing technologies at short wavelength, namely InAs/AlSb on InAs, GaInAs/AlAs(Sb) on InP, GaInAs/AlInAs on InP. The InAs based material with high conduction band offset of ~2.1eV and large separation of Γ and L conduction bands are attractive for short wavelength QCLs. However, large broadening due to interface scattering, and incompatibility of InAs material growth and waveguide technology with the existing InP technology are negative points. The GaInAs/AlAs(Sb) on InP is attractive due to its large conduction band offset of 1.6eV and compatibility with InP technology. But Sb based materials are not standard component in an MBE, and heat extraction from active region becomes difficult due to poor conductivity of Sb compound. The GaInAs/AlInAs on InP has a conduction band offset of only 0.52 eV in lattice matched condition. This value can be increased to as high as 1.2 eV by incorporating strain in the active region so that the net strain per period is close to zero. Two QCLs with cores of Ga0.2In0.8As/ Al0.82In0.18As superlattice were grown with slight difference in their layer thicknesses [6

6. N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “High power continuous wave, room temperature operation of λ~3.4 µm and λ~3.55µm InP-based qantum cascade lasers,” Appl. Phys. Lett. 100(21), 212104 (2012). [CrossRef]

]. Intervalley scattering is a major challenge in this wavelength as activation energy of electron escaping from the upper level to the lowest L valley state in the quantum well is reduced. Degradation of high temperature operation is a direct effect of this phenomenon. At present, this effect is minimized by increasing the Indium concentration inside GaInAs, which pushes up the lower L valley state. Thus, highly strained GaInAs reduces the intervalley carrier escape. No dislocation is found in the core region even at this high strain. To verify the material quality of the superlattice at this large strain, λ~3.56µm electroluminescence (EL) sample was prepared and tested with 200ns pulses at 10 kHz frequency. The linewidth is found to be 47.4 meV, showing a good material quality. Figure 5 shows the pulsed EL spectrum of the QCL at RT.
Fig. 5 RT, pulsed mode EL spectrum of λ~3.56µm QCL as a function of voltage

The two wafers were processed into double-channel laser with widths of 10.5µm and 8.6 µm for λ~3.56µm and λ~3.39µm QCL, respectively. Maximum RT, CW powers of 437 mW and 403mW were obtained at λ~3.56µm and λ~3.39µm respectively. Figure 6 shows P-I-V curve for CW operation at different heat sink temperatures. The inset shows the respective emission spectrum.
Fig. 6 (a) P-I-V curve of a HR coated 10.5 µm wide and 5mm long QCL emitting at 3.56µm, at RT. (b) P-I-V curve of a HR coated 8.6 µm wide and 5mm long QCL emitting at 3.39µm, at RT. The insets of both show their respective CW, RT emission spectrum.

CW, RT operation of 3-3.2 µm QCL

For 3-3.2µm QCL, even higher conduction band offset is required for electron confinement. This is achieved by increasing Al composition in the AlInAs barrier to 85% and 89% for emission at 3.2 and 3 µm, respectively [4

4. N. Bandyopadhyay, Y. Bai, S. Tsao, S. Nida, S. Slivken, and M. Razeghi, “Room temperature continuous wave operation of λ ~ 3-3.2 μm quantum cascade lasers,” Appl. Phys. Lett. 101(24), 241110 (2012). [CrossRef]

]. The Ga composition inside GaInAs well was 23% and 21% respectively. However, extremely high tensile strain level in AlInAs can cause relaxation in the core region, if the global strain is not zero or the thickness of individual layer is larger than the critical thickness. In order to neutralize the net strain per period, Al0.6In0.4As/Al0.89In0.11As composite barriers was used as in the injector region instead of incorporating more Indium inside GaInAs, as pseudomorphic growth of extremely high strained GaInAs is very difficult. In this approach the coupling of the wavefunction in the injectors was ensured by choosing proper thickness of the composite barrier, while the addition of tensile strain was minimized by tuning the composition of lesser strained AlInAs composition. Conduction band edge and wave function diagram for a 3µm QCL is shown in Fig. 7.
Fig. 7 Conduction band edge and wave function diagram for a 3µm QCL

A 3µm broad area QCL having ridge width and cavity length of 200 µm and 5 mm, respectively, shows a maximum pulsed output power of 10 W at RT (Fig. 8(a)).
Fig. 8 (a) Pulsed P-I-V curve of a HR coated 200 µm wide and 5mm long QCL emitting at 3.02 µm, at RT. (b) CW P-I-V curve of a HR-PHR coated 7 µm wide and 5mm long QCL emitting at 3.23 µm, at RT. (c) CW P-I-V curve of a HR-PHR coated 3 µm wide and 5mm long QCL emitting at 3.02µm, at RT. The insets of both show their respective CW, RT emission spectrum.
For RT, CW operation, a combination of buried ridge processing, narrow ridge width, epi-down bonding on diamond submount is done. As short wavelength QCLs have a small differential gain, to reduce the threshold current, the front facet is partially high reflection (PHR) coated and back side is high reflection (HR) coated. The PHR has a reflectivity of 85%, hence reduces the mirror loss from 1.4cm−1 for a HR coated back and uncoated front facet, to only 0.21cm−1 for HR coated back and PHR coated front facet. A 3.2 µm QCL having a ridge width and cavity length of 7µm and 5 mm, respectively, shows a maximum CW output power of 20mW at RT (Fig. 8(b)). The emission spectrum at a driving current of 0.5A is shown, in inset, to have a wavelength of 3.23 µm. A 3µm QCL having a ridge width and cavity length of 3µm and 5 mm, respectively, shows a maximum CW output power of 2.8 mW at RT (Fig. 8(c)). The emission spectrum at a driving current of 0.4A is shown, in inset, to have a wavelength of 3.02 µm. The minimization of lasing volume, efficient heat removal, and reduction of threshold current density by partial reflective coating on the front facet to decrease the mirror loss enable CW operation at RT. The 3.02 µm emission is the shortest wavelength demonstration of CW operation at RT, for any QCL.

The composite barriers have been generally used in short wavelength QCLs. The pros of using them in longer wavelength QCLs are larger conduction band offset in the active region and decrease in thermally activated carrier escape from the upper laser level into the continuum. The cons are that the higher strained material is more difficult to grow than milder strained ones and may lead to rougher interfaces. This may lead to broadening of the transition, due to higher interface scattering, and a decrease in the differential gain.

Broad Area QCL with 190 W of pulsed output power at RT

High power, CW operation at RT, is possible only with narrow ridge widths. However, in some applications where very high peak power is required, the QCL peak power can be scaled up by increasing the ridge width, thus increasing the laser gain volume. This trend can be observed for scaling of power from 50µm to 400µm [7

7. Y. Bai, S. Slivken, S. R. Darvish, A. Haddadi, B. Gokden, and M. Razeghi, “High power broad area quantum cascade lasers,” Appl. Phys. Lett. 95(22), 221104 (2009). [CrossRef]

]. Both the threshold current and roll over current, increases linearly with ridge width. However, WPE decreases with ridge width from 16% at a width of 50µm to 12% at a width of 200µm and then saturates. This behavior may be attributed to heating which increases with ridge width and then saturates when the ridge width approaches the thickness of the substrate. P-I-V curve for a BA QCL with a ridge width of 400µm wide and cavity length of 3 mm is shown in Fig. 9 (a).
Fig. 9 (a) RT, P-I-V curve of a BAQCL having a ridge width of 400µm and cavity length 4mm, showing a peak power output of 120W. (b) RT, P-I-V curve of a BAQCL having a ridge width of 800µm and cavity length 3mm, showing a peak power output of 190W.
A high peak power of 116 W is obtained. The lasing far field consists of distinct dual lobes at angles around ± 38°C which suggest that near field is composed of, periodic local maxima. This also indicates that the near field is stable and there is no random formation of filament, which creates many narrow, bright spots at the near field. This is of course expected, as the linewidth enhancement factor which is one of the main reasons for filament formation, is much less than conventional interband lasers. To test if even more peak power can be achieved, another BAQCL with a ridge width of 800µm and cavity length of 3mm was processed from a separate λ~4.5µm QCL wafer and tested with 200 ns pulses of 0.02% duty cycle at RT. Power was measured by placing the thermopile directly in front of the laser facet. The peak power was 190W (Fig. 9 (b)). The kinks at the high current were due to change in the lateral modes.

2.4W RT, CW power output of distributed feedback grating QCL

Widely Tunable, dual section, sampled grating DFB (SGDFB) QCL

A SGDFB QCL with front and rear section of ~1.6mm and ~1.4mm is fabricated. Both sections use 30 period grating which is sampled 3 times, where the grating period is 753nm.The wafer is processed into double channel ridge waveguide with ridge width of 10µm and cleaved into a device cavity length of 3mm. The tuning characteristic, output power and SMSR is shown in Fig. 11.
Fig. 11 The bottom figure shows single-mode emission spectra for an electrically-tuned, sampled grating laser at RT. Top most figure shows continuous wave output power, the middle one shows SMSR as a function of emission wavelength.
Single mode tuning of more than 50cm−1 is obtained. More than 100 mW of CW power, with a mean SMSR of 24 dB is obtained at RT over the tuning range.

Conclusion

In summary, InP based quantum cascade laser technology has demonstrated its versatility as a source in the mid-infrared. However, many challenges remain which limit the performance of QCLs. For example, non-idealities such as intervalley scattering, carrier escape to the continuum, heat removal from the core region, and interface scattering still effects the RT, performance of QCL, especially at short wavelengths. Some of these can be lessoned or removed by better gain medium and waveguide design strategies, while the quality of strained material growth can be improved to reduce linewidth of transition. QCLs with spatially and spectrally pure single mode, and wide tuning, are also being explored for practical applications, by borrowing some successful ideas from the near infrared laser technology. Thus InP based QCL technology is currently observing a rapid progress to achieve even more efficient compact, room temperature, spectrally pure sources for different applications

Acknowledgments

References and links

1.

M. Razeghi, “High-Performance InP-based Mid-IR Quantum Cascade Lasers,” IEEE J. Sel. Top. Quantum Electron. 15(3), 941–951 (2009). [CrossRef]

2.

Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. 98(18), 181102 (2011). [CrossRef]

3.

Y. Bai, S. Slivken, S. Kuboya, S. R. Darvish, and M. Razeghi, “Quantum cascade lasers that emit more light than heat,” Nat. Photonics 4(2), 99–102 (2010). [CrossRef]

4.

N. Bandyopadhyay, Y. Bai, S. Tsao, S. Nida, S. Slivken, and M. Razeghi, “Room temperature continuous wave operation of λ ~ 3-3.2 μm quantum cascade lasers,” Appl. Phys. Lett. 101(24), 241110 (2012). [CrossRef]

5.

N. Bandyopadhyay, Y. Bai, B. Gokden, A. Myzaferi, S. Tsao, S. Slivken, and M. Razeghi, “Watt level performance of quantum cascade lasers in room temperature continuous wave operation at λ ~ 3.76 μm,” Appl. Phys. Lett. 97(13), 131117 (2010). [CrossRef]

6.

N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “High power continuous wave, room temperature operation of λ~3.4 µm and λ~3.55µm InP-based qantum cascade lasers,” Appl. Phys. Lett. 100(21), 212104 (2012). [CrossRef]

7.

Y. Bai, S. Slivken, S. R. Darvish, A. Haddadi, B. Gokden, and M. Razeghi, “High power broad area quantum cascade lasers,” Appl. Phys. Lett. 95(22), 221104 (2009). [CrossRef]

8.

Q. Y. Lu, Y. Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “2.4W room temperature continuous wave operation of distributed feedback quantum cascade lasers,” Appl. Phys. Lett. 98(18), 181106 (2011). [CrossRef]

9.

S. Slivken, N. Bandyopadhyay, S. Tsao, S. Nida, Y. Bai, Q. Y. Lu, and M. Razeghi, “Sampled grating, distributed feedback quantum cascade lasers with broad tunability and continuous operation at room temperature,” Appl. Phys. Lett. 100(26), 261112 (2012). [CrossRef]

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.3490) Lasers and laser optics : Lasers, distributed-feedback
(140.3600) Lasers and laser optics : Lasers, tunable
(230.5590) Optical devices : Quantum-well, -wire and -dot devices
(140.5965) Lasers and laser optics : Semiconductor lasers, quantum cascade

ToC Category:
Laser Materials

History
Original Manuscript: July 8, 2013
Revised Manuscript: August 10, 2013
Manuscript Accepted: August 14, 2013
Published: October 10, 2013

Virtual Issues
Mid-IR Photonic Materials (2013) Optical Materials Express

Citation
Manijeh Razeghi, Neelanjan Bandyopadhyay, Yanbo Bai, Quanyong Lu, and Steven Slivken, "Recent advances in mid infrared (3-5µm) Quantum Cascade Lasers," Opt. Mater. Express 3, 1872-1884 (2013)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-11-1872


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. M. Razeghi, “High-Performance InP-based Mid-IR Quantum Cascade Lasers,” IEEE J. Sel. Top. Quantum Electron.15(3), 941–951 (2009). [CrossRef]
  2. Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett.98(18), 181102 (2011). [CrossRef]
  3. Y. Bai, S. Slivken, S. Kuboya, S. R. Darvish, and M. Razeghi, “Quantum cascade lasers that emit more light than heat,” Nat. Photonics4(2), 99–102 (2010). [CrossRef]
  4. N. Bandyopadhyay, Y. Bai, S. Tsao, S. Nida, S. Slivken, and M. Razeghi, “Room temperature continuous wave operation of λ ~ 3-3.2 μm quantum cascade lasers,” Appl. Phys. Lett.101(24), 241110 (2012). [CrossRef]
  5. N. Bandyopadhyay, Y. Bai, B. Gokden, A. Myzaferi, S. Tsao, S. Slivken, and M. Razeghi, “Watt level performance of quantum cascade lasers in room temperature continuous wave operation at λ ~ 3.76 μm,” Appl. Phys. Lett.97(13), 131117 (2010). [CrossRef]
  6. N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “High power continuous wave, room temperature operation of λ~3.4 µm and λ~3.55µm InP-based qantum cascade lasers,” Appl. Phys. Lett.100(21), 212104 (2012). [CrossRef]
  7. Y. Bai, S. Slivken, S. R. Darvish, A. Haddadi, B. Gokden, and M. Razeghi, “High power broad area quantum cascade lasers,” Appl. Phys. Lett.95(22), 221104 (2009). [CrossRef]
  8. Q. Y. Lu, Y. Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “2.4W room temperature continuous wave operation of distributed feedback quantum cascade lasers,” Appl. Phys. Lett.98(18), 181106 (2011). [CrossRef]
  9. S. Slivken, N. Bandyopadhyay, S. Tsao, S. Nida, Y. Bai, Q. Y. Lu, and M. Razeghi, “Sampled grating, distributed feedback quantum cascade lasers with broad tunability and continuous operation at room temperature,” Appl. Phys. Lett.100(26), 261112 (2012). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited