PROCEEDINGS OF SPIE SPIEDigitalLibrary.org/conference-proceedings-of-spie New structure of semiconductor lasers: quantum-cascade vertical- cavity surface-emitting laser (QC VCSEL) Włodzimierz Nakwaski, Sandra Grzempa, Maciej Dems, Tomasz Czyszanowski Włodzimierz Nakwaski, Sandra Grzempa, Maciej Dems, Tomasz Czyszanowski, "New structure of semiconductor lasers: quantum-cascade vertical-cavity surface-emitting laser (QC VCSEL)," Proc. SPIE 10974, Laser Technology 2018: Progress and Applications of Lasers, 109740A (4 December 2018); doi: 10.1117/12.2519617 Event: Thirteenth Symposium on Laser Technology, 2018, Jastarnia, Poland Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 12/4/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use New structure of semiconductor lasers: quantum cascade vertical- cavity surface-emitting laser (QC VCSEL) Włodzimierz Nakwaski*, Sandra Grzempa, Maciej Dems, and Tomasz Czyszanowski Photonics Group, Institute of Physics, Lodz University of Technology, 219 Wolczanska, 93-005 Lodz, Poland ABSTRACT A new structure of semiconductor lasers called the quantum-cascade vertical-cavity surface emitting laser (QC VCSEL) is proposed in the present paper. A structure of the QC VCSEL is a cross of the quantum-cascade laser (QCL) and the vertical-cavity surface-emitting laser (VCSEL). The QC VCSEL is expected to demonstrate important advantages of laser emission of both the QCL and the VCSEL without their drawbacks. In the QC VCSEL, the monolithic high- contrast grating (MHCG) structure is applied to cope with the fundamental requirement of the polarization direction of the electro-magnetic radiation perpendicular to the quantum cascade (QC) necessary to initiate within it the stimulated emission. The QC VCSEL structure recommended in the present paper is a result of the advanced modeling with the aid of our comprehensive self-consistent optical-electrical model. Keywords: Sub-wavelength grating structure, quantum cascade laser, vertical-cavity surface emitting laser 1. INTRODUCTION LASER is an abbreviation of Lightwave Amplification by Stimulated Emission of Radiation. But usually we are using this notion as a name of electronic devices emitting stimulated laser radiation. The laser radiation is: - monochromatic, which means that its all radiation particles (photons) are characterized by the same wavelength, - collimated - all laser beams are unidirectional and are parallel to each other and - coherent – all laser photons are in phase. It is interesting to note that nature itself has been successful in creating and taking advantage of nearly all later human inventions and achievements including even nuclear and thermonuclear energy. But it could not create and/or applied laser radiation. Therefore our eyes (and eyes of all other animals) are not sensitive to different nature of laser radiation than common features of ordinary one. The above means that all animals including human creatures are not able to distinguish laser emission from other kinds of radiation. For our eyes laser radiation is practically the same as all kinds of other radiation. Semiconductor lasers are lasing devices built of semiconductor materials. There are very many possible configurations of these lasers The most important ones for infrared communication are considered and compared in the second section of this paper. Then expected properties of these lasers are defined and accurately described. None of currently existing semiconductor lasers can match all these requirements. Therefore a new semiconductor structure, namely the high- contrast grating structure is considered in the present paper as a possible solution of this problem. For that reason, a new semiconductor laser structure, called the quantum-cascade vertical-cavity surface-emitting laser (QC VCSEL), has been proposed. The QC VCSEL structure recommended in the present paper is a result of advanced modelling of anticipated performance of possible laser structures with the aid of our comprehensive self-consistent optical-electrical model. [1-3]. *[email protected]; phone: (4842) 631-3641 fax: (4842) 631-3639 Laser Technology 2018: Progress and Applications of Lasers, edited by Jan K. Jabczyński, Ryszard S. Romaniuk, Proc. of SPIE Vol. 10974, 109740A · © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2519617 Proc. of SPIE Vol. 10974 109740A-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 12/4/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use 2. STRUCTURES OF SEMICONDUCTOR LASERS There are numerous possible structures of semiconductor lasers. First ones are built of both the n-type and p-type materials and contain the p-n junction. They are usually called diode lasers or laser diodes. Historically the first one and still very popular diode laser is the edge-emitting laser (EEL) proposed in 1962. Its manufacturing is relatively easy but its operation characteristics are far from expected ones. Diode lasers are emitting very high outputs but their output beams are very divergent as going out from a thin optical cavity. Besides their output beams are not symmetrical, manifest astigmatism and their radiation contains many longitudinal modes. Much better operation characteristics are demonstrated by later vertical-cavity surface-emitting lasers (VCSELs). Their radiation contains inherently a single longitudinal mode (if any), their output beam is narrow, symmetrical and without astigmatism. Besides, quality of their structures may be tested before more complicated technology processes, called jointly – processing, which enables casting off incorrect structures in good time and drastically reduces manufacturing costs. The most essential disadvantage of VCSEL radiation is connected with relatively low their outputs, therefore they should be used in applications not requiring higher radiation power. All earlier structures of semiconductor lasers, i. e. diode lasers, suffer from very essential limitations connected with inter-band carrier recombination used in these devices – in all diode lasers photons are emitted as a result of recombination of the electrons from the conduction band and the holes from the valence band, which are separated by the energy gap [4]. Therefore emission of radiation of strictly defined energy (which means also of its strictly defined wavelength) requires finding first a semiconductor material of an energy gap very close to this energy. Besides, taking into account many additional requirements associated with expected values of other material parameters as electrical resistivities, thermal conductivities, possible doping, etc., there are very few semiconductor materials, which may be used to manufacture efficient diode lasers taking advantage of hitherto known semiconductor structures. Hence disappointingly very limited number of accessible laser energies (and wavelengths) emitted by diode lasers follows from the above feature. Some years ago a new structure of semiconductor lasers has been proposed. There are quantum-cascade semiconductor lasers (QCLs) [5,6]. It is important to note, that a completely new kind of radiative recombination is used in these lasers. It is not the inter-band recombination as in all earlier semiconductor lasers but the intra-band recombination of carriers during their transitions between successive energy levels in quantum wells (QWs). This kind of recombination is completely not connected with semiconductor energy gap. It is even possible to design QW structures for emission of strictly specified expected energy, i.e. the defined wavelength of laser emission. Besides, in such devices we do not have the electron-hole recombination as in all earlier semiconductor lasers - therefore QCLs do not need to have a p-n junction, they may be produced as unipolar devices. They are definitely not diode lasers. In QCLs a laser emission follows jumping down carriers of one kind (electrons or holes) between QW energy levels. Usually they are electrons. These devices do not need to have even the p-n junction. With some obvious limits associated with limited QW depths, quantum-cascade lasers may be designed for an emission of laser radiation of any infra-red wavelength. Let us consider an optimal structure of a semiconductor laser used as a source of laser infra-red radiation. It should demonstrate a low threshold current, its emission beam should be narrow, symmetrical and without astigmatism, its radiation should contain a single longitudinal mode, its technology could not be costly and complicated and it should enable high modulation speed. Comparing operation characteristics of all known semiconductor lasers, we should conclude that VCSEL structure is the closest to the optimal semiconductor laser structure demonstrating however not too high output power. 3. HIGH-CONTRAST GRATING STRUCTURES The laser cavity in semiconductor lasers is always composed of an active region between two resonator mirrors. Until very recently VCSEL cavity mirrors have been practically always manufactured as distributed Bragg reflectors (DBRs). To increase their reflectivity, very high number of DBR layer pairs of alternating (high and low) refractive indices as Proc. of SPIE Vol. 10974 109740A-2 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 12/4/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use well as high contrast between their values are necessary. Then thicknesses of such high-reflective