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New structure of : quantum-cascade vertical- cavity surface-emitting (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

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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 () 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 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

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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 . 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 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 , 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 -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 monolithic DBRs are very high, especially in lasers emitting infra-red radiation, many times larger than semiconductor active areas themselves. In the case of semiconductor lasers emitting infra-red radiation of relatively long wavelengths, such thick laser DBR mirrors create a serious problem. Fortunately it has been found recently, that thick DBR mirrors may be replaced by new sub-wavelength highcontrast-grating (HCG) structures (e.g. [7-9]). In the subwavelength structures grating period is shorter than the wavelength. It seems to be difficult to believe that such a subwavelength HCG VCSEL mirror [10-12] of nearly perfect reflectivity (close to one) may be as thin as only half the wavelength [8,13]. HCG structure [14,15] may be manufacture putting stripes of a high-refractive-index (HRI) material on a substrate manufactured from a low-refractive index material (LRI) [16,17]. Besides these stripes should be completely surrounded by a LRI material. Then optical reflection from HCG structures is caused by a destructive interference of two surface-normal modes resulting from abrupt and large index contrast [8,18]. However, when vacuum or oxides are used as LRI materials, serious problems with the heat-flux spreading from the place of light emission as well as with the current spreading towards this place are generated. To cope with this problem, new monolithic HCG (MHCG) structures have been proposed [19], in which an underlying layer of the exact same material as that used to form the grating [13,20] is applied. It is important to note that the MHCG structure may be manufactured in a variety of transparent materials of refractive indices higher than 1.75 [21,22]. Besides MHCG structures have been found not to scatter radiation [23,24]. On the contrary, MHCG mirrors suppress of the incident wave into higher diffraction orders, therefore they practically reflect only the zero-order plane waves. Currently VCSELs with MHCG cavity mirrors seem to be the best semiconductor lasers emitting infra-red radiation.

4. THE QUANTUM-CASCADE VERTICAL-CAVITY SURFACE-EMITTING LASER From all currently known structures of semiconductor lasers, two of them seem to be the most interesting ones as possible sources of infra-red radiation – VCSELs (because of their excellent narrow low-divergent emission band, a single-mode operation and a possibility to test structure quality before more complex and relatively costly technological operations jointly named processing) and QCLs (because of a possibility to manufacture their structures for an emission of radiation of strictly defined wavelength). Therefore it is interesting to consider a possible cross of VCSELs and QCLs, which may lead to devices of expected advantages of both constituent devices. However, there is a fundamental restriction concerning interaction of a VCSEL radiation with that of QCL: stimulated emission within quantum wells of the QCL laser is possible only for the electro-magnetic radiation polarized perpendicularly to the QC plane [5], whereas, in conventional VCSELs, the radiation is polarized parallelly to the active-area surface.

a) b)

Fig. 1 Wave vectors (red arrows), electric-field vectors (blue arrows) and magnetic-field vectors (green arrows) in a vertical resonant cavity created by the sub-wavelength high contrast grating (HCG) mirror and the distributed-Bragg- reflector (DBR) mirror in the case of the TE polarization (a) and the TM polarization (b) [25]

Proc. of SPIE Vol. 10974 109740A-3 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 12/4/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use Let us, however, consider in detail an interaction of radiation of the VCSEL laser equipped with the MHCG cavity mirrors with radiation of the QCL laser. In such a semiconductor structure, the quantum cascade (QC) is embedded within the sub-wavelength monolithic high-contrast-grating (MHCG) over the distributed-Bragg-reflector (DBR) (Fig. 1). Let us also assume the following co-ordinate system: its z-axis is perpendicular to the edges of the DBR layers, the y-axis is along the HCG bars, and the x-axis is perpendicular to the HCG bars. Then edges of the DBR layers are in the 0xy plane. Following [8], HCG grating is assumed to be infinite in the 0y direction and infinitely periodic in the 0x direction. It is well known, that electrical (E) and magnetic (H) components of the incident electromagnetic plane wave are perpendicular to each other and also to the propagation direction k. The electromagnetic wave is propagated in the 0z direction. It demonstrates the transverse electric field polarization (TE mode) when the electric field E is directed along the grating HCG bars (Fig. 1a), whereas, when this vector is perpendicular to the above bars (Fig. 1b), it is the transverse magnetic field polarization (TM mode) [9]. The quantum cascade within the MHCG stripe may be shifted to increase its interaction with the optical field, which may reduce lasing thresholds. Besides it is also possible in lasers with longer optical cavities to create more QC active regions in successive anti-node positions of the laser standing electro-magnetic wave. Then one electron jumping down between levels in successive quantum wells may create more than just one . As it has been stated a moment ago, stimulated emission within quantum wells of the QC laser requires polarization of the electro-magnetic radiation directed perpendicularly to the QC plane [5]. Then, for radiation propagated within the DBR part of the postulated QC VCSEL, the TE mode radiation (Fig. 1a), i.e. radiation, for which the wave vector k is perpendicular to the edges of the DBR, contains only the kz component. But within the HCG part of this laser, on the other hand, this vector is perpendicular to the HCG bars, so it contains only the kx component. Therefore in the considered case, the magnetic field is only composed of the Hx component within the DBR part which is transformed into the Hz component within the HCG part. The electric field of the TE radiation is then reduced to the Ey component within both the DBR part and the HCG part.

The electric field of the TM radiation (Fig. 1b), on the other hand, is only composed of the Ex component within the DBR part. Then this radiation is transformed into the Ez component within the HCG part, whereas the magnetic field of the TM radiation is reduced to the Hy component within both the DBR part and the HCG part of the QC VCSEL. It is important to note that, as you can see, for the considered here the TM radiation propagated through the quantum cascade (QC) embedded within the sub-wavelength HCG structure, it is possible to induce the Ez electric-field component perpendicular to the epitaxial layers. Therefore we can see that the necessary condition for generation of the stimulated emission [5] within quantum wells of the quantum-cascade laser is fulfilled. Then the quantum-cascade vertical-cavity surface-emitting laser (QC VCSEL) [26] with the upper sub-wavelength MHCG mirror and the bottom DBR mirror seems to be an expected structure of a new semiconductor laser for the TM radiation. This laser is expected to exhibit advantages of both VCSELs and QC lasers without most of their drawbacks.

5. The InP-BASED QC VCSEL Taking into account conclusions of the previous chapter, let us consider a possible structure of the QC VCSEL (Fig. 2) emitting the 9-µm radiation. The laser is equipped with the upper active top mirror in a form of the InP-based MHCG and a bottom wafer-fused [1] GaAs/Al0.9Ga0.1As DBR. The gold layer beneath DBR is add to reduce DBR pairs to 16. The following set of the MHCG parameters (see Fig.3) is applied: L = 4.889 µm, h = 8.995 µm and F = a/ L = 0.402. A thickness hp of the phase-matching layer should be selected separately for each of the MHCG because each of them generates different phase of a reflected wave. The laser cavity is relatively long which enables introducing two 1-µm

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separate Al0.477In0.523As/In0.533Ga0.467As quantum cascades embedded within the MHCG stripes at two anti-node positions of the Ez component of the optical mode. Lateral optical confinement is introduced increasing the period of the last three MHCGs from 4.889 µm to 5.52 µm. Because the length of the MHCG stripes is exactly equal to the product of the stripe number and the stripe periods, the device MHCG is square-shaped in the 0xy plane. To reduce the free-carrier absorption in metal contacts and in highly doped CLs, they are located at nodes of the electro-magnetic standing wave. top quantum insulator contact cascades subwavelength grating bottom contact \k

DBR Au layer

heat -sink

Fig. 2 Cross-section of the MHCG VCSEL configuration consisting of top MHCG with side contacts enabling current injection to each stripe, two QC layers embedded within the MHCG, bottom side contacts, bottom DBR and gold layer. [26]

L a

h

hP I

Fig. 3 Dimensions of a stripe MHCG structure: a – the stripe width, L – the MHCG period, h – the stripe thickness, hp – the thickness of the phase-matching layer. As one can see in Fig. 4, laser operation characteristics are distinctly dependent on a number p of the MHCG stripes: dependence on p of the threshold current density jth and of the power reflectance R is shown in Fig. 4a, whereas Fig. 4b presents analogous dependences of the output power P and the wall-plug efficiency η. Proposed structure of the QC VCSEL is rather complex. Its manufacturing will require a very advanced technology. Fig. 5 shows an impact of deviations of real laser parameters from design ones on a laser threshold current density regarded here as a measure of laser proper operation. It is clearly seen in the picture that exact manufacturing of two laser MHCG parameters, namely the duty cycle F and the thickness of the phase-matching layer hp, has rather insignificant impact on lasing operation whereas exactness of two other parameters, i.e. the grating period L and the stripe height h, is much

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more important. Besides it is necessary to add that, in the proposed QC VCSEL emitting an infra-red radiation, a thickness of the DBR bottom mirror would be very high, probably not acceptable high. Therefore replacing DBR bottom mirrors with the HCG ones should be the first step to modify its design. 5 5.5 w 98.0 0.6 4 all -2] 5.0 - [k 97.5 0.4 3 R pl A [ 4.5 P 2 ug c 97.0 j 0.2 [% 1 4.0 96.5 0.0 0 20 40 60 20 40 60 p p a) b) Fig. 4 Properties of the QC VCSELs as a function of the numbers p of MHCG periods: (a) the threshold current density j (squares, left axis) and optimal optical reflectance R of the MHCG (circles, right axis), (b) emitted optical power P (squares, left axis) and the wall-plug efficiency η (circles, right axis). [26]

10 F ] % L [ h j 5 hp

0 -10 -1 -.3 0 .3 1 10 [%] Fig. 5 Relative deviation of threshold current density (Δj) of QC VCSEL shown as a function of relative deviations of the MHCG parameters: the period (L), the duty cycle (F = a/L) and the height (h) of the MHCG stripes as well as the thickness of the phase matching layer (hp). [26].

6. CONCLUSIONS A role of researchers engaged in theoretical investigations is to create new theoretical approaches to some generally accepted understanding of physical phenomena or to show possible modifications of known scientific solutions. In particular, they are expected. to propose new modifications of known designs of existing electronic devices leading to their improved performance or even to present completely new device structures anticipating their useful new characteristics. The structure of the new semiconductor laser proposed in the present paper is probably too complex to manufacture it using currently existing technology. However its anticipated performance characteristics would be very helpful in many applications. Hence using proposed here methodology, the structure similar to that presented in the present paper may be used to create similar new devices of performance currently unachievable by existing semiconductor lasers.

ACKNOWLEDGEMENTS This work is supported by the Polish National Science Centre (NSC) through the project 2017/25/B/ST7/02380.

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