- Email: [email protected]
High-efficiency electron-beam-pumped semiconductor laser emitters
Physica B 185 (1993) North-Holland
PHYSICA II
505-507
High-efficiency emitters
electron-beam-pumped
A.L. Gurskii, E.V. Lutsenko, B.I.
semiconductor
laser
A.I. Mitcovets and G.P. Yablonskii
Stepanov Institute of Physics, Academy
of Science of Belarus, Minsk, Belarus
The operational parameters of semiconductor electron-beam-pumped lasers made by a new technology are described. The lasing regimes and the parameters of electron-beam-pumped semiconductor lasers based on &Se, CdS,Se,_,, Zn,Cd, _.S, CdTe, GaAs crystals in the 460-900 nm range have been studied. The radiation from these lasers was used to excite generation in a number of media.
One of the problems in creating powerful ‘radiating-mirror’ type lasers is increasing their output power. At the present time, the restricting factor of the increase is the low beam stability of the cavity mirrors which are usually prepared by vacuum deposition. For semiconductor lasers with longitudinal electron-beam-pump destruction of mirrors begins at power densities 10MW/cm2 [l] and less. Another disadvantage of the traditional reflective coatings is the nonselectivity of their reflective properties in the direction which requires special measures to suppress spurious modes. In the present paper, by the example of a multicomponent semiconductor laser with a longitudinal electron-beam pump, it is shown how we can overcome, to a certain extent, the above disadvantages and increase the laser efficiency and power if we abandon the deposition of reflecting coatings. To make multi-element lasers with longitudinal excitation by an electron beam, commercial semiconductor compounds were used. They were 200 km-thick monocrystal plates, 5 cm in diameter, cemented on quartz substrates. Preliminarily dielectric output mirrors with reflection coefficient of 30-70% were placed on the surface of the plate to be cemented. Then the plates were Correspondence to: A.L. Gurskii, B.I. Stepanov Institute of Physics, Academy of Science of Belarus, Scaryna pr., 70, Minsk 220602, Belarus. 0921-4526/93/$06.00
0
1993 - Elsevier
Science
Publishers
cut into 0.5-l mm square elements to a depth of 180 pm. A totally reflecting silver mirror was deposited onto the surface facing the electron beam or this surface was subjected to chemical etching by special technology [2] to create a certain micro-relief on the surface. For example, for the CdS crystals the treated surface represents a set of densely spaced, cut etch hills with sizes from fractions to dozens of micrometers [3]. Such a relief in the first approximation is an analog of a multitude of corner reflectors and exhibits the property of selective reflection, extracting the spurious mode radiation from the cavity. Semiconductor lasers were excited by an electron beam with electron energy 200 keV, halfheight pulse duration 1.5 ns and current density of the beam j = 20-200 A/cm’, The beam was brought into the atmosphere through a 20 pm thick titanium foil 14 or 22 mm in diameter. The luminescence and generation spectra of the crystals were recorded by a spectrum analyzer with a diffraction grating of 1200 mm-’ and linear inverse dispersion of 2.4nm/mm. The duration and shape of the light pulses were determined by a photodetector and an oscilloscope, the energy characteristics were measured by an acoustooptical meter and a pyroelectric detector. Semiconductor lasers were created using ZnSe, Zn,Cd,_,S, CdS,Se,_,, CdTe, GaAs, which provide lasing over a wide spectral range
B.V. All rights
reserved
A. L. Gurskii et al. I High-efficiency
506
electron-beam-pumped
(from 470 to 900 nm). The characteristic halfwidth of the radiation line was 2-6 nm depending on the composition of the semiconductor compound and the exciting electron beam power. Characteristically, when the excitation power density is increased to about 15 MW/cm2, the maximum of the induced radiation spectrum is not shifted and corresponds to the maximum of the luminescence spectrum. Further increase in the level of excitation leads to a considerable longwave shift of the radiation spectrum maximum. For instance, at power density 70MW/ cm’, the value of the shift of the band maximum relative to the initial spectrum is 5.5 nm. The longwave shift is also accompanied by broadening of the radiation band from 2.2 to 5 nm and a change in its form. In our opinion, such behavior of the radiation line is explained by the fact that under powerful excitation of semiconductor crystals, the mechanisms of stimulated radiation are associated with degeneration of electron-hole plasma and due to this, broadening of the band gap. Indeed, as follows from the literature data, the band gap narrowing manifests itself at charge carrier concentrations comparable to the critical Mott concentration n = 7 X 1Or’ cme3. In our case, a shift is observed at a power density >13.5 MWlcm’, which corresponds to a charge carrier concentration y1> 2.5 x 1018cmP3. A Table 1 Parameters
of semiconductor
N
Semiconductor compounds
1 2 3 4 5 6 7 8 9 10 11 4 7 8 9 10 11
CdSSe CdSSe CdSSe CdSSe CdS CdS CdS ZnSe ZnCdS CdTe GaAs CdSSe CdS ZnSe ZnCdS CdTe GaAs
electron-beam-pumped
semiconductor
laser emitters
similar behavior of the radiation line is also characteristic of other crystals under study. The energy and spatial characteristics of the radiation parameters of semiconductor lasers with longitudinal excitation by an electron beam are known to substantially depend on the methods of preparing the crystal surfaces. In addition, the radiation efficiency is also determined by the technology of crystal growth in which, in particular, depends the value of the internal quantum yield of luminescence. Therefore, to compare the influence of the method of working the crystal surfaces with the energy characteristics of lasers, the semiconductor plates were cut from neighboring parts of the boule. Table 1 gives the following parameters for different samples of semiconductor radiators: the plate thickness d, the cut step h, the reflection coefficient of the output face with a dielectric coating R (without coating, R = 30%), the wavelength of the induced radiation band maximum, the power density of the output radiation W,, and the efficiency of these radiators with reference to electron beam energy. The last column gives additional characteristics of these samples: the presence of a microrelief or a silver mirror on the input surface. The radiant energy of the plates was measured at a constant value of W, = 26 MWlcm2 (t = 1.5 ns) which correspon-
lasers.
d
h
R
km) 200 150 110 210 185 110 210 200 200 200 200 210 210 200 200 200 200
(mm)
(%I
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5 1.0 1.0 1.0 1.0 1.0 0.5
70 50 50 30 70 70 30 50 70 70 50 30 30 50 70 70 50
619 619 619 586 529 529 529 471 518 856 894 586 529 471 518 856 894
$*/cm’)
Efficiency (%)
Note
0.16 0.23 0.25 0.25 0.21 0.16 0.28 0.22 0.05 0.17 0.42 2.3 3.02 2.26 0.53 0.35 0.86
0.8 1.2 1.3 1.2 1.03 0.8 1.4 1.1 0.24 0.86 2.1 11.5 15.1 12.5 2.6 1.74 4.3
Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Microrelief Microrelief Microrelief Microrelief Microrelief Microrelief
A. L. Gurskii et al. I High-ej‘iciency
electron-beam-pumped
of semiconductor-laser
N
Active element
1 1 1 2 3 4 5 6 7
LiF( OH) : F; LiF(OH) : F; LiF(OH) : F; LiF( OH) : FL’ LiF( Mg) : Fl KGW: Nd YAG : Nd KNFS AI>O, : Ti
pumped
507
References [l]
0.V Bogdankevich, N.D. Vorob’ev, M.M. Zverev, S.P. Kopyt, E.M. Krasavina, 1.V Kryukova, VP. Petrov, V.A. Usakin and V.K. Yakushin, Sov. .I. Quant. Electron. 12 (1985) 1519. [2] V.P. Gribkovskii, V.V. Grusinskii, A.L. Gurskii et al., Inventor’s certificate SU No. 1653514, IPC HOlS 3118. [3] G.P. Yablonskii, Phys. Stat. Sol. A 92 (1985) 431.
solid lasers. max
7e
Size
Efficiency
(ns)
(nm)
(ns)
(mm)
(%)
Concentration
2.0 2.0 2.0 8.0 2.0 2.0 2.0 2.0 8.0
923 923 923 923 678 1076 1064 1060 776
4.0 4.0 4.0 5.0 6.0 150 250 940 7.0
11.5 x 13 x 27 11.5 x 13 x 27 11.5 x 13 x 27 4 x 12 x 18 3~6x11 1.4x4.5x12 3 x 4.5 x 12 3X5X14 3X5X11
2.5 12.3 15.3 30.0 1.8 0.27 0.14 0.15 Effect
1.2 X 1.2 X 1.2 X 1.8 X 1.8 x 10% 1% 2.5% 0.12%
7P 529 586 619 619 586 586 586 586 529
laser emitters
In the LiF : Fl(OH) crystal, pumped at a wavelength of 586 nm, tunable room-temperature generation between 0.895 and 1.05 Km with output power P = 270 kW, and radiation line halfwidth = 2.4 nm has been obtained. Simultaneous multifrequency generation was realized with polymer emitters based on rhodamine 6G, C, 640, and oxasine in polymethylmetacrylate arranged on one semiconductor plate. The highest generation of power with rhodamine 6G was 0.5 MW for pumping by radiation of wavelength 529 nm. Thus, the use of a microrelief with directionselective reflective properties in place of a totally reflecting mirror, has made it possible to provide an effective suppression of closed nonaxial modes, lowering the requirements for optical isolation of elements, removes the problem of destruction and detachment of mirrors and, in the final analysis, improves the power characteristics and efficiency of semiconductor lasers with a longitudinal electron-beam pump and makes them suitable for obtaining generation with a high efficiency in a number of promising active media.
ded to the linear portion of the dependence W,(K). As seen from table 1, N = 1-7, for CdSSe compounds the change in the reflection coefficient of the output mirror R by 30-70% weakly affects the output parameters. Therefore, the output surface of the plate can be used without a reflecting mirror (R = 30%). It is seen from the above data that the application of a microrelief instead of a silver mirror permits an increase in the output radiation power of all the semiconductor compounds under study. This is most clearly pronounced on crystals radiating in the visible range, where the power density increases by about one order of magnitude. The smaller positive effect obtained on compounds CdTe and GaAs is primarily due to the insufficient perfection of the etch profile. The excitation of semiconductor lasers by an electron beam with a duration of 20 ns is characterized by somewhat lower transformation coefficients. It should be noted that while in the first case the radiation pulse practically follows the shape of the beam current pulse and is equal to 2 ns at half-height, in the case of beampumped electrons with a duration of 20 ns, the light pulse is equal to 8 ns at half-height. The power of our uncooled semiconductor lasers is sufficient to create a laser effect in a number of media. The principal results of the experiments are given in table 2, where the following symbols are used: hp = pump radiation wavelength, rP = pump radiation pulse duration, max = wavelength at the generation band maxih, mum, 79 = half-height duration of the generation pulse. Table 2 Parameters
semiconductor
A,
lOI cm-’ 1Ol6 crnm3 lOI cmm3 10” cmm3 1Ol6 cm-3
PHYSICA II
505-507
High-efficiency emitters
electron-beam-pumped
A.L. Gurskii, E.V. Lutsenko, B.I.
semiconductor
laser
A.I. Mitcovets and G.P. Yablonskii
Stepanov Institute of Physics, Academy
of Science of Belarus, Minsk, Belarus
The operational parameters of semiconductor electron-beam-pumped lasers made by a new technology are described. The lasing regimes and the parameters of electron-beam-pumped semiconductor lasers based on &Se, CdS,Se,_,, Zn,Cd, _.S, CdTe, GaAs crystals in the 460-900 nm range have been studied. The radiation from these lasers was used to excite generation in a number of media.
One of the problems in creating powerful ‘radiating-mirror’ type lasers is increasing their output power. At the present time, the restricting factor of the increase is the low beam stability of the cavity mirrors which are usually prepared by vacuum deposition. For semiconductor lasers with longitudinal electron-beam-pump destruction of mirrors begins at power densities 10MW/cm2 [l] and less. Another disadvantage of the traditional reflective coatings is the nonselectivity of their reflective properties in the direction which requires special measures to suppress spurious modes. In the present paper, by the example of a multicomponent semiconductor laser with a longitudinal electron-beam pump, it is shown how we can overcome, to a certain extent, the above disadvantages and increase the laser efficiency and power if we abandon the deposition of reflecting coatings. To make multi-element lasers with longitudinal excitation by an electron beam, commercial semiconductor compounds were used. They were 200 km-thick monocrystal plates, 5 cm in diameter, cemented on quartz substrates. Preliminarily dielectric output mirrors with reflection coefficient of 30-70% were placed on the surface of the plate to be cemented. Then the plates were Correspondence to: A.L. Gurskii, B.I. Stepanov Institute of Physics, Academy of Science of Belarus, Scaryna pr., 70, Minsk 220602, Belarus. 0921-4526/93/$06.00
0
1993 - Elsevier
Science
Publishers
cut into 0.5-l mm square elements to a depth of 180 pm. A totally reflecting silver mirror was deposited onto the surface facing the electron beam or this surface was subjected to chemical etching by special technology [2] to create a certain micro-relief on the surface. For example, for the CdS crystals the treated surface represents a set of densely spaced, cut etch hills with sizes from fractions to dozens of micrometers [3]. Such a relief in the first approximation is an analog of a multitude of corner reflectors and exhibits the property of selective reflection, extracting the spurious mode radiation from the cavity. Semiconductor lasers were excited by an electron beam with electron energy 200 keV, halfheight pulse duration 1.5 ns and current density of the beam j = 20-200 A/cm’, The beam was brought into the atmosphere through a 20 pm thick titanium foil 14 or 22 mm in diameter. The luminescence and generation spectra of the crystals were recorded by a spectrum analyzer with a diffraction grating of 1200 mm-’ and linear inverse dispersion of 2.4nm/mm. The duration and shape of the light pulses were determined by a photodetector and an oscilloscope, the energy characteristics were measured by an acoustooptical meter and a pyroelectric detector. Semiconductor lasers were created using ZnSe, Zn,Cd,_,S, CdS,Se,_,, CdTe, GaAs, which provide lasing over a wide spectral range
B.V. All rights
reserved
A. L. Gurskii et al. I High-efficiency
506
electron-beam-pumped
(from 470 to 900 nm). The characteristic halfwidth of the radiation line was 2-6 nm depending on the composition of the semiconductor compound and the exciting electron beam power. Characteristically, when the excitation power density is increased to about 15 MW/cm2, the maximum of the induced radiation spectrum is not shifted and corresponds to the maximum of the luminescence spectrum. Further increase in the level of excitation leads to a considerable longwave shift of the radiation spectrum maximum. For instance, at power density 70MW/ cm’, the value of the shift of the band maximum relative to the initial spectrum is 5.5 nm. The longwave shift is also accompanied by broadening of the radiation band from 2.2 to 5 nm and a change in its form. In our opinion, such behavior of the radiation line is explained by the fact that under powerful excitation of semiconductor crystals, the mechanisms of stimulated radiation are associated with degeneration of electron-hole plasma and due to this, broadening of the band gap. Indeed, as follows from the literature data, the band gap narrowing manifests itself at charge carrier concentrations comparable to the critical Mott concentration n = 7 X 1Or’ cme3. In our case, a shift is observed at a power density >13.5 MWlcm’, which corresponds to a charge carrier concentration y1> 2.5 x 1018cmP3. A Table 1 Parameters
of semiconductor
N
Semiconductor compounds
1 2 3 4 5 6 7 8 9 10 11 4 7 8 9 10 11
CdSSe CdSSe CdSSe CdSSe CdS CdS CdS ZnSe ZnCdS CdTe GaAs CdSSe CdS ZnSe ZnCdS CdTe GaAs
electron-beam-pumped
semiconductor
laser emitters
similar behavior of the radiation line is also characteristic of other crystals under study. The energy and spatial characteristics of the radiation parameters of semiconductor lasers with longitudinal excitation by an electron beam are known to substantially depend on the methods of preparing the crystal surfaces. In addition, the radiation efficiency is also determined by the technology of crystal growth in which, in particular, depends the value of the internal quantum yield of luminescence. Therefore, to compare the influence of the method of working the crystal surfaces with the energy characteristics of lasers, the semiconductor plates were cut from neighboring parts of the boule. Table 1 gives the following parameters for different samples of semiconductor radiators: the plate thickness d, the cut step h, the reflection coefficient of the output face with a dielectric coating R (without coating, R = 30%), the wavelength of the induced radiation band maximum, the power density of the output radiation W,, and the efficiency of these radiators with reference to electron beam energy. The last column gives additional characteristics of these samples: the presence of a microrelief or a silver mirror on the input surface. The radiant energy of the plates was measured at a constant value of W, = 26 MWlcm2 (t = 1.5 ns) which correspon-
lasers.
d
h
R
km) 200 150 110 210 185 110 210 200 200 200 200 210 210 200 200 200 200
(mm)
(%I
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5 1.0 1.0 1.0 1.0 1.0 0.5
70 50 50 30 70 70 30 50 70 70 50 30 30 50 70 70 50
619 619 619 586 529 529 529 471 518 856 894 586 529 471 518 856 894
$*/cm’)
Efficiency (%)
Note
0.16 0.23 0.25 0.25 0.21 0.16 0.28 0.22 0.05 0.17 0.42 2.3 3.02 2.26 0.53 0.35 0.86
0.8 1.2 1.3 1.2 1.03 0.8 1.4 1.1 0.24 0.86 2.1 11.5 15.1 12.5 2.6 1.74 4.3
Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Ag-coating Microrelief Microrelief Microrelief Microrelief Microrelief Microrelief
A. L. Gurskii et al. I High-ej‘iciency
electron-beam-pumped
of semiconductor-laser
N
Active element
1 1 1 2 3 4 5 6 7
LiF( OH) : F; LiF(OH) : F; LiF(OH) : F; LiF( OH) : FL’ LiF( Mg) : Fl KGW: Nd YAG : Nd KNFS AI>O, : Ti
pumped
507
References [l]
0.V Bogdankevich, N.D. Vorob’ev, M.M. Zverev, S.P. Kopyt, E.M. Krasavina, 1.V Kryukova, VP. Petrov, V.A. Usakin and V.K. Yakushin, Sov. .I. Quant. Electron. 12 (1985) 1519. [2] V.P. Gribkovskii, V.V. Grusinskii, A.L. Gurskii et al., Inventor’s certificate SU No. 1653514, IPC HOlS 3118. [3] G.P. Yablonskii, Phys. Stat. Sol. A 92 (1985) 431.
solid lasers. max
7e
Size
Efficiency
(ns)
(nm)
(ns)
(mm)
(%)
Concentration
2.0 2.0 2.0 8.0 2.0 2.0 2.0 2.0 8.0
923 923 923 923 678 1076 1064 1060 776
4.0 4.0 4.0 5.0 6.0 150 250 940 7.0
11.5 x 13 x 27 11.5 x 13 x 27 11.5 x 13 x 27 4 x 12 x 18 3~6x11 1.4x4.5x12 3 x 4.5 x 12 3X5X14 3X5X11
2.5 12.3 15.3 30.0 1.8 0.27 0.14 0.15 Effect
1.2 X 1.2 X 1.2 X 1.8 X 1.8 x 10% 1% 2.5% 0.12%
7P 529 586 619 619 586 586 586 586 529
laser emitters
In the LiF : Fl(OH) crystal, pumped at a wavelength of 586 nm, tunable room-temperature generation between 0.895 and 1.05 Km with output power P = 270 kW, and radiation line halfwidth = 2.4 nm has been obtained. Simultaneous multifrequency generation was realized with polymer emitters based on rhodamine 6G, C, 640, and oxasine in polymethylmetacrylate arranged on one semiconductor plate. The highest generation of power with rhodamine 6G was 0.5 MW for pumping by radiation of wavelength 529 nm. Thus, the use of a microrelief with directionselective reflective properties in place of a totally reflecting mirror, has made it possible to provide an effective suppression of closed nonaxial modes, lowering the requirements for optical isolation of elements, removes the problem of destruction and detachment of mirrors and, in the final analysis, improves the power characteristics and efficiency of semiconductor lasers with a longitudinal electron-beam pump and makes them suitable for obtaining generation with a high efficiency in a number of promising active media.
ded to the linear portion of the dependence W,(K). As seen from table 1, N = 1-7, for CdSSe compounds the change in the reflection coefficient of the output mirror R by 30-70% weakly affects the output parameters. Therefore, the output surface of the plate can be used without a reflecting mirror (R = 30%). It is seen from the above data that the application of a microrelief instead of a silver mirror permits an increase in the output radiation power of all the semiconductor compounds under study. This is most clearly pronounced on crystals radiating in the visible range, where the power density increases by about one order of magnitude. The smaller positive effect obtained on compounds CdTe and GaAs is primarily due to the insufficient perfection of the etch profile. The excitation of semiconductor lasers by an electron beam with a duration of 20 ns is characterized by somewhat lower transformation coefficients. It should be noted that while in the first case the radiation pulse practically follows the shape of the beam current pulse and is equal to 2 ns at half-height, in the case of beampumped electrons with a duration of 20 ns, the light pulse is equal to 8 ns at half-height. The power of our uncooled semiconductor lasers is sufficient to create a laser effect in a number of media. The principal results of the experiments are given in table 2, where the following symbols are used: hp = pump radiation wavelength, rP = pump radiation pulse duration, max = wavelength at the generation band maxih, mum, 79 = half-height duration of the generation pulse. Table 2 Parameters
semiconductor
A,
lOI cm-’ 1Ol6 crnm3 lOI cmm3 10” cmm3 1Ol6 cm-3