688
23 Electricity-to-Light Conversion
(a)
(b)
(c)
400
300
200
100
0
5
atmospheric
windows
10
50 100
2
2 0
(d)
Fig. 23.74 a Schematic band diagram of quantum cascade laser. b Cross sectional TEM of cascade layer sequence. The
periodicity of the vertical layer sequence is 45 nm. From [2030]. c Laser emission wavelengths and operation temperatures
for various realized quantum cascade lasers (squares: cw, circles: pulsed operation, solid symbols: InP-, empty symbols:
GaAs-based). Data from [2031]. d SEM image of a quantum cascade DFB laser (grating period: 1.6 µm). From [2031]
23.4.16 Quantum Cascade Lasers
In a quantum cascade laser (QCL), the gain stems from an intersubband transition. The concept was
conceived in 1971 [145, 2032] and realized in 1994 [149]. In Fig. 23.74a, the schematic conductionband structure at operation is shown. The injector supplies electrons into the active region. The electron
is removed quickly from the lower level in order to allow inversion. The electron is then extracted into
the next injector. The laser medium consists of several such units as shown in Fig. 23.74b. Since every
unit can deliver a photon per electron (with efficiency η 1 ), the total quantum efficiency of N units
η = N η 1 can be larger than 1.
The emission wavelength is in the far- or mid-infrared, depending only on the designed layer
thicknesses and not on the band gap of the material (Fig. 23.74d). In the mid-infrared, room-temperature
operation has been achieved while operation in the far-infrared requires cooling so far. Extensions to
the THz-range and the infrared spectral region (telecommunication wavelengths of 1.3 and 1.55 µm)
seem feasible. The cascade laser concept can also be combined with the DFB technology to create
single-mode laser emission (Fig. 23.74d).
23.4.17 Hot-Hole Lasers
The hot-hole laser, which is mostly realized with p-doped Ge, is based on a population inversion
between the light- and heavy-hole valence subbands. The laser operates with crossed electric and
magnetic fields (Voigt configuration, typically E = 0.5–3 kV/cm, B = 0.3–2 T) and at cryogenic
temperatures (T = 4–40 K) [2033–2035].
23 Electricity-to-Light Conversion
(a)
(b)
(c)
400
300
200
100
0
5
atmospheric
windows
10
50 100
2
2 0
(d)
Fig. 23.74 a Schematic band diagram of quantum cascade laser. b Cross sectional TEM of cascade layer sequence. The
periodicity of the vertical layer sequence is 45 nm. From [2030]. c Laser emission wavelengths and operation temperatures
for various realized quantum cascade lasers (squares: cw, circles: pulsed operation, solid symbols: InP-, empty symbols:
GaAs-based). Data from [2031]. d SEM image of a quantum cascade DFB laser (grating period: 1.6 µm). From [2031]
23.4.16 Quantum Cascade Lasers
In a quantum cascade laser (QCL), the gain stems from an intersubband transition. The concept was
conceived in 1971 [145, 2032] and realized in 1994 [149]. In Fig. 23.74a, the schematic conductionband structure at operation is shown. The injector supplies electrons into the active region. The electron
is removed quickly from the lower level in order to allow inversion. The electron is then extracted into
the next injector. The laser medium consists of several such units as shown in Fig. 23.74b. Since every
unit can deliver a photon per electron (with efficiency η 1 ), the total quantum efficiency of N units
η = N η 1 can be larger than 1.
The emission wavelength is in the far- or mid-infrared, depending only on the designed layer
thicknesses and not on the band gap of the material (Fig. 23.74d). In the mid-infrared, room-temperature
operation has been achieved while operation in the far-infrared requires cooling so far. Extensions to
the THz-range and the infrared spectral region (telecommunication wavelengths of 1.3 and 1.55 µm)
seem feasible. The cascade laser concept can also be combined with the DFB technology to create
single-mode laser emission (Fig. 23.74d).
23.4.17 Hot-Hole Lasers
The hot-hole laser, which is mostly realized with p-doped Ge, is based on a population inversion
between the light- and heavy-hole valence subbands. The laser operates with crossed electric and
magnetic fields (Voigt configuration, typically E = 0.5–3 kV/cm, B = 0.3–2 T) and at cryogenic
temperatures (T = 4–40 K) [2033–2035].