Klaus Biermann

Passively mode-locked Tm,Ho:YAG laser at 2 µm based on saturable absorption of intersubband transitions in quantum wells

We report the first demonstration of a solid state laser passively mode-locked through the saturable absorption of short-wavelength intersubband transitions in doped quantum wells: a continuous wave Ti:sapphire laser end-pumped Tm,Ho:YAG laser at the center wavelength of 2.091 mum utilizing intersubband transitions in narrow In(0.53)Ga(0.47)As/Al(0.53)As(0.47)Sb quantum wells. Stable passive mode-locking operation with maximum average output power of up to 160 mW for 2.9 W of the absorbed pump power could last for hours without external interruption and a mode-locked pulse with duration of 60 ps at repetition rate of 106.5 MHz was generated.

Femtosecond pump-probe spectroscopy of intersubband relaxation dynamics in narrow InGaAs/AlAsSb quantum well structures

Intersubband relaxation dynamics in InGaAs∕AlAsSb multiquantum wells (QWs) is studied by single-color femtosecond pump-probe measurements. At early delay times, all samples show an exponential decay of the transient transmission occurring with time constants of the order of a picosecond. The relaxation dynamics at later delay times strongly depend on both QW thickness and doping location. A non-single-exponential decay behavior indicates extra competing relaxation channels, as further confirmed by solving three-level rate equations. It is shown that slowly decaying components are due to electron transfer to states related to indirect valleys in the wells or in the barriers.

Intersubband relaxation dynamics in single and double quantum wells based on strained InGaAs∕AlAs∕AlAsSb

Intersubband relaxation dynamics in single and coupled double quantum well (QW) structures based on strained InGaAs∕AlAs∕AlAsSb are studied by femtosecond pump probe spectroscopy at wavelengths around 2μm. For single QWs, the transient transmission was observed to decay exponentially with a time constant of 2ps, showing that side valleys have negligible influence on the intersubband relaxation dynamics for strained InGaAs QWs. For double QWs, the pump-probe signal at the intersubband energy involving the two electronic levels located at the wider QW exhibits an induced absorption component attributed to the population of the second subband (associated with the narrow QW) by hot electrons.

Low temperature MBE-grown In(Ga,Al)As/InP structures for 1.55 μm THz photoconductive antenna applications

Low temperature MBE-grown In(Ga,Al)As materials on InP incorporating Be-doping feature extremely short optical response times down to the sub-ps range which is adequate for THz generation and detection in photoconductive antennas. However, very low resistivity of low temperature InGaAs is the main obstruction for the implementation in real THz antennas in the 1.55 mum wavelength range. The high conductive behaviour is due to the incorporation of growth induced defect states close to the conduction band acting as donors. Resistivity was increased in a twofold manner: firstly, by compensation with carefully balanced Be incorporation, and secondly by using additionally a novel layer sequence exploiting trapping centres in cladding layers. Specially designed periodic InGaAs/InAlAs multi-layer structures were grown, where the deep electron traps in the InAlAs material increase the resistivity of the multi-layer structure beyond 106 Omega/sq. The combination of high resistivity behaviour and simultaneously sub-ps response time allows the fabrication of photoconductive THz antennas for the 1.55 mum range based on low temperature InGaAs/InAlAs multi-layer structures. These elements, similar to low temperature GaAs in the 850 nm range, represent a key element for compact THz systems making use of mature telecom components.

THz Photoconductive Antennas for 1.55 µm Telecom Wavelengths

We present low temperature grown InGaAs multiple-quantum well structures which are successfully applied as transmitters and receivers in THz time domain systems. Operation at telecom wavelengths of 1.55µm with a femtosecond fiber laser is demonstrated.

Saturable absorption mode-locking based on intersubband transitions in quantum wells at 2 µm

Solid-state lasers emitting in the 2 µm eye-safe region are promising candidates for various applications in laser ranging, spectroscopy, metrology, medicine, or optical communication. Mode-locked pulse trains are especially interesting for time-resolved spectroscopy or as pump source for OPOs operating at 3–5 µm and 8–12 µm. A Tm3+ and Ho3+ ion doped solid-state laser crystal, such as YAG, is the most popular way to obtain laser radiation around 2µm. Until now, continuous wave, Q-switched, and active mode-locked operation of Tm3+ and Ho3+ doped lasers around 2 µm have been demonstrated at picosecond pulse durations [1, 2]. However, due to the lack of appropriate saturable absorbers, there have been no related reports on passively mode-locked 2 µm rod lasers, although Schepler et al. proposed a passive mode-locking method for a Tm-Ho co-doped 2 µm femtosecond laser by using a semiconductor saturable absorber mirror based on InGaAs in 1993 [3]. Intersubband transitions (ISBTs) in semiconductor quantum wells (QWs) have been involved in several midinfrared optoelectronics devices such as quantum cascade lasers, detectors, and optical modulators [4, 5]. Short wavelength intersubband absorption has been investigated in several III–V material systems, such as InGaAs/AlAs [6] and InGaAs/AlAsSb [7]. Very recently, with femtosecond pump-probe measurements on narrow InGaAs/AlAsSb QWs with the peak absorption around 2 µm, an ISBT relaxation time of 1.2 ps has been observed [8], which shows exciting potential for realizing saturable absorbers for the generation of ultrashort pulses. Here, we report the first demonstration of passive mode locking in a Ti-sapphire laser end-pumped Tm3+,Ho3+:YAG laser utilizing ISBTs in InGaAs/AlAsSb quantum wells grown on an InP substrate. The experimental setup is schematically shown in Fig. 1. The pump source is a tunable Ti-sapphire laser with a maximum output power of 2.2 W at 785 nm, corresponding to the peak absorption of the Tm3+, Ho3+:YAG crystal. A 4.9 mm-long 5at.%-Tm3+, 0.4at.%-Ho3+:YAG crystal is employed and water-cooled to 15°C. The n-doped InGaAs/AlAsSb QWs are inserted in the cavity under Brewster angle in order to achieve an electric field component along the growth direction of the QWs. The whole cavity length is 53.3 cm. The output coupler has a transmission of 0.5% at 2 µm. A maximum output power of 40 mW is obtained at a center wavelength of 2.09 µm. Fig. 2 shows the temporal pulse trains on the oscilloscope obtained with a 60 MHz bandwidth InGaAs PIN photodiode.

Impact of Interface Formation on Intersubband Transitions in MBE GaInAs:Si/AlAsSb Multiple Coupled DQWs

The impact of indium segregation and diffusion of antimony on intersubband transition (IST) wavelengths in AlAsSb/GalnAs multiple quantum well (MQW) and double quantum well (DQW) structures has been evaluated. By means of 8-band k-p calculations the effect of non-abrupt interfaces on IST wavelengths in such structures is elucidated. Comparison of measured and calculated absorption spectra reveals the sacrificial character of AlAs interfacial layers for diffused or segregated atoms and allows for estimating the limits of the intersubband approach as regards short wavelength relaxation transitions.

Intersubband Relaxation Dynamics in Narrow InGaAs/AlAsSb Quantum Well Structures Studied by Femtosecond Pump‐Probe Spectroscopy

Intersubband relaxation dynamics in modulation doped InGaAs/AlAsSb multiquantum well (QW), with different QW thicknesses was investigated by femtosecond pump‐probe Spectroscopy. At early delay times, the samples show an exponential decay of the transient transmission with picosecond time constants. At later delay times, the relaxation dynamics depends on QW thickness. The presence of a slowly decaying component indicates the transfer of electrons to states related to indirect valleys in the barriers. This result is further confirmed by solving three‐level rate equations.