Lars Liebermeister

Accurate characterization of thin semiconductor layers is crucial for quality control in the production of semiconductor devices. Traditional methods, like the four-point probe technique, require direct contact and are thus destructive. This study explores terahertz time-domain spectroscopy (THz TDS) as a promising non-contact alternative for measuring the electrical properties of semiconductor layers. We investigated 1 μm to 2 μm thick highly doped indium gallium arsenide (InGaAs) layers on indium phosphide (InP) substrates, with electrical sheet conductance between 10-7 and 10-1 S/sq. Utilizing a fiber-coupled THz-TDS system, we performed measurements in both reflection and transmission geometry. For the first time, we apply the self-referenced method to thin semiconductor layers as well as systematically compare self-referenced and referenced based methods to extract the electrical sheet conductance of thin semiconductor layers. In reflection geometry, self-referencing improves accuracy by minimizing operator error and eliminating frequent re-referencing. Agreement with contact-based four-point probes validates THz-TDS for nondestructive evaluation of thin semiconductor layers and extends its measurable conductance range, enabling reference-free deployments.

Ultrafast solid-state glass lasers are frequently mode-locked using a semiconductor saturable absorber mirror (SESAM), which can provide reliable laser self-start and stable mode-locked operation. However, state-of-the-art GaAs-based SESAMs for the 1.55 µm wavelength range require highly strained InGaAs quantum well absorbers with a lattice-mismatch of ≥ 2% relative to the GaAs substrate, which leads to defect formation, reduced damage threshold, and limited design freedom. Here, we present the first fully strain-free SESAMs for solid-state glass lasers at 1.55 µm wavelength, which overcome these constraints. Our devices are grown on InP with a very low residual lattice-mismatch of < 0.1% and incorporate an iron-doped InGaAs bulk absorber. This enables continuous tunability of the SESAM modulation depth and precise control over the ultrafast SESAM recovery time – an unprecedented degree of design freedom. Furthermore, the SESAM structure combines an anti-resonant design with a highly reflective InAlAs/InGaAlAs bottom DBR and a TiO ₂ /SiO ₂ top DBR, which results in record-low non-saturable losses for InP-based SESAMs of 0.6% for a modulation depth of 0.6%. With this approach, we demonstrate the first stable continuous-wave mode-locked operation of a solid-state Er,Yb:glass laser using an InP-based SESAM. We achieve 224 fs pulse duration at a maximum output power of 102.5 mW and a 79.1 MHz repetition rate, in combination with excellent noise properties of 0.005% integrated RIN over [100 Hz, 1 MHz] and 3 fs integrated timing jitter over [1 kHz, 1 MHz].

Spectrum analyzers and spectrometers are essential for designing sources, analyzing material properties, layer structures and fingerprinting substances. We present an ultra-wideband, continuous-wave photonic receiver with kHz-level spectral resolution in the terahertz domain for both heterodyne and homodyne detection. Employed as a spectrum analyzer front end, it records the emitted spectrum of a source under test, assessing spectral purity, spectral shape and undesired frequency components. It outperforms state-of-the-art electronic systems in terms of frequency coverage and system cost with a competitive spectral resolution and noise floor on the few aW/Hz level at room temperature. It covers the important frequencies above 1.5 THz, yet commercially inaccessible, where sources like quantum cascade lasers operate. When combined with a comb-based photonic source, we demonstrate hetero- and homodyne spectroscopy over an unprecedented frequency range from below 100 GHz to 6.5 THz and a very low noise floor. Locking the photonic system to GPS enables tracing back the measured parameters to SI units, being of key importance for metrological applications. The presented setups offer the broadest continuous-wave frequency coverage to date, combined with a sharp spectral resolution, enabling diverse applications ranging from fast non-destructive testing, astronomic high-resolution spectroscopy, to frequency-modulated RADAR.

Continuous wave (CW) terahertz spectroscopy systems are a proven and convenient solution for industrial non-destructive testing and multi-layer thickness determination. For these systems to find use in real-time inline monitoring applications, a high acquisition and data evaluation rate is required. One approach that can increase and potentially enable kHz acquisition rates is to scan through only a few selected frequencies instead of a full spectrum. However, the data analysis can become a bottleneck when realizing measurement systems capable of operating at these speeds. Here we show the feasibility of extracting multi-layer thicknesses from a sparsely sampled spectrum by using a real-time evaluation scheme capable of runtimes below a millisecond with a thickness uncertainty comparable to a full spectrum evaluation. This is achieved by reducing the number of computations by three orders of magnitude. The proposed measurement scheme requires complete knowledge of the sample composition and the refractive index of each layer. Additionally, achieving these high evaluation rates assumes that individual layer thicknesses deviate by no more than 200 µm from their nominal values. However, both conditions are often met in an industrial setting. The sparse evaluation is demonstrated on a three layer sample, the achieved standard deviation of the layer thicknesses remains below 5 µm for each layer. These measurements confirm the effectiveness of the sparse sampling approach for THz spectroscopy and demonstrate the ability to address industrial applications with measurement rates in the kilohertz range.

Continuous wave (cw) terahertz (THz) radiation has a wide array of applications, ranging from sensing to next-generation wireless communication links. Industrial applications frequently require THz systems that are broadband, highly efficient, and compact. Photomixer-based solutions hold promise in meeting these demands, offering extremely broadband operation and the potential for miniaturization through photonic integration. However, current photoconductive antenna (PCA) receivers used in these systems are top-illuminated, which strongly limits their efficiency and renders them incompatible with photonic integration. To overcome these limitations, we developed optical waveguide-integrated photoconductive antennas (win-PCAs) for cw-THz detection. These antennas not only facilitate integration into photonic integrated chips (PICs) but also allow us to explore new device geometries to optimize the PCA’s responsivity. By optimizing the absorber geometry of the win-PCAs, we achieve a 22-fold increase in photoresponse, a 500-fold improvement in THz responsivity, and a 4.7-fold reduction in noise-equivalent power compared to state-of-the-art top-illuminated PCAs. In a coherent cw-THz spectrometer, these improvements enable measurements with a peak dynamic range of 123 dB for 300 ms averaging, which is 11 dB higher than what is achievable with comparable top-illuminated receivers. The presented win-PCAs represent a significant step toward fully integrated, high-performing photonic cw-THz systems for both spectroscopy and high-capacity wireless links.

Spectrum analyzers and spectrometers are essential for exploring extended spectral ranges, designing sources, analyzing material properties, layer structures and fingerprinting substances. We present an ultra-wideband, continuous-wave photonic receiver with kHz-level spectral resolution in the terahertz domain (100 GHz-10 THz) for both heterodyne and homodyne detection. Employed as a spectrum analyzer front end, it records the emitted spectrum of a source under test assessing spectral purity, spectral shape and undesired frequency components. It outperforms state-of-the-art electronic systems in terms of frequency coverage and system cost with a competitive noise floor and spectral resolution. It covers the important frequencies above 1.5 THz, that are not yet commercially accessible, where sources like quantum cascade lasers operate. When combined with a comb-based photonic source, we demonstrate hetero- and homodyne spectroscopy over an unprecedented frequency range from below 100 GHz to 6.5 THz. The presented setups offer the broadest frequency coverage of their kinds combined with extremely sharp resolution, enabling diverse applications ranging from fast non-destructive testing, astronomic high-resolution spectroscopy, to frequency-modulated RADAR.

For the first time, we present photoconductive, continuous wave (cw) terahertz (THz) detectors for 1550 nm excitation based on rhodium- (Rh) doped indium gallium arsenide (InGaAs) grown by molecular beam epitaxy. Compared to iron- (Fe) doped material, the Rh-doped InGaAs shows higher carrier mobilities with similar carrier lifetimes. Therefore, these photoconductive antennas outperform InGaAs:Fe-based detectors by a factor of 10 in terms of responsivity and noise-equivalent-power (NEP) while maintaining the same bandwidth. In a homodyne spectrometer configuration, we achieve a record peak dynamic range (DNR) of 132 dB, which constitutes an improvement of 20 dB.

We investigate terahertz time-domain spectroscopy using a low-noise dual-frequency-comb laser based on a single spatially multiplexed laser cavity. The laser cavity includes a reflective biprism, which enables generation of a pair of modelocked output pulse trains with slightly different repetition rates and highly correlated noise characteristics. These two pulse trains are used to generate the THz waves and detect them by equivalent time sampling. The laser is based on Yb:CALGO, operates at a nominal repetition rate of 1.18 GHz, and produces 110 mW per comb with 77 fs pulses around 1057 nm. We perform THz measurements with Fe-doped photoconductive antennas, operating these devices with gigahertz 1 µm lasers for the first time, to our knowledge, and obtain THz signal currents approximately as strong as those from reference measurements at 1.55 µm and 80 MHz. We investigate the influence of the laser’s timing noise properties on THz measurements, showing that the laser’s timing jitter is quantitatively explained by power-dependent shifts in center wavelength. We demonstrate reduction in noise by simple stabilization of the pump power and show up to 20 dB suppression in noise by the combination of shared pumping and shared cavity architecture. The laser’s ultra-low-noise properties enable averaging of the THz waveform for repetition rate differences from 1 kHz to 22 kHz, resulting in a dynamic range of 55 dB when operating at 1 kHz and averaging for 2 s. We show that the obtained dynamic range is competitive and can be well explained by accounting for the measured optical delay range, integration time, as well as the measurement bandwidth dependence of the noise from transimpedance amplification. These results will help enable a new approach to high-resolution THz-TDS enabled by low-noise gigahertz dual-comb lasers.

We use a high-sampling rate terahertz (THz) homodyne spectroscopy system to estimate thoracic movement from healthy subjects performing breathing at different frequencies. The THz system provides both the amplitude and phase of the THz wave. From the raw phase information, a motion signal is estimated. An electrocardiogram (ECG) signal is recorded with a polar chest strap to obtain ECG-derived respiration information. While the ECG showed sub-optimal performance for the purpose and only provided usable information for some subjects, the signal derived from the THz system showed good agreement with the measurement protocol. Over all the subjects, a root mean square estimation error of 1.40 BPM is obtained.

Material characterisation and imaging applications using terahertz radiation have gained interest in the past few years due to their enormous potential for industrial applications. The availability of fast terahertz spectrometers or multi-pixel terahertz cameras has accelerated research in this domain. In this work, we present a novel vector-based implementation of the gradient descent algorithm to fit the measured transmission and reflection coefficients of multilayered objects to a scattering parameter-based model, without requiring any analytical formulation of the error function. We thereby extract thicknesses and refractive indices of the layers within a maximum 2% error margin. Using the precise thickness estimates, we further image a 50 nm-thick Siemens star deposited on a silicon substrate using wavelengths larger than 300 µm. The vector-based algorithm heuristically finds the error minimum where the optimisation problem cannot be analytically formulated, which can be utilised also for applications outside the terahertz domain.