Improving the dynamic range of InGaAs-based THz detectors by localized beryllium doping: up to 70 dB at 3 THz.

In this Letter, we report on photoconductive terahertz (THz) detectors for 1550 nm excitation based on a low-temperature-grown InGaAs/InAlAs superlattice with a localized beryllium doping profile. With this approach, we address the inherent lifetime-mobility trade-off that arises, since trapping centers also act as scattering sites for photo-excited electrons. The localized doping of the InAlAs barrier only leads to faster electron trapping for a given mobility. As a result, we obtain THz detectors with more than 6 THz bandwidths and 70 dB dynamic ranges (DNRs) at 3 THz and 55 dB DNR at 4 THz. To the best of our knowledge, this is the highest DNR for photoconductive THz time-domain spectroscopy systems published so far.

Terahertz quasi time-domain spectroscopy based on telecom technology for 1550 nm.

We present a fiber-coupled terahertz quasi time-domain spectroscopy system driven by a laser with a central wavelength of 1550 nm. By using a commercially available multimode laser diode in combination with state-of-the-art continuous wave antennas, a bandwidth of more than 1.8 THz is achieved. The peak signal-to-noise ratio is around 60 dB. A simulation based on the optical spectrum of the laser diode and the transfer function of the THz path is in agreement with the experimental results. The system is used to extract the refractive index from two different samples and the results indicate that the performance is up to 1.8 THz comparable to a terahertz time-domain spectroscopy system.

Fiber-coupled transceiver for terahertz reflection measurements with a 4.5 THz bandwidth.

We present a fiber-coupled transceiver for THz time-domain spectroscopy, which combines an emitter and a receiver on a single photoconductive chip. With a bandwidth of 4.5 THz and a peak dynamic range larger than 70 dB, it allows for THz reflection measurements under normal incidence. This THz reflection head is a promising device for applications in such fields as material inspection and nondestructive testing.

Absolute terahertz power measurement of a time-domain spectroscopy system.

We report on, to the best of our knowledge, the first absolute terahertz (THz) power measurement of a photoconductive emitter developed for time-domain spectroscopy (TDS). The broadband THz radiation emitted by a photoconductor optimized for the excitation with 1550-nm femtosecond pulses was measured by an ultrathin pyroelectric thin-film (UPTF) detector. We show that this detector has a spectrally flat transmission between 100 GHz and 5 THz due to special conductive electrodes on both sides of the UPTF. Its flat responsivity allows the calibration with a standard detector that is traceable to the International System of Units (SI) at the THz detector calibration facility of PTB. Absolute THz power in the range from below 1 µW to above 0.1 mW was measured.

All fiber-coupled THz-TDS system with kHz measurement rate based on electronically controlled optical sampling.

We demonstrate a completely fiber-coupled terahertz (THz) time-domain spectrometer (TDS) system based on electronically controlled optical sampling with two erbium-doped femtosecond fiber lasers at a central wavelength of 1560 nm. The system employs optimized InGaAs/InAlAs photoconductive antennas for THz generation and detection. With this system, we achieve measurement rates of up to 8 kHz and up to 180 ps scan range. We further achieve 2 THz spectral bandwidth and a dynamic range of 76 dB at only 500 ms measurement time.

Influence and adjustment of carrier lifetimes in InGaAs/InAlAs photoconductive pulsed terahertz detectors: 6 THz bandwidth and 90dB dynamic range.

We investigate the influence of Beryllium (Be) doping on the performance of photoconductive THz detectors based on molecular beam epitaxy (MBE) of low temperature (LT) grown In(0.53)Ga(0.47)As/In(0.52)Al(0.48)As multilayer heterostructures (MLHS). We show how the optical excitation power affects carrier lifetime, detector signal, dynamic range and bandwidth in THz time domain spectroscopy (TDS) in dependence on Be-doping concentration. For optimal doping we measured a THz bandwidth in excess of 6 THz and a dynamic range of up to 90 dB.

Multichannel terahertz time-domain spectroscopy system at 1030 nm excitation wavelength.

We present Terahertz (THz) imaging with a 1D multichannel time-domain spectroscopy (TDS) system which operates with a photoconductive array of 15 detection channels excited by a 1030 nm femtosecond fiber laser. The emitter and detector are photoconductive antennas based on InGaAs/InAlAs multi-layer heterostructures (MLHS). We characterized the THz optics and the resolution of the system. The performance is demonstrated by the multichannel imaging of two samples. A simultaneous measurement of 15 THz pulses with a pixel pitch of 1 mm increases the measurement speed of the TDS system by factor 15.

Telecom technology based continuous wave terahertz photomixing system with 105 decibel signal-to-noise ratio and 3.5 terahertz bandwidth.

A modified photoconductive receiver significantly improves the performance of photomixing-based continuous wave (cw) THz systems driven at the optical telecommunication wavelength of 1.5 µm. The achieved signal-to-noise ratio of 105 dB at 100 GHz and 70 dB at 1 THz, both for an integration time of 200 ms, are to our knowledge the highest numbers reported in literature for any optoelectronic cw THz system, including classical setups operating at 800 nm. The developed receiver allows for combining low cost and high performance in one system for the first time to our knowledge.

Large area photoconductive terahertz emitter for 1.55 µm excitation based on an InGaAs heterostructure.

We present scalable large area terahertz (THz) emitters based on a nanoscale multilayer InGaAs/InAlAs heterostructure and a microstructured electrode pattern. The emitters are designed for pump lasers working at the telecommunication wavelength of 1.55 µm. Electric THz fields of more than 2.5 V cm⁻¹ are reached with moderate pump powers of 80 mW, the corresponding spectrum extends up to 3 THz. The saturation characteristics have been investigated for different pump laser spot sizes. For small pump powers of less than 50 mW the emitted THz field is nearly independent of the spot size, for higher pump powers and small spot sizes a clear saturation of the generated THz pulse can be observed. Hence the use of scalable emitters is especially promising for high power fibre laser systems. The spectral content of the generated radiation is nearly independent of the parameters spot size, pump power, and bias voltage, which allows for stable operation in spectroscopic applications.

THz generation at 1.55 µm excitation: six-fold increase in THz conversion efficiency by separated photoconductive and trapping regions.

We present first results on photoconductive THz emitters for 1.55µm excitation. The emitters are based on MBE grown In0.53Ga0.47As/In0.52Al0.48As multilayer heterostructures (MLHS) with high carrier mobility. The high mobility is achieved by spatial separation of photoconductive and trapping regions. Photoconductive antennas made of these MLHS are evaluated as THz emitters in a THz time domain spectrometer (THz TDS). The high carrier mobility and effective absorption significantly increases the optical-to-THz conversion efficiency with THz bandwidth in excess of 3 THz.