The amplification of spontaneous emission is used to initiate laser action. As the phase of spontaneous emission is random, the phase of the coherent laser emission (the carrier phase) will also be random each time laser action begins. This prevents phase-resolved detection of the laser field. Here, we demonstrate how the carrier phase can be fixed in a semiconductor laser: a quantum cascade laser (QCL). This is performed by injection seeding a QCL with coherent terahertz pulses, which forces laser action to start on a fixed phase. This permits the emitted laser field to be synchronously sampled with a femtosecond laser beam, and measured in the time domain. We observe the phase-resolved buildup of the laser field, which can give insights into the laser dynamics. In addition, as the electric field oscillations are directly measured in the time domain, QCLs can now be used as sources for time-domain spectroscopy.

2 AbstractThe spectral gain of bound-to-continuum terahertz quantum cascade lasers (QCLs) is measured as a function of current density using terahertz time-domain spectroscopy.During lasing action the full width at half maximum (FWHM) of the gain is found to monotonically decrease with increasing current density until lasing action stops at which point the FWHM reaches a minimum (0.22THz for a laser operating at 2.1THz).Bandstructure calculations show that the spectral gain narrowing is due to the alignment and misalignment of the injector with the active region as a function of the applied bias field.3 Important progress on terahertz quantum cascade lasers (QCLs) has been achieved in the last few years leading to long wavelength 1 , high power 2 , low current operation 3 , and working temperatures up to 178K 4 . In order to realize further improvements, a better understanding of the gain formation mechanism, and its limiting factors are needed. To this end it is necessary to perform detailed measurements of the gain including its spectral shape. Previous studies have been performed for mid-infraredQCLs. For these measurements, electro-luminescence from a non-lasing cavity is coupledinto an adjacent cavity that shows laser action. 5 However, in the terahertz regime such electro-luminescence based studies are difficult, because of the reduced spontaneous emission at longer wavelengths. Electro-luminescence from the laser cavity has also been used to provide information on upper state lifetimes of terahertz QCLs. 6 In this case to avoid the effect of laser emission, the electro-luminescence from the laser cavity must be collected from a cleaved edge running though the middle of the laser. 7 Multiple probe pulses coupled into the QCL's end facets can also be used to investigate the temporal dynamics of the gain. Coherent population transfer and gain saturation have been observed with this technique at mid-infrared frequencies. 8,9 Recently, terahertz timedomain spectroscopy (TDS) has been shown to be a powerful technique to measure the gain spectra in terahertz QCLs. 10,11,12 Here, a broadband terahertz probe pulse is coupled into the QCL, and the electric field of the transmitted pulses is measured using electrooptic sampling. 13In this letter terahertz TDS is used to investigate the line-width of the spectral gain as a function of current density. Two terahertz QCLs lasers with different bound-tocontinuum designs are studied. One laser emits at 2.1THz 14 and the other emits at 2.9THz. 15 For both devices, as the current density is increased from threshold, we observe a monotonic decrease of the full width at half maximum (FWHM) of the gain. After the 4 laser reaches maximum power, this gain narrowing increases sharply, until laser action ceases. By calculating the band structure for different bias fields, we show the gain narrowing is a consequence of a misalignment of the upper state of the laser transition with the injector miniband.The 2.1THz (2.9THz) sample has an active region thickness of 14µm (12µm), and a...

International audienceTerahertz (THz) time domain spectroscopy (TDS) is widely used in a broad range of applications where knowledge of both the amplitude and phase of a THz wave can reveal useful information about a sample. However, a means of amplifying THz pulses which would be of great benefit for improving the applicability of TDS is lacking. While THz quantum cascade lasers (QCL) are promising devices for THz amplification, gain clamping limits the attainable amplification. Here we circumvent gain clamping and demonstrate amplification of THz pulses by ultrafast gain switching of a QCL via the use of an integrated Auston switch. This unclamps the gain by placing the laser in a nonequilibrium state that allows large amplification of the electromagnetic field within the cavity. This technique offers the potential to produce high field THz pulses that approach the QCL saturation field

Frequency tunable terahertz (THz) interdigitated photoconductive antennas (PCAs) are realised by adjusting electrode spacing. An interdigitated geometry allows fabrication of PCAs with small gaps and large surface area for optical excitation. The pulsed electric field and emission spectra are measured using THz-time domain spectroscopy (TDS). It is observed that the peak frequency of the emitted spectra is shifted to higher frequencies (from 0.73 to 1.33 THz) as the electrode gap decreases (from 20 to 2mm). Measurements are in good agreement when compared to Drude-Lorentz simulations that include the interdigitated capacitance and gap size. The simulations highlight that as the electrode spacing is reduced, faster space-charge screening of the bias field occurs, leading to the higher frequency emission. As a result, the frequency response of an interdigitated PCA can be easily designed with an appropriate electrode spacing geometry.Introduction: The need for efficient THz sources has become a major research interest since the potential applications for this type of radiation are wide ranging, from materials studies to life sciences [1]. Emitters such as photoconductive antennas (PCAs) are commonly used to generate a large spectral bandwidth that is useful for spectroscopy studies in the THz range using TDS based techniques [1]. PCA emission is based on the ultra-fast acceleration and deceleration of optically generated carriers under an applied bias field across the antenna gap. The fast deceleration is attributed to space-charge screening of the bias field by the carrier depletion [2, 3]. An interesting variant of PCAs is that based on an interdigitated geometry. This structure was demonstrated by Dreyhaupt et al. and is now known to be an efficient way to generate THz pulses [4]. (Work on interdigitated antennas in CW mode has also been reported [5].) Interdigitated PCAs provide a strong emitted electric field, little diffraction of the THz beam owing to a large illumination area ( 500 mm diameter) and, in addition, a low bias is needed owing to the possibility of a small electrode spacing. It has been shown previously that the geometry of the antenna structure, coupled with the excitation density and relative position from the anode, can be used to enhance the emitted power of these structures [6][7][8]. However, only simple geometries have been investigated (dipole, bowtie, etc.), and a study of the geometry of an interdigitated antenna has not been previously reported. Indeed, unlike simple antennas, the interdigitated geometry allows the total surface excited to be the same even with different gap spacing. Therefore, the effect of only the gap geometry on the generated THz pulse can be studied and decoupled from the (constant) power excitation density. In this Letter, we study the relation between the gap width of interdigitated PCAs (between electrodes) and the emitted THz spectrum and how the latter can be shifted to higher frequencies by reducing the gap separation.