White-light Pump Research Articles

Rapid scan white light pump–probe spectroscopy with 100 kHz shot-to-shot detection

We demonstrate a femtosecond pump–probe spectrometer that utilizes a white light supercontinuum as input and relies on mutual synchronization of the laser repetition rate, acousto-optical chopper, pump–probe delay stage, and the CCD camera to record shot-to-shot pump–probe spectra while the pump–probe delay is scanned synchronously with the laser repetition rate. The unique combination of technologies implemented here allows for electronically controllable and repetition-rate scalable detection throughput that is only limited by the camera frame rate. Despite high probe RMS fluctuations due to sample scatter (from ∼1.8% with solvent to 7.9% with sample scatter), a combination of fast and slow averaging with a fine sampling of pump–probe delay leads to reduction of RMS noise without multichannel referencing down to ∼0.4 mOD for a scattering nanotube sample. Throughput and limitations of the rapid versus stepwise scanning approaches are analyzed. Experimental comparison with stepwise scan shows ∼1.9x noise reduction in a significantly faster experiment, suggesting an additional suppression of 1/f noise enabled by rapid scan data collection. The particular combination of technologies implemented here makes our approach especially suitable for high throughput impulsive pump–probe micro-spectroscopy of highly scattering samples, without added cost and complexity of light sources, multichannel detection, or long sample exposure.

Rod-Type Ce/Cr/Nd : YAG Ceramic Lasers with White-Light Pump Source

Ceramic is promising for use as a solid-laser material pumped with solar or lamp light. We developed a Cr3+ ion doped Nd : YAG ceramic laser that converts white light into near-infrared laser light more efficiently. Investigation of its optical properties has revealed that large gain can be realized with excitation power that is one order of magnitude less than that in the case of Nd : YAG. Ce3+ ion doping also makes it possible to utilize the excitation light components with wavelengths of 350 nm or less, preventing generation of color centers. A rod-type Ce3+/Cr3+/Nd : YAG ceramic pumped by white light such as solar light or flash lamp light was developed. Fluorescence lifetime of ceramic was measured. Laser oscillations at free running mode were observed. Also, numerical calculation for output laser power and gain at lasing threshold was performed. Fluorescence lifetime increased as temperature rose, which was observed in Cr/Nd : YAG ceramic. This increase suggests the existence of a cross-relaxation effect. Maximum output laser energy of 73 mJ with the peak power of 330 W was obtained. Obtained output laser energy was around twice more than that in case of Cr3+/Nd : YAG ceramic with the same Nd and Cr ion concentration.

Amplification properties of KW Nd/Cr:YAG ceramic multi-stage active-mirror laser using white-light pump source at high temperatures

An output peak power of over 1kW for a Nd/Cr3+:YAG ceramic multi-amplifier system with white-light pumping was obtained. A four-stage active-mirror amplifier system was adapted for obtaining kW laser peak power. Output spectrum of pumping light used was close to that of solar light. Enhancement of saturated output laser power at high temperatures was observed owing to the phonon-assisted cross-relaxation effect. An output laser peak power of 1.6kW were achieved at a ceramic disk temperature of 100°C. The evaluated optical-optical conversion efficiency for volume of ceramics through which laser passed was 63% for the maximum output laser peak power of 1.6kW. The laser system will be applicable to solar-pumped lasers with high optical-optical conversion efficiency.

Time-resolved ARPES with sub-15 fs temporal and near Fourier-limited spectral resolution.

An experimental setup for time- and angle-resolved photoelectron spectroscopy with sub-15 fs temporal resolution is presented. A hollow-fiber compressor is used for the generation of 6.5 fs white light pump pulses, and a high-harmonic-generation source delivers 11 fs probe pulses at a photon energy of 22.1 eV. A value of 13 fs full width at half-maximum of the pump-probe cross correlation signal is determined by analyzing a photoemission intensity transient probing a near-infrared interband transition in 1T-TiSe2. Notably, the energy resolution of the setup conforms to typical values reported in conventional time-resolved photoemission studies using high harmonics, and an ultimate resolution of 170 meV is feasible.

Suppressing spontaneous polarization of p-GaN by graphene oxide passivation: augmented light output of GaN UV-LED.

GaN-based ultraviolet (UV) LEDs are widely used in numerous applications, including white light pump sources and high-density optical data storage. However, one notorious issue is low hole injection rate in p-type transport layer due to poorly activated holes and spontaneous polarization, giving rise to insufficient light emission efficiency. Therefore, improving hole injection rate is a key step towards high performance UV-LEDs. Here, we report a new method of suppressing spontaneous polarization in p-type region to augment light output of UV-LEDs. This was achieved by simply passivating graphene oxide (GO) on top of the fully fabricated LED. The dipole layer formed by the passivated GO enhanced hole injection rate by suppressing spontaneous polarization in p-type region. The homogeneity of electroluminescence intensity in active layers was improved due to band filling effect. As a consequence, the light output was enhanced by 60% in linear current region. Our simple approach of suppressing spontaneous polarization of p-GaN using GO passivation disrupts the current state of the art technology and will be useful for high-efficiency UV-LED technology.

Development of a multiplex stimulated Raman microscope for spectral imaging through multi-channel lock-in detection

We report the development of a multiplex stimulated Raman microscope for spectral imaging through multi-channel lock-in detection with a single light source. A white pump beam is prepared with a piece of photonic crystal fiber (PCF). The system does not require the synchronization of plural light sources or the scanning of their wavelengths, and thus a jitter-free pair of pump and Stokes beams is obtained, and a high degree of temporal synchronization is attained in the spectra. The multi-channel lock-in detection (extended to 128 channels) enables the observation of pseudo-continuous stimulated Raman spectra, demonstrating the strong ability of qualitative analysis to identify various types of C-H stretching modes such as the symmetric and asymmetric modes of the methylene∕methyl and aromatic groups. Images of a mixed film of polystyrene and polymethylmethacrylate are presented to demonstrate the system's spectral imaging ability. The spatial distribution of these materials is successfully captured through one-time imaging, although the noise of the white light pump beam generated with the PCF limits the system's imaging speed.

Cross-relaxation and spectral broadening of gain for Nd/Cr:YAG ceramic lasers with white-light pump source under high-temperature operation

Spectral broadening of the fluorescence of a Nd/Cr:YAG ceramic at the 1064 nm lasing wavelength was observed, and the amplification properties at a high temperature were investigated by considering cross-relaxation. These ceramics are promising for use as a solid-laser material pumped with solar or lamp light. It has been found that whenever the temperature of a laser medium is high and a spectral shift occurs, a high small-signal gain remains owing to the broad spectral band and the cross-relaxation. This optical property is remarkably different from that of a Nd:YAG laser. For a conventional Nd:YAG laser, the bandwidth at 1064 nm is 0.45 nm, and a reduction in small-signal gain occurs at a temperature of 373 K because the spectral peak shift is 0.005 nm/K. However, for the Nd/Cr:YAG ceramic, the bandwidths are 1.2 and 1.9 nm in the case of 0.1% Cr ion doping and 3% Cr ion doping, respectively, owing to the existence of excited Cr ions and the shortening of the effective Nd ion coherence time. It is prospected that the laser medium can be used at a high temperature of 600 K.

Measurement of White Light Chirp with Kerr Lens

The instantaneous transient Kerr lens in a transparent material offers a simple and efficient method to measure the spectrogram of a white light continuum. Even if the kernel of the spectrogram is not known very well, the instantaneous frequency may be extracted with high accuracy. It is shown that the knowledge of the phase modulation of a white light continuum simplifies the design of the prism compressors in a NOPA system. It also makes cross-correlation measurements at different frequencies in white light pump probe experiments obsolete.

Photoreflectance spectroscopy with white light pump beam

Using a dual chopping scheme where both the pump beam and the probe beam are chopped, we show that it is possible to perform photoreflectance spectroscopy with a broadband white light source as the pump beam instead of a laser source. We show that although the signal strength is reduced by a factor of 1/π in the dual chopping scheme, it nevertheless has several advantages. A white light pump beam provides a wide range of excitation energy values, enabling one to characterize semiconductors of different band gaps spread over a wide energy range with a single modulation source. In the case of semiconductor heterostructures, simultaneous as well as selective excitation of different interfaces can be achieved easily. This technique is not plagued by the problem of background due to luminescence often encountered in low-temperature photoreflectance measurements. In this article, we present the details of this technique and demonstrate its usefulness by applying it to selected semiconductor heterostructure samples.