During his Japan sojourn Dey stayed with Taikan Yokoyama, the Japanese artist, who encouraged the younger Indian artist to make a career in art. The letters and diary entries recorded by Dey provide a glimpse into his daily activities, routines and invitations at various Japanese households. They tell us about the artists Dey lived with or met during his travels.

In a letter he wrote to Gaganendranath Tagore on 8 June, 1916, for instance, he wrote:
‘This afternoon we lunched at Mr. Katsuta’s house. It felt good to see so many of your and chhotobabu’s (Abanindranath) pictures hanging at their house. Mr. Katsuta is a wonderful man… he’s planning to visit Calcutta soon. I’m staying at Taikan’s house here. We are all doing very well and love the place so far. It would have been great if you had come as well. Gurudev (Rabindranath) will soon go to Okakura’s Summer House. I’ll probably have to stay back here. I’m taking lessons here as well.'

As a historian recounted, ‘Arai Kampo had to stay at Ajanta for some days to copy the frescos. During Kampo’s stay in Ajanta in July 1917, Mukul Dey visited Ajanta to heal the sorrow of losing his father. They unexpectedly met each other at Ajanta. Arai Kampo wrote about this meeting with Mukul Dey in the following manner, ‘… screaming Arai san, Arai san, and suddenly one person jumped in front of me, the person was Mukul Dey.’ During this visit to Ajanta Mukul Dey experienced the beauty of nature, flowers, and waterfall at Ajanta which opened up a new door of art to him. He lived with Arai san at Ajanta and helped him in his endeavour as much as possible. In Ajanta's first cave, he was touched by the method of Arai san when copying the ‘temptation of Buddha’. Mukul Dey was moved by Arai san’s hardworking spirit day and night. Influenced by Arai Kampo, Mukul Dey too decided to copy Ajanta's fresco murals. After a few days, Mukul Dey left Ajanta and Arai Kampo also returned to Japan.’ Dey would return in 1919 and spend a year and a half making copies of the Ajanta murals.

Hiroshi's artistic style was heavily influenced by Western techniques, which he incorporated into the traditional Japanese woodblock printing process. His mastery of the medium allowed him to create prints of remarkable detail and depth, capturing the beauty and essence of his subjects with precision and elegance. Dorothy Blair calls him a ‘progressive experimenter’, and shows how he sought to create a synthesis between Japanese and western artistic techniques: ‘He studied western methods; he traveled extensively in foreign countries; and he took from the west what seemed to him good for his own creative effort. Certainly his own wood-block prints profited in depth without losing the qualities of charm inherent in the wood-block process and without loss of the significant spirit of Japanese art.’

Hiroshi's Indian prints depict a wide range of subjects, from majestic Himalayan peaks to bustling city streets and serene rural landscapes. He had a keen eye for capturing the intricate architectural details of India's temples, palaces, and monuments, as well as the everyday life of its people. Through his prints, Hiroshi conveyed a sense of reverence for the spiritual and cultural heritage of India, portraying its landscapes and landmarks with a sense of awe and admiration.

One of Hiroshi's most famous Indian prints is Benares, depicting the sacred city (now Varanasi) on the banks of the Ganges River. In this print, Hiroshi masterfully captures a regular scene at its famous ghats. The play of light and shadow adds depth and atmosphere to the scene, imbuing it with a sense of mystery and spirituality.

Another notable work is Taj Mahal Under the Moonlight, showcasing the iconic mausoleum in Agra, India. Hiroshi's rendition of the Taj Mahal is both majestic and intimate, conveying the monument's grandeur while also capturing its exquisite architectural details and serene beauty, under the romantic glow of moonlight.